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We cast a bridge's concrete deck and kept samples for concrete strength tests at 7 and 28 days.

We took a test at 7 days, and the test result were over the design strength (designed for 30 MPa, result was 33 MPa). The control engineer insisted that we have to also do the 28-day test.

This seems pointless to me, as I believe concrete gains strength during curing, and doesn't lose it. Can the results of the 28-day test be worse than those of the 7-day test?


I am asking this question because I want to get paid for the job. The control engineer (who signs the documents) is the one who is insisting that we have to wait until the 28-day test.

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  • $\begingroup$ Have a look at the project specifications as well as the national standards regarding concrete strength and the testing of cubes. Usually one is allowed a percentage of the results be below the required strength, as long as enough results are available overall. For this specification to apply, the standard of 28 day test cubes is important. Also, the 7 day cubes are used as an indicator, while the 28 day results are the legal proof that you achieved the required standard. They are also a check for incorrect testing. If there is a discrepancy in the results, on can also address that. $\endgroup$ – SlydeRule Jun 27 '16 at 16:12
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With the exception of anything extremely unusual: the concrete will continue to gain strength until 28 days. In fact, it will continue to gain strength for something like 50 years, but the main strength gain happens early on.

The motivation for waiting for the 28-day test is therefore nothing to do with strength. Far more likely: the later you are paid, the better it is for the main contractor. This helps the contractor manage cash flow, and/or allows them to make interest by having money in their bank account for longer. I have no idea what your contract says in terms of when you get paid, but it is completely normal for the control engineer to pay you as late as he is able.

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    $\begingroup$ Paying later reduces the NPV of the payment for them. Also, it may very well be a simple matter of "hey, I need to show my bosses the results for the 28-day results, regardless of what the 7-day tests say. So yeah, wait until you do the 28-days to get paid." $\endgroup$ – Wasabi Jun 27 '16 at 16:36

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