Concrete cube testing

    Compressive strength of concrete is a highly variable property. Standard deviation ratio for cube strength may usually varies from 6 to 20%. Statistical quality control can work where sample are at least 30 & may be much more. Hence we are in a field where statistical control is also not applicable.

    We design concrete for its characteristic strength, which is value for 95% confidence, and only 5% chance that the strength can be less. Average strength is a value for which 50% chance is that an individual sample strength may exceed it and also 50% chance that a sample strength may be less.

    For designing / proportioning the concrete mix, to this characteristic strength we add a margin for variations such that for 95% probability strength will not be less than characteristic value. Thus we determine from characteristic strength to the average strength, which is called a target mean strength.

    Criteria for acceptance of concrete based on sample cube-strength is given by IS456 or other international codes. A test sample should consist of minimum 3 cubes from particular type and grade of concrete work taken in a day. Average strength (say at 28 days) of the 3 cubes is the strength of the sample. A sample to be acceptable, the strength of sample should not be less than f_ck - 3 and also the average of 4 consecutive test samples should not be more than f_ck + 3. If these two criteria are not satisfied, the test sample under consideration is not acceptable i.e. quality of concrete represented by the sample is not satisfactory.

Further few actions are required.

    There may be chance that sampling was not proper or it may not represent the concrete poured in the related members of the structure. If quality of concrete represented by the sample in general appears to be satisfactory and there is no other reason for quality to be not satisfactory, one option is to undertake non-destructive testing (normally a combination of rebound hammer & ultrasonic) of the concerned members of the structure. If these NDT tests indicates that concrete is up to mark, the concrete can be accepted. If NDT test results are marginal and does not enough confidence cores can be taken out for further testing.

    Core is a cylindrical specimen. For compressive strength of concrete the results of cubes and cylindrical specimens are not same, and also changes with size of specimen. For normal range of concrete grades the cylindrical strength is about 85 % of the cube strength. Where cylinder is tested the equivalent cube strength will be cylinder strength divided by 0.85. This ratio of strength changes with the type of concrete, and increases with the grade of concrete. For high strength concrete, or for highly cohesive concrete the factor is almost unity (in place of 0.85). Some such considerations need to be taken in to account. These factors should be known for the type of concrete being dealt. Further a decision is to be drawn on acceptability of concrete.

    If quality of concrete is found to be not acceptable, further actions are required as per contract i.e. imposing the penalty and taking corrective actions for future concrete works, like revising the mix proportioning (mix design) and other procedures of concreting operations. One more decision is required regarding the acceptability of concrete members represented by the sample of concrete which is found sub-standard. Concrete members may require strengthening or rebuilding depending upon severity of sub-standard of the concrete.

    In the design procedure, we apply a partial material factor 1.5 to concrete strength, i.e. for characteristic strength M30, the design strength is taken as M20 only i.e. 67% (some take this as 70 or 75%). This margin is kept for reduction of strength of concrete on the members because of many operations required after pouring concrete, such as segregation, compaction, curing, effect of temperature variation in immature and young concrete etc. By core tests and other tests if we ensure that the concrete strength is more than two-third of characteristic strength, the strength of structure will be acceptable, though the strength was below acceptable limit. Note one taken accept the structure with penalty but without strengthening, and no need to redo the concrete.

    A core strength >85% indicates that concrete is acceptable, between 60 to 85% indicates that quality is not acceptable, but structure may not need strengthening if there are no construction defects.

It is all a game of probabilities.

 Acceptance Criteria of Concrete as per IS: 456