Important Factors as per IS 456-2000

The following are some of the important factors to be followed properly as per the stipulations in IS 456 even for the design mix concrete with materials free from impurities in order to achieve the desired strength and quality of concrete. The relevant clause numbers of IS 456 are also mentioned as ready references for each of the factors.

(a) Mixing (cl. 10.3)

Concrete is mixed in a mechanical mixer at least for two minutes so as to have uniform distribution of the materials having uniform colour and consistency.

It’s ideal to use the concrete from a RMC Plant certified by Quality Council of India (QCI) as per Ready Mixed Concrete Plant Certification Scheme.

(b) Formwork (cl. 11)

Properly designed formwork shall be used to maintain its rigidity during placing and compaction of concrete. It should prevent the loss of slurry from the concrete. The stripping time of formwork should be such that the concrete attains strength of at least twice the stress that the concrete may be subjected at the time of removing the formwork. As a ready reference IS 456 specifies the minimum period before striking formwork. There is a scope for good design of formwork system so that stripping off is efficient without undue shock to concrete and facilitating reuse of formwork.

(c) Assembly of reinforcement (cl. 12)

The required reinforcement bars for the bending moment, shear force and axial thrust are to be accommodated together and proper bar bending schedules shall be prepared. The reinforcement bars should be placed over blocks, spacers, supporting bars etc. to maintain their positions so that they have the required covers. High strength deformed steel bars should not be re-bent. The reinforcement bars should be assembled to have proper flow of concrete without obstruction or segregation during placing, compacting and vibrating.

12.1 Reinforcement shall be bent and fixed in accordance with procedure specified in IS 2502. The high strength deformed steel bars should not be re-bent or straightened without the approval of engineer-incharge. Bar bending schedules shall be prepared for all reinforcement work.

12.2 All reinforcement shall be placed and maintained in the position shown in the drawings by providing proper cover blocks, spacers, supporting bars, etc.

12.2.1 Crossing bars should not be tack-welded for assembly of reinforcement unless permitted by engineer-in-charge.

12.3 Placing of Reinforcement: Rough handling, shock loading (prior to embedment) and the dropping of reinforcement from a height should be avoided. Reinforcement should be secured against displacement outside the specified limits.

12.3.1 Tolerances on Placing of Reinforcement

Unless otherwise specified by engineer-in-charge, the reinforcement shall be placed within the following tolerances:

• for effective depth 200mm or less: ± 1Omm
• for effective depth more than 200 mm: ±15mm

(d) Transporting, placing, compaction and curing (cl. 13)

Concrete should be transported to the formwork immediately after mixing to avoid segregation, loss of any of the ingredients, mixing of any foreign matter or loss of workability. Proper protections should be taken to prevent evaporation loss of water in hot weather and loss of heat in cold weather.

To avoid rehandling, concrete should be deposited very near to the final position of its placing. The compaction should start before the initial setting time and should not be disturbed once the initial setting has started. While placing concrete, reinforcement bars should not be displaced and the formwork should not be moved.

The compaction of concrete using only mechanical vibrators is very important, particularly around the reinforcement, embedded fixtures and the corners of the formwork to prevent honeycomb type of concreting. Excessive vibration leads to segregation.

Proper curing prevents loss of moisture from the concrete and maintains a satisfactory temperature regime. In moist curing, the exposed concrete surface is kept in a damp or wet condition by ponding or covering with a layer of sacking, canvas, hessian etc. and kept constantly wet for a period of 7-14 days depending on the type of cement and weather conditions. Blended cement needs extended curing. In some situations, polyethylene sheets or similar impermeable membranes may be used to cover the concrete surface closely to prevent evaporation.

(e) Sampling and strength of designed concrete mix (cl. 15)

Random samples of concrete cubes shall be cast from fresh concrete, cured and tested at 28 days as laid down in IS 516. Additional tests on beams for modulus of rupture at 3 or 7 days, or compressive strength tests at 7 days shall also be conducted. The number of samples would depend on the total quantity of concrete as given in cl. 15.2.2 and there should be three test specimens in each sample for testing at 28 days, and additional tests at 3 or 7 days.

(f) Acceptance criteria (cl. 16)

Concrete should be considered satisfactory when both the mean strength of any group of four consecutive test results and any individual test result of compressive strength and flexural strength comply the limits prescribed in IS 456.

(g) Inspection and testing of structures (cl. 17)

Inspection of the construction is very important to ensure that it complies the design. Such inspection should follow a systematic procedure covering materials, records, workmanship and construction.

All the materials of concrete and reinforcement are to be tested following the relevant standards. It is important to see that the design and detailing are capable of execution maintaining a standard with due allowance for the dimensional tolerances. The quality of the individual parts of the structure should be verified. If needed, suitable quality assurance schemes should be used. The concrete should be inspected immediately after the removal of formwork to remove any defective work before concrete has hardened.

Standard core tests (IS 516) are to be conducted at three or more points to represent the whole concrete work in case of any doubt regarding the grade of concrete during inspection either due to poor workmanship or unsatisfactory results on cube strength obtained following the standard procedure. If the average equivalent cube strength of cores is equal to at least 85 per cent of the cube strength of that grade of concrete at that age and each of the individual cores has strength of at least 75 per cent, then only the concrete represented by the core test is considered acceptable. For unsatisfactory core test results, load tests should be conducted for the flexural members and proper analytical investigations should be made for non-flexural members. Such load tests should be done as soon as possible after expiry of 28 days from the date of casting of the flexural members subjected to full dead load and 1.25 times the imposed load for 24 hours and then the imposed load shall be removed. The maximum deflection of the member during 24 hours under imposed load in mm should be less than 40 l2/D, where l is the effective span in m and D is the overall depth of the member in mm. For members showing more deflection, the recovery of the deflection within 24 hours of removal of the imposed load has to be noted. If the recovery is less than 75 per cent of the deflection under imposed load, the test should be repeated after a lapse of 72 hours. The structure is considered unacceptable if the recovery is less than 80 per cent.

There are further provisions of conducting non-destructive tests like ultrasonic pulse velocity (UPV), rebound hammer, probe penetration, pull out and maturity, as options to core tests or to supplement the data obtained from a limited number of cores. However, it is important that the acceptance criteria shall be agreed upon prior to these non-destructive testing. There are reports that UPV tests conducted three days after casting after removal of side formwork give very dependable insight about the quality of concrete.

Conclusions:

The reinforced concrete consisting of plain concrete and steel reinforcement opened a new vista fulfilling the imaginations of architect with a unified approach of the architect and structural engineer. This has been made possible due to mouldability and monolithicity of concrete in addition to its strength in both tension and compression when reinforced with steel. However, concrete is produced by mixing cement, sand, gravel, water and mineral admixtures, if needed. Therefore, the final strength of concrete depends not only on the individual properties of its constituent materials, but also on the proportions of the material and the manner in which it is prepared, transported, placed, compacted and cured. Moreover, durability of the concrete is also largely influenced by all the steps of its preparation.

Steel reinforcement though produced in steel plants and made available in form of bars and rods of specific diameter also influences the final strength of reinforced concrete by its quality and durability due to environmental effects.

Concrete cover provides the protective environment to embedded steel from rusting that would need presence of both oxygen and moisture. Not only the extent of cover but the quality of cover is important for this reason.

Accordingly, inspection of concrete work, sample testing of specimens, core tests, load tests and non-destructive tests are very important to maintain the quality, strength and durability of reinforced concrete structures. Moreover, it is equally important to remove small defects or make good of it after removing the formwork before it has thoroughly hardened.

Thus, starting from the selection of each constitutive material to the satisfactory construction of the structure, the designer's responsibility will only produce the desired concrete structure which will satisfy the functional requirements as well as will have its aesthetic values exploiting all the good properties of this highly potential material.