Some concrete specification requires the testing of compressive strength for both 7 days and 28 days. Why?

7-day compressive strength test results are usually not used for acceptance purpose but for information only. Instead, 28-day compressive strength test results are commonly adopted for acceptance purpose.

7-day 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-day strength. As such, it serves as a warning signal to both concrete producers and contractors should 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-day 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.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.