04-CONCRETE WORK

CONCRETE WORK FOR BRIDGES AND CULVERTS

GENERAL

This section includes the quality control requirements for imparting prestress to structural members by stressing of wires/strands/tendons/cables with jacks of required capacity and holding them between approximately designed anchorages fixed internally or externally to the members.

MATERIALS:

CEMENT

The quality control requirements such as cement identification, storage, sampling and testing of cement shall be as per, IRC: 112 and MoRTH Specifications for Road  and Bridge works.

    AGGREGATE

1. The quality control requirements, storage, sampling and testing of fine and coarse aggregate for concrete shall be as per, IRC: 112 and MoRTH Specifications for Road and Bridge works.
2. The size (Maximum nominal) of coarse aggregate for concrete to be used in various components shall be as given.

The quality control requirements, sampling and testing of water used for mixing of concrete and curing shall be as per, IRC: 112 and MoRTH Specifications for Road and Bridge works.

ADMIXTURES

1. Chemical admixtures such as super-plasticizers, or air entraining, water reducing, accelerating and retarding agents for concrete may be used with the approval of the Engineer.
2. As the selection of an appropriate concrete is an integral part of the mix design, the Contractor shall ensure that any admixture shall be used only after obtaining complete information of all the actual constituents of concrete as well as methodologies of manufacture, transportation and compaction of concrete proposed to be used in the work.
3. The quality control requirements, sampling and testing of admixtures shall be as per, IRC: 112 and MoRTH Specifications for Road and Bridge works.
4. Compatibility of the admixtures with the cement and any other pozzolana or hydraulic addition shall be ensured for avoiding the problems such as:

1. Requirement of large dosage of super-plasticizer for achieving the desired workability.
2. Excessive retardation of setting.
3. Excessive entrainment of large air bubbles.
4. Unusually rapid stiffening of concrete.
5. Rapid loss of slump.
6. Excessive segregation and bleeding.

STEEL REINFORCEMENT

1. The quality control requirements, storage, handling, inspection, sampling and testing of steel reinforcement for concrete work shall be as per IRC: 112 and MoRTH Specifications for Road and Bridge works.
2. Fusion bonded epoxy coated reinforcement shall conform to IS: 13620 or International Standards as approved by the Engineer.

1. PVC coated GI binding wires of 18G shall only be used in conjunction with fusion bonded epoxy bars.
2. Chairs for supporting the reinforcement shall be of fusion bonded epoxy coated bars.
3. The cut ends and damaged portions shall be touched up with repair patch up material.
4. The bars shall be cut by saw-cutting and not by flame cutting.
5. While bending the bars, the pins of work benches shall be provided with PVC or plastic sleeves.
6. The coated steel shall not be directly exposed to sun rays or rains and shall be protected with opaque polyethylene sheets or such other approved materials.
7. While concreting, the workman or trolley shall not move directly on coated bars but shall move only on wooden planks placed on the bars.
8. Hot dipped galvanized reinforcing steel shall be provided wherever specified. The coating shall conform to IS: 12594.

REQUIREMENTS FOR CONCRETE

1.  GRADE OF CEMENT

1. The characteristic strength is defined as the strength of material below which not more than 5% of the test results are expected to fall.
2. The type and grade of concrete for bridge works shall be as designated (Ref: Table 6.4 of IRC: 112-2011).
3. Ordinary concrete is made on the basis of nominal mix proportioned by weight of its main ingredients.

  Table: Types and grades of concrete

   Requirement of concrete for different exposure conditions using 20 mm aggregate

4. 1. All three provisions given in the above table for a particular exposure condition, shall be satisfied.
   2. The maximum limit of fly ash and  ground granulated blast furnace slag in the blended cement shall be as specified in IS: 1489 (Part-1) and IS: 455 respectively.
   3. For plain cement concrete with or without surface reinforcement, the minimum grade of concrete can be lowered by 5 MPa and maximum water- cement ratio exceeded by 0.05.
   4. Cement content shown in the above table shall be increased by 40 kg/m3 for use of 12.5 mm nominal size aggregates and decreased by 30 kg/m3 for use of 40 mm nominal size aggregate.
5. For concrete subjected to sulphate attack the minimum grades of concrete, minimum cement content and maximum water-cement ratio for different concentration of sulphate content shall be as (Ref: Table 14.4 of IRC: 112).
6. The maximum cement content excluding any mineral admixtures (Portland cement component alone) shall not exceed 450 kg/m3.
7. For all items of concrete, only design mix shall be used, except where nominal mix concrete is permitted as per Drawing or by the Engineer. Nominal mix may be permitted only for minor works or for other incidental construction, where strength requirements are upto M20 only. Nominal mix may also be permitted for non-structural concrete or for screed below open foundations.

Table : Requirement of concrete exposed to sulphate attack

8. Concrete shall meet any other requirements as specified in the Drawing or as directed by the Engineer. The overall limits of deleterious substances in concrete shall be as follows:

1. Total acid soluble chloride content in the concrete mix expressed as chloride ions shall not exceed the following values by mass of cement:
   1. Prestressed concrete:  - 0.10%
   2. Reinforced concrete:
      1. In severe, very severe or extreme exposure condition- 0.20%

1. 1. 2.    In moderate exposure condition- 0.30%

2. The total water soluble sulphate content of the concrete mix expressed as SO₃, shall not exceed 4% by mass of cement in the mix.

For concrete made with PPC, PSC or mineral admixtures, the setting time and rate of gain of strength are different from those for concrete made with OPC alone.

MIX  DESIGN

1. The target mean strength of specimen shall exceed the specified characteristic compressive strength by at least the current margin:

1. The current margin for a concrete mix shall be determined by the Contractor and shall be taken as 1.64 times the standard deviation of sample test results taken from at least 40 separate batches of concrete of normally similar proportions produced at site by the same plant under similar supervision, over a period exceeding 5 days, but not exceeding 6 months.
2. Where there is insufficient data to satisfy the above, the current margin for the initial design mix shall be taken as given in table.

Table : Current margin for initial design mix

2. The mix design within the limitations of parameters and other stipulations laid down in IS: 10262

3. Based on test results of material, a draft mix design calculation for all grades of concrete to be used in the works, shall be prepared after taking into account the provisions in the technical specifications and Guidelines of IS: 10262, IS: SP-23 and IRC: 112 and submitted to the Engineer for approval.
4. During concreting with the approved trial mix design, if source of any constituents is changed, the mix design shall be revised and tested for satisfying the strength requirements.
5. Slump test shall be used to measure consistency of the concrete as per IS: 516.
6. The optimum consistency for various types of structures shall be as indicated in table. 

Table : Requirements of Consistency

NOTE:  Where the concrete is to be transported through transit mixer and placed using concrete pump, the optimum consistency during placement for the items of work of Sl. Nos. 1 to 3 can be considered ranging from 75 mm to 150 mm, subject to satisfying the other essential criteria of strength, durability etc., and approval of the Engineer.

DESIGN OF FORMWORK

1. The design, erection and removal of formwork shall conform to IRC: 87. The forms shall be such as to ensure that they can be conveniently removed without disturbing the concrete. The design shall facilitate proper and safe access to all parts of formwork for inspection.

2. Precautions to be taken while removing formwork shall be followed as per IRC:87.

READY MIXED CONCRETE

1. Use of Ready Mixed Concrete (RMC) proportioned and mixed off the project site and delivered to site in a freshly mixed and unhardened state conforming to IS: 4926.

ACCEPTANCE CRITERIA

1. Acceptance or rejection of concrete is mostly based on compressive strength. However, other properties of the concrete in fresh and hardened states including durability are also important.
2. Apart from meeting the acceptance criteria, 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 met. However, the hardened concrete may be accepted after carrying out suitable remedial measures approved by the Engineer.
3. The test results of concrete shall deemed to comply with the criteria specified in table .

Table : Acceptance criteria for concrete

If the concrete is deemed not to comply as per the above criteria, the structural adequacy of the bridge elements affected shall be investigated and any consequential action as needed shall be taken.

RELATED CODES :

1.IRC:87-Guidelines for the design and erection of false work for road bridges.

2.IRC:112-Code of practice for concrete road bridges.

3.IS:516-Methods of tests for strength of concrete.

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

5.IS:2751-Code of practice for welding of mild steel plain and deformed bars for reinforced concrete construction.

6.IS:4926-Code of practice for ready-mixed concrete.

7.IS:9013-Method of making, curing and determining compressive strength of accelerated cured concrete test specimen.

8.IS:9417-Recommendations for welding cold worked bars for reinforced concrete construction.

9.IS:10262-Guidelines for concrete mix design proportioning.

10.IS:12594-Hot-dip zinc coating on structural steel bars for concrete reinforcement.

11.IS:13620-Fusion bonded epoxy coated reinforcing.

12.IS:14959-Determination of water soluble and acid soluble chlorides in mortar and concrete - method of test.

     (Part 1)-Fresh mortar and concrete.

13.ASTMC1202-Standard test method for electrical indication of concrete’s ability to resist chloride ion penetration.

14.MoRTH-Specification for Road and Bridge works (5th revision).

*****