CONCRETE



CONCRETE

Concrete works involves correct procedures of proportioning,batching, mixing, transporting, placing, compacting and curing.

1. Requirements

  1. For building works, the concrete shall be in grades designated as per table (Ref: Table 2 of IS: 456-2000) and is classified as ordinary concrete, standard concrete and high strength concrete.
  2. The characteristic strength is defined as the strength of material below which not more than 5% of the test results are expected to fall.
  3. Ordinary concrete is made on the basis of nominal mix proportioned by weight of its main ingredients. Proportion by volume may be allowed with the permission of the Engineer, only where proportion by weight is not practical.
  4. Standard concrete is made on the basis of design mix proportioned by weight of its ingredients and may contain chemical admixtures to achieve certain target values of various properties in fresh condition.
  5. High strength/performance concrete is similar to standard concrete but contains additional one or more mineral admixtures providing binding characteristics and partly acting as inert filler material which increase its strength, reduce its porosity and modify its other properties in fresh as well as hardened condition.

Grades of concrete: 

Group

Grade

Designation

Characteristic compressive

strength in 28 days (MPa)

Ordinary concrete

M10

M15

M20

10

15

20

Standard concrete

M25

M30

M35

M40

M45

M50

M55

25

30

35

40

45

50

55

High strength   concrete

M60

M65

M70

M75

M80

60

65

70

75

80

2 Workmanship

2.1 Proportioning

  1. The proportions of cement, aggregates and water to attain the required characteristic strengths shall be made by designing the concrete mix (Design mix concrete) or by adopting nominal concrete mix (Nominal mix concrete).
  2. Design mix concrete is preferred to nominal mix. Design mix shall invariably be used for grade M25 and higher.
  3. The proportions of materials for nominal mix concrete shall be in accordance (Ref: Table 9 of IS: 456-2000)

Grade ofconcrete

Total quantity of dryaggregate   (CA+FA)by mass (kg)

Quantityof water (litres)

M5

M7.5

M10

M15

M20

800

625

480

330

200

60

45

34

32

30

NOTE: Proportion of fine aggregate (FA) to coarse aggregate (CA) is generally 1:2, butvaries from 1:1.5 (Zone-I) to 1:2.5 (Zone-III).

Example,1:1.5 for Maximum size of aggregate (MSA) = 10 mm,

1:2.0 (Zone-II) for MSA = 20 mm,

1:2.5 (Zone-III) for MSA = 40 mm for an average grading of FA (Zone-II of

Table 7.4 of IS: 383).

  1. The mix design should be as per IS: 10262 and the mix so designed shall be approved by the Engineer.

  2. Mix design shall be revised, if there is either change of the source or in the quality of the materials.

  3. The materials and their proportion as established by the mix design shall invariably be followed.

2.2 Volume batching

  1. Volume batching may be allowed with the permission of Engineer, only where weigh-batching is not practical.
  2. In such cases, accurate bulk densities of materials to be used in concrete shall be determined earlier. Allowance for bulking shall be made in accordance with IS: 2386 (Part 3).
  3. The mass volume relationship should be checked as frequently as necessary, the frequency for the given job being determined by Engineer to ensure that the specified grading is maintained.
  4. Volume batching shall be done by measuring boxes having capacity in multiples of volumes of one bag of cement.
  5. It is important to maintain the water-cement ratio constant at its correct value. For this, determination of moisture contents in both fine and coarse aggregates shall be made as frequently as possible, according to weather conditions.
  6. The quantity of water to be added shall be adjusted to compensate for any observed variations in the moisture content determined as per IS: 2386(Part 3).

2.3 Plant batching and mixing

  1. Ready-mixed concrete (RMC) supplied by ready-mixed concrete plant shall bepreferred. For large and medium project sites the concrete shall be sourcedfrom ready mixed concrete plants or from on site or off site batching andmixing plants.
  2. The quantity of both cement and aggregate shall be determined by mass, solidadmixture by mass, liquid admixture measured in volume or mass, water shallbe weighed or measured by volume in a calibrated tank (Also refer IS: 4925).
  3. The accuracy of the weigh batcher shall be periodically checked and the needleshould be adjusted to zero when the hopper is empty.
  4. The mix recipe for the mixes to be produced shall be readily available to themixer operator.
  5. Sequencing and blending of the ingredients during charging of the mixers shallbe carried out in such a way as to obtain uniformity and homogeneity in theconcrete.
  6. During batching, aggregates shall be measured in a manner to maintain theirdesired grading, and all materials shall be weighed to the tolerances required toproduce the approved design mix.
  7. The coarse aggregate shall be controlled to minimise segregation andundersized material.
  8. Fine aggregate shall be controlled to minimise variations in gradation, givingspecial attention for keeping finer fractions uniform and exercising care toavoid excessive removal of fines during processing.
  9. Avoid blending two sizes of fine aggregate by placing alternate amounts in binsor stockpiles or when loading in trucks. Satisfactory results are achieved whendifferent size fractions are blended as they flow into a stream from regulatinggates or feeders.
  10. Stockpiling of coarse aggregate shall be kept to a minimum because fines tendto settle and accumulate. When stockpiling is necessary, use proper methodsto minimize the problems with fines, segregation, aggregate breakage, excessivevariation in gradation, and contamination.
  11. Stockpiles shall be built up in horizontal or gently sloping layers, not by end-dumping. Trucks, loaders, and dozers, or other equipment shall not beoperated on the stockpiles because, in addition to breaking the aggregate, theyfrequently track dirt onto the piles.
  12. Storage facilities for bulk cement shall include separate compartments for eachtype of cement used. The interior of a cement silo shall be smooth, with aminimum bottom slope of 50  from the horizontal for a circular silo and 55 to60  for a rectangular silo.
  13. Silos shall be equipped with non-clogging air diffuser flow pads through whichsmall quantities of dry, oil free, lowpressure air can be introducedintermittently at approximately 20 to 35 kPa to loosen cement that has settledtightly in the silos. Storage silos shall be drawn down frequently, preferablyonce per month, to prevent cement caking.
  14. Each bin compartment from which cement is batched shall include a separategate, screw conveyor, air slide, rotary feeder, or other conveyance thateffectively allows both constant flow and precise cut off to obtain accuratebatching of cement.
  15. All bins and silos shall be properly tagged at silos, bins and near charginghose.
  16. Bags of cement should be stacked on pallets or similar platforms to permitproper circulation of air. For a storage period of less than 60 days, stack thebags in not more than 14 layers, and for longer periods, in not more than 7layers.
  17. The quantity of water actually entering the mixing drum should be checkedwith the reading of the gauge or valve setting, when starting a job.
  18. The water batcher and the water pipes should be leak free.

2.4 Process of plant batching and mixing

  1. Manual control batching: Manual plants are acceptable for small jobs havinglow batching rate requirements.
  2. Semi-automatic control batching: In this system, aggregate bin gates forcharging batchers are opened by manually operated push buttons or switches.Gates are closed automatically when the designated weight of material hasbeen delivered.
  3. Automatic control batching: Automatic batching of all materials is electricallyactivated by a single starter switch. However, interlocks shall interrupt thebatching cycle when the scale has not returned to ±0.3% of zero balance orwhen weighing tolerances are exceeded.
  4. Batch plant bins shall be of sufficient size to effectively accommodate theproduction capacity of the plant. Compartments in bins separate the variousconcrete materials, and the shape and arrangement of aggregate bins shallprevent aggregate segregation and leakage.
  5. Weigh batchers shall be charged with easy-operating clam shells or undercutradial-type bin gates.
  6. Gates used to charge semi-automatic and fully automatic batchers shall bepower operated and equipped to obtain the desired weighing accuracy. Theyshall be calibrated by the plant supplier for the types of aggregate used at thestandard range of moisture contents.
  7. Weigh batchers shall be accessible for obtaining representative samples, andthey shall be arranged to obtain the proper sequencing and blending ofaggregates during charging of the mixer.
  8. The amount of concrete mixed in any one batch shall not exceed the ratedcapacity of the mixer.
  9. All mixing and batching plants shall be maintained free of set concrete orcement and shall be clean before commencing mixing.
  10. For each different type of cement at use at the plant, a separate silo shall beprovided.
  11. Each batch shall be so charged into the mixer that some of the water will enterin advance of the cement and aggregates. Controls shall be provided to preventbatched ingredients from entering the mixer before the previous batch has beencompletely discharged.
  12. The first batch of concrete through the mixer shall contain an excess of cementto allow for coating of the inside of the mixing drum without reducing therequired mortar content of the mix.
  13. All water should be in the drum by end of the first 15 seconds of the specifiedmixing time. Each batch would be mixed until the concrete is uniform in colourat least for a period of two minutes after all the materials and water are in thedrum.
  14. Mixing plant that has been out of action for more than 30 minutes shall bethoroughly cleaned before any fresh concrete is mixed in it.
  15. When a change of mix is made to one using a different type/grade of cement,the mixing plant shall be thoroughly cleaned of all traces of the previously usedcement, whatever is the time interval between successive mixes.

2.5 Tolerances in batching

  1. The accuracy of the measuring equipment shall be within +2% of the quantityof cement being measured and within +3% of the quantity of aggregate,admixtures and water being measured. Standard test weights shall be availableto permit the checking of scale accuracy.
  2. Testing of the weighers shall be at three month intervals. If water is dispensedby flow meter, the frequency of testing shall be at three-month intervals. Suchtesting shall be undertaken by a calibration agency approved bythedepartment.
  3. Test certificates shall be displayed in the plant in prominent positions.
  4. Cement supplied in bags shall be placed directly from the bag into the intake ofthe mixing plant and each batch must contain one or more complete bags ofcement. No mixer having a rated capacity of less than a one-bag batch shall beused and the mixer shall not be charged in excess of its rated capacity.

2.6 Hand mixing

  1. Hand mixing shall be avoided as far as possible.
  2. Engineer may permit hand mixing for small jobs when mixing by mixer is notpossible due to practical reasons.
  3. It shall be done on a smooth water tight platform large enough to allow efficientturning over of the ingredients of concrete before and after adding water.
  4. Mixing platform shall be so arranged that neither foreign material shall getmixed with concrete nor the water for mixing shall flow out.
  5. Cement bags of required number shall be placed in a uniform layer on top ofthe measured quantity of fine aggregate, which shall also be spread in a layerof uniform thickness on mixing platform.
  6. Dry sand and cement shall then be mixed thoroughly by turning over to get amixture of uniform colour.
  7. Water required for the mix shall then be added gradually and the mass turnedover till the mixture obtained is of uniform colour and required consistency.
  8. Measured quantity of aggregate shall then be placed on the mixing platformand mixed till the mortar and mixture obtained is of uniform colour andrequired consistency.
  9. In hand mixing, quantity of cement shall be increased by 10% more than thequantity actually required by the design mix at the cost of Contractor.

 2.7 Pumped concrete

  1. Access for the pump shall be checked prior to the pour. If access cannot beassured, the Contractor shall not continue with concreting operations.
  2. If approval is obtained for pumped concrete, the Contractor shall ensure thatshock is not transferred from the pipeline to the formwork and previously laidconcrete.
  3. During placing of concrete by pumping, the end hose must never reach into theconcrete. All measures shall be taken to avoid blockage of the delivery hosesystem.
  4. Grout shall be pumped through the concrete pump to provide initiallubrication. The initial discharge of any pumped concrete shall not beincorporated in the permanent works.
  5. Where concrete is conveyed by chuting or pumping, the plant shall be of a sizeand design to ensure continuous flow in the chute or pipe.
  6. The slope of the chute or the pressure of the pump shall allow the concrete toflow without the use of any water additional to that approved by the Engineerto produce the required consistency and without segregation of the ingredients.
  7. The delivery end of the chute or pipe shall be thoroughly flushed with waterbefore and after each working period and kept clean. The water used for thispurpose shall be discharged outside and away from any permanent works.

2.8 Placing of concrete

  1. Concrete shall be placed in its final position before initial set has commencedand shall not be subsequently disturbed. All concrete shall be placed within 15minutes of mixing unless carried in purpose made agitators.
  2. Concrete shall be carefully placed in horizontal layers which shall be kept at aneven height throughout the work. The depth of layers and time between placingof layers shall be such that each layer can be properly merged into thepreceding layer before initial set takes place, the depth of layer shall bedetermined from the type of plant the Contractor proposes to use.
  3. Concrete shall be allowed to slide or flow down sloping surfaces like chute orpipe or shovelled into position.
  4. Concrete placed in horizontal slabs from barrows or other tipping vehicles shallbe tipped into the face of the previously placed concrete.
  5. Concrete dropped into place shall be dropped vertically. It shall not strike theformwork between the point of its discharge and its final place in the work, and except by approval of the Engineer it shall not be dropped freely through aheight greater than 1.5 m.
  6. Chutes and conveyor belts shall be so designed that there is no segregation orloss of mortar and shall be provided with a vertical tapered down pipe, or otherdevice, to ensure that concrete is discharged vertically into place.
  7. Concrete shall not be placed in standing water in the formwork.
  8. Concrete that has attained its initial set or has contained its water content formore than 1.5 hours or 300 drum revolutions, whichever comes first, shall notbe deposited in the work.
  9. During wet weather, the concrete shall be adequately protected as soon as it isin position.
  10. No concreting shall be carried out during periods of continuous heavy rainunless it is completely covered during mixing, transporting and placing.
  11. Underwater placing of concrete is allowed only for unreinforced components,the placing being effected exclusively with stationary tremies or with a bottom-opening watertight boxes and shall be in accordance with the requirements ofdesign or equivalent as accepted.
  12. Underwater concrete is to be placed continuously without interruption. Forwater depths up to 1m the concrete may be placed without tremie. In the caseof water depths exceeding 1m the concrete is to be placed in such a way that itdoes not fall freely through the water.
  13. The tremies must at all times, dip sufficiently far into the freshly placedconcrete to ensure that the concrete emerging from the tremie does not comeinto contact with the water.
  14. All works connected with the placing of concrete under water shall be designed,directed and inspected with due regard to local circumstances and purposes.
  15. Stops in concrete, at the end of a period of work, shall be made only atconstruction joint locations shown on the drawings and/or positions accepted.
  16. Where the positions of construction joints are not indicated on the drawings,these may be assumed, for estimating purposes, to occur at 5 m intervals infoundations and retaining walls and at one-third to one-quarter of span inslabs and beams subject to a maximum spacing of approximately 9 m.
  17. At construction joint location, the surface of the completed concrete shall beprepared by wire brushing or chipping so that it is free from all laitance, scumand loose material and shows a slightly roughened texture and tips of thecoarse aggregate exposed. Before continuing concreting the exposed concreteface shall be thoroughly wetted and grouted with rich mix.
  18. Engineer’s acceptance shall be obtained by the Contractor, prior to start ofwork, on the casting sequence and the layout of joints.
  19. Water stops shall be carefully maintained in position prior to concreting onaccurately profiled stop boards to create rigid conditions.
  20. The type of waterstops to be used shall suit the joint and purpose according towater bar manufacturer’s recommendations.

2.9 Compaction

  1. Mechanical vibrators used shall comply with IS: 2506, IS: 2514 and IS: 4656.
  2. Concrete shall be thoroughly compacted by vibration during the operation ofplacing and thoroughly worked around the reinforcement, around embeddedfixtures and into corners or the formwork to form a solid mass free from voids.
  3. When vibrators are used to compact the concrete, vibration shall be appliedcontinuously during the placing of each batch of concrete until the expulsion ofair has practically ceased and in a manner that does not promote segregationof the constituents of the concrete.
  4. A sufficient number of vibrators in serviceable condition shall be on site toensure that spare equipment is always available in the event of breakdown.
  5. Operators shall be trained in the use of vibrators. Foremen shall have sufficientexperience in the supervision of placing concrete.
  6. The diameter of needle vibrator shall be decided from the consideration of thespacing between the reinforcing bars in the formwork.
  7. The preferred frequency of needle vibrator shall be between 3000 and 6000 rpmwith an acceleration of 4 gm to 10 gm.
  8. The spacing of vibrator immersion depends on the radius of influence ofvibrator and it shall be preferable to immerse the vibratorinto concrete at intervals of not more than 600 mm or 8 to 10 times thediameter of the needle vibrator.
  9. The period of vibration required shall be of the order of 30 seconds to 2 minute.The concrete shall be placed in layers not more than 600 mm high.
  10. Plunge the vibrator vertically into the concrete and do not use the vibrator tomove concrete horizontally. Vibration of the concrete shall not be applied byway of the reinforcement.
  11. Allow the vibrator to sink by means of its own weight to the bottom of the liftand penetrate the previous lift about 100 mm.
  12. Manipulate the vibrator with an up-and-down motion for 5 to15 seconds to knitthe two layers together.
  13. Withdraw the vibrator gradually with a series of up-and-down motions so thatair bubbles are worked upward in front of the vibrator.
  14. Rapidly extract the vibrator from the concrete when the head becomes partiallyexposed. If the vibrator hole does not close, partially and quickly reinsert thevibrator a short distance away from the hole.
  15. During the placing of all reinforced concrete, a competent steel fixer and acompetent carpenter shall be in attendance on each concreting gang. Theyshall ensure the reinforcement, embedded fittings and forms are kept inposition as work proceeds.

2.10 Curing.

  1. Freshly placed concrete shall be protected from sun, wind, exposure andexcessive drying out. Moisture curing shall be commenced at the earliest afterplacing of concrete.
  2. Immediately following form removal, the surfaces shall be kept continuouslywet by a water spray or water-saturated fabric or until the membrane-formingcuring compound is applied.
  3. Shall adopt curing measures that preclude the possibility ofthermal shock to the concrete during curing. This may be achieved by ensuringthat the temperature of the water used for curing does not differ from that ofthe concrete by more than 15°C.
  4. Curing shall continue for at least 7 days and until it attains an in-placecompressive strength of the concrete of at least 70% of the specifiedcompressive or flexural strength, whichever period is longer.
  5. When low water to cement ratio is used, the concrete shall be preferably curedby water.
  6. Special attention shall be given to the curing of vertical and overhangingsurfaces to ensure satisfactory curing.
  7. All fresh concrete shall be cured by liquid membrane curing compound orequivalent as soon as possible, provided that the surface water has evaporated.The liquid membrane curing shall not be applied if bleed water is forming or ispresent on the concrete surface.
  8. If the concrete is cured by liquid membrane, the applied film should beprotected from rain for at least 3 hours and care should be taken to ensurethat the film is not broken.
  9. All concrete shall be cured for a period of time required to obtain the fullspecified strength, but not less than seven consecutive days. The method ofcuring shall be by water for the first seven days and by water or membraneuntil the concrete has reached the full specified strength.
  10. For mixtures with a low to zero bleeding rate, or in the case of aggressivelyevaporative environments, or both, the curing shall start at early anytimebetween placement and final finishing of the concrete. The curing shall be byreducing the moisture loss from surface using fogging systems and the use ofevaporation reducers such as monomolecular water curing compound.
  11. The method of curing shall ensure that sufficient moisture is present tocomplete the hydration of the cement, and shall be to the approval of theEngineer. The method of curing shall not:

i) Disfigure permanently exposed surfaces.

ii) Affect bonding of subsequent coatings.

iii) Increase the temperature of the concrete.

l. During the curing period, exposed concrete surface shall be protected from thedirect rays of the sun.

 Curing of formed surfaces:

  1. Formed surfaces, including the underside of beams, girders, supported slabsand the like, by moist curing with the forms in place for the full curing period,or until the forms are removed.
  2. In case of formed concrete if liquid membrane curing is used, it should beapplied immediately after de-shuttering (Removal of the form). In such a casethe concrete surface must be dampened with clean water prior to theapplication.

Moisture curing:

  1. Moisture curing shall be performed by:
  1. Covering the surface of the concrete with water and keeping it continuouslywet.
  2. Continuous use of fine fog water sprays.
  3. Covering the surface with a saturated absorptive cover and keeping itcontinuously wet.
  4. Jute mats, cotton mats, and other absorbent materials can be used to holdwater on horizontal or vertical surfaces.
  5. Wet straw or hay can be used for wet-curing small areas, but there is thedanger that wind might displace it unless it is held down with screen wire,burlap, or other means.
  1. Where method (i) is employed, the bunds used shall not be made from fill fromexcavations or any other areas where there is the possibility of chloridecontamination.

Moisture retaining cover curing:

  1. The concrete surface shall be covered with a suitable absorptive covering.
  2. The absorptive covering shall be overlaid with a 1000 gauge polythene sheet.
  3. The cover shall be in the widest practical widths and shall have 200 mm sideand end laps.
  4. The laps shall be sealed with adhesive tape.
  5. Any penetrations or tear in the covering shall be repaired with the samematerial and waterproof tape.

Liquid membrane curing:

Membrane forming curing compounds shall be applied in accordance with themanufacturer's recommendations immediately after any water sheen whichmay develop after finishing has disappeared from the surface and within 2 h ofstripping formwork on formed surfaces.

Membrane forming curing compounds shall not be used on surfaces againstwhich additional concrete or other material is to be bonded unless:

  1. It is proven that the curing compound will not prevent bond, or
  2. Positive measures are taken to remove it completely from those areas whichare to receive bonded applications on fair faced concrete surfaces.

Steam curing:

  1. An enclosure shall be formed around the concrete using tarpaulin or othersuitable means.
  2. Application of steam shall not be commenced until at least 2 hours after finalplacement of concrete.
  3. Steam shall be applied at a temperature between 65°C and 80°C.
  4. Excessive rates of heating and cooling shall be prevented during steam curingand temperatures in the enclosure shall not be allowed to increase or decreaseby more than 22°C per hour.
  5. The maximum steam temperature shall be maintained in the enclosure untilconcrete has reached its specified strength.

3.  Field checks to be effected during concrete work

  1. In proportioning concrete, the quantity of both cement and aggregate shall bedetermined by weight.
  2. Where the weight of cement bag as given by the manufacturer is accepted, areasonable number of bags shall be weighed separately to check the net weight.
  3. Where cement is weighed from bulk stock at site and not by bag, it shall beweighed separately from the aggregates.
  4. Water shall either be measured by volume in calibrated tanks or weighedseparately from the aggregates.
  5. All measuring equipment shall be maintained in a clean and serviceablecondition. Their accuracy shall be periodically checked.
  6. The specified water-cement ratio shall always be kept constant and at itscorrect value.
  7. Moisture content in both fine and coarse aggregates shall be determined asfrequently as possible, the frequency for a given job being determined by theEngineer according to the weather conditions. The amount of water to be addedshall then be adjusted to compensate for variations in the moisture content.
  8. For the determination of moisture content in the aggregates IS: 2386 (Part 3)shall be referred. Suitable adjustments shall be made in the weight ofaggregates to allow for the variation in weight due to variation in their moisturecontent.
  9. It shall be ensured that any cut-outs or openings provided in any structuralmember to facilitate erection of formwork are closed with the same grade ofconcrete as that of the structure, after formwork is removed.
  10. Provision for safe access to the formwork shall be made at all levels as required.
  11. Close watch shall be maintained to check for settlement of formwork duringconcreting and any settlement shall be promptly rectified.
  12. Natural ground shall be checked for bearing capacity and likely settlementbefore erection of the staging.
  13. It shall be ensured that water used for curing or rain water does not stagnatenear the base plate of the staging.
  14. For shutters used for deep and narrow member, temporary openings in the sideshall be provided to facilitate pouring and compaction of concrete.

4. Sampling and testing:

Test

Code

Min. quantity of

material/work for

carrying out test

Minimum

frequency of

sampling

Slump test

IS: 1199

--

Every 5 m3.

Cube test

IS: 516

20 m3 in slab,

beam

Every 20 m3 of a

day’s concrete.

5 m3 in columns

Every 5 m3.

NOTE: For all other small items and where RCC done in a day is less than5 m3, test may be carried out as required by Engineer.

  1. Samples from fresh concrete shall be taken as per IS: 1199 and cubes shall bemade, cured and tested at 28 days in accordance with IS: 516.
  2. In order to get a relatively quicker idea of the quality of concrete, optional testson beams for modulus of rupture at 72+2 h or at 7 days, or compressivestrength tests at 7 days may be carried out in addition to 28 days compressivestrength test.
  3. For this purpose the values should be arrived at based on actual testing. In allcases, the 28 days compressive strength as specified shall alone be thecriterion for acceptance or rejection of the concrete.
  4. A random sampling procedure shall be adopted to ensure that each concretebatch shall have a reasonable chance of being tested, that is, the samplingshould be spread over the entire period of concreting and cover all mixingunits.
  5. The minimum frequency of sampling of concrete of each grade shall be inaccordance with table above.
  6. Three test specimens shall be made for each sample for testing at 28 days.Additional samples may be required for various purposes such as to determinethe strength of concrete at 7 days or at the time of striking the formwork, or todetermine the duration of curing, or to check the testing error. The specimenshall be tested as described in IS: 516.
  7. Additional samples may also be required for testing samples cured byaccelerated methods as described in IS: 9103.
  8. The test results of the sample shall be the average of the strength of three specimens. The individual variation should not be more than ±15% of the average. If more, the test results of the sample are invalid.
  9. The concrete shall be deemed to comply with the strength requirements whenboth the following condition are met:
  10. The mean strength determined from any group of four consecutive test results compiles with the appropriate limits in table.
  11. Any individual test result complies with the relevant limits in table .
  12. To ensure that the construction complies with the design, an inspectionprocedure should be set up covering materials, records, workmanship andconstruction.

Concrete is liable to be rejected if it is porous or honey-combed, its placing has been interrupted without providing a proper construction joint, there inforcement has been displaced beyond the tolerances specified, or construction tolerances have not been met. However, the hardened concretemay be accepted after carrying out suitable remedial measures to the satisfaction of the Engineer.

Characteristic compressive strength compliance requirements

Specified

grade

Mean of the group of 4 non-overlapping

consecutive test results (MPa)

Individual test

results

(MPa)

M15

≥ fck + 0.825 x established standard

deviation (rounded off to nearest 0.5 MPa)

or

fck +3 MPa, whichever is greater.

≥ fck - 3

M20 or

above

≥ fck + 0.825 x established standard

deviation (rounded off to nearest 0.5 MPa)

or

fck + 4 MPa, whichever is greater.

≥ fck - 4

 

NOTE: In the absence of established value of standard deviation, values givenin table 8 of IS: 456-2000 may be assumed and attempt should be made toobtain result of 30 samples as early as possible to establish the value ofstandard deviation.

5. Inspection and testing of structures

5.1. Inspection

  • Immediately after stripping the formwork, all concrete shall be carefullyinspected and any defective work or small defects shall be either removed ormade good before concrete has thoroughly hardened.
  • Concrete members shall be inspected within 15 days for occurrence of cracksdue to shrinkage, temperature, localrestraint, undue deflection and deformation.

5.2. Testing of concrete in structures

In case of doubt regarding the grade of concrete in the structure, either due to poorworkmanship or based on results of cube strength tests, compressive strength tests of concrete shall be carried out by core tests and/or non-destructive tests.

5.2.1 Core test

  1. Cores shall be prepared and tested as described in IS: 516.
  2. The location from which samples are to be taken and the number of samplesrequired shall be decided so as to be representative of the whole concreteconcerned. In no case the number of samples shall not be less than three.
  3. Concrete tested shall be acceptable if the average equivalent cube strength ofthe cores is equal to at least 85% of the characteristic strength of the grade ofconcrete specified for the corresponding age and no individual core hasstrength less than 75%.

5.2.2 Non-destructive tests

  1. Non-destructive tests provide alternatives to core tests for estimating thestrength of concrete in a structure, or can supplement the data obtained from alimited number of core specimen tested.
  2. The methods used for non-destructive tests are Schmidt’s Rebound Hammertest (IS: 13311Part-2) and ultrasonic pulse velocity test (IS: 13311 Part-1).

5.3. Load test on structures

In case the core test and non-destructive test do not satisfy the strengthrequirements, load test may be carried out on superstructures as per SP: 51.

RELATED CODES

1.IS: 456-Code of practice for plain and reinforced concrete.

2.IS: 516-Method of test for strength of concrete.

3.IS: 716-Specification for pentachlorophenol.

4.IS: 1199-Method of sampling and analysis of concrete.

5.IS: 1791-Specification for batch type concrete mixes.

6.IS: 2502-Code of practice for bending and fixing bars for concretereinforcement.

7.IS: 2506-General requirements for concrete vibrators, screed board type

8.IS: 2514-Specification for concrete vibrating tables.

9.IS: 4656-Specification for form vibrators for concrete.

10.IS: 2751-Recommended practice for welding of mild steel, plain anddeformed bars for reinforced construction.

11.IS: 4925-Specification for concrete batching and mixing plant.

12.IS: 10262-Recommended guidelines for concrete mix design.

13.IS: 13311-Non-destructive testing of concrete:

(Part-1)-Method of test for ultrasonic pulse velocity.

(Part-2) -Method of testing by rebound hammer.

7. Conduits

  1. Conduits or pipes shall be located so as not to reduce the strength of theconstruction.
  2. In no case shall pipes other than conduits be placed in a slab 125 mm or lessin thickness.
  3. Conduits embedded in a concrete slab shall not have an outside diametergreater than one-third the thickness of the slab nor be placed below the bottomreinforcing steel or over the top reinforcing steel.
  4. Pipes and conduits where embedded shall be placed as nearly as possible tothe centre line of the concrete section.

8.  Stripping of formwork

  1. Forms shall not be released until the concrete has achieved strength of at leasttwice the stress to which the concrete may be subjected at the time of removalof formwork.
  2. The period of time elapsing between the placing of the concrete and the strikingof the formwork shall be approved by the Engineer after consideration of theloads likely to be imposed on the concrete and shall in any case be not lessthan the periods as shown in table 600-15 (Clause 11.3.1 of IS: 456).

Stripping time of formwork (When OPC is used)

No.

Type of formwork

Minimum period before

striking formwork

1

Vertical formwork to columns, walls,beams

16 to 24 hrs

2

Soffit formwork to slabs (Props to berefixedimmediately after removal offormwork)

3 days

3

Soffit formwork to beams (Props tobe refixed immediately after removalof formwork)

7 days

4

Props to slabs:

i) Spanning up to 4.5 m

ii) Spanning over 4.5 m

 

7 days

14 days

5

Props to beams and arches:

i) Spanning up to 6 m

ii) Spanning over 6 m

 

14 days

21 days

NOTE: For other cements and lower temperature, the stripping time recommended above may be suitably modified.

  1. The number of props left under, their sizes and disposition shall be such as tobe able to safely carry the full dead load of the slab, beam or arch as the casemay be together with any live load likely to occur during curing or further construction.
  2. Where the shape of the element is such that the formwork has re-entrantangles, the formwork shall be removed as soon as possible after the concrete has set to avoid shrinkage cracking occurring due to the restraint imposed.
  3. Stripping of the formwork within the time limits listed above does not relieve the Contractor from successfully crushing test cubes and achieving the compressive strength as specified.
  4. Not withstanding the foregoing, the Contractor shall be held responsible for any damage arising from removal of formwork before the structure is capable ofcarrying its own weight and any incidental loading.

8.1 Tolerances

  1. Deviations of specified dimensions of cross sections of beams and columnsshall be +12 or -6 mm.
  2. Deviations of specified plan dimensions of footings shall be +50 or -12 mm.
  3. Deviations of specified thickness of footings shall be ± 0.05 times the specifiedthickness.

8.2 RELATED CODES

1.IS: 14687-False work for concrete structures - guidelines.

*****