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.