CEMENT & CONCRETE
The vital importance of laboratory concrete testing All the raw materials used to produce concrete are variable. And the readymix supplier must absorb these variances in his production process to produce concrete that meets the specification. This is inherently the risk that readymix suppliers will carry, making testing a vital part of the quality validation process. By Jacques Smith, Pr Eng*
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he first party that needs to know whether the readymix supplier is producing concrete within the strength requirement is the supplier himself. To that end, he will take samples normally at the plant from the back of the truck to have them tested as process control samples. It is up to the client to take samples for cube-making and testing for acceptance control for quality assurance purposes. These two types of testing are very similar, yet there are fundamental differences between the two. The main difference in testing is the sampling that is done from the back of the truck at the plant for process control testing, whereas on-site sampling is done from a moving stream after the first 15% and before the last 15% of concrete has been offloaded for acceptance control for quality assurance testing (SANS 5878 para. 12.2). The criteria for assessing these results are then also different. Normally, process control cubes are not (but may be in certain circumstances) accepted as acceptance control testing for quality assurance.
1. Process control cubes are too high. 2. Process control cubes are too low. 3. Acceptance control cubes are too high. 4. Acceptance control cubes are too low. In number 1 above, this would give a false sense of security and, even unknowingly, substandard concrete may be produced that could result in a failure and huge claim. In number 2 above, the readymix supplier will increase cement contents because he is scared of a claim. The customer will be very happy with this concrete, which will give higher strengths and even earlier strengths, but this comes at the readymix supplier’s expense. In number 3 above, it is great for everyone, until something goes wrong and cores are requested. This could then result in huge failures. In number 4 above, the specification rules that the engineer may call for the process control test results and will then call for cores to be drilled, if the situation cannot be resolved. Ultimately, the concrete will be accepted but with mistrust and unhappiness due to delays and penalties – and the additional cost of the drilling and testing of cores. The readymix supplier will probably increase cement contents to prevent possible failures.
Potential problems
Concrete laboratory testing
The following things can go wrong with the testing (of perfectly good concrete):
Due to our involvement in disputes and court cases, Go Consult has been asked on a
IMIESA March 2022
number of occasions to investigate the state of testing at a laboratory. On one occasion, eight of the eleven laboratories visited in a sample run were doing testing commercially at the time. Not one of the laboratories included in this article was accredited by Sanas (the South African National Accreditation Systems) for concrete testing. Sanas accreditation would imply that they have a management system in place to ensure the proper documentation of everything they do, plus systems in place to ensure a repeatable test that will reproduce a similar test result every time.
Incorrect testing At every laboratory, there were signs of incorrect crushing of cubes with tensile cracks and the cubes only being crushed on one side, with the other side failing in tension. This error will definitely give a lower result and create the impression that the concrete is of substandard quality. This, of course, would raise the following logical questions (as the advocate would certainly ask in court): How many of the cubes were tested like that? Which ones were they and what would the effect of that be? This should result in a rejection of all test results, as we know that some results are lower, but we do not know which and we do not know by how much, so we cannot use any of these results. We, therefore, might have to drill cores to determine the
