CONCRETE WORK -1



CONCRETE WORK

4.0 The concrete can be designed in grades denoting by volumetric proportion of the constituents’ characteristic compressive strength. The concrete by volumetric proportion or nominal mix concrete of the constituents as well as Design Mix denoting compressive strength as detailed in this section.

4.1. Materials.:

Water, cement, lime, fine aggregate or sand, surkhi, cinder and fly ash shall be as specified in Section 0.

Coarse aggregate

4.1.2.1. General - Aggregate most of which is retained on 4.75 mm IS Sieve and contains only as much fine material as is permitted in IS 383 for various sizes and grading is known as coarse aggregate. Coarse aggregate shall be specified as stone aggregate, gravel or brick aggregate and it shall be obtained from approved / authorised sources

a)  Stone aggregate -It shall consist of naturally occurring (uncrushed, crushed or broken) stones. It shall be hard, strong, dense, durable and clean. It shall be free from veins, adherent coating, and injurious amounts of disintegrated pieces, alkali, vegetable matter and other deleterious substances. It shall be roughly cubical in shape. Flaky and elongated pieces shall be avoided. It shall conform to IS:  383 unless otherwise specified.

b) Gravel - It shall consists of naturally occurring (uncrushed, crushed or broken ) river bed shingle or pit gravel. It shall be sound, hard and clean. It shall be free from flat particles of shale or similar laminated material, powdered clay, silt, and loam adherent coating, alkali vegetable, matter and other deleterious substances. Pit gravel shall be washed if it contains soil materials adhering to it. These shall soil materials soil materials adhering to it. These shall conform to IS: 383 unless otherwise specified.

c) Brick aggregate - Brick aggregate shall be obtained by breaking well burnt or over burnt dense bricks / brick bats. They shall be homogenous in texture, roughly cubical in shape and clean. They shall be free from unburnt clay particles. Soluble salt, silt, adherent coating of soil vegetable matter and other deleterious substances. Such aggregate should not contain more than one percent of sulphate and should not absorb more than 10% of their own mass of water, when used in cement concrete and 20% when used in lime concrete. It shall conform to IS: 383 unless otherwise specified.

d) Lightweight aggregates such as sintered fly ash aggregate may also be used provided the engineer is satisfied with the data on the proportion of concrete made with them.

4.1.2.2. Deleterious material - Course aggregate shall not contain any deleterious material, such as pyrites, coal, lignite, shale or similar laminates material, clay, alkali, soft fragments, sea shells and organic impurities in such quantity as to affect the strength or durability of the concrete. Coarse aggregate to be used for reinforced cement concrete shall not contain any material liable to the steel reinforcement. Aggregates which are chemically reactive with alkali of cement shall not be used. The maximum quantity of deleterious material shall not more than five per cent of the weight of coarse aggregate when determined in accordance with IS: 2386 part II.

4.1.2.3. Size and grading

(i)  Stone aggregate and gravel - It shall be either graded or single sized as specified. Normal size and grading shall be as under  --(a)Nominal sizes of graded stone aggregate or gravel shall be 40, 20, 16, or 12.5 mm as specified. For any one of the nominal sizes, the proportion of other sizes shall be in accordance with Table 1.

Table 1 -Graded stone aggregate or gravel

IS Sieve Designation

Percentage passing (by weight) for nominal size of

40 mm

20 mm

16 mm

12.5 mm

75 mm

100

-

-

-

37.5 mm

95 to 100

100

-

-

19 mm

-

95 to 100

100

100

16 mm

-

-

90 to 100

-

11.2 mm

-

-

-

90 to 100

9.5 mm

10 to 35

25 to 55

30 to 70

40 to 85

4.75 mm

0 to 5

0 to 10

0 to 10

0 to 10

2.36 mm

-

-

-

-

Concrete work 

(b). Normal sizes of single sized stone aggregate or gravel shall be 63, 40, 20, 16, 12.5 or 10 mm as specified. For any one of the nominal sizes the proportion of other sizes shall be in accordance with Table 2.

Table 2 -Single sized (ungraded) stone aggregate or gravel

IS Sieve Designation

Percentage passing (by weight) for nominal size of

63 mm

40 mm

20 mm

16 mm

12.5 mm

10 mm

75 mm

100

-

-

-

-

-

63 mm

85-100

100

-

-

-

-

37.5 mm

0-30

85-100

100

-

-

-

19 mm

0-5

-20

85-100

100

-

-

16 mm

-

-

-

-85-100

100

-

11.2 mm

-

-

-

-

85-100

100

9.5

-

0-5

0-20

0-30

0-45

85-

100

 

 

 

 

 

 

4.75 mm

-

-

0-5

0-5

0-10

0-20

2.36 mm

-

-

-

-

-

0-5

c). When stone aggregate or gravel brought to site is single sized (ungraded), it shall be mixed with single sizes aggregate of different sizes in the proportion to be determined by field tests to obtain graded aggregate of specified nominal size. For the required nominal size, the proportion of other sizes in mixed aggregate shall be in accordance with Table 1. Recommended proportions by volume for mixing of different sizes of single size (ungraded) aggregate to obtain the required nominal size of graded aggregate are given in Table 3.

Table 3 -Single sized (ungraded) stone aggregate or gravel

Cement

Concrete

Nominal size of graded aggregate required

Parts of single size aggregate of size

50 mm

40 mm

20 mm

12.5 mm

10 mm

1: 6:12

63

9

-

3

-

-

1: 6: 12

40

-

9

3

-

-

1: 5: 10

63

7 ½

-

2 ½

-

-

1: 5: 10

40

-

7 ½

2 ½

-

-

1: 4: 8

63

6

-

2

-

-

1: 4: 8

40

-

6

2

-

-

1: 3: 6

63

4 ½

-

1 ½

-

-

1: 3: 6

40

-

4 ½

1 ½

-

-

1: 3:6

20

-

-

4 ½

-

-

1: 2: 4

40

-

2 ½

1

-

½

1: 2: 4

20

-

-

3

-

1

1: 2: 4

12.5

-

-

-

3

-

1: 1 ½ : 3

20

-

-

2

-

1

Note-(i) The proportions indicated in Table 3 above are by volume when considered necessary, these proportions may be varied marginally by engineer after making sieve analysis of aggregate brought to site for obtaining required graded aggregate. No adjustments in rate shall be made for any variation in the proportions so ordered by the engineer. If single size coarse aggregates are not premixed at site to obtain the graded coarse aggregate required for mix, the volume of single size aggregates required for the mix shall be suitably increased to account for reduction in total volume at the site of mixing.

(ii) Brick aggregate - Nominal size of brick aggregate shall be40 mm and its grading shall be as specified in the Table 4 when tested for sieve.

Table 4 -Brick aggregate

IS Sieve Designation(by weight)

Percentage passing

75 mm

100

37.5 mm

95-100

19.0 mm

45-100

4.75

0-5

Note -Coarse aggregate for cement concrete shall generally conform to para 4.2.1 of IS: 456 and fine aggregate shall conform to IS: 383.

4.1.2.4. Stacking - Aggregate shall be stacked on a hard, dry and level patch of ground. When stack piling, the aggregate shall not form pyramids resulting in segregation of different sized materials. It shall be stacked separately according to nominal size of coarse aggregates. Stacking shall be done in regular stacks, of height not exceeding 100 cm.

4.1.2.5.  Testing - Coarse aggregate shall be tested for the following (as per IS: 2386 )

  1. Determination of particle size and shape
  2. Estimation of organic impurities (as per IS: 2386-Part II )
  3. Surface moisture
  4. Determination of 10% fine value

Measurements - The aggregates shall be measured in stacks and paid for after making a deduction of 7.5% of the gross measurements of stacks in respect of aggregates of nominal size 40 mm and above. No deduction from the gross measurements of the stacks is to be made in respect of aggregates nominal size below 40 mm.

Admixtures - When required, admixtures of approved quality shall be mixed with concrete, as specified. The admixtures shall conform to IS: 9103.

4.2. SPECIFICATIONS FOR CEMENT CONCRETE

4.2.0.  This shall be prepared by mixing graded stone or brick aggregate of nominal size as specified with fine aggregate and cement in specified proportions with required quantity of water. The grading and quality of aggregates shall be such as to give minimum compressive strength of 140 kg/cm² and 210 kg / cm² at 7 days and 28 days respectively in case of mix 1:2:4, (One cement - two Coarse sand - four stone aggregate). One sample consisting of 6 cubes 15x15x15 cm shall be taken for every 15 cubic meter or part thereof cement concrete 1:2:4. The cube tests shall not be carried out in case the quantity of cement concrete placed on any day is less than 15 cubic meter unless otherwise specific. For other details, refer section on R.C.C. work.

4.2.1. Proportioning -   It shall be done by volume. Boxes of suitable size shall be used for measuring sand and aggregate. The internal dimensions of the boxes shall be generally 35 X 25 X40 cm deep or as otherwise approved by the engineer. The unit of measurement of cement shall be a bag of 50 kg. and this shall be taken as 0.035 cubic meter. While measuring the aggregate, shaking, ramming or heaping shall not be done. The proportioning of sand shall be on the basis of its dry volume and in case of damp sand, allowances for bulk age shall be made as given for mortar.

4.2.2. Preparation - This shall be prepared by mixing coarse aggregate, fine aggregate and cement in specified proportions with required quantity of water. Nominal size and quality of aggregate shall be as specified. Except where brick aggregate is used in cement concrete, minimum compressive strength on works test for different concrete mixes shall be as specified for various grades prepared by volume basis, in Table 5 below. The work test shall be carried out for every 15 cum of a day’s concreting unless otherwise specified. 

Table 5

Concrete mix Min compressive strength on15 cm cube in  Kg / cm²
7 days strength 28 days strength

1:1:2

210

315

1:1½ :3

 

265

1:2:4

140

175

4.2.2.1. Mixing - Concrete shall be mixed in mechanical batch type concrete mixers conforming to IS: 1791 having two blades and fitted with power loader (lifting hopper type). Half bag mixers and mixers without lifting hoppers shall not be used for mixing concrete. In exceptional circumstances, such as mechanical break down of mixer, work in remote areas or power breakdown and when the quantity of concrete work is very small, hand mixing may be done with the specific prior permission of the engineer in writing subject to adding 10% extra cement. When hand mixing is permitted, it shall be carried out on a watertight platform and care shall be taken to ensure that mixing is continued until the concrete is uniform in colour and consistency. Before mixing the brick aggregate shall be well soaked with water for a minimum period of two hours and stone aggregate or gravel shall be washed with water to remove, dirt, dust and other foreign materials. For guidance, the mixing time may be 1½ to 2 minutes, for hydrophobic cement it may be taken as 2½ to 3 minutes.

4.2.2.2. Power loader - Mixer will be fitted with a power loader complying with the following requirements.

a). The hopper shall be of adequate capacity to receive and discharge the maximum nominal batch of unmixed materials without spillage under normal operating conditions on a level site. Note -  In such a case the volume of the maximum nominal batch of mixed material is 50% greater than the nominal mixed batch capacity.

b). The minimum inside width of the feeding edge of the hopper shall be as specified below in Table 6.

Table 6

Nominal size of mixer (T, NT or R), litre

Minimum inside width of hopper feeding edge in mm

140

1.0

200

1.1

280

1.2

375

1.4

500

1.5

1000

2.0

T = tilting; NT = non-tilting; R = Reverse

The design of the loader shall be such that it allows the loading hopper to be elevated to such a height that the center line of the chute plate of the hopper when in discharge position, is at an angle of not less than 50º to the horizontal. A mechanical device to aid discharge of the contents as quickly as possible from the hopper to the drum may also be provided. Even when a mechanical device is provided, it is recommended that the angle of center line of the chute plate of the hopper when in discharge position, should be as large as practicable, preferably not less than 40º to horizontal.

When the means of raising and lowering the loading hopper includes flexible wire ropes winding on to a drum or drums, the method of fastening the wire to rope to the drums shall be such as to avoid, as far as possible any tendency to cut the strands of the ropes and the fastening should preferably be positioned clear of the barrel of the drum for example, outside the drums flange. When the loading hopper is lowered to its normal loading position, there should be at least one and half drums of rope on the drum.

Clutch brake and hydraulic control lever shall be designed so as to prevent displacement by liberation or by accidental contact with any person.

The clutch and brake control arrangements shall also be so designed that the operator can control the falling speed of the loader.

Safety device shall be provided to secure the hopper in raised position when not in use

4.2.2.3. Mixing efficiency - The mixer shall be tested under normal working conditions in accordance with the method specified in IS - 4643 with a view to check its ability to mix the ingredients to obtain concrete having uniformity within the prescribed limits. The uniformity of mixed concrete shall be evaluated by finding the percentage variation in quantity (mass in water) of cement, fine aggregate and coarse aggregate in a freshly mixed batch of concrete. The percentage variation between the quantities of cement, fine aggregate and coarse aggregates (as found by weighing in water) in the two halves of a batch and average of the two halves of the batch shall not be more than the following limits -

Cement  - 8%

Fine aggregate  -  6%

Coarse aggregate - 5%

4.2.2.4. Machine mixing - The mixer drum shall be flushed clean with water. Measured quantity of coarse aggregate shall be placed first in the hopper. This shall be followed with measured quantity of fine aggregate and then cement. In case fine aggregate is damp, half the required quantity of coarse aggregate shall be placed in the hopper, followed by fine aggregate and cement. Finally the balance quantity of coarse aggregate shall be fed in the hopper, & then the dry materials are slipped into the drum by raising the hopper. The dry material shall be mixed for at least four turns of the drum. While the drum is rotating, water shall be added gradually to achieve the water cement ratio as specified or as required by the engineer. After adding water, the mixing shall be continued until concrete of uniform colour, uniformly distributed material and consistency is obtained. Mixing shall be done for at least two minutes after adding water. If there is segregation after unloading from the mixer, the concrete should be remixed. The drum shall be emptied before recharging. When the mixer is closed down for the day or at any time exceeding 20 minutes, the drum shall be flushed clean with water.

4.2.2.5 Hand mixing - When hand mixing has been specifically permitted in exceptional circumstances by the engineer in writing, subject to adding 10% extra cement, it shall be carried out on a smooth, clean and water tight platform of suitable size. Measured quantity of sand shall be spread evenly on the platform and the cement shall be dumped on the sand and distributed evenly. Sand and cement shall be mixed intimately with spade until mixture is of even colour throughout. Measured quantity of coarse aggregate shall be spread on top of cement sand mixture and mixing done by shoveling and turning till the coarse aggregate gets evenly distributed in the cement sand mixture. Three quarter of the total quantity of water required shall be added in a hollow made in the middle of the mixed pile and the material is turned towards the middle of pile with spade. The whole mixture it turned slowly over and again and the remaining quantity of water is added gradually. The mixing shall be continued until concrete of uniform colour and consistency is obtained. The mixing platform shall be washed and cleaned at the end of the day.

4.2.3.  Workability - The quantity of water to be used for each mix shall be such that the concrete is of adequate workability for the placing conditions of the concrete and can properly be compacted with the means specified. Generally, the quantity of water to be used for each mix of 50 Kgs cement shall not be more than 34 litres for 1:3:6 mix, 30 litres for 1:2:4 mix, 30 litres for 1:1½:3 mix and 25 litres for 1:1:2 mix. In case of vibrated concrete, the quantity of water may be suitably reduced to avoid segregation. The quantity of water shall be regulated by carrying out regular slump tests as described in Annexure 4.A.1. The slump and workability for different kind of works shall be as per Table 7 below

Table 7

Placing conditions.

Degree of workability

Value of workability

Concreting of shallow Sections with vibration

Very low

0.75-0.80 Compacting factor.

Concreting of lightly reinforced section with vibration.

Low

Slump up to 25 mm, 10-5 Seconds, vee bee time 0.8-0.85 compacting factor.

Concreting of lightly reinforced Section without vibration or heavily reinforced sections with vibration.

Medium

25-75 mm, slump for 20 mm aggregate.

Concreting of heavily reinforced sections without vibration.

High

75-125 mm slump for 20 mm aggregate.

Note - Where considered necessary, the workability of the concrete my also be ascertained by compacting factor test and vee-bee censistometer method as specified in IS: 1199. For suggested ranges of value of workability of concrete by the above methods, reference may be made to IS: 456-2000.

4.2.4.  Transportation - Concrete shall be transported from the mixer to the place of laying as rapidly as possible by methods which will prevent the segregation or loss of any of the ingredients and maintaining the required workability.

4.2.5. Placing - The concrete shall be deposited as nearly as practicable in its final position to avoid rehandling. It shall be laid gently (not thrown) and shall be thoroughly vibrated and compacted before setting commences and should not be subsequently disturbed. Method of placing shall be such as to preclude segregation. Care shall be taken to avoid displacement of reinforcement or movement of form work and damage due to rains.

4.2.6. Compaction - Concrete shall be thoroughly compacted and fully worked around embedded fixtures and into corners of the form work. Compaction shall be done by mechanical vibrator of appropriate type till a dense concrete is obtained. The mechanical vibrators shall conform to IS: 2505 specifications for concrete vibrators (immersion type). To prevent segregation, over vibration shall be avoided. The use of mechanical vibrator may be relaxed by the engineer at his discretion for certain items and permit hand compaction.  Hand compaction shall be done with the help of tamping rods. Compaction shall be completed before the initial setting starts. For the items where mechanical vibrators are not to be used, the contractor shall take permission of the engineer in writing before the start of the work. After compaction the top surface shall be finished even and smooth with wooden trowel before the concrete begins to set.

4.2.7.  Construction joints - Connecting shall be carried out continuously up to construction joints. The position and arrangement of construction joints shall be as shown in the structural drawings or as directed by the engineer. Number of such joints shall be kept minimum and shall be kept as straight as possible.

4.2.7.1. When the work has to be resumed on a surface which has hardened, such surface shall be roughened. It shall then be swept clean and thoroughly wetted. For vertical joints, neat cement slurry, of workable consistency by using 2kgs of cement per sq m shall be applied on the surface before it is dry.  For horizontal joints, the surface shall be covered with a layer of mortar about 10-15 mm thick composed of cement and sand in the same ratio as the cement and sand in concrete mix. This layer of cement slurry of mortar shall be freshly mixed and applied immediately before placing of the concrete

4.2.7.2. Where the concrete has not fully hardened, all laitance shall be removed by scrubbing the wet surface with wire or bristle brushes, care being taken to avoid dislodgement of particles of coarse aggregate. The surface shall be thoroughly wetted and all free water removed. The surface shall then be coated with neat cement slurry @ 2 kgs of cement per sqm. On this surface, a layer of concrete not exceeding 150 mm in thickness shall first be placed and shall be well rammed against corners and close spots; work, thereafter, shall proceed in the normal way.

4.2.8.  Concreting under special conditions

4.2.8.1 Work in extreme weather conditions - During hot and cold weather, the concreting shall be done as per the procedure set out in IS: 7861(Part-I) and IS: 7861(Part II) respectively. Concreting shall not be done when the temperature falls below 4.5º C. In cold weather, the concrete placed shall be protected against frost. During hot weather, it shall be ensured that the temperature of wet concrete does not exceed 38ºC.

Under water concreting - Concrete shall not be deposited under water if it is practicable to de-water the area and place concrete in the regular manner. The concrete shall contain at least 10% more cement than that required for the same mix placed in dry conditions, the quantity of extra cement varying with conditions of placing with prior written permission of the engineer. Such extra cement will be paid extra. The volume of coarse aggregate shall not be less than 1½ times nor more than twice the fine aggregate and slump not less than 100 mm nor more than 180 mm. Where found necessary to deposit any concrete under water, the method, equipment, materials and mix shall first be got approved by the engineer. Concrete shall be deposited continuously until it is brought to required height. While depositing, the top surface shall be kept as nearly level as possible and the formation of heaps shall be avoided. The concrete shall be deposited under water by one of the approved methods such as Tremie method, drop bottom bucket, bags, grouting etc. as per details given in IS: 456-2000. If it is necessary to raise the water after placing the concrete, the level shall be brought up slowly without creating any waves or commotion tending to wash away cement or to disturb the fresh concrete in any way

4.2.9.  Curing - When the concrete begins to harden i.e. two to three hours after compaction, the exposed surfaces shall be kept damp with moist gunny bags, sand or any other material approved by the engineer 24 hours after compaction, the exposed surface shall be kept continuously in damp or wet conditions by ponding or by covering with a layer of sacking, canvass, Hessian or similar absorbent materials and kept constantly wet for at least 7 days where ordinary Portland cement is used and 10 days, where Portland pozzolana cement is used from the date of placing of concrete. For concrete work with other types of cement, curing period shall be as directed by the engineer.

Approved curing compounds may be used in lieu of moist curing with the permission of the engineer. Such compounds shall be applied to all exposed surfaces of the concrete as soon as possible after the concrete has set

4.2.9.1 Freshly laid concrete shall be protected from rain by suitable covering.

4.2.9.2 Over the foundation concrete, the masonry work may be started after 48 hours of its compaction but the curing of exposed surfaces of cement concrete shall be continued along with the masonry work for at least 7 days. And where cement concrete is used as base concrete for flooring, the flooring may be commenced before the curing of period of base concrete is over but the curing of base concrete shall be continued along with top layer of flooring for a minimum period of 7 days. 

4.2.10.Testing of concrete will be done as described in section on R.C.C

4.2.11. Form work - Form work shall be as specified in R.C.C section and shall be paid for separately unless otherwise specified.

4.2.12.Finishes -  Plastering and special finishes other than those, obtained through form work shall be specified and paid for separately unless otherwise specified.   

4.2.13.Measurements

4.2.13.1. Dimensions of length, breadth and thickness shall be measured correct to nearest cm. Except for the thickness of slab and partition which shall be measured to nearest 5 mm. Area shall be worked out to nearest 0.01 square meter and the cubic contents of consolidated concrete shall be worked out nearest 0.001 cubic meters. Any work done in excess over the specified dimension or as required by engineer is ignored.

4.2.13.2. Concrete work executed in the following conditions shall be measured separately

  1. At or near the ground level
  2. Work in liquid mud
  3. Work in or under foul positions

4.2.13.3. Cast-in-situ concrete and or precast concrete work shall be measured in stages described in the item of work, such as -

  1. At or near the ground level
  2. Up to specified floor level
  3. Between two specified floor levels
  4. Up to specified height above or depth below plinth level/ defined datum level
  5. Between two specified heights or depths with reference to plinth level / defined datum level

4.2.13.4. No deduction shall be made for the following -

a. Ends of dissimilar materials for example beams, girders, rafters, purlins trusses corbels and steps up to 500sq. cm in cross sections.

b. Opening up to 0.1sq meter (1000sq.cm).

c. Volume occupied by pipes, conduits, sheathing etc. not exceeding 100sq cm each in cross sectional areas.

d. Small voids such as shaded portions in Figure when these do not exceed 40sq cm each in cross section.

Note - In calculating area of opening, the thickness of any separate lintel or still shall be included in the height. Nothing extra shall be payable for forming such openings or voids.

4.2.13.5. Cast-in-situ concrete shall be classified and measured as follows -

  1. Foundation, footings, bases for columns
  2. Walls (any thickness) including attached pilasters, buttresses, plinth and string courses, fillets etc.
  3. Shelves
  4. Slabs
  5. Chajjas including portions bearing on the wall
  6. Lintels, beams and Bressemmers
  7. Columns, piers abutments, pillars, post and struts
  8. Stair case including stringer beams but excluding landings.
  9. Balustrades, newels and sailing
  10. Spiral staircase (including landing)
  11. Arches
  12. Domes, vaults
  13. Shell roof, arch ribs and folded plates
  14. Chimneys and shaft.
  15. Breast walls, retaining, walls, return walls
  16. Concrete filling to precast components
  17. Kerbs, steps and the like
  18. String or lacing courses, parapets, copings, bed block, anchor blocks, plain window sills and the like
  19. Cornices and moulded windows sills.
  20. Louvers, fins, fascia.

4.2.13.6. Precast cement concrete solid articles shall be measured separately and shall include muse of moulds, finishing the top surfaces even and smooth with wooden trowel, before setting in position in cement mortar 1:2 (1 cement -2 coarse sand). Plain and moulded work shall be measured separately and the work shall be classified and measured as under  -

Classification

Method of measurement

 a.   Wall panels In square meters stating the thickness

In square meters stating the thickness

 b.   String or lacing courses, coping, bed plats, plain windows sills, shelves, louvers,  steps etc.

 In cubic meters

 c.   Kerbs, edgings etc. In cubic meters

In cubic meters

 d.  Solid block work                                         

In square meters stating the thickness or in cubic meters.                                                        

 e.  Hollow block work                                      

In square meters stating the thickness or in cubic meters.

 f.  Light weight Partitions                                

In square meters stating the thickness or in cubic meters.                                                  

4.2.14. Rate - The rate is inclusive of the cost of labour and materials involved in all the operations described above.                  

4.3. SPECIFICATIONS FOR LIME CONCRETE

4.3.0.   This shall be prepared by mixing coarse aggregate and lime mortar in the specified proportions with required quantity of water. Lime shall conform to IS: 712 and shall be classified as below - 

Class A -Eminently hydraulic lime used for structural purposes.

Class B - Semi hydraulic lime used for lime concrete.

4.3.1.  Proportioning - The proportions of aggregate to lime mortar shall be done by volume. Generally lime is prepared by mixing 100 parts of 40 mm nominal size graded stone aggregate, gravel or brick aggregate as specified and 40 parts of lime mortar of the specified mix.

4.3.2.  Mixing - Concrete shall be mixed in a mechanical mixer. In exceptional circumstances specified in 4.2.2.1 hand mixing may be done with the specific permission of the engineer in writing. Before mixing, bricks aggregate shall be well soaked with water for minimum period of hours and stone aggregate or gravel shall be washed with water to remove dirt, dust or any other foreign materials.

4.3.2.1. Machine mixing - The mixture shall be flushed clean with water. Mixing shall be done by pouring measured quantity of coarse aggregate and wet ground mortar for one batch in the drum of the mixer, while it is revolving. The quantity of materials loaded in the drum shall not exceed the rated capacity of the mixer. The water shall be added gradually up to the required quantity and the wet mixing of batch shall continued for at least two minutes in the drum till concrete of uniform colour uniformly distributed materials and consistency is obtained. The consistency of the concrete shall be such that the mortar does not tend to separate from, the coarse aggregate. If there is segregation after unloading from the mixer the concrete should be removed. The entire concrete of batch shall be discharged before the materials for the new batch are poured into the drum. Before suspending the work, the mixer shall be cleaned by revolving the drum with plenty of water each time.

4.3.2.2. Hand mixing - When hand mixing is specifically permitted by engineer, it shall be done on a clean and water tight masonry platform of sufficient size to provide ample mixing space. The he specified wet lime mortar shall be laid on the top of the aggregate and turning shall be done over and again with addition of necessary quantity of water till a uniform mix of required consistency is obtained. The consistency of the concrete shall be such that the mortar shall not tend to separate from the coarse aggregate.

4.3.2.3. Placing compaction - Concrete shall be laid (and not thrown) in layers while it is quite fresh. Each layer and shall be thoroughly rammed and consolidated before the succeeding layer is placed. Consolidated thickness of each layer shall not exceed 15 cm. joints where necessary shall be staggered in different layers unless otherwise specified. Ramming shall be done by heavy iron rammers of 4.5 kg to 5.5 kg. The area of each rammer shall not be more than 300 sq. cm Ramming shall be continued till a skin of mortar covers the surface completely. Concrete laid on a particular day shall be consolidated thoroughly on the same day before the work is stopped. Ramming on the following day shall not be done. Freshly laid concrete shall be protected from rains by suitable coverings.

4.3.4   Curing -  After the concrete has begun to harden i.e. about 24 hours after its placing and compaction, the curing shall be done by keeping the concrete damp with moist gunny bags, sand, or any other method approved by the engineer for a minimum period of 7 days. Till then, masonry and flooring work over the foundation or base concrete shall not be started.

4.3.5  Measurements 

4.3.5.1 Length, breadth and depth or thickness shall be measured correct to a cm and the consolidated cubic contents of the concrete shall be calculated net to the nearest 0.01 cubic meter. Concrete laid in excess of the dimensions shown in the drawings shall not be measured.

4.3.5.2 Deductions shall be as specified in 4.2.13.4.

4.3.6.  Rate - The rate shall include the cost of materials and labour involved in all operation described above.

4.4  SPECIFICATIONS FOR CEMENT-FLY ASH CONCRETE

Fly Ash concrete shall be prepared by mixing graded coarse aggregate of nominal size as specified with fine aggregate, ordinary Portland cement and fly ash in specified proportions with required quantity of water. The recommended composition of cement fly ash concrete are as under  -

Table 8 Fly ash concrete mixes

Composition (Dry volume)

 

Compressive strength at seven days

Lean concrete (1:5:10)

 

 

Cement (ordinary port land)

1.0

 

Fly ash

2.5

28 kg / cm2

Sand

4.0

 

Stone aggregate

11.0

 

Lean concrete (1:4:8)

 

 

Cement (ordinary Portland)

1.0

 

Fly ash

2.0

37 kg / cm2

Sand

3.5

 

Stone aggregate

9.0

 

Note - No fly ash is to be added to pozzolana Portland cement in any case which itself contains fly ash. Proportioning shall be as specified in 4.2.1.

4.4.1.1 Mixing shall be as specified in 4.2.2 except that the fly ash shall be placed in the hopper before cement in case of machine mixing.

Placing and compaction shall be as specified in 4.2.5 and 4.2.6.

Measurements shall be as specified in 4.2.13.

Curing shall be as specified in 4.2.9.

4.4.5 Rate - Rate shall include the cost of materials and labour involved in all the operations described above.

4.5  SPECIFICATIONS FOR READY MIXED CONCRETE

4.5.1 Ready Mixed Concrete - Concrete delivered at site or into the purchaser’s vehicle in a plastic condition and requiring no further treatment before being placed in the position in which it is to set and harden.

4.5.1.1 Agitation-The process of continuing the mixing of concrete at a reduced speed during transportation to prevent segregation.

4.5.1.2 Agitator-Truck mounted equipment designed to agitate concrete during transportation to the site of delivery.

4.5.1.3 Truck Mixer-A mixer generally mounted on a self-propelled chassis, capable of mixing the ingredients of concrete and of agitating the mixed concrete during transportation.

4.5.2 Types-For the purpose of this standard, the ready-mixed concrete shall be one of the two types, according to the method of production and delivery as specified in 4.5.3.1 and 4.5.3.2.

4.5.2.1 Centrally-mixed concrete – Concrete produced by completely mixing cement, aggregates, admixtures, if any and water at a stationary central mixing plant and delivered in containers fitted with agitating devices, except that when so agreed to between the purchaser and the manufacturer, the concrete may be transported without being agitated.

4.5.2.2 Truck-mixed concrete - Concrete produced by placing cement, aggregates and admixtures, if any, other then those to be added with mixing water, in a truck mixer at the batching plant, the addition of water and admixtures to be added along with mixing water, and the mixing being carried out entirely in the truck mixer either during the journey or on arrival at the site of delivery.  No water shall be added to the aggregate and cement until the mixing of concrete commences.

4.5.3. Materials

4.5.3.1 Cement - The cement used shall be ordinary Portland cement or low heat Portland cement conforming to IS: 269-1989 or 8112-1989 or 1226:1987 or Portland slag cement conforming to IS: 455-1989 or ‘Portland-pozzolana cement conforming to IS: 1489-1991 or rapid hardening Portland cement conforming to IS: 8041-1976 as may be specified by the purchaser at the time of placing the order.  If the type is not specified, ordinary Portland cement shall be used.

Fly ash when used for partial replacement of cement, shall conform to the requirements of IS:3812 -1981

4.5.3.2. Aggregates - Unless otherwise agreed to between the purchaser and the manufacturer, the aggregates shall conform to IS: 383-1970.  Fly ash when used as fine aggregate shall conform to the requirements of IS: 3812-1981.

4.5.3.3. Water used for concrete shall conform to the requirements of IS: 456-2000.

4.5.3.4, Admixtures – Admixtures shall only be used when so agreed to between the purchaser and the manufacturer.  The admixtures shall conform to the requirements of IS: 456-2000, and their nature, quantities and methods of use shall also be specified.  Fly ash when used as an admixture for concrete shall conform to IS: 3812-1981.

4.5.3.5, Measurement and storage of materials – Measurement and storage of materials shall be done in accordance with the requirements of IS: 456-2000.

4.5.4 Basis of supply

4.5.4.1 Depending upon the agreement between the purchaser and the manufacturer, the ready-mixed concrete shall be manufactured and supplied on either of the following basis:

a) Specified strength based on 28-day compressive strength of 15-cm cubes tested in accordance with IS: 456-2000.

b) Specified mix proportion.

Note - Under special circumstances and subject to the agreement between the purchaser and the supplier, strength of concrete in (a) above may be based on 28-day or 7-day flexural strength of concrete instead of compressive strength of 15-cm cube tested in accordance with IS: 456-2000. When the concrete is manufactured and supplied on the basis of specified strength, the responsibility for the design of mix shall be that of the manufacturer and the concrete shall conform to the requirements. When the concrete is manufactured and supplied on the basis of specified mix proportion, the responsibility for the design of the mix shall be that of the purchaser and the concrete shall conform to the requirements.

4.5.4.2 Measurement of Ready-mixed concrete

The basis of purchase shall be the cubic meter of plastic concrete as delivered to the purchaser.

The volume of plastic concrete in a given batch shall be determined from the total mass of the batch divided by the actual mass per m³ of concrete. The total mass of the batch shall be calculated either as the sum of the masses of all materials, including water, entering the batch or as the net mass of concrete in the batch as delivered.  If the purchaser wishes to verify the total mass, of the batch, this shall be obtained from the gross and tare masses of the vehicle on a stamped weigh bridge.  The mass per m³ shall be determined in accordance with the method given in IS:1199-1959.

4.5.5 General requirements

4.5.5.1. In addition to the requirements specified in this standard and subject to such modifications as may be agreed to between the purchaser and the manufacturer at the time of placing order, the ready-mixed concrete shall generally comply with the requirements of IS:456-2000. Unless otherwise agreed to between the purchaser and the supplier, the minimum quantity of cement and the details regarding proportioning and works control shall be in accordance with IS:456-2000.

When a truck mixer agitator is used for mixing or transportation of concrete, no water from the truck-water system or from elsewhere shall be added after the initial introduction of the mixing water for the batch, except when on arrival at the site of work, the slump of the concrete is less than that specified; such additional water to bring the slump within required limits shall be injected into the mixer under such pressure and direction of flow that the requirements for uniformity specified in Appendix. A are met.

Unless otherwise agreed to between the purchaser and the supplier, when a truck mixer or agitator is used for transporting concrete, the concrete shall be delivered to the site of work and discharge shall be complete within 1½ hour (when the prevailing atmospheric temperature is above 20º C) and within 2 hours (when the prevailing atmospheric temperature is at or below  20º C ) of adding the mixing water to the dry mix of cement and aggregate or of adding the cement to the aggregate, whichever is earlier.

4.5.5.2 Temperature - The temperature of the concrete at the place and time of delivery shall be not less than 5º C.  Unless otherwise required by the purchaser, no concrete shall be delivered, when the site temperature is less than 2.5º C and the thermometer reading is falling. The temperature of the concrete shall not exceed 5º C above the prevailing shade temperature, when the shade temperature is over 20º C.  The temperature of concrete mass on delivery shall not exceed 40º C.

4.5.5.3. Sampling and testing  - Adequate facilities shall be provided by the manufacturer for the purchaser to inspect the materials used, the process of manufacture and the methods of delivery of concrete.  He shall also adequate facilities for the purchaser to take samples of the materials used. Unless otherwise agreed to between the purchaser and the supplier, the sampling and testing of concrete shall be done in accordance with the relevant requirements of IS: 456-2000, IS:1199-1959 and IS: 516-1959

Consistency or workability – The tests for consistency or workability shall be carried out in accordance with requirements of IS: 1199-1959 or by such other method as may be agreed to between the purchaser and the manufacturer.

4.5.5.4. Strength test – The compressive strength, and flexural strength tests shall be carried out in accordance with the requirements of IS: 516-1959 and the acceptance criteria for concrete whether supplied on the basis of specified strength or on the basis of mix proportion, shall conform to the requirements mentioned below.

Compressive strength - The concrete shall be deemed to comply with the strength requirements when both the following conditions are met:

a) The mean strength determined from any group of four consecutive test results compiles with the appropriate limits in col. 2 of Table.

b) Any individual test result complies with the appropriate limits in col.3 of Table.

Flexural strength - When both the following conditions are met, the concrete complies with the specified flexural strength.

a) The mean strength determined from any group of four consecutive test results exceeds the specified characteristic strength by at least 0.3 N/mm².

b) The strength determined from any test result is not less than the specified characteristic strength less 0.3 N/mm².

4.5.5.5. Quantity of concrete represented by strength test results - The quantity of concrete represented by a group of four consecutive test results shall include the batches from which the first and last samples were taken together with all intervening batches. For the individual test result requirements given in col.2 of Table 9 or in item (b) of 16.2 only the particular batch from which the sample was taken shall be at risk. Where the mean rate of sampling is not specified the maximum quantity of concrete that four consecutive test results represent shall be limited to 60m³ of the concrete is deemed not to comply, the structural adequacy of the parts affected shall be investigated and any consequential action as needed shall be taken Concrete of each grade shall be assessed separately.

Concrete is liable to be rejected if it is porous or hone-combed, its placing has been interrupted without providing a proper construction joint, the reinforcement has been displaced beyond the tolerances specified, or construction tolerances have not been met.  However, the hardened concrete may be accepted after carrying out suitable remedial measures to the satisfaction of the engineer-in-charge.

Table 9 Characteristic compressive strength compliance requirement

Specified Grade

Mean of Group of 4 Non-Overlapping Consecutive Test Results in N/mm².

Individual Test Results in N/mm².

(1)

(2)

(3)

M15

           +0.825 x established standard deviation (rounded off to nearest 0.5 N/mm².

N/mm².

M 20 or above

       + 3 N/mm², whichever is greater + 0.825 x established standard deviation (rounded off to nearest 0.5 N/mm² )  or +4 N/mm², whichever is greater

N/mm².

Note:- In the absence of established value of standard deviation, the value given in Table 8 of IS:456-2000 may be assumed, and attempt should be made to obtain results of 30 samples as early as possible to establish the value of standard deviation.

4.5.5.6 Cost of testing – Unless otherwise agreed to between the purchaser and the manufacturer, the cost of the tests carried out in accordance with the requirements of this specification shall be borne as follows:

a) By the manufacturer if the results show that the concrete does not comply with the requirements of this standard.

b) By the purchaser if the results show that the concrete complies with the requirements of this standard.

4.5.5.7 Manufacturer’s records and certificates – The manufacturer shall keep batch records of the quantities by mass of all the solid materials, of the total amount of water used in mixing and of the results of all tests. If required by the purchaser, the manufacturer shall furnish certificates, at agreed intervals, giving this information.

4.5.6.  Concrete manufactured and supplied on the basis of specified strength

4.5.6.1 The purchaser shall supply the following information for guidance of the manufacturer :

a) The type of cement to be used;

b) The maximum size and type of the aggregate;

c) The type of admixtures to be used;

d) The minimum acceptable compressive strength of flexural strength or both, determined from samples of plastic concrete taken at the place and time of delivery, in accordance with requirements of IS:456-2000.

e) The slump or compacting factor or both, or other requirements for consistency or workability at the place and time of delivery of the concrete;

f) The ages at which the test cubes or beams are to be tested, and the frequency and the number of tests to be made; and

g) Any other requirements.

4.5.6.2 Tolerances – Unless otherwise agreed to between the purchaser and the manufacturer, the concrete shall be deemed to comply with the requirement of these standard, if the results of tests where applicable, lie within the tolerances specified.

4.5.6.3. Consistency of workability – The slump (average of two tests) shall not differ from the specified value by ± 10 mm for a specified slump of 75mm or less and  ± 25mm when the specified slump is greater than ± 75m.  The compacting factor average of two tests shall be within 0.03 of the value specified. If any other method of determining consistency is to be used, a suitable tolerance shall be agreed to between the purchaser and the manufacturer.  The test for consistency or workability shall be completed within 15 minutes of the time of receipt of the ready-mixed concrete at the site.

4.5.6.4. Aggregates – When tested in accordance with IS: 2386(Part I)-1963, the quantity of aggregate larger than the maximum size specified by the purchaser shall not exceed 5 percent of the quantity of coarse aggregate and all such excess shall pass through sieve (conforming to IS: 460 (Part 1-3)-1985 of the next higher size.

4.5.7.  Concrete manufactured and supplied on the basis of mix proportion

4.5.7.1 The purchaser shall supply the following information for guidance of the manufacturer:

a) The type of the cement to be used;

b) The sizes and types of the aggregate;

c) The type of admixtures to be used;

d) The proportions of the mix including the maximum water cement ration at the place and time of delivery of the concrete;

e) The minimum mixing time after addition of the water; and

f) Any other requirements.

Tolerances Unless otherwise agreed to between the purchaser and the manufacturer, the concrete shall be deemed to comply with the requirements of this standard, if the result of tests where applicable, lie within the tolerance specified.

Cement content – The cement content, as shown by the samples taken, shall be not less than 95 percent of that specified.

Ratio of coarse to fine aggregates – The ratio of coarse to fine aggregates, as indicated by the sample taken, shall neither exceed nor fall below the ration specified by the purchaser by more than 10 percent.

Water/ cement ratio - ± 5 percent of the specified value.

Consistency or workability – The slump shall not differ from the amount specified by  10mm for a specified slump of 75 mm or less and ï 25mm when the specified a slump is greater than 75mm.  The compacting factor shall be within ï 0.03 of the value specified. If any other method of determining consistency is used, a suitable tolerance shall be agreed to between the purchaser and the supplier.

APPENDIX A

Concrete uniformity requirement

A-1  Tests

A-1.1 The variation within a batch as provided in Table 10 shall determined for each property listed as the difference between the highest value and the lowest value obtained from the different portions of the same batch. For this specification the comparison shall be between two samples, representing the first and last portions of the batch being tested.  Test results conforming to the limits of five of the six tests listed in Table I shall indicate uniform concrete within the limits of this specification.  Analysis of concrete samples shall be made in accordance with the relevant requirements of IS: 1159-1959.

A.2. Coarse aggregate content

A-2.1 Coarse aggregate content shall be determined using the following equation:

Where 

P= Percentage of coarse aggregate by mass in concrete;

c= saturated surface dry mass in kg of aggregate retained on 4.75 mm IS Sieve, resulting from washing all material finer than this sieve from the fresh concrete; and

b= mass of sample, in kg of fresh concrete in unit mass container.

Table 10 Requirements for uniformity of concrete

Sl. No.

Test

Requirement expressed as maximum permissible difference in results of tests or samples representing the first and last portions or concrete batch

1

2

3

i)

Mass per cubic meter calculated to an air-free basis

16 kg/m³

ii)

Air-content, percent by volume of concrete

1.0

iii)

Slump:

 

 

If average slump is 10cm or less

2.5 cm

 

If average slump is 10 to 15 cm

3.8 cm

iv)

Coarse aggregate content, percent (portion by mass of each sample retained on 4.75-mm IS Sieve)

6.0

v)

Unit mass of air-free mortar, percent based on average for all comparative samples tested

1.6

VI)

Average compressive strength at 7 days for each comparative test specimens, percent

7.5

A-3. Unit mass of air free mortar

A-3.1 Unit mass of air free mortar shall be calculated as follows:

Where,

M= Unit mass of air free mortar in Kg/m³

b= mass of concrete sample in unit mass container in kg,

c= saturated-surface-dry mass of aggregate in kg retained on 4.75mm IS Sieve,

V= Volume of unit mass container in  m³

A= air content of concrete in percent measured in accordance with the relevant requirements of IS:1199-1959*, and

G = specific gravity of coarse aggregate.

4.6  SPECIFICATIONS FOR Reinforced cement concrete work

General - Reinforced cement concrete work may be cast-in-situ or Precast as may be directed by engineer according to the nature of work. Reinforced cement concrete work shall comprise of the following which may be paid separately or collectively as per the description of the item of work.

  1. Form work ( Centering and shuttering )
  2. Reinforcement
  3. Concreting - 1) Cast-in-situ  2) Precast

4.6.1  Materials

4.6.1.1 Water, cement, fine and coarse aggregate shall be as specified under respective clauses of mortars and section 04-concrete work as applicable.

4.6.1.2 Steel for reinforcement

The steel used for reinforcement shall be any of the following types -

Mild steel sand medium tensile bars conforming to IS: 432 (part  I)

Hard drawn steel write conforming to IS: 432 (part  II)

High strength deformed steel bars conforming to IS: 1786

Hard drawn steel wire fabric conforming to IS: 1566

Structural steel section conforming to IS: 2062-1999

Types and grades - Reinforcement supplied in accordance with this standard shall be classified into the following types -

a) Mild steel bars - It shall be supplied in the following two grades

i) Mild steel bars grade I designated as Fe 410-S

ii) Mild steel bars grade II designated as Fe 410-O.

b) Medium tensile steel bars, grade II designated as Fe-540-W-HT.

Mild steel and medium tensile steel - Physical requirement are given in Table 11.

Table 11

Sl No

Type and nominal size Of bars

Ultimate tensile Stress N/mm2 minimum

Yield stress N/mm2 minimum

Elongation Percent

1

Mild steel grade I For bars up to and including 20 mm

410

250

23

 

For bars over 20 mm up to and Including 50 mm

410

240

23

2

Mild steel grade I For bars up to and including 20 mm

370

225

23

 

For bars over 20 mm up to and Including 50 mm

370

215

23

3

Medium tensile steel For bars up to & including 16 mm

540

350

20

 

For bars over 16 mm, up to And including 32 mm

540

340

20

 

For bars over 32 mm, up to And including 50 mm

510

330

20

Elongation percent on gauge length 5.65 Öso where so is the cross section area of the test piece.

Note-1. Grade (II) Mild steel bars are not recommended for the use in structures located in the earthquake zone subjected to serve damage and for structures subjected to dynamic loading (other than wind loading) such as railway and highway bridges.

2.  Welding of reinforcement bars covered in this specification shall be done in accordance with the requirements of IS: 2751.

Nominal mass / weight - The tolerance on mass/weight for round and square bars shall be the percentage given in Table.12 of the mass/weight calculated on the basis that the masses of the bar/wire of nominal diameter and of density 0.785 kg / cm3 or 0.00785 kg / mm3.

Table 12 (Tolerance on nominal mass)

Nominal size In mm

Tolerance on the nominal  mass percent

Batch

Individual Sample +

Individual sample for coil(-x-)

 a) up to and including 10

± 7

± 8

± 8

 b) over 10, up to and including 16     

+5

-6

+6

 c) over 16

± 3

-4

± 4

for individual sample plus tolerance in not specified

(x) for coil batch tolerance is not applicable

Tolerance shall be determined in accordance with method given in IS 1786-1985

Tests - Following type of lab test shall be carried out

  1. Tensile test - This shall be done as per IS: 1608
  2. Bend test - This shall be done as per IS: 1599
  3. Re-test - This shall be done as per IS: 1786
  4. Rebend test -This shall be done as per IS: 1786

Should any one of the test pieces first selected fail to pass any of the tests specified above, two further samples shall be selected for testing in respect of each failure. Should the test pieces from both these additional samples pass, the materials represented by the test samples shall be deemed to comply with the requirement of the particular test. Should the test piece from either of these additional samples fail, the material represented by the test samples shall be considered as not having complied with standard. High strength deformed bars & wires shall conform to IS: 1786. The physical properties for all sizes of steel bars are mentioned below in Table 13.                         

Table 13

Sl. No

Property

Grade

Fe 415

Fe 500

Fe 550

1.

0.2% proof Stress/Yield stress, in. N/mm²

415

500

550

2.

Elongation, percent min. on gauge Length 5.65 A, Where A is the X-sectional Area of the test piece

14.5

12

8

3.

Tensile strength

10 % more than actual 0.2 % proof stress but not less than 465 N/mm²

8 % more than actual 0.2 % proof stress but not less than 545 N/mm²

6 % more than actual 0.2 % proof stress but not less than 585 N/mm²

Tests - Selection and preparation of test sample. All the tests pieces shall be selected by the engineer or his authorised representative either-

  1. From cutting of bars or
  2. If he so desires, from any after it has been cut to the required or specified size and the test piece taken from any part of it.

In neither case, the test pieces shall be detached from the bar or coil except in the presence of the engineer or his authorised representative.

The test pieces obtained in accordance with as above shall be full sections of the bars as rolled and subsequently cold worked and shall be subjected to physical tests without any further modifications. No deductions in size by machining or otherwise shall be permissible. No test piece shall be enacted or otherwise subject to heat treatment. Any straightening which a test piece may require shall be done cold.

Tensile test - This shall be done as per IS: 1599.

Re-test -This shall be done as per IS: 1786.

4.6.1.3 Stacking and storage - Steel for reinforcement shall be stored in such a way as to prevent distorting and corrosion. Bars of different classifications, sizes and lengths shall be stored separately to facilitate issue in such sizes and lengths to cause to minimum wastage in cutting from standard length.

4.6.2 SPECIFICATIONS FOR FORMWORK (CENTRING & SHUTTERING)

4.6.2.1 -  Form work shall include all temporary or permanent forms or moulds required for forming the concrete which is cast-in-situ, together with all temporary construction required for their support.

4.6.2.2 - Design & tolerance in construction - Form work shall be designed and constructed to the shapes, lines and dimensions shown on the drawings with the tolerances given below.

a)

Deviation from specified dimensions of cross section of columns and beams

+ 12 mm

b)

Deviation from dimensions of footings

+ 12 mm

 

i)

Dimension in plan

+ 50 mm

 

ii)

Eccentrically in plan

0.02 times the width of the footings in the direction of deviation but not more than 50 mm

 

iii)

Thickness

+ 0.05 times the specified thickness.

(Note – Tolerance apply to concrete dimensions only, and not to positioning of vertical steel or dowels.)

4.6.2.3. General requirement - It shall be strong enough to withstand the dead and live loads and forces caused by ramming and vibrations of concrete and other incidental loads, imposed upon it during and after casting of concrete. It shall be made sufficiently rigid by using adequate number of ties and braces, Screw jacks or hard board wedges where required shall be provided to make up any settlement in the form work either before or during the placing of concrete. Forms shall be so constructed as to be removable in sections in the desired sequence, without damaging the surface of concrete or disturbing other sections. Care shall be taken to see that no piece is keyed into the concrete. See also Annexure 4-A.7

4.6.2.4. Material for form work

Propping and centering - All propping and centering should be either of steel tubes with extension pieces or built up sections of rolled steel.

Centering / Staging - Staging should be as designed with required extension pieces as approved by engineer to ensure proper slopes, as per design for slabs /beams etc. and as per levels as shown in drawings. All the staging to be either  tubular steel structure with adequate bracings as approved or made of built up structural sections made from rolled structural steel sections 

a) In case of structures with two or more floors, the weight of concrete, centering and shuttering of any upper floor being cast shall be suitably supported on one floor below the top most floor already cast.

b) Form work and concreting of upper floor shall not be done until concrete of lower floor has set at least for 14 days.

Shuttering - Shuttering used shall be of sufficient stiffness to avoid excessive deflection and joints shall be tightly butted to avoid leakage of slurry. If required, rubberized lining of material as approved by the engineer shall be provided in the joints. Steel shuttering used for concreting should be sufficiently stiffened. The steel shuttering should also be properly repaired before use and properly cleaned to avoid stains, honey combing, seepage of slurry through joints etc.

  1. Runner joints RS, MS Channel or any other suitable section of the required size shall be used as runners.
  2. Assembly of beam head over props, Beam head is an adopter that fits snugly on the head plates of props to provide wider support under beam bottoms.

Form work shall be properly designed for self weight, weight of reinforcement, weight of fresh concrete, and in addition, the various live loads likely to be imposed during the construction process (such as workmen, materials and equipment). In case the height of centering exceeds 3.50 meters, the prop may be provided in multi-stages. Typical arrangements of form work for ‘Beams, columns and walls, and forms secured by wall ties are shown in Figure 1 to 8: and typical detail of multistage shuttering is given in Fig. 9.

Camber - Suitable camber shall be provided in horizontal members of structure, especially in cantilever spans to counteract the effect of deflection. The form work shall be so assembled as to provide for camber. The camber for beams and slabs shall be 4 mm per meter (1 to 250) or as directed by the engineer, so as to offset the subsequent deflection. For cantilevers the camber at free end shall be 1/50th of the projected length or as directed by the engineer.

Walls - The forms faces have to be kept at fixed distance apart and an arrangement of wall ties with spacer tubes or bolts is considered best. A typical wall form with the components identified is given in Fig.1, 2, & 3. The two shutters of the wall are to be kept in place by appropriate ties, braces and studs. Some of the accessories used for wall forms are shown in Fig.3. surrounding concrete or any fixture attached to the steel or concrete.

Fig. 1 Wall Form

Fig. 2 Adjustable curve wall form (Double sided)

Fig. 3

Fig. 4 Typical standard units of form work

Fig. 5 Typical components of form work

Fig. 6 Typical arrangement of column form work

Fig. 7 Typical column shuttering

Fig. 8 Typical detail of beam head and stiffener

Fig. 9 Typical details of multi stage shuttering

Removal  of form work (stripping time) - In normal circumstance and where ordinary Portland cement is used, forms may generally be removed after the expiry of the following periods  - 

a)  Walls ,columns and faces of all structural members 24 to 48 hours as many be decided by the engineer

b)  Slab

i)    Spanning up to 4.50 M - 7 days

ii)   Spanning over 4.50 M - 14 days

c)   Beams and arches

i)    Spanning up to 6 M  - 14 days

ii)   Spanning over 6 M & up to 9 m  - 21 days

iii)  Spanning over 9 M - 28 days

Note 1-For the other types of cement, the stripping time recommended for ordinary Portland cement may be suitably modified. If Portland pozzolana or low heat cement has been used for concrete, the stripping time will be 10/7 of the period stated above.

Note 2-The number of props left under, their sizes and disposition shall be such as to be able to safely carry the full dead of the slabs, beam or arch as the case may be together with any live load likely to occur during curing of further construction.

Note 3 -For rapid hardening cement, 3/7 of above periods will be sufficient in all cases except for vertical side of slabs, beams and columns which should be retained for at least 24 hours.

Note 4 -In case cantilever slabs and beams, the centering shall remain till structures for counter acting or bearing down have been erected and have attained sufficient strength.

Note 5 -Proper precautions should be taken to allow for the decrease in the rate of hardening that occurs with all types of cement in cold weather and accordingly stripping time shall be increased.

Note 6 -Work damaged through premature or careless removal of forms shall be reconstructed.

4.6.2.5. Surface treatment

Oiling the surface - Shuttering gives much longer service life in the surfaces are coated with suitable mould oil which acts both as a parting agent and also gives surface protections.  Typical mould oil is heavy mineral oil or purified cylinder oil containing not less than 5% pentachlorophenol conforming to IS 716 well mixed to a viscosity of 70-80 centipoises. After 3-4 uses and also in case when shuttering has been stored for a long time, it should be recoated with mould oil before the next use. The design of form work shall conform to sound engineering practices and relevant IS codes.

4.6.2.6. Inspection of form work - The completed form work shall be inspected and approved by the engineer before reinforcement bars are placed in position. Proper from work should be adopted for concreting so as to avoid honey combing, blow holes, grout loss, stains or discolouration of concrete etc. Proper and accurate alignment and profile of finished concrete surface will be ensured by proper designing and erection of form work which will be approved by engineer. Shuttering surface before concreting should be free from any defect / deposits and fully cleaned so as to give perfectly straight smooth concrete surface. Shuttering surface should be therefore checked for any damage to its surface and exclusive roughness before use.

4.6.2.7. Erection of form work (centering and shuttering) - Following points shall be borne in mind while checking during erection.

  1. Any member which is to remain in position after the general dismantling is done, should be clearly marked.
  2. Material used should be checked to ensure that, wrong items / rejects are not used.
  3. If there are any excavations nearby which may influence the safety of form works, corrective and strengthening action must be taken.

i)     The bearing soil must be sound and well prepared and the sole plates shall bear well on the ground.

a) Sole plates shall be properly seated on their bearing pads or sleepers.

b) The bearing plates of steel props shall not be distorted.

c) The steel parts on the bearing members shall have adequate bearing areas.

d) Safety measures to prevent impact of traffic; scour due to water etc. should be taken. Adequate precautionary measures shall be taken to prevent accidental impacts etc. 

e) Bracing, struts and ties shall be installed along with the progress of form work to ensure strength and stability of form work at intermediate stage. Steel sections (especially deep sections) shall be adequately restrained against tilting, over turning and form work should be restrained against horizontal loads. All the securing device and bracing shall be tightened.

f) The stacked materials shall be placed as catered for, in the design.

 g) When adjustable steel props are used, they should -            

i) Be undamaged and not visibly bent.

ii) Have the steel pins provided by the manufacturers for use.

iii) Be restrained laterally near each end.

iv) Have means for centralizing beams placed in the fork heads.

h)   Screw adjustment of adjustable props shall not be over extended.

i)   Double wedges shall be provided for adjustment of the form to the required position wherever any settlement / elastic shortening of props occur. Wedges should be used only at the bottom end of single prop. Wedges should not be too steep and one of the pair should be tightened / clamped down after adjustment to prevent their shifting.

j)    No member shall be eccentric upon vertical member.

k)  The number of nuts and bolts shall be adequate.

l)   All provisions of the design and / or drawings shall be complied with.

m)  Cantilever supports shall be adequate.

n)   Props shall be directly under one another in multistage constructions as far as possible.

o)   Guy ropes or stays shall be tensioned property.

p)   There shall be adequate provision for the movement and operation of vibrators and other construction plant and equipment.

q)   Required camber shall be provided over long spans.

r)   Supports shall be adequate, and in plumb within the specified tolerances.   

4.6.2.8  Measurements

4.6.2.8.1. General - The form work shall include the following;

a)  Splayed edges, notching, allowance for overlaps and passing at angles, sheathing battens, strutting, bolting, nailing, wedging, easing, striking and removal.

b)  All supports, struts, braces, wedges as well as mud sills, piles or other suitable arrangements to support the form work.

c)  Bolts, wire ties, clamps, spreaders, nails or any other items to hold the sheathing together.

d)   Working scaffolds ladders, gangways, and similar items.

e)  Filling to form stop chamfered edges of splayed external angles not exceeding 20 mm wide to beams, columns and the like.

f)   Where required, the temporary openings provided in the forms for pouring concrete, inserting vibrators, and cleaning holes for removing rubbish from the interior of the sheathing before concrete.

g)   Dressing with oil to prevent adhesion and

h)  Raking or circular cutting.

4.6.2.8.2. Classification of measurements - Where it is stipulated that the form work shall be paid for separately, measurements shall be taken of the area of shuttering in contact with the concrete surface. Dimensions of the form work shall be measured correct to a cm. The measurements shall be taken separately for the following -

a). Foundations, footings, bases of columns etc. and for mass concrete and precast shelves,

b). Walls (any thickness) including attached pilasters, buttresses, plinth and string courses etc.

c). Suspended floors, roofs, landings, shelves and their supports and balconies.

d). Lintels, beams, girders, Bressummers and cantilevers.

e). Columns, pillars, posts and struts.

f). Stairs (excluding landing) except Spiral staircase.

g). Spiral staircase (including landing).

h). Arches.

i). Domes, vaults, shells roofs, arch ribs and folded plates.

j). Chimneys and shafts.

k). Well steining.

l). Vertical and horizontal fins individually nor forming box, louvers and bands.

m). Waffle or ribbed slabs.

n). Edges of slabs and breaks in floors and walls (to be measured in running meters where below 200 mm in width or thickness).

o). Cornices and mouldings.

p). Small surfaces, such as cantilevers ends, brackets and end of steps, caps and boxes to pilasters and columns and like.

q). Chula hoods, weather shades, Chajjas, corbels etc. including edges and

r). Elevated water reservoirs.

4.6.2.8.3 Centering, and shuttering where exceeding 3.5 meter height in one floor shall be measured and paid for separately.

4.6.2.8.4 Where it is not specifically stated in the description of the item that form work shall be paid for separately, the rate of the RCC item shall be deemed to include the cost of form work.

4.6.2.8.5. No deductions from the shuttering due to the openings / obstructions shall be made if the area of such openings / obstructions does not exceed 0.1 square meters. Nothing extra shall be paid for forming such openings.

4.6.2.8.7 Rate - The rate of the form work includes the cost of labour and materials required for all the operations described above.

4.6.3. SPECIFICATIONS FOR REINFORCEMENTS IN CONCRETE

4.6.3.1. General requirements - Steel conforming to para 4.6.1.2. for reinforcement shall be clear and free from loose mill scales, dust, loose rust, coats of paints, oil or other coatings which may destroy or reduce bond. It shall be stored in such a way as to avoid distortion and to prevent deterioration and corrosion. Prior to assembly of reinforcement on no account any oily substance shall used for removing the rust.

(1). Assembly of reinforcement - Bars shall be bent correctly and accurately to the size and shape as shown in the detailed drawing or as directed by engineer. Preferably bars of full length shall be used. Necessary cutting and straightening is also included. Over lapping of bars, where necessary shall be done as directed by the engineer. The overlapping bars shall not touch each other and these shall be kept apart with concrete between them by 25 mm or 1 ¼ times the maximum size of the coarse aggregate whichever is greater. But where this is not possible, the overlapping bars shall be bound together at intervals not exceeding twice the dia. Of such bars with two strands annealed steel wire of 0.90 mm to 1.6 mm twisted tight. The overlaps / splices shall be staggered as per directions of the engineer. But in no case the over lapping shall be more than 50% of cross sectional area at one section.

(2). Bonds and hooks forming end anchorages - Reinforcement shall be bent and fixed in accordance with procedure specified in IS 2502, code of practice for bending and fixing of bars for concrete reinforcement. The details of bends and hooks are shown below for guidance.

a)  U-Type hook - In case of mild steel plain bars standard U-type hook shall be provided by bending ends of rod into semicircular hooks  having clear diameter of the bar         

Note-In case of work in seismic zone, the size of hooks at the end of the rod shall be eight times the diameter of bar or as given in the structural drawing.

b)  Bends - Bend forming anchorage to a M.S. plain bar shall be bent with an internal radius equal to two times the diameter of the bar with a minimum length beyond the bend equal to four times the diameter of the bar.

(3). Anchoring bars in tension - Deformed bars may be used without end anchorages provided, development length requirement is satisfied. Hooks should normally be provided for plain bars in tension. Development length of bars will be determined as per clause 25.2.1 of IS:  456-2000.

(4). Anchoring bars in compression - The anchorage length of straight bar in compression shall be equal to the ‘Development length’ of bars is compression as specified in of IS: 456-2000. The projected length of hooks, bends and straight lengths beyond bend, if provided for a bar in compression, shall be considered for development length.

(5). Binders, stirrups, links and the like - In case of binders, stirrups, links etc. the straight portion beyond the curve at the end shall be not less than eight times the nominal size of bar.

(6). Welding of bars - Whenever facility for electric arc welding is available, welding of bars shall be done in lieu of overlap. The location and type of welding shall be got approved by the engineer. Welding shall be as per IS: 2751 for mild steel bars and for cold worked bars.

4.6.3.2 Placing in position - Fabricated reinforcement bars shall be placed in position as shown in the drawings or as directed by the engineer. The bars crossing one another shall be tied together at every intersection with two stands of annealed steel wire 0.9 to 1.6 mm thickness twisted tight to make the skeleton of the steel work rigid so that the reinforcement does not get displaced during deposition of concrete.

Track welding in crossing bars shall also be permitted in lieu of bending with steel wire if approved by engineer.

The bars shall be kept in correct position by the following methods -

a)  In case of beam and slab construction precast cover blocks of cement mortar 1:2 about 4x4 cm section and of thickness equal to the specified cover shall be placed between the bars and shuttering, so as to secure and maintain the requisite cover of concrete over reinforcement.

b)  In case of cantilevered and doubly reinforced beams or slabs, the vertical distance between the horizontal bars shall be maintained by introducing chairs, spacers or support bars of steel at 1.0 meter or at shorter spacing to avoid sagging.

c)  In case of columns and walls, the vertical bars shall be kept in position by means of timber templates with slots accurately cut in them; or with block of cement mortar 1:2 of required size suitably tied to the reinforcement to ensure that they are in correct position during concreting.

d)   In case of R.C.C. structure such arches, domes, shells, storage tanks etc. a combination of cover blocks, spaces and templates shall be used as directed by engineer.     

Tolerance on placing of reinforcement  - Unless otherwise specified by the engineer, reinforcement shall be placed within the following tolerances -Tolerance in spacing

 

Tolerance in spacing

a)

For effective depth 200 mm or less

± 10

b)

For effective depth More than 200 mm

± 15

The cover shall in no case be reduced by more than one third of specified cover or 5 mm which ever is less.

Bending at construction joints - Where reinforcement bars are bent aside at construction joints and afterwards bent back into their original position care should be taken to ensure that at no time the radius of the bend is less than 4 bars diameters for plain mild steel or 6 bar diameters for deformed bars. Care shall also be taken when bending back bars to ensure that the concrete around the bars in not damaged.

4.6.3.3. Measurements - Reinforcement including authorised spacer bars and laps shall be measured in length of different diameters, as actually (not more than as specified in the drawings.) used in the work nearest to a centimeter and their weight calculated on the basis of standard weight given in Table 14 below. Wastage and unauthorized overlaps shall be paid for. Annealed steel wire required for binding or tack welding shall not be measured, its cost being included in the rate reinforcement.

Wherever tack welding is used in lieu of binding, such welds shall not be measured. Chairs separators etc. shall be provided as directed by the engineer and measured separately and paid for.

Table 14 Cross-sectional area and mass of steel bar

Nominal size mm

Cross sectional area sq.mm

Mass per meter run kg

6

28.3

0.222

7

38.5

0.302

8

50.3

0.395

10

78.6

0.617

12

113.1

0.888

16

201.2

1.58

18

254.6

2.00

20

314.3

2.47

22

380.3

2.98

25

491.1

3.85

28

616.0

4.83

32

804.6

6.31

36

1018.3

7.99

40

1257.2

9.85

45

1591.1

12.50

50

1964.3

15.42

Note - These are as per clause 5.2 of IS 1786.

4.6.3.4. Rate - The rate for reinforcement shall include the cost of labour and materials required for all operations described above such as cleaning of reinforcement bars, straightening, cutting, as required of directed including tack welding on crossing of bars in lieu of binding with wires.

4.6.4  SPECIFICATIONS FOR CONCRETING

The concrete shall be done as specified. The proportion by volume of ingredients shall be as specified.

4.6.4.1Consistency - The concrete which will flow sluggishly into the forms and around the reinforcement without any segregation of coarse aggregate from the mortar shall be used. The consistency shall depend on whether the concrete is vibrated on or hand tamped. It shall be determined by slump test as n[prescribed in chapter “ concrete under para 4.2.3  workability”

Where considered necessary, the workability of the concrete may also be ascertained by compacting factor test and VEE BEE censistometer method specified in IS: 1199. For suggested ranges of values of workability of concrete by the above two methods, reference may be made to IS: 456.

4.6.4.2  Placing of concrete

Concreting shall be commenced only after engineer has inspected the centering, shuttering and reinforcement as placed and passed the same. Shuttering shall be clean and free from all shaving, saw dust, pieces of wood, or other foreign material and surfaces shall be treated as prescribed.

In case of concreting of slabs and beams, wooden plank or cat walks of chequered MS plates or bamboo chlies or any other suitable material supported directly on the centering by means of wooden blocks or lugs shall be provided to convey the concrete to the place of deposition without disturbing the reinforcement in any way. Labour shall not be allowed to walk over the reinforcement.

 In case of columns and walls, it is desirable to place concrete without construction joints. The progress of concreting in the vertical direction shall be restricted to one meter per hour.

The concrete shall be deposited in its final position in a manner to preclude segregation of ingredients. In deep trenches and footings concrete shall be placed through chutes or as directed by the engineer. In case of columns and walls, the shuttering shall be so adjusted that the vertical drop of concrete in not more than 1.5 meters at a time.

During cold weather, concreting shall not be done when the temperature falls below 4.5° c. the concrete placed shall be protected against frost by suitable converting. Concrete damaged by frost shall be removed and work redone.

During hot weather precaution shall be taken to see that the temperature of wet concrete does not exceed 38°C. no concrete shall be laid within half of the closing time of the day, unless permitted by the engineer.

It is necessary that the time taken between mixing and placing of concrete shall not exceed 30 minutes so that the initial setting process is not interfered with

4.6.4.3 Compaction - Concrete shall be compacted into dense mass immediately after placing by means of mechanical vibrators designed for continuous operations. The engineer may however relax this conditions at his discretion for certain items, depending on the thickness of the members and feasibility of vibrating the same and permit hand compaction instead. Hand compaction shall be done with the help of tamping rods so that concrete is thoroughly compacted and completely worked around the reinforcement, embedded fixtures, and into corners of the from. The layers of concrete shall be so placed that the bottom layer does not finally set before the top layer is placed. The vibrators shall maintain the whole of concrete under treatment in an adequate state of agitation, such that de-aeration and effective compaction is attained at a rate commensurate with the supply of concrete from the mixers. The vibration shall continue during the whole period occupied by placing of concrete, the vibrators being adjusted so that the centre of vibrations approximates to the centre of the mass being compacted at the time of placing.

Concrete shall be judged to be properly compacted, when the mortar fills the spaces between the coarse aggregate and begins to cream up to form an even surface. When this condition has been attained, the vibrator shall be stopped in case of vibrating tables and external vibrators. Needle vibrators shall be withdrawn slowly so as to prevent formation of loose pockets in case of internal vibrators. In case both internal and external vibrators are being used, the internal vibrator shall be first withdrawn slowly after which the external vibrators shall be stopped so that no loose pocket is left in the body of the concrete. The specific instructions of the makers of the particular type of vibrator used shall be strictly complied with. Shaking of reinforcement for the purpose of compaction should be avoided. Compaction shall be completed before the initial setting starts, i.e. within 30 minutes of addition of water to the dry mixture.

4.6.4.4 Construction joints - Concreting shall be carried out continuously up to the construction joints, the position and details of which shall be as shown in structural drawing or as indicated in Fig. 26 or as directed by engineer. Number of such joints shall be kept to minimum. The joints shall be kept at places where the shear force is the minimum. These shall be straight and shall be at right angles to the direction of main reinforcement.

In case of columns the joints shall be horizontal and 10 to 15 cm below the bottom of the beam running into the column head. The portion of the column between the stepping off level and the top of the slab shall be concreted with the beam.

When stopping the concrete on a vertical plane in slabs and beams, an approved stop-board (see Fig.26C) shall be placed with necessary slots for reinforcement bars or any other obstruction to pass the bars freely without bending. The construction joints shall be keyed by providing a triangular or trapezoidal fillet nailed on the stop-board. Inclined or feather joints shall not be permitted. Any concrete flowing through the joints of stop-board shall be removed soon after the initial set. When concrete is stopped on a horizontal plane, the surface shall be roughened and cleaned after the initial set.

When the work has to be resumed, the joint shall be thoroughly cleaned with wire brush and loose particles removed. A coat of neat cement slurry at the rate of 2.75 kg of cement per square meter shall then be applied on the roughened surface before fresh concrete is laid.

4.6.4.5 Expansion joints - Expansion joints shall be provided as shown in the structural drawings or as indicated in Fig. 10 to 25 or as directed by engineer, for the purpose of general guidance. However it is recommended that structures exceeding 45 m in length shall be divided by one or more expansion joints. The filling of these joints with bitumen filler, bitumen felt or any such material and provision of copper plate, etc. shall be paid for separately in running meter. The measurement shall be taken up to two places of decimal stating the depth and width of joint.

4.6.4.6  Curing - After the concrete has begun to harden i.e. about 1 to 2 hours after its laying, it shall be protected from quick drying by covering with moist gunny bags, sand, canvass Hessian or any other material approved by the engineer. After 24 hours of laying of concrete, the surface shall be cured of ponding with water for a minimum period of 7 days from the date of placing of concrete.

4.6.4.7 Finishing - In case of roof slabs the top surface shall be finished even and smooth with wooden trowel, before the concrete begins to set.

Immediately on removal of forms, the R.C.C work shall be examined by the engineer, before any defects are made good.

a) The work that has sagged or contains honey combing to an extent detrimental to structural safety or architectural concept shall be rejected as given for visual inspection test.

b) Surface defects of a minor nature may be accepted. On acceptance of such a work by the engineer, the same shall be rectified as follows -

1)  Surface defects which require repair when forms are removed, usually consist of bulges due to movement of forms, ridges at form joints, honey combed areas, damage resulting from the stripping of forms and bolt holes, bulges and ridges are removed by careful chipping or tooling and the surface is then rubbed with a grinding stone. Honey-combed and other defective areas must be chipped out, the edges being cut as straight as possible and perpendicularly to the surface, or preferable slightly undercut to provide a key at the edge of the path.

2)  Shallow patches are first treated with a coat of thin grout composed of one part of cement and one part of fine sand and then filled with mortar similar to that used in the concrete. The mortar is placed in layers not more than 10 mm thick and each layer is given a scratch finish to secure bond with the succeeding layer. The last layer is finished to match the surrounding concrete by floating, rubbing or tooling on formed surfaces by pressing the form material against the patch while the mortar is still plastic.

3)  Large and deep patches require filling up with concrete held in place by forms. Such patches are reinforced and carefully dowelled to the hardened concrete.

4)  Holes left by bolts are filled with mortar carefully packed into places in small amounts. The mortar is mixed as dry as possible, with just enough water so that it will be tightly compacted when forced into place.

5)  Tiered holes extending right through the concrete may be filled with mortar with a pressure gun similar to the gun used for greasing motor cars.

6)  Normally, patches appear darker than the surrounding concrete, possibly owing to the presence on their surface of less cement laitance. Where uniform surface colour is important, this defect shall be remedied by adding 10 to 20 percent of white Portland cement to the patching mortar, the exact quantity being determined by trial.

7)  The same amount of care to cure the material in the patches should be taken as with the whole structure. Curing must be started as soon as possible, after the patch is finished to prevent early drying. Damp Hessian may be used but in some locations it may be difficult to hold it in place. A membrane curing compound in these cases will be most convenient.

c). The exposed surface of R.C.C work shall be plastered with cement mortar 1 -3 (1 cement  - 3 fine sand) of thickness not exceeding 6 mm to give smooth and even surface true to line and form. Any RCC surface which remains permanently exposed to view in the completed structure shall be considered exposed surface for the purpose of this specification.

Where such exposed surface exceeding 0.5 sq.m in each location is not plastered with cement mortar 1:3 (1 cement to 3 fine sand) 6 mm thick, necessary deduction shall be made for plastering not done.

d). The surface which is to receive plaster or where it is to be joined with brick masonry wall, shall be properly roughened immediately after the shuttering is removed, taking care to remove the laitance completely without disturbing the concrete. The roughening shall be done by hacking. Before the surface is plastered, it shall be cleaned and wetted so as to give bond between concrete and plaster.

e). The surface of RCC slab on which the cement concrete of mosaic floor is to be laid shall be roughened with brushes while the concrete is green. This shall be done without disturbing the concrete.

4.6.4.8  Strength of concrete - The compressive strength on work tests for different mixes shall be as given in Table 15 below -

Table 15

Concrete mix (Nominal mix on volume basis)

Compressive strength in (kg/sq cm)

 

7 days

28 days

1:1:2

210

315

1:1 ½ : 3

175

265

1:2:4

140

210

4.6.4.9  Testing of concrete

(1). Regular mandatory tests on the consistency and workability of the fresh concrete shall be done to achieve the specified compressive strength of concrete. These will be of two types

  1. Mandatory Lab. Test
  2. Mandatory Field Test

(3). Results of Mandatory Field Test will prevail over Mandatory Lab. Test.

a)  Work Test-Mandatory Lab. Test shall be carried out as prescribed.

b)  Mandatory Field Test (Hammer Test), shall be carried out as prescribe in Annexure 4.A.2

(4). Additional test - Additional test, if required, shall be carried out as prescribed in Annexure 4.A.7

(5). Slump test - This test shall be carried out as prescribed in Annexure 4.A.1

(6). Visual inspection test - The concrete will be inspected after removal of the form work as described. The question of carrying out mandatory test or other tests described in Annexure 4-A.2 and 4-A.4 will arise only after satisfactory report of visual inspection. 

The concrete is liable to be rejected, if,

(i)   It is porous or honeycombed.-

(ii)  Its placing has been interrupted without providing a proper construction joint;

(iii) The reinforcement has been displaced beyond tolerance specified; or construction tolerance has not been met.

However, the hardened concrete may be accepted after carrying out suitable remedial measures to the satisfaction of the engineer at the risk and cost of the contractor.    

4.6.4.10 Standard of acceptance

(1).  Mandatory lab test - For concrete sample and tested as prescribed in Annexure 4- A.2 the following requirement shall apply.

Out of six sample cubes, three cubes shall be tested at 7 days and remaining three cubes at 28 days, if found necessary.

(2). 7days’ tests

(a). Sampling - The average of the strength of three specimens shall be accepted as the compressive strength of the concrete provided the variation In strength of individual specimen is not more than ± 15% of the average. Difference between the maximum and minimum strength should not exceed 30% of average strength of three specimen. If the difference between maximum and minimum strength exceeds 30% of the average strength, then 28 days’ test shall have to be carried out.

(a). Strength -  If the actual average strength of sample accepted in para ‘sampling’ above is equal to or higher than specified strength up to 15% then strength of the concrete shall be considered in order. In case the actual average strength of sample accepted in the above para is lower than the specified or higher by more than 15% then 28 days’ test shall have to be carried out to determine the compressive strength of concrete cubes.

(3). 28 days’ test

(a) The average of the strength of three specimen be accepted as the compressive strength of any individual cube shall neither be less than 70% nor higher than 130% of the specified strength.

(b) If the actual average strength of accepted sample exceeds specified strength by more than 30%, the engineer, if he so desires may further investigate the matter. However, if the strength of any individual cube exceeds more than 30% of specified strength, it will be restricted to 130% only for computation of strength.

(c) If the actual average strength of accepted sample is equal to or higher than specified strength upto30% then strength of the concrete shall be considered in order and the concrete shall be accepted at full rates.

(d) If the actual average strength of accepted sample is less than specified strength but not less than specified strength but not less than 70% of specified strength, the concrete may be accepted at reduced rate at the discretion of engineer.

(e) If the actual average strength of accepted sample is less than 70% of specified strength, the engineer shall reject the defective portion of work represented by sample and nothing shall be paid for the rejected work. Remedial measures necessary to retain the structure shall be taken at the risk and cost of contractor. If, however, the engineer so desires, he may order additional tests (see Annexure 4-A.4) to be carried out to ascertain if the structure can be retained. All the charges in connection with these additional tests shall be borne by the contractor.

(4). Acceptance criteria of mandatory field test

(A) Preparation of standard test cubes for calibration of rebound hammer at site

(a) In the beginning the standard test cubes of specified mix shall be prepared by field units before undertaking any concrete work in each project.

(b) At least 18 standard cubes necessary for formation of one specimen of specified mix, shall be cast by site staff well in advance. From these 18 cubes any 3 cubes may be selected at random to be tested for crushing strength of 7 days. The crushing strength obtained should satisfy the specified strength for the mix as per specification or agreement. If the strength is satisfactory then the remaining cubes will form the standard samples for calibration of rebound hammer. In case of failure, the site staff should totally reject the samples and remove them also and then make another set of samples by fresh mixing or alternatively, out of the remaining 15 cubes 3 cubes will be tested on 28 days. If the 28 days’ tests are found satisfactory then remaining 12 cubes will form the standard sample for calibration at 28 days’ strength otherwise all samples shall be rejected and whole procedure repeated to form a fresh specimen. All the results shall be recorded in a register.

(c) No concreting will be allowed unless the standard specimen cubes are obtained. The criteria for acceptance and calibration of hammer will be 28 days’ strength. the 7 days’ strength is only to facilitate the work to start.

(d) No work (for the concrete cast between 8th day) shall be allowed to be paid unless 28 days’ cube strength is obtained. For the concrete cast between 8th and 28th day, the decision to make the payment may be taken by the engineer on the basis of existing criteria. Concrete work will be rejected if 28 days’ strength falls short as per acceptance criteria. No further work will be allowed till the acceptable standard cubes are obtained.

(e) Frequency - It will be once in each quarter or as per the direction and discretion of engineer. Whenever the acceptance criteria is changed or concrete mix or type of cement is changed or engineer feels it necessary for recorded reasons with the approval of the authority according technical sanction, fresh specimen shall be prepared.

(B) Calibration of hammer

(a) Simultaneously, same three cubes to be tested on 28 days as referred in para A (b) above shall be used to correlate the compressive strength of their concrete with rebound number as per procedure described in para 5.2 of the IS: 13311 (Part 2) “Indian standard for non-destructive testing of concrete Method of test by rebound hammer which is given below in para B (b). the average of values of the rebound number (minimum readings) obtained in respect of same three cubes passing on 28 days’ work test shall form the datum reference for remaining cubes for the strength of cubes.

(b) The concrete cubes specimens are held in a compression testing machine under a fixed load, measurements of rebound hammer taken and then compressive strength determined as per IS: 516. The fixed load required is of the order of 7N / mm² when the impact energy of the hammer is about 2.2 NM.

If the specimens are wet cured, they should be removed from wet storage & kept in the laboratory atmosphere for about 24 hours before testing. Only the vertical faces of the cubes as cast should be tested for rebound number. At least nine readings should be taken on each of the three vertical faces accessible in the compression testing machine when using rebound hammers. The points of impact on the specimen must not be nearer than 20 mm from each other. The same points must not be impacted more than once.

(c) The rebound number of hammer will be determined on each of the remaining (18-3-3=12) cubes. Whenever the rebound number of hammer of any individual cube varies by more than ± 25% from the datum readings referred to in para B(a) above, that cube will be excluded and will not be considered for standard specimen cubes for calibration. It must be ensured that at least 8 cubes out of 12 that is 66.6% are within the permissible range of variation of rebound number i.e. ± 25% or otherwise whole procedure shall have to be repeated and fresh specimen prepared.

These 8 cubes will form one standard sample in the beginning before commencement of work and shall be kept carefully for the visiting officers who will calibrate their hammers on these cubes.

(d) This calibration will be done by field staff with their hammer and then chart of calibration giving the details of the average readings, date & month of casting, mix of the concrete etc. shall be prepared and signed by engineer and will be duly preserved for future reference as and when required.

(C) Preservation of cubes at site - Standard sample cubes cast shall be carefully preserved at site under the safe custody of AE or his representative for making them available together with the charts, to the any other senior departmental officers, during their inspection of the work.

(D) Testing at site - (D-2) Testing will be done generally by non-destructive methods like rebound hammers etc. Each field Division / Sub Division / Unit will purchase rebound hammers and keep them in working order at work site. Testing will be done only by hammers, which are dully calibrated.

(D-3) The relative strength of actual field work will be tested with reference to strength of these standard cubes and calibration charts of a hammer for determining the rebound number on the field work. The hammer will be used as per manufacturer’s guidelines at various locations chosen at random. The number of location / reading on each wall, beam or column etc. shall not be less than 12. All the readings should be within the ± 25% range of values prescribed in calibration chart normally. However, reading indicating good strength will be when it is at par with calibrated value between 100% & 125% and very good if more than 125%. Any value between 100% & 75% of calibrated value shall be considered satisfactory. Values from 75% to 50% shall be considered for fragment at rates reduced on prorata basis. The concrete indicating rebound number less than 50%of calibrated value shall be rejected and not paid for.

(E) Acceptance of field tests and strength - If the relative strength of actual field work is found satisfactory considering the calibration charts with reference to the standard cube test kept at site, the representative work will be considered satisfactory. If the work is considered below satisfactory, the same will be dealt as stated in para D-3 above.

(F)  7 days’ Strength in rare cases only - Normally cube crushing strength on 28 days’ test shall form the basis of acceptance. However in rare cases of time bound projects / urgent repairs 7 days’ cube test strength criteria may be adopted on similar lines using 7 days’ standard test cubes and calibration graphs / curves /charts for 7 days’ in lieu of 28 days’ and testing work done at 7 days’.

(G) Precautions

(G-1) The testing shall be done generally as per the guidelines of manufacturer of the apparatus and strictly in accordance with the procedure laid down in clause 6 of IS: 13311 (part 2) Indian Standard for Non-Destructive Testing of concrete-Method of Test by Rebound Hammer.

(G-2) The rebound hammers are influenced by number of factors like type of cement aggregate, surface conditions, moisture content, age of concrete etc. Hence care shall be taken to compare the cement, aggregate etc. and tested under the similar surface conditions having more or less same moisture content and age. However effect of age can be ignored for concrete between 3 days & 3 months old.

4.6.4.11 Measurement

4.6.4.11.1. Dimensions shall be measured nearest to a cm except for the thickness of slab which shall be measured correct to 0.5 cm.

4.6.4.11.2. The areas shall be worked out nearest to 0.01 sq. mt. The cubical contents shall be worked out to nearest 0.01 cubic meters.

4.6.4.11.3. Reinforced cement concrete whether cast-in-situ or present shall be classified and measured separately as follows.

(a)  Raft, footing, bases of columns etc. and mass concrete. (b) walls (any thickness) including attached pilasters, buttresses, plinth and string course, fillets etc. (c) suspended floors, roofs, landings and balconies. (d) Shelves (e) Chajjas (f) Lintel, beams and Bressummers. (g) Columns, pillars, piers, abutments, posts and struts. (h) Stair-cases including waist or waist less slab but excluding landing except in (I) below. (j) Spiral stair-case (including landing). (k)  Arches, arch ribs, domes and vaults. (l) Chimneys and shafts. (m) Well steining. (n) Vertical and horizontal fins individually or forming box, louvers and fascias. (o) Kerbs, steps and the like. (p) String course, bands, coping, bed plates, anchor blocks, plain window sills and the like. (q) Moldings as in cornices window sills etc.

Shell, dome and folded plates.  (r) Extra for shuttering in circular work in plan.

4.6.4.11.4 No deduction shall be made for the following -

(a)  Ends of dissimilar materials (e.g. joists, beams post girders, rafters, purlin trusses, corbels steps etc.) up to 500 sq cm in cross-section

(b)  Opening up to 0.1sq.m.

Note-In calculating area of openings up to 0.1sq.m the size of opening shall include the thickness of any separate lintels or sills. No extra labour for forming such opening or voids shall be paid for.

(c)   The volume occupied by reinforcement.

(d)  The volume occupied by water pipes, conducts etc. not exceeding 25 sq cm each in cross sectional area. Nothing extra shall be paid for leaving and finishing such cavities and holes.

4.6.4.11.5 Measurement shall be taken before any rendering is done in concrete members. Measurement will not include rendering. The measurement of R.C.C. work between various units shall be regulated as below -

(a) Slabs shall be taken as running continuously through except when slab is monolithic with the beam. In that case it will be from the face to face of the bream.

(b) Beams shall be measured from face to face of columns and shall include haunches, if any, between columns and beam. The depth of the bottom of beam shall be from the bottom of slab to the bottom of beam and slabs are not monolithic. In case of monolithic construction where slabs are integrally connected with beam, the depth of beam shall be from the top of the slab to the bottom of beam.

(c) The columns measurement shall be taken through.

(d) Chajjas along with its bearing on wall shall be measured in cubic meter nearest to two places of decimal. When Chajjas is combined with Lintel, slab or beam, the projecting portion shall be measured as Chajjas, built in bearing shall be measured as per item of Lintel, slab or beam in which chhajja bears.

(e)  Where the band and Lintels are of the same height and the band serves as Lintel, the portion of the band to be measured as lintel shall be for clear length of opening plus twice the over all depth of band.

4.6.4.12. Tolerances - Subject to the condition that structural safety is not impaired and architectural concept does not hamper, the tolerances in dimensions of R.C.C members shall be as specified in the drawing by the designer. Whenever these are not specified, the permissible tolerance shall be decided by the engineer after consultations with the Designer, if necessary.

When tolerances in dimensions are permitted, following procedure for measurements shall apply.

(a). If the actual dimensions of R.C.C members do not exceed or decrease the design dimensions of the members plus or minus tolerance limit specified above, the design dimensions shall be taken for the purpose of measurements.

(b). If the actual dimensions exceed the design dimensions by more than the tolerance limit, the design dimensions only shall be measured for the purpose of payment.

(c). If the actual dimensions decrease more than the tolerance limit specified, the actual dimensions of the RCC members shall be taken for the purpose of measurement and payment.

(d). For acceptance of RCC members whose dimensions are not exactly as per design dimension of engineer shall be final. For the purpose of payment, however, the clarification as given in para a, b & c above shall apply

4.6.4.13 Rate

The rate includes the cost of materials and labour involved in all the operations described above except for the cost of centering and shuttering. On the basis of mandatory lab tests, in case of actual average compressive strength being less than specified strength but upto 70% of specified strength, the rate payable shall be in the same proportion as actual average compressive strength bears to the specified compressive strength.

Example

1. Average compressive strength in 80% of specified strength. Rate payable shall be 80% of agreement rate.

2. In case average compressive strength in less than 70% of the specified strength, the work represented by the sample shall be rejected.

3. However, on the basis of mandatory field test, where they prevail, the rates of the work represented by samples showing actual compressive strength less than specified strength shall be worked out as per para above. In addition, engineer may order for additional tests (see Annexure 4-A.4) to be carried out at the cost of contractor to ascertain if the portion of structure where in concrete represented by the samples has been used, can be retained on the basis of these test. Engineer may take further remedial measures as necessary to retain the structure at the risk and cost of the contractor.

Where throating or plaster drip or molding is not required to be provided in RCC Chajjas, deduction for not providing throating or plaster drip or molding shall be made from the item of R.C.C. In Chajjas. The measurement for deduction item shall be measured in running meters direct to a cm of the edge of chhajja.

No extra payment for richer mix which projects into any meter  from another member during concreting of junctions of beams and columns etc. will be made except to the extent structurally considered necessary and when so indicated in the structural drawing. The payments for work done under items of different mixes shall be limited strictly to what is indicated in the structural drawings.

4.6.5  SPECIFICATIONS FOR ENCASING ROLLED STEEL SECTIONS

Before concrete work is started, the engineer shall check that all rolled steel sections to be encased, have been erected truly in position. The sections shall be unpainted and shall be wire brushed to remove the lose rust / scales etc. Where so specified, ungalvanised metal, having mesh or perforations large enough to permit the free passage of 12.5 mm nominal size aggregate through them, shall be wrapped round the section to be encased in concrete and paid for separately.

4.6.5.2  Wrapping

In case of columns, the wrapping shall be arranged as illustrated in Fig. 27 to pass through the centre of the concrete covering. The wrapping of the entire length of the columns be carried out in stages and no stage shall cover more than 1.5 meter of height of columns. Successive wrapping shall be carried out only after the immediate adjacent wrapping has been encased in concrete. The surface and edges of the flanges of the steel columns shall have a concrete cover of not less than 50 mm. The wrapping of successive stages shall be tied together.

In the case of beams and grillages, the wire mesh or expanded metal shall be wrapped round the lower flange of the beam as illustrated in Fig.28 and the wrapping shall be suspended by wire hangers 5 mm diameter placed at about 1.2 meters centres. The surfaces and edges of the steel sections shall have a concrete cover of not less than 50 mm. The wrapping shall pass through the centre of the concrete covering at the edges and soffits of the flanges.

4.6.5.3. Form Work shall be as prescribed.

4.6.5.4 Concreting - Concrete shall consist of mix of 1:2:4(1 cement - 2 coarse sand - 4 graded stone aggregate of 12.5 mm nominal size) unless a richer mix is specified. The mix shall be poured solidly around the steel sections and around the wrapping by vibrating the concrete, placing of concrete into position. Consistency of concrete, placing of concrete and its compaction, curing, finishing and strength of concrete shall be as described above.

4.6.5.5 Measurements - The length shall be measured correct to one cm and other dimensions correct to 0.5 cm. The cement concrete shall be measured as per gross dimensions of the encasing exclusive of the thickness of plaster. No deduction shall be made for the volume of steel sections, expanded metal, mesh or any other reinforcement used therein. However, in case of boxed stanchions or girders, the boxed portion only shall be deducted.

Fabric reinforcement such as expanded metal shall be measured separately in square meters stating the mesh and size of strands.

The description shall include the bending of the fabric as necessary, ranking or circular cutting and waste shall be included in the description.          

4.6.5.6 Rate -The rate shall include the cost of materials and labour required for all the operations described above except the cost of fabric reinforcement. The cost of providing and erecting steel section and wire hangers shall be paid for separately.

4.6.6  SPECIFICATIONS PRECAST REINFORCED CONCRETE

4.6.6.1 General requirements - Precast reinforced concrete units such as columns, fencing post, door and window frames, lintels, Chajjas, copings, sills, shelves, slabs, louvers etc. shall be of grade of mix as specified and cast in forms or moulds. The form / moulds shall be of fibre glass or of steel sections for better finish. Provision shall be made in the forms and moulds to accommodate fixing devices such as nibs, clips, hooks, bolts and forming of notches and holes. The contractor may precast the units on cement or steel platform which shall be adequately oiled provided the surface finish is of the same standard as obtained in the forms. Each unit shall be cast in one operation.

4.6.6.2 Concrete used for precasting the units should be well proportioned, mixed, placed and thoroughly compacted by vibrations or tamping to give a dense concrete free from voids and honeycombing.

Precast articles shall have a dense surface finish showing no coarse aggregate and shall have no cracks or crevices likely to assist in disintegration of concrete or rusting of steel or other defects that would interfere with the proper placing of the units. All angles of precast units with the exception of the angles resulting from the splayed or chamfered faces shall be true right angles. The arises shall be clean and sharp except those specified or shown to be rounded. The wearing surface shall be true to the lines. On being fractured, the interior of the units should present a clean homogeneous appearance.

The longitudinal reinforcement shall have a minimum cover of 12 mm or twice the diameter of the main bar, whichever is more, unless otherwise directed in respect of all items except fencing posts or electric posts where the minimum cover shall be 25 mm.

4.6.6.5 Curing - After having been cast in the mould or form the concrete shall be adequately protected during setting in the first stage of hardening from shocks and from harmful effects of frost, sunshine, drying winds and cold. The concrete shall be cured at least for 7 days from the date of casting.

4.6.6.6 The precast articles shall be matured for 28 days before erection of being built in so that the concrete shall have sufficient strength to prevent damage to units when first handled.

4.6.6.7 Marking - Precast units shall be clearly marked to indicate the top of member and its location and orientation in the structure.

Precast units shall be stored, transported and placed in position in such manner that they will not be overstressed or damaged.

4.6.7. SPECIFICATIONS FOR PRECAST CEMENT CONCRETE JALI.

The jali shall be of cement concrete 1:2:4 (1 cement: 2 coarse sand: 4 aggregate 6mm nominal size) reinforced with 1.6 mm thick mild steel wire, unless otherwise specified.

4.6.7.1. Fixing - The jali shall be set in the position true to plumb and level before the joints sills and soffits of the openings are plastered. It shall then be properly grouted with cement mortar 1:3 (1 cement: 3 coarse sand) and rechecked for levels. Finally the jambs, sills and soffits shall be plastered embedding the jali uniformly on all sides.

4.6.7.2 Measurements - The jali shall be measured for its gross superficial area. The length and breadth shall be measured correct to a cm. The thickness shall not be less than specified.

4.6.7.3 Rate - The rate shall be inclusive of materials and labour involved in all the operations described above except plastering of jambs, sills and soffits, which will be paid for under relevant items of plastering.

4.6.8. SPECIFICATIONS FOR DESIGN MIX CONCRETE.

Definition - Design mix concrete is that concrete in which the design of mix i.e. the determination of proportions of cement, aggregate & water is arrived as to have target mean strength for specified grade of concrete.

It will be designed based on the principles given in IS 456-2000 and 23 “Hand book for design mix concrete”.

In order to ensure that not more than the specification proportion of test results is likely to fall below the characteristic strength, the concrete mix has to be designed for higher average compressive strength for a specified grade of concrete is defined as target mean strength.

4.6.8.1. Materials

Cement - One of the following types of cement as specified shall be used -

1.  Ordinary Portland Cement 33 grade conforming to IS: 269.

2.  Ordinary Portland Cement 43 grade conforming to IS: 8112.

3.  Ordinary Portland Cement 53 grade conforming to IS: 2269.

4.  Rapid hardening Portland Cement Conforming to IS: 8041.

5.  Blast Furnace slag cement conforming to IS: 455.

However for severe conditions of sulphate content in sub soil water, special literature on use of sulphate resisting cement may be referred to.

Coarse aggregate - This shall be specified in para 4.1.2 and subparas.

Fine aggregate - This shall be grading zone I, II, or III as specified under para 3.1.4 and  subparas.

Water -   It shall conform to the requirement as laid down in IS: 456 para and para 4.6.1.1. of this section.

Grades of concrete  - The compressive strength of various grades of designation concrete shall be as given in table 16 below -

Table 16

Grades designation 

Compressive strength on 15 cm cubes min at 7 days (N/mm2)

Specified characteristic compressive strength at 28 days (N/mm2)

M 15

10.0

15

M 20

13.5

20

M 25

17.0

25

M 30

20.0

30

M 35

23.5

35

Note -   In the designation of a concrete mix letter M refer the mix and the number to the specified characteristic compressive strength of 15 cm-cubes at 28 days expressed in N/mm².                                                                                              

4.6.8.2 Scope - The procedure described below for design mix is for concrete up to grade M-35 which are generally used for reinforced concrete structure. Minimum grade of concrete for design mix will be M-20 normally. However in cases of projects having some parts of M-15 also in addition to M-20 to M-35 grade, then design mix concrete will cover M-15 grade as an exception only.                        

4.6.8.3 Data for mix design - The following basic data are required to be specified for design of concrete mix.

  1. Characteristic compressive strength of concrete at 28 days.
  2. Degree of workability desired.
  3. Limitation on water cement ratio and minimum cement content to ensure adequate durability.
  4. Type of maximum size of aggregate to be used.
  5. Standard deviation of compressive strength of concrete.

Minimum cement content required in Reinforced cement concrete to ensure durability under specified conditions of exposure, will be in accordance with IS: 456. However it shall not be less than 300 Kgs /m3 of concrete for 33 grade cement.

(a).  Standard Deviation of concrete for each grade shall depend upon the degree of quality control expected to be exercised at site. As per IS: 10262 the values of standard deviation for various grades of concrete for different degree of control shall be specified in Table. 17.

Table 17

Grade of concrete

Standard Deviation for different degree of control in N/mm²

Very good

Good

Fair

M-15

2.5

3.5

4.5

M-20

3.6

4.6

5.6

M-25

4.3

4.3

6.3

M-30

5.0

6.0

7.0

M-35

5.7

6.7

7.7

???Degree of quality control expected under different site conditions are described in Table18

Table 18

Degree of          Control

Condition of production of concrete

Very good

Fresh cement from single source and regular test, weigh batching of all materials, aggregates grading and moisture content, control of water added, frequent supervision, regular workability and strength tests and field laboratory facilities,

Good

Carefully stored cement and periodic test, weigh batching of all materials, controlled water, graded aggregate supplied, occasional grading and moisture tests, periodic check of workability and strength, intermittent supervision and experienced workers.           

Fair

Proper storage of cement, volume batching of all aggregates allowing for bulking of sand, weigh batching of cement, water content controlled by inspection of mix and occasional supervision and tests

4.6.8.4. Target strength for mix design - The target mean strength for a specified grade concrete depends upon the quality control (expressed by standard deviation) and accepted proportion of results of the strength tests below the characteristic strength (Fck) and is given by relation,

TcK = fck + t.s

Tck – target mean compressive strength at 28 days

Fck – characteristic compressive strength at 28 days

    s – standard Deviation

    t – a statistical figure depending upon the accepted proportion of low test results and number of tests.

Note - According to IS: 456 & IS: 1343 the characteristic strength is defined as that value below which not more than 5% (1 in 20) results are expected to fall. In such case value of t will be 1.65 and equation will reduce to Tck = fck+1.65 s.

Selection of proportions - Since different cement, aggregate, of different maximum size, grading surface texture shape, produce concrete of different compressive strength for the same free water cement ratio, the relationship between strength and free water cement ratio corresponding to 28 days’ strength of cement of various grades is given in Fig.1 of IS: 10262 and is reproduced below in chart 1.

28 days strength of cement tested according IS: 4031-1968

A = 31.9 – 36.8 N/mm² (325-375 kg /cm²)

B = 36.8 – 41.7 N/mm²  (375-425 kg /cm²)

C = 41.7 – 46.6 N /mm² (425-475 kg /cm²)

D = 46.6 – 51.5 N /mm² (475-525 kg /cm²)

E = 51.5 – 56.4 N/mm²  (525-575 kg/cm²)

F = 56.4 - 61.3 N /mm² (575-625 kg /cm²)

Chart 1- Relationship between free water cement ratio and concrete strength for different cement strengths.

(a) The free water cement ratio selected from Chart 1 above should be checked against the limiting water cement ratio for requirement of durability as given in IS: 456 and the lower of the two values is to be adopted.

(b)  Estimate of air control - The amount of entrapped air for normal mix (non air entrained) concrete as per IS: 10262 are given in Table 19.

Table 19.

Nominal maximum size of aggregate

Entrapped air as percentage of volume of  concrete

10 mm

3.0

20 mm

2.0

40 mm

1.0

(c) Selection of water content and fine to total aggregate ratio - Based on experience, empirical relationship have been established between quantity of water per unit volume of concrete and ratio of fine aggregate to total aggregate by absolute volume for desired workability. The estimated values for concrete up to M35 grade are given in Table 20.

Table 20.

Nominal maximum size of aggregate in mm

Water content in kgs per cubic meter of concrete

Sand as % age of total aggregate by absolute volume

10

208

40

20

186

35

40

165

30

A)  The values given in Table 19. are based on the following conditions -

  1. Crushed coarse aggregate conforming to IS: 383 and para 4.1.2 of this specification
  2. Fine aggregate consisting of natural sand conforming to grading zone II of IS: 383 water cement ratio (by mass) of 0.6 and
  3. Workability corresponding to compacting factor of 0.8.

B)  For other conditions of workability, water cement ratio, grading of fine aggregate and for round aggregate, certain adjustment in quantities of mixing water and fine to total aggregate ratio as given in Table 19 are to be made as per IS: 10262. These are explained in Table 21 below -

Table  21.

Change of conditions stipulated for

Adjustment required in

Water content

Percentage of fines to total aggregate

For sand conforming to grading 

Zone I & III of IS  -383

 

0

+1.5% for Zone I                                                    -1.5% for Zone III

Increase or decrease in the value of compacting factor by 0.1

For increase

For decrease

 

 

+3.0 %

-3.0%

 

 

0

For each 0.05 increase or

decrease in free water-cement ratio

For increase

For decrease

 

 

 

0

0

 

 

 

+1.0 %

-1.0 %

For rounded aggregates

-15 kg / mm3

-7

C)  Comparison of consistency measurement by various methods-

Workability description

Slump mm

Compacting factor

Extremely dry

--

--

Very stiff

--

0.70

Stiff

0-25

0.75

Stiff plastic

25-50

0.85

Plastic

75-100

0.90

Flowing

150-175

0.95

Calculation of aggregate content - With the quantities of water and cement per unit volume of concrete and ratio of fine to total aggregate content per unit volume of concrete to be calculated from the following equations -

V = absolute volume of fresh concrete which is equal to gross volume (m3),  minus the volume of entrapped air.

W = mass of water (kg) per m3     of concrete

C = mass cement (kg) per m3 of concrete        

P  = ratio of fine aggregate to total aggregate by absolute volume

Sc = specific gravity of cement

Fa, Ca = aggregate (kg) per mof concrete respectively (total masses of fine aggregate and coarse aggregate )

Sfa, Sca = Specific gravities of saturated surface dry fine aggregate and coarse aggregate respectively.                             

Calculation of batch masses - The masses of various ingredients for concrete for design mix of a particular batch size may be calculate as described above.

4.6.8.5 Production of controlled concrete - The calculated mix proportion shall be checked by means of trial batches. Quantities of materials worked out as described above shall be termed as trial mix no.1. The quantities of materials for each trial mix shall be sufficient for at least three 150 mm size cube concrete specimens and concrete required to carry out workability test according to IS: 1199.

Workability of Trial Mix No.1 shall be measured. The mix shall be carefully observed for freedom from segregation and bleeding and its finishing properties. If the measured workability of Trial Mix No.1 is different from the stipulated value, the water content shall be adjusted according to Table 22 corresponding to the required changes in compacting factor. With this adjustment in water content, the mix proportions shall be recalculated keeping the free water-cement ratio at the preselected value which will comprise Trial Mix No.2. In addition, two more Trial Mixes No 3 and 4 shall be made with the water content same as Trial Mix No.2 and varying the free water cement ratio by (+) 10 per cent and (-) 10 per cent of the preselected value. For these two additional trial mixes No.3 and 4, the mix proportions are to be recalculated for the altered condition of free water-cement ratio with suitable adjustments in accordance with Table 22. Fresh trial mixes are to be made for different types and brands of cement, alternative source of aggregates, maximum size and grading of aggregates.

4.6.8.6. Batching - In proportioning concrete, the quantity of both cement and aggregate should be determined by mass. Cement shall be used on the basis of mass and should be weighed separately from the aggregate. Water should be either measured by volume in calibrated tanks or weighed. Any solid admixture that may be added may be measured by mass. Liquid and paste admixture by volume or mass. Batching plant where used should conform to IS: 4925. All measuring equipment should be maintained in a clean serviceable condition and their accuracy periodically checked.

Except where it can be shown to the satisfaction of engineer that supply of properly graded aggregate of uniform quality can be maintained over the period of work, the grading of aggregate should controlled by obtaining the coarse aggregate in different sizes and blending them in the right proportions when required, the different sizes being stocked in separate stock piles. The material should be stock-piled for several hours preferably a day before use. The grading of coarse and fine aggregate should be checked as frequently as possible, the frequency for a given job being determined by engineer to ensure that the specified grading in maintained.

It is important to maintain the water-cement ratio constant at its correct value. To this end, determination of moisture contents in both fine and coarse aggregate shall be made as frequently as possible, the frequency for a given job being determined by the engineer according to weather conditions. The amount of the water to be added shall be adjusted to compensate for any observed variations in the moisture contents. For the determination of moisture content in the aggregates, IS: 2386 (part 3) may be referred to. The allow for the variation in mass of aggregate due to variation in their moisture content, suitable adjustments in the masses of aggregates shall also be made. In the absence of exact data, only in the case of nominal mixes, the amount of surface water may be estimated from the values given in the Table 22.

Table 22 (Surface water carried by aggregate) (Clause 4.6.8.4)

Aggregate

Approximate quantity of surface water

Percent by mass

Litres/m3

Very wet sand

7.5

20

Moderately wet sand

5.0

80

Moist sand                               

2.5

40

Moist gravel to crushed rock

1.25-2.5

20-40

4.6.8.7. Mixing - Concrete shall be mixed in mechanical mixer. The should mixer comply with IS -1791. It shall be fitted with hopper. The mixing shall be continuous until there is uniform distribution of the material and the mass is uniform in colour and consistency. If there is segregation after unloading from the mixer, the concrete should be remixed. The mixing time shall be not less than 2 minutes.

4.6.8.8. Laying - It shall be done as specified under para 4.2.4 of this specification.

4.6.8.9. Curing - It shall be done as specified under para 4.3.4 of this specification

4.6.8.10. Approval of design mix - The preliminary test for approval of design mix shall consists of three sets of separate tests and each set of test shall be conducted on six specimens. Not more than one set of six specimens shall be made on any particular day. Of the six specimens of each set, three shall be tested at seven days and remaining three at 28 days. The preliminary tests at seven days are intended only to indicate the strength to be attained at 28 days.

4.6.8.11. Work strength test - Work strength test shall be conducted in accordance with        IS - 516 on random sampling. Each test shall be conducted on ten specimens, five or which shall be tested at 7 days and remaining five at 28 days. Not less than one work test consisting of testing of test on 10 cubes shall be carried out for every 30 cubic meter of concrete or less as per the lot size as specified below -

Lot size - Concrete under acceptance shall be notionally divided into lots for the purpose of sampling, before commencement of work. The delimitation of lots shall be determined by the following -

  1. No individual lot shall be more than 30 m3 in volume.
  2. At least one cube forming an item of the sample representing the lot shall be taken from the concrete of same grade and mix proportions cast in any day.
  3. Different grades or mixes of concrete shall be divided into separate lots.
  4. Concrete of a lot shall be used in the same identifiable unit of the structure.

4.6.8.12.  Standard of acceptance

  1. The average strength of group of cubes cast for each day shall not be less than the specified work cube strength. 20 per cent of cubes cast for each day may have values less than the specified strength provided that the lowest value is not less than 85% of the specified strength.
  2. Concrete strength less than specified may as a special case be accepted in a member with the approval of engineer provided that the maximum stress in the member under the maximum design live load does not exceed the permissible safe stress appropriate to the lower strength of the concrete.
  3. Concrete which does not meet the strength requirements as specified but has a strength greater than that of the lowest value of 85% may, at the discretion of the designer, be accepted as being structurally adequate without further testing.
  4. Concrete of each grade shall be assessed separately.
  5. Concrete shall be assessed daily for compliance.
  6. Concrete is liable to be rejected if it is porous or honey combed, its placing has been interrupted without providing a proper construction joint, the reinforcement has been displaced beyond the tolerances specified, or construction tolerances have not seen met. However, the hardened concrete may be accepted after carrying out suitable remedial measures to the satisfaction of the engineer.

4.6.8.13. An example illustration the mix design for concrete mix M 20 grade is given below -

Design stipulation

a

Characteristic compressive strength required in the field at 28 days

20N/mm²

b

Maximum sizes of aggregate

20 MM (angular crushed)

c

Degree of workability

0.9 compacting factor (slump 75 mm)

d

Degree of quality control

Good

e

Type of exposure

Mild

Test data of material

a

Cement used - ordinary Portland cement satisfying the requirements of IS: 269-1989

 

b

Specific gravity of cement

3.15

c

Specific gravity of

 

i)

Coarse aggregate

2.60

ii)

Fine aggregate (natural sand)

2.60

d

Water absorption of

 

i)

Coarse aggregate

0.5 percent

ii)

Fine aggregate (natural sand)

1.0 percent

e

Free surface moisture of

 

i)

Coarse aggregate

Nil (absorbed moisture also nil)

ii)

Fine aggregate (natural sand)

2.0 percent

Sieve analysis

a)  Coarse aggregate

IS sieve Size mm

Analysis of course aggregate fraction (Percent passing)

Percentage of different fraction 

 

 

I

II

Combined

20

100

100

60%

40%

100%

10

0

71.2

60%

40%

100%

4.75

 

9.4

0

28.5%

28.5%

2.63

 

0

 

3.7%

3.7%

The grading of combined fraction I and II in the ratio of 60 and 40 conform to Table 10 described above.

b) Fine aggregate

IS sieve sizes

Fine aggregate (percent passing)

100

-

2.36 mm

100

1.18 mm

93

600 micron

60

300 micron

12

150 micron

2

The sand conforms to grading zone III.

Target mean strength - As described earlier for degree of quality control ‘good’ the value of standard deviation is 4.6, therefore with a tolerance factor of 1.65 the value of target mean strength for specified characteristic cube strength = 20 + 1.65 x 4.6 = 27.6 N/mm².

Selection of water cement ratio - From chart 1, the free water cement ratio required for target mean strength of 27.6 N/mm² is 0.50. This is lower than the maximum value of 0.65 prescribed for mild exposure.

Selection of water and sand content - From Table 8 for 20 mm nominal maximum size aggregate and sand conforming to grading zone II water content as per cum concrete is 186 kg and sand content percentage of total aggregate by absolute volume is equal to 35%. For change in value of water cement ratio compacting factor, and sand belonging to zone III the following adjustment is required.

Change in condition

Adjustment required in

Water content

Percentage in total aggregate

    For decrease in water cement Ratio by (0.6-0.5) i.e.0.10

0

-2

For increase in compacting Factor by (0.9-0.8) I.e. 0.10

+3

0

For the conforming Grading zone III

0

-1.5

Total

3

-3.5

Therefore, the required water content = 186+186/100 x3 =186+3.58 =191.6 kg / m3

And required sand content = 35 – 3.5 = 31.5 percent

Determination of Cement Content

Water-Cement ratio = 0.5

Water = 191.6 kgs

Cement = 191.6 / 0.5 = 383 kg / m3

Thus cement content is adequate for mild exposure condition as per IS: 456-2000 as described in table below.

Determination of coarse and fine aggregate content

From Table 18  for specified maximum size of aggregate of 20 mm, the amount of entrapped air in wet concrete is 2 per cent. Taking this into account and applying equations given above.

0.98 m3     = 191.6 +383/3.15 + 1/0.315. fa / 2.60) x 1/ 1000

and

0.98 m3 = 191.6 +383/3.15 + 1/0.315. Ca / 2.60) x 1/ 1000

or fa = 546 kg / m3 and ca = 1187 kg / m3

The mix proportion now works out -

Water

Cement

Fine aggregate

Coarse aggregate

191.6

383 kg

546 kg

1187 kg

or 0.5

1

1.42

3.0

For 50 kg cement, the quantity of materials are worked out as below -

a)

Cement

= 50 kg.

b)

Sand

= 71 kg

c)

Coarse aggregate

154.5 kg.

 

Fraction I - 92.7

 

 

Fraction II - 61.8

 

d)

Water

 

1

For water cement ratio of 0.5 quantity

= 25.0 kg.

2

Extra quantity of water to be added for absorption in coarse aggregate at 0.5% by mass

= 154.5 / 100x0.5 = 0.77 kg.

3

Quantity of water to be deducted for free moisture in sand at 2% by mass

= (-) 171.0/100x2=(-)1.42 kg.

Therefore actual quantity of water = 25.00 + 0.77 – 1.42 = 24.35 kg

Actual quantity of sand required after allowing for mass of free moisture

= 71.0 +1.42=72.42 kg

Actual quantity of Coarse aggregate

Fraction I = 92.7 – (0.6 x 0.77) = 92.24

Fraction II = 61.8 – (0.4 x 0.77) = 61.49

Therefore the actual quantities of different constituent required for mix are -

Water = 24.35 kg

Cement = 50 kg

Sand = 72.42 kg

Coarse aggregate      Fraction I = 92.42 kg      Fraction II = 61.49 kg

Measurements shall be done in accordance with paras above.

Tolerances - Paras above shall apply.

Rate – Paras above shall apply with the exception regarding limitations for actual average compressive strength being less than specified strength which shall be governed by para above for acceptance and prorata rates worked out accordingly.

4.6.9  SPECIFICATIONS FOR Precast reinforced conrete door and window frames

4.6.9.1. Manufacture of precast reinforced concrete door and window frames is described here. These will conform to IS: 6523 in all respects unless otherwise specified. Frames shall be manufactured in an approved factory with all necessary arrangements for fixing hinges or hinges fixed at position as specified with hole for receiving tower bolt, sliding bolt etc. as specified.

4.6.9.2  Shape and dimensions - Precast reinforced concrete door and window frames shall be 60 x 100 mm or 70 x 75 mm in cross section for single shutter and 60 x 120 mm for double shutter door, cross section generally conforming to fig. 29 and 30. Where specified, suitable groove for receiving wall plaster shall be provided.

The over all sizes (width and height) shall be as per drawing or as specified.

4.6.9.3.1 Materials

4.6.9.3.2. Aggregate - The aggregate used shall be of well-graded mixture of clean coarse and fine aggregates. The nominal size of coarse aggregate shall not exceed 10 mm.

4.6.9.3.3. Concrete - Mix of concrete shall be as specified or as directed by the engineer. But the mix shall not be weaker than M 20 controlled mix or 1:1 ½ :3 (1 cement  - 1 ½ coarse sand  - 3 stone aggregate 10 mm nominal size by volume Mix) and shall be suitable for producing a dense concrete without voids after proper vibration.  

4.6.9.3.4. Reinforcement - There shall be minimum of three bars of 6 mm dia or equivalent reinforcement for each vertical or horizontal member shall be one piece and shall be firmly held by 3 mm dia ties spaced at not more than 300 mm center to center. The longitudinal reinforcement shall have a maximum cover of 12 mm or twice the diameter of main bar, which is higher.

4.6.9.4. Casting - The entire frame may be cast complete in one piece or each of the vertical and horizontal members of the frame may be cast separately to be assembled into the complete frame at site. When the frame is cast in separate parts, one of the reinforcing bars of the vertical members of the frame shall be kept projecting so as to tennon into the corresponding hole in the horizontal member. The holes in the horizontal member for taking the projecting reinforcement from the vertical members shall be slightly larger than the bar diameter to facilitate easy insertion of the projecting bar. After assembly at site, the holes shall be grouted with cement slurry of cement 1:2 coarse sand.

4.6.9.5. Mould - The mould for casting shall be preferably be of steel to ensure better surface finish of the cast frame. Provision shall be made in the mould to accommodate fixing devices for hinges and the hold fasts. Where specified, suitable rebates may also be provided to act as plaster groove.

4.6.9.6. Protection and curing  - After casting in moulds, during setting and in first stage of hardening the concrete shall be protected from shocks, running or surface water and the harmful effect of frost, sunshine drying winds and cold. The concrete shall be cured for at least 7 days unless special curing methods are adopted which shall conform to IS: 6523

The frames shall be matured before testing or dispatch for the following periods -

Type of cement used

Period

Ordinary Portland cement, Portland blast furnace slag cement Portland pozzolana cement

28 days

Rapid hardening cement (to be used with approval of engineer)           

14 days

The frames after maturing shall have sufficient strength to prevent damage when handled.

4.6.9.7 Arrangements for fixing of hinges to frames - Suitable arrangement for fixing hinges shall be provided in the frame by one of the following methods as directed (Ref. Figure 30) -

  1. Hardwood fixture - Hardwood blocks of well seasoned teak or other suitable timber 150 mm long, 45 to 50 mm x 30 to 40 mm in cross section, one block for each of the hinge, shall be fixed in position with 6 mm mild steel bolts, nut and washers, after the frame has been cast, cured and matured. After tightening the nuts, the bolt heads and the nuts shall be suitably covered with hard wood fillets, finished flush with concrete surfaces of the frame.

  2. Hinge directly attached to frame - L Type flap hinge may be attached may be attached directly to the frame with the help of 6 mm dia, mild steel bolts and nuts.

  3. Hinge welded to frame - the hinge may be welded to 3 mm thick mild steel flat embedded in frame.

4.6.9.8 Arrangements for door and window fixtures - Suitable arrangements shall be provided in the frame for receiving tower bolts, sliding bolts and other door and window fixtures as indicated.

4.6.9.9 Fasteners

4.6.9.9.1 Arrangements for fixing the frames with hold fasts or metallic fasteners shall be provided in vertical members of frames as specified. In case of door frame, there will be 3 Nos. hold fasts and in case of window, there will be 2 Nos. hold fasts on each vertical member in contact with the opening where the frame is to be fixed. Holes to accommodate 10 mm dia bolts to be fixed to hold fasts and the nuts shall be left at appropriate locations.

4.6.9.10 Erection

When a three piece frame is used, the vertical members shall be held in position with top member placed over them, the whole frame plumbed and firmly supported till the concrete around the hold fasts in the masonry has properly set and hardened. Cement and coarse sand mortar slurry 1 -2 shall be used in grouting the joints between the vertical and horizontal members of door frame. In case where four members are used, the bottom member shall be first placed in position and other erected on this base.

4.7 SPECIFICATIONS FOR PRE-STRESSED CONCRETE

Symbols – For the purpose of this specifications following letter symbols shall have the meaning indicated against each; where other symbols are used, they are explained at the appropriate place:

A

Area

B

Breadth of beam

bw

Breadth of web or rib

D

Overall depth of beam

DL

Dead load

D

Effective depth of beam

dt

Effective depth of beam in shear

Ec

Modulus of elasticity of concrete

EL

Earthquake load

Es

Modulus of elasticity of steel

e

Eccentricity

F

Characteristic load

Fbst

Bursting tensile force

Fd

Design load

¦

Characteristic strength of material

¦ci

Cube strength of concrete at transfer

¦ck

Characteristic compressive strength of concrete

¦cp

Compressive stress at centroidal axis due to prestress or average intensity of effective prestress in concrete

¦cr

Modulus of rupture of concrete (flexural tensile strength)

¦d

Design strength

¦p

Characteristics strength of prestressing steel

¦pl

Maximum prestress after losses

¦pi

Maximum initial prestress

¦pq

Ultimate tensile stress in the tendons

¦t

Maximum principal tensile stress

¦y

Characteristic strength of steel

LL

Live load or imposed load

M

Bending moment

m

Modular ratio

s

Spacing of stirrups

T

Torsional moment

V

Shear force

Vc

Ultimate shear resistance of a section of concrete

Vcu

Ultimate shear resistance of a section uncracked in flexure

Vcr

Ultimate shear resistance of a section cracked in flexure

WL

Wind load

xu

Depth of neutral axis

gt

Partial safety factor for load

gm

Partial safety factor for material

dm

Percentage reduction in moment

tc

Shear stress in concrete

f

Diameter of tendon or bar

4.7.1 Materials, workmanship, inspection and testing

4.7.1.1 Materials

4.7.1.1.1 Cement - The cement used shall be any of the following conforming to accepted standards with the prior approval of the engineer.

a) Ordinary Portland cement,

b) Portland slag cement with not more than 50 percent slag content,

c) Rapid-hardening Portland cement, and

d) High strength ordinary Portland cement.

4.7.1.2 Aggregates - All aggregates shall conform to accepted standard.

The nominal maximum size of coarse aggregate shall be as large as possible subject to the following:

a) In no case greater than one-fourth the minimum thickness of the member, provided that the concrete can be placed without difficulty so as to surround all prestressing tendons and reinforcements and fill the corners of the form;

b) It shall be 5 mm less than the spacing between the cables, strands or sheathings where provided; and

c) Not more than 40 mm: for aggregates having a maximum nominal size of 20 mm or smaller are generally considered satisfactory.

Coarse and fine aggregate shall be batched separately.

4.7.1.3 Water - The requirements of water used for mixing and curing shall conform to the requirements given in IS: 456-2000. However use of seawater is prohibited.

4.7.1.4 Admixtures - Admixtures may be used with the approval of the Engineer. However, use of any admixture containing chlorides in any form is prohibited. he admixture shall conform to the accepted standards IS: 456-2000.

4.7.1.5. Prestressing steel

4.7.1.5.1 The prestressing steel shall be any one of the following conforming to accepted standards IS: 456-2000:

a) Plain hard-drawn steel wire,

b) Cold-drawn indented wire,

c) High tensile steel bar, and

d) Uncoated stress relieved strand.

All prestressing steel shall be free from splits, harmful scratches: surface flaws: rough jagged and imperfect edges and other detects likely to impair its use in prestressed concrete: slight rust may be permitted provided there is no surface pitting visible to the naked eye.

Coupling units and other similar fixtures used in conjunction with the wires or bars shall have an ultimate tensile strength of not less than the individual strengths of the wires or bars being joined.

4.7.1.5.2. Modulus of elasticity – The value of the modulus of elasticity of steel used for the design of prestressed concrete members shall preferably be determined by tests on samples of steel to be used for the construction, for the purposes of this clause a value given by the manufacturer of the prestressing steel shall be considered as fulfilling the necessary requirement.

Where it is not possible to ascertain the modulus of elasticity by test or from the manufacturer of the steel, the following values may be adopted:

Type of steel

Modulus of elasticity, E, (kN/mm2)

Plain cold-drawn wire

210

High tensile steel bars rolled or heat – treated

200

Strands

195

4.7.1.6 Un-tensioned steel - Reinforcement used as untensioned steel shall be any of the following conforming to accepted standards.

a) Mild steel and medium tensile steel bar,

b) Hot rolled deformed bars,

c) Cold-twisted bars, and

d) Hard-drawn steel wire fabric.

4.7.1.7 Storage of materials – Storage of materials shall be done in accordance with good practice.

4.7.2 Concrete

4.7.2.1 Grades - The concrete shall be in grade designated as per Table 23.

Table 23 Grades of concrete

Grade designation

Specified characteristic compressive strength at 28 days, N/mm2

M 30

30

M 35

35

M 40

40

M 45

45

M 50

50

M 55

55

M 60

60

Note 1 - In the designation of a concrete mix, letter M refers to the mix and the number to the specified characteristic compressive strength of 15-cm cube at 28 days expressed in N/ mm2.

Note 2 - For pre-tensioned prestressed concrete, the grade of concrete shall be not less than M 40.

The characteristic strength of concrete is defined, as the strength of the concrete is below which not more than 5 percent of the ten results are expected to fall.

4.7.2.2 Properties of concrete

(1). Increase in strength with age – where it can be shown that a member will not receive its full design stress with a period of 28 days after the casting of the member (for example, in foundations and lower columns in multi storey buildings); the characteristic compressive strength given in Table A may be increased by multiplying by the factors given below.

Minimum age of member when full design stress is expected (Months)

Age factor

1

1.0

3

1.10

6

1.15

12

1.20

Note 1- Where members are subjected to lower dire load during construction, they should be checked for stresses resulting from combination of direct load and bending during construction.

Note 2 - The design strength shall be based on the increased value of compressive strength.

(2). Tensile strength of concrete - The flexural strength shall be obtained in accordance with good practice.. When the designer wishes to use an estimate the flexural strength from the compressive strength, the following formula may be used

Flexural Strength ¦cr, = 0.7 Ö ¦ck   N/mm2

where ¦ck is the characteristic compressive strength of concrete.

(3). Elastic deformation – The modulus of elasticity is primarily influence by the elastic properties of the aggregate and to a lesser extent by the conditions of curing and age of the concrete, the mix proportions and the type of cement. The modulus of elasticity is normally related to the compressive strength of concrete.

In the absence of test data the modulus of elasticity for structural concrete may be assumed as follows:

Ec = 5000 Ö¦ck

Where Ec is the short term static modulus of elasticity in   N/mm2, and ¦ck is the characteristic cube strength of concrete in N/mm2.

(4). Shrinkage - The shrinkage of concrete depends upon the constituents of concrete, size of the member and environmental conditions. For a given environment, the shrinkage of concrete is most influenced by the total amount of water present in the concrete at the time of mixing and, to a lesser extent, by the cement content.

In the absence of test data, the approximate value of shrinkage strain for design shall be assumed as follows:

For pre-tensioning    = 0.0003

0.0002

For post-tensioning  =  ---------------- 

Log10 (t +2)

t = age of concrete at transfer in days.

Note - The value of shrinkage strain for design of post tensioned concrete may be increased by 50 percent in dry atmospheric conditions, subject to a maximum value of 0.000 3.

For the calculation of deformation of concrete at some stage before the maximum shrinkage is reached, it may be assumed that half of the shrinkage takes place during the first month-and that about three quarters of the shrinkage takes place in first six months after commencement of drying.

(5). Creep of concrete - Creep of concrete depends, in addition to the factors listed in 4.7.2.2., on the stress in the concrete, age at loading and the duration of loading. As long as the stress in concrete does not exceed one-third of its characteristic compressive strength, creep may be assumed to be proportional to the stress.

In the absence of experimental data and detailed information on the effect of the variables, the ultimate creep strain may be estimated from the following values of creep coefficient (that is, ultimate creep strain/elastic strain at the KM of loading):

Age at loading

Creep coefficient

7 days

2.2

28 days

1.6

1 year

1.1

Note - The ultimate creep strain estimated does not include the elastic strain.

For the calculation of deformation at some stage before the total creep is reached, it may be assumed that about half the total creep takes place in the first month after loading and that about three-quarters of the total creep takes place in the first six months after loading.

(6). Thermal expansion – The coefficient of thermal expansion depends on nature of cement, the aggregate, the cement content, the relative humidity and the size of sections.

4.7.3. Workability of concrete - The concrete mix proportions chosen should be such that the concrete is of adequate workability for the placing conditions of the concrete and can properly be compacted with the means available.

4.7.4. Durability - The durability of concrete depends on its resistance to deterioration and the environment in which it is placed. The resistance of concrete to weathering, chemical attack, abrasion, frosts and Fire depends largely upon its quality and constituent materials. The strength alone is not a reliable guide to the quality of durability of concrete; it must also have adequate cement content and a low water-cement ratio.

One of the main characteristics influencing the durability of concrete is its permeability. With strong, dense aggregates, a suitably low permeability is achieved by having a sufficiently low water-cement ratio, by ensuring as thorough compaction of the concrete as possible and by ensuring sufficient hydration of cement through proper curing methods. Therefore, for given aggregates, the cement content should be sufficient to provide adequate workability with a low water cement ratio so that concrete can be thoroughly compacted with the means available.

Appendix A given below provides guidance regarding minimum cement content and permissible limits of chloride and sulphate in concrete.

4.7.5. Concrete mix proportioning

4.7.5.1 Mix proportion - The mix proportions shall be selected to ensure that the workability of the fresh concrete is suitable for the conditions of handling and placing, so that after compaction surrounds all prestressing tendons and reinforcements if present and completely Fills the formwork. When concrete is hardened, it shall have the required strength, durability and surface finish.

The determination of the proportions of cement, aggregates and water to attain the required strengths shall be made by designing the concrete mix. Such concrete shall be called 'design mix concrete'.

For prestressed concrete construction, only 'design mix concrete' shall be used. The cement content in the mix should preferably not exceed 530 kg/m3.

Information required - In specifying a particular grade of concrete the information to be included shall be: a) Grade designation. b) Type of cement, c) Maximum nominal size of aggregates, d) Minimum cement content, e) Maximum water-cement ratio, and f) Workability. 

In appropriate circumstances, the following additional information may be specified: a) Type of aggregate. b) Maximum cement content and c) whether an admixture shall or shall not be used and the type of admixture and the conditions of use.

4.7.5.2 Design mix concrete

The mix shall be designed to produce the grade of concrete having the required workability and characteristic strength not less than appropriate values given in Table 23.

4.7.6. Production and control of concrete.

Quality of materials - It is essential for designers and construction engineers to appreciate that the most effective use of prestressed concrete is obtained only when the concrete and the prestressing steel employed are of high quality and strength.

The normal provisions shall apply: except that no hand mixing shall be permitted in prestressed concrete work.

4.7.7. Formwork - Moulds for pre-tension work shall be sufficiently strong and rigid to withstand without distortion, the effects of placing and compacting concrete as well as those of prestressing in the case of manufacture by the individual mould process where the prestressing tendon is supported by the mould before transfer.

4.7.8. Assembly of pre-stressing and reinforcing steel

4.7.8.1.. Pre-stressing steel

(1). Straightening - The wire as supplied shall preferably be self-straightening when uncoiled. If it is not so the wire may need to be mechanically straightened before use. In this event; care shall be taken to avoid alteration in the properties of the wire during the straightening process and preferably a test shall be made on a sample of the wire after straightening.

In the case of high tensile alloy steel bars any straightening (or bending if the design provided for curved bars) shall be carried out by means of a bar-bending machine. Bars shall not be bent when their temperature is less than 10° C.

In no case heat shall be applied to facilitate straightening or bending of prestressing steel.

(2). Arrangement of wires and positioning - All prestressing steel shall be carefully and accurately located in the exact positions shown in the design drawings. The permissible tolerance in the location of the prestressing tendon shall be ± 5 mm. Curves or bends in prestressing tendon required by the designer shall be gradual and the prestressing tendon shall not be forced around sharp bends or. be formed in any manner which is likely to set up un-desirable secondary stresses. 

The relative position of wires in cable, whether curved or straight shall be accurately maintained by suitable means, such as sufficiently 'rigid and adequately distributed spacers.

In the case of post-tension work, the spacing of wires in a cable shall be adequate to ensure the free flow of grout.

The method of fixing and supporting the steel in the mould or the form work shall be such that it is not displaced during the placing or compaction of the concrete or during tensioning of the steel.

The type of Fixtures used for positioning the steel shall be such that it does not give rise to friction greater than that assumed in the design.

(3). Jointing - High tensile wire other than hard-drawn wire may be joined together by suitable means provided the strength of such joints is not less than the individual strengths of the wires being joined. Hard-drawn wire used in prestressed concrete work shall be continuous over the entire length of the tendon.

 High tensile steel bars may be joined together by means of couplings, provided the strength of the coupling is such that in a test to destruction, the bar shall fail before the coupling.

Welding shall not be permitted in either wires or bars.

(4). Cutting - All cutting to length and trimming of the ends of wires shall be done by suitable mechanical or flame cutters. Where flame cutters arc used, care shall be taken to ensure that the flame does not come into contact with other stressed wires or concrete.

Bars shall preferably be ordered to the exact length required. Any trimming required shall be done only after the bar has been tensioned and the grout has set: it shall then be carried out.

(5). Protection of pre-stressing steel and anchorage - In all constructions of the post-tensioned type, whore; prestressing is initially carried out without bond, the prestressing tendon shall, at a subsequent date and generally not later than one week after prestressing, be given an adequate protection against corrosion.

4.7.8.2. Internal pre-stressing steel - Internal prestressing steel is best protected by a cement or cement-sand grout preferably in colloidal form. Care shall be taken to prevent segregation and, for that purpose, only fine sand shall be used.

The grout shall be placed under pressure, and it shall be ensured that the entire space between the duct and the prestressing tendon is properly filled with grout. Where small ducts are encountered, it is advisable that water is flushed through prior to grouting, care being taken to see that all water is subsequently displaced by grout. In the case of but led assemblies, flushing with water shall be carried out only after the jointing material has properly hardened.

Injection shall proceed from one end or preferably in case of curved ducts from the lowest point of the curve, and shall be continued until the grout overflows from the other end.

4.7.8.3. External pre-stressing steel - The protection of external prestressing steel is usually best done by encasing the tensioned wires, cables or bars in a dense concrete secured to the main concrete, for example, by wires left projecting from the latter. If a cement sand mix is used, the cover-provided 'and its density should be adequate to prevent corrosion.

Alternatively, the steel may be encased in bitumen or, where the steel is accessible for inspection and maintenance, paint protection may be provided.

The anchorage shall be adequately protected against damage or corrosion soon after the completion of the final stressing and grouting operations.

4.7.8.4. Cover - In pre-tensioned work the cover of concrete measured from the outside of the prestressing tendon shall be at least 20 mm.

In post-tensioned work, where cables and large sized bars are used, the minimum clear cover from sheathing/duct shall be at least 30 mm or the size of the cable or bar whichever is bigger. Where prestressed concrete members are located in aggressive environment, the cover specified shall be increased by 10 mm.

4.7.8.5. Spacing - In the case of single wires used in pre-tension system, the minimum clear spacing shall not be less than greater of the following:

a) 3 times the diameter of wire, and 

b) 1 1/3 times the maximum size of aggregate.

In the case of cables or large bars, the minimum clear spacing (measured between sheathings/ ducts wherever used) shall not be less than greater of the following:

a) 40 mm, b) Maximum size of cable or bar, and c) 5 mm plus maximum size of aggregate.

4.7.8.6. Grouped cables - Cables or ducts may be grouped together in-groups of not more than four.

The minimum clear spacing between groups of cables or ducts of grouped cables shall be greater of the following: a) 40mm, and b) 5 mm plus maximum size of aggregate.

The vertical distance between groups shall not be less than 50 mm

4.7.8.7. Sheaths and extractable cores - Sheaths shall be sufficiently watertight to prevent concrete laitance penetrating in them in quantities likely to increase friction. Special care shall be taken to ensure water- tightness at the joints.

They shall be preferably machine manufactured and have bores sufficiently large to allow being easily threaded on the cable or bar in long lengths.

The tubes or sheaths shall be of such strength as not to be dented or deformed during handling or concreting.

The alignment of all sheaths and extractable cores shall be correct to the requirements of the drawings and maintained securely to prevent displacement during placing and compaction of concrete. The permissible tolerance in the location of the sheaths and extractable cores shall be ± 5 mm.  Any distortion of the sheath during concreting may lead to additional friction.

4.7.8.8 Reinforcing steel - Assembly of reinforcement and requirements of cover and spacing between bars shall conform to prescribed standards.

4.7.9. Prestressing

4.7.9.1 Prestressing equipment

4.7.9.1.1 Tensioning apparatus - The requirements of 4.7.9.1.1 shall apply to both the pre-tensioned and the post- tensioned methods of prestressing concrete except where specifically mentioned otherwise.

Prestressing steel may be tensioned by means of levers, screw jacks, hydraulic jacks or similar mechanical apparatus. The method of tensioning steel covered by this section is generally by means of hydraulic or similar mechanical jacks. The type of tensioning apparatus shall be such that a controlled force can be applied.

The tensioning apparatus shall not induce dangerous secondary stresses or torsional effects on the steel, concrete, or on the anchorage. The anchorage provided for the temporary gripping of wires or bars on the tensioning apparatus shall be secure and such as not to damage the wire or bar. Devices attached to the tensioning apparatus for measuring the applied force 

shall be such that they do not introduce errors exceeding Five percent.

4.7.9.1.2 Temporary gripping device - Prestressing tendons may be gripped by wedges, yokes, double cones or any other approved type of gripping devices. The prestressing wires may be gripped singly or in groups. Gripping devices shall be such that in a tensile test, the wire or wires fixed by them would break before failure of the grip itself.

4.7.9.1.3 Releasing device - The releasing device shall be so designed that during the period between the tensioning and release, the tension in the prestressing elements is fully maintained by positive means, such as external anchorage. The device shall enable the transfer of prestress to be carried out gradually and so as to avoid large difference of tension between wires in a tendon, severe eccentricities of prestress or the sudden application of stress to the concrete.

4.7.9.1.4 Anchorage - The anchorage may consist of any device, patented or otherwise, which complies with the requirements laid down in this para.

The anchoring device shall be capable of holding, without more than nominal slip, the prestressing tendon subjected to a load midway between the proposed initial prestressing load and the ultimate strength of the prestressing tendon.

The anchoring device shall be strong enough to resist in all respects a force equal to at least the breaking strength of the prestressing tendon it anchors.

The anchorage shall transfer effectively and distribute, as evenly as possible, the entire force from the prestressing tendon to the concrete without inducing undesirable secondary or local stresses.

The anchorage shall be safe and secure against both dynamic and static loads as well as against impact.

The anchorage shall have provision for the introduction of a suitable protective medium, such as cement grout, for the protection of the prestressing steel unless alternative arrangements are made.

4.7.9.2 Procedure for tensioning and transfer

4.7.9.2.1 Stressing - The tensioning of prestressing tendons shall be carried out in a manner that will induce a smooth and even rate of increase of stress in the tendons.

The total tension imparted to each tendon shall conform to the requirements of the design. No alteration in the prestressing force in any tendon shall be allowed unless specifically approved by the designer. 

Any slack in the prestressing tendon shall first be taken up by applying a small initial tension. The initial tension required to remove-slackness shall be taken as the starting point for measuring the elongation and a correction shall be applied to the total elongation to compensate for the initial tensioning of the wire.  The extent of correction shall be arrived at by plotting on a graph the gauge reading as abscissa and extensions as ordinates; the intersection of the carve with the y-axis when extended shall be taken to give the effective elongation daring initial tensioning and the effective elongation shall be added to the measured elongation to arrive at the actual total elongation as shown in fig 2.

When two or more prestressing tendons are to be tensioned simultaneously care shall be taken to ensure that all such tendons are of the same length from grip to grip. This provision shall be more carefully observed for tendons of length smaller than 7.5 m.

The placement of cables or ducts and the order of Messing and grouting shall be so arranged that the prestressing steel, when tensioned and grouted, does not adversely affect the adjoining ducts.

4.7.9.2.2 Measurement of prestressing force - The force induced in the prestressing tendon shall be determined by mean of gauges attached to the tensioning apparatus as well as by measuring the extension of the steel and relating it to its stress-strain curve. It is essential that both methods be used jointly so that the inaccuracies to which each is singly susceptible are minimized. Due allowance shall be made for the frictional losses in the tensioning apparatus.

The pressure gauges or devices attached to the tensioning apparatus to measure the force shall be periodically calibrated to ensure that they do not at any time introduce errors in reading exceeding 2 percent.

In measuring the extension of prestressing steel, any slip, which may occur in the gripping device, shall be taken into consideration.

4.7.9.2.3 Breakage of wires – The breakage of wire in any one prestressed concrete member shall not exceed 2.5 percent during tensioning. Wire breakages after anchorage, irrespective of percentage, shall not be condoned without special investigations.

4.7.9.2.4 Transfer of prestressing force - The transfer of the prestress shall be carried out gradually so as to avoid large differences of tension between wires in a tendon, severe eccentricities of prestressing force and the sudden application of stress to the concrete.

Where the total prestressing force in a member is built up by successive transfers to the force of a number of individual tendons on to the concrete, account shall be taken of the effect of the successive prestressing.

In the long line and similar methods of prestressing, when the transfer is made on several moulds at a time, care shall be taken to ensure that the prestressing force is evenly applied on all the moulds and that the transfer of prestress to the concrete is uniform along the entire length of the tension line.

4.7.9.3 Grouting - The requirements of the grout are fluidity and low sedimentation (or bleeding) in the plastic state, in the hardened state, it shall be dense, have low shrinkage and be durable. The grouting technique adopted should be such that it can be earned out easily and effectively.

Grout shall be made from any of the cements specified in 4.1 and water conforming to 43. Fine sand passing 150 Micron IS Sieve may be added only for ducts of very large size. If permitted by the engineer, admixtures may be added to improve the performance of the grout. The water-cement ratio for neat cement grouts should be approximately 0.50 by mass, but should in no case exceed 0.55 by mass.

The compressive strength of 100 mm cubes of the grout shall not be less than 17 N/mm at 7 days. Cubes shall be cured in a moist atmosphere for the first 24 hours, and subsequently in water.

4.7.9.4. Grouting equipment - The mixer shall be of high speed mixing type, capable of mixing with high local turbulence while imparting only a slow motion to the body of the grout. A grout screen should preferably be fitted.

The pump and the injection equipment shall be capable of continuous operation with little, if any pressure variation and shall have a system for recirculating the grout while actual grouting is not in progress. No compressed air system should be used for Grouting work. The pumping equipment shall be able to deliver the grout at a nozzle pressure 'of at least 0.7 N/mm

All piping to and from the grout pump shall have a minimum of bends, valves and changes in diameter, and the delivery hose shall be as short as practicable.

All piping, pumping And mixing equipment should be thoroughly washed with clean water after each series of operations or more frequently, if necessary. In any case, the intervals between the washings shall not exceed 3 hours.

4.7.9.5 Mixing - Water shall be measured and added to the mixer first, followed by cement. When these are thoroughly mixed, the additive and sand if any, shall be added. When all the ingredients have been added, mixing shall continue for at least two minutes

4.7.9.6. Duct preparation - Ducts shall be kept clean at all times. Unwanted opening at anchorages and in any other location shall be sealed before grouting commences. In all long ducts, or in any duet where considerable changes of level occur and in any large diameter ducts, grout vent shall be provided at all crests and at interval of 20m to 30m so that grout can be injected successively through vents as the grout flows along the ducts. Where water is likely to enter ducts, valley vents shall also be provided for drainage.

4.7.9.7 Grout injection - Grouts should be injected from the lowest point or 'uphill' wherever practicable so that air and water in the duct, being less dense than the grout, will be pushed ahead of the grout mix and be less liable to become entrapped in the grout mix. Grout mix shall be allowed to flow through vent openings until its consistency is equivalent to that of the grout injected. Vent openings shall then be firmly closed one after the other in the direction of flow. Once good grout mix has commenced to flow freely from the end or ends of the duct, that end or ends shall be closed and the pressure built up inside the duct to 0.7 N/mm' before closing the injection end.

In the case of large ducts where pressure grouting cannot be used, a stand pipe or vent pipe shall be provided and kept topped up with cement for an hour or two to replace grout losses due to wastage and subsidence it the termination of grouting operation.

4.7.10. Transporting, placing, compacting and curing

4.7.10.1. Provisions given in Section 4 shall apply.  The use of construction joints in prestressed concrete work should preferably be avoided. But if found necessary, their position and arrangement shall be predetermined by the designer.

4.7.10.2. Jointing of butted assemblies - he joints of butted assemblies shall be made of either cement grout or cement mortar or concrete. Grouting shall be used for joints up to 12 mm thick. For joints thicker than 12 mm and preferably for thicknesses between 18 and 25 mm, mortal shall be used.

The mortar which may be made of one part cement and one and a half part sand, shall be of a dry consistency and shall be packed hard in layers so that it rings true. Where joints exceeding 75 mm are encountered, the joint shall be made of concrete.

The stressing operations may be carried out in case of mortar joints immediately after placing the mortar but the stress in the mortar shall not exceed 7.0 N/rnm2 In the case of grouted joints and concrete joints the allowable stress in the First 24 hours after placing of the grout or concrete in the joint shall approximate as closely as possible to the strength of the grout or concrete used.

The holes for the prestressing tendons shall be accurately located and shall be in true alignment when the units are put together.

Full tensioning shall not be carried out until the strength of the concrete or mortar in the joint has reached twice the transfer stress.

4.7.11. Concreting under special conditions

Work in extreme weather conditions - During hot or cold weather, the concreting should be done in accordance with good practice

4.7.12. Sampling and strength test of concrete

4.7.12.1. The provisions given in Section 4 shall apply; but the optional test requirements of concrete and values of assumed standard deviation shall be as given in Table 24 and Table 25 respectively. 

Table 24 Optional tests requirements of concrete

Grade of concrete

Compressive strength on 15 cm cubes.  Min at 7 days

Modulus of rupture by beam test, Min

At 72 ± 2h

At 7 days

(1)

(2)

N/mm2

(3)

N/mm2

(4)

N/mm2

M 30

20.0

2.1

3.0

M 35

23.5

2.3

3.2

M 40

27.0

2.5

3.4

M 45

30.0

2.7

3.6

M 50

33.5

2.9

3.8

M 55

37.5

3.1

4.0

M 60

40.0

3.3

4.2

Table 25 Assumed standard deviation

Grade of concrete

Assumed standard deviation, N/mm2

M 30

6.0

M 35

6.3

M 40

6.6

M 45

7.0

M 50

7.4

M 55

7.7

M60

7.8

4.7.12.2 Concrete strength at transfer - In addition to the tests required as per 4.7.12.1, additional cube tests should be conducted at appropriate intervals to ensure that the concrete strength in the member at transfer conforms to the design requirements. The frequency of sampling and number of cubes should be decided by the engineer. The sampling of concrete should preferably be at the point of placing and the cubes should be stored as far as possible under the same conditions as the concrete in the members.

4.7.12.3 Acceptance criteria - The provisions of Section 4 shall apply.

4.7.12.4. Inspection and testing of structures - The provisions of Section 4 shall apply, except for the following:

a) For type 1 and 2 structures (see 19.3.2), if within 24 hours of removal of the imposed load, the structure does not recover at least 85 percent of the deflection under superimposed load, the test may be repeated after a lapse of 72 hours. If the recovery is less than 90 percent, the structure shall be deemed to be unacceptable.

b) For type 3 structures (see 19.3.2), if within 24 hours of the imposed load, the structure does not recover at least 75 percent of the deflection under superimposed load, the test may be repeated after a lapse of 72 hours. If the recovery is less than 80 percent, the structure shall be deemed to be unacceptable.

4.7.12.5. General design requirements - The general design requirements for design of prestressed concrete structures shall be as given in Section 4 except as modified and supplemented. 

The effects of prestress shall also be taken into account in assessing loads and forces.

The deductions for prestressing tendons shall be considered for the determination of area, centroid and moment of inertia of the cross section.

4,7,12.6. Deductions for prestressing tendons - In calculating area, centroid and moment of inertia of a cross-section, deduction for prestressing tendons shall be made as follows:

a) In the case of pre-tensioned members, where the prestressing tendons arc single wires distributed on the cross section or strands of wires of relatively small cross sectional area, allowance for the prestressing tendons need not be made. Where allowance is made it shall be on the basis of (m - 1) times the area of the prestressing tendons, m being the modular ratio. 

b) In the case of post-tensioned members, deductions shall invariably be made for prestressing tendons, cable ducts or sheaths and such other openings whether they are formed longitudinally or transversely. These deductions need not however be made for determining the effect of loads applied after the ducts, sheaths or openings have been grouted or Filled with concrete. Where such deductions are not made, a transformed area equivalent to (m -1) times the area of the pre-stressing tendon shall be taken in calculation, m being the modular ratio.

4.7.12.7. Instability during erection – In evaluating the slenderness effects during lifting of slender beams, the following factors require consideration;

a) Beam geometry,

b) Location of lifting points,

c) Method of lifting, and

d) Tolerances in construction.

All beams which are lifted on vertical or inclined slings, shall be checked for lateral stability and lateral moment on account of tilling of beam due to inaccuracies in location of lifting points, and due to the lateral bow. 

For calculating the factor of safety against lateral instability y, reference may be made to specialist literature. This factor shall not be less than two.

For determining the lateral moment due to tilling, realistic values, which are not likely to be exceeded in practice, shall be assumed for the eccentricity of lifting points and the lateral bow.  The maximum tensile stress for gi / (gi – 1) times the lateral moment due to lilting, shall not exceed 1.5 N/mm2.

4.7.13. Prestressing requirements

4.7.13.1. Maximum initial prestress – At the time of initial tensioning, the maximum tensile stress,  ¦py immediately  behind  the anchorages shall not exceed 80 percent of the ultimate tensile strength of the wire or bar or strand.

4.7.13.2. Losses in prestress – While assessing the stresses in concrete and steel during tensioning operations and later in service, due regard shall be paid to all losses and variations in stress resulting from creep of concrete, shrinkage of concrete, relaxation of steel, the shortening (elastic deformation) of concrete at transfer, and friction and slip of anchorage. Unless otherwise determined by actual tests, allowance for these losses shall be made in accordance with the values specified.

In computing the losses in prestress when untensioned reinforcement is present, the effect of the tensile stresses developed by the untensioned reinforcement due to shrinkage and creep shall be considered, 

4.7.13.3. Loss of prestress due to creep of concrete - The loss of prestress due to creep of concrete under load shall be determined for all the permanently applied loads including the prestress.

The creep loss due to live load stresses, erection stresses and other stresses of short duration may be ignored. The loss of prestress due to creep of concrete is obtained as the product of the modulus of elasticity of the prestressing steel {see 4.53) and the ultimate creep strain of the concrete fibre (see 5.2.5.1) integrated along the line of centre of gravity of the prestressing steel over its entire length.

The total creep strain during any specific period shall be assumed for all practical purposes to be the creep strain due to sustained stress equal to the average of the stresses at the beginning and end of the period.

4.7.13.4. Loss of prestress due to shrinkage of concrete - The loss of prestress due to shrinkage of concrete shall be the product of the modulus of elasticity of steel and the shrinkage strain of concrete.

4.7.13.5. Loss of prestress due to relaxation of steel - The relaxation losses in prestressing steels vary with type of steel, initial prestress, age and temperature and therefore shall be determined from experiments. When experimental values are not available, the relaxation losses may be assumed as given in Table 26.

Table 26 Relaxation losses for prestressing steel at 1000 hours at 270C

Initial stress

Relaxation loss N/mm2

0.5 ¦p

0

0.6 ¦p

35

0.7 ¦p

70

0.8 ¦p

90

Note ¦p is the characteristic strength of prestressing steel.

For tendons at higher temperatures or subjected to large lateral loads, greater relaxation losses as specified by the engineer shall be allowed for. No reduction in the value of the relaxation losses should be made for a tendon with a load-equal to or greater than the relevant jacking force that has been applied for a short time prior to the anchoring of the tendon.

4.7.13.6. Loss of prestress due to shortening of concrete - This type of loss occurs when the prestressing tendons upon release from tensioning devices cause the concrete to be compressed. This loss is proportional to the modular ratio and initial prestress in the concrete and shall be calculated as below, assuming that the tendons are located at their centroid:

a) For pre-tensioning, the loss of prestress in the tendons at transfer shall be calculated on a modular ratio basis using the stress in the adjacent concrete.

b) For members with post-tensioned tendons, which are not stressed simultaneously, there is a progressive loss of prestress during transfer due to the gradual application of the prestressing forces. This loss of prestress should be calculated on the basis of half the product of the stress in the concrete adjacent to the tendons averaged along their lengths and the modular ratio. Alternatively, the loss of prestress may be exactly computed based on the sequence of tensioning.

4.7.13.7. Loss of prestress due to slip in anchorage - Any loss of prestress, which may occur due to slip of wires during anchoring or due to the strain of anchorage, shall be allowed for in the design. Loss due to slip in anchorage is of special importance with short members and the necessary additional elongation should be provided for at the time of tensioning to compensate for this loss.

4.7.13.8. Loss of prestress due to friction -The design shall take into consideration all losses in prestress that may occur during tensioning due to friction between the prestressing tendons and the surrounding concrete or any fixture attached to the steel or concrete.

4.8. SPECIFICATIONS FOR SELF-COMPACTING CONCRETE

1. Self compacting concrete is concrete that is able to flow under its own weight and completely fill the formwork, even in the presence of dense reinforcement, without segregation, whilst maintaining homogeneity.

2. Application Area - Self compacting concrete may be used in precast-applications or for concrete placed on site.  It can be manufactured in a site batching plant or in a ready mix concrete plant and delivered to site by truck.  It can then be placed either by pumping or pouring into horizontal or vertical structures.  In designing the mix, the size and the form of the structure, the dimension and density of reinforcement and cover should be taken in consideration.

3. Characteristics of fresh self compacting concrete - The level of fluidity of self compacting concrete are governed chiefly by the dosing and type of super-plasticizer.  Due to the high fluidity of self compacting concrete, the risk of segregation and blocking is very high.  Preventing segregation is therefore an important feature of the control regime.  The tendency to segregation can be reduced by the use of sufficient amount of fine (< 0.125 mm) or using a Viscosity Modifying Admixture (VMA).

Features of fresh self compacting concrete

  1. Slump about 600 mm
  2. Sufficient amount of fines ( < 0.125 mm)
  3. Use of Viscosity Modifying Admixture
  4. Segregation resistance.

Fig. 25 Seismic separation joints detail at roof

Fig. 26 Details of construction joints, A, B, C, D, E, F, And H

Fig. 29 Reinforced concrete door and window frame

Fig. 30 RCC Door & Window Frames

 

                               

* * * *