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Cube tests are considered as one of the prime and basic gauge to measure the strength of concrete after the pour. Though there are several methods and tests to ensure that the mix poured meets all required parameters, the strength of the concrete is judged primarily by cube test only. Nowadays, for all major works, concrete is batched centrally at a batching plant, and delivered to site by transit mixers. In such situations, site engineers are usually left with no options other than counting the batching report received along with the concrete as reference, on mix proportions.
Today is the 7th day after the concreting… the cube tests ….err….failed.
The first thing we need to understands is that the compressive strength test on 7th day is only an indicator; just to give a quicker idea about the strength of concrete and also, what may be in store for us after 28th day test. According to IS 456:2000, the compressive strength on 28th day is the sole criterion for accepting or rejecting the concrete.
Failure of a cube test at 7th day doesn’t mean that the cubes should fail at its 28th day tests also, though it happens in most cases.There might be various reasons for a cube test failure apart from the inadequate strength of concrete mix. There are situations, where the cube test results show non conformity even though the concrete is having sufficient strength.
Let us investigate the possible causes for a cube test failure.
Wrong mix poured: The mix poured is not as per the approved mix design for that particular strength.
Wrong mix design: The mix design does not give the required strength.
Improper sample collection: If the sample is not collected by a skilled worker, and is not supervised by a technical person, there are possibilities that the collected sample is not truly representing the fresh concrete supplied at site. The samples should be collected in accordance with IS 1199 provisions.
Improper cube preparation: The cubes to be prepared as per the guidelines given in IS 516. Compressive strength of the cube might be affected by several errors during cube making, such as,
Improper alignment of mould – resulting in distorted cube shape.
Improper / inadequate tamping- leaving the cube porous.
Improper filling of the moulds – creating weak zones of mortar without aggregates.
Inadequate curing of the cube samples: The cubes to be cured properly as per the guidelines given in IS 516. Once prepared, the cubes to be covered with wet gunny bags for a period of 24hrs from the time when water is first added to the concrete, and then removed from moulds and kept under clean water until the time of test. At all time the temperature should be in the range of 24 to 300C.
Testing wrong samples: The identification numbering system should be unique so that exactly the intended cubes are retrieved and tested at every time.
Errors during testing: like placing of moulds over the CTM, cleaning the surface, seating the cube properly, rate of load application etc.
Errors with the testing apparatus: This could include errors with display, loading or hydraulic systems or a manual error in reading and interpreting the displayed value.
Unauthorized w/c correction of the mix: unlawful addition of water to the mix without consulting the structural consultant.
Change in quality of materials: The quality of materials used for preparing the concrete may be different from those used while preparing the trial mix.
Batching plant error: error in preparation of the mix in the batching plant, due to any mechanical / software error, which went unnoticed.
Now, what are the actions to be taken after the cube failure at 7th day?
As we have already discussed, the 7th day test is a warning bell. We have 21 more days to finally accept or reject the member.
The first and foremost action to be taken on receiving a failed cube test report is to suspend further activities on the member/members in question.
Secondly, before continuing concreting activity on any member, recheck the mix design and ensure that the mix design is accurate.
Also, reconfirm that it is as suggested by the structural consultant for that particular strength, if this is the first batch mixed with that particular mix design.
The above two steps are critical to ensure that the error, if at all happened, is not extending to other members or parts of the structure.Now, each of the above possibilities shall be investigated in detail to find a possible cause for the non conformity.
When we closely observe the possibilities listed above, we can identify two major chances after a cube failure.
1. The concrete poured is good - the cube failed due to some other reason.
2. The concrete poured is not good - hence the cube failed.
Trial mixes shall be taken with the same mix design, well supervised to eliminate all workmanship errors, and tested after 7th day at actual site conditions in the same testing machines and also at an external laboratory to ensure that the mix design is conforming to the strength requirement. Also, the structural consultant and architect should be alerted to the situation.
Further actions can be taken only after the cube test on the 28th day.
