7-days compressive strength test results are usually not used for acceptance purpose but for information only .Instead, 28-days compressive strength test results are commonly adopted for acceptance purpose.
7-days compressive strength test results are often used to monitor the gain of early strength and they are estimated to be about 64% to 70% of the 28-days strength . As such, it serves as a warning signal to both concrete producers and contractors if the 7-day compressive strength test results are far less than 75% of the 28-day strength.
Nowadays, most concrete placement schedule are very tight and it is of paramount importance for contractors to get to know as soon as possible the occurrence of low 7-days compressive strength test results. As such, the contractor could implement suitable measures promptly to get better quality control procedures at construction site and to monitor closely on sampling, molding and testing of the test cubes so as to avoid the recurrence of the production of low-strength concrete in the coming concrete batches.
