Q. ACI 301-99, Section 188.8.131.52, regarding core test results, maintains that concrete represented by core tests is considered adequate when the average compressive strength of the cores is equal to at least 85% of the specified strength. I’m looking for any information that explains the basis for this claim.
A. The commentary of ACI 318 (Section R5.6.5—Investigation of low-strength test results) states that "Core tests having an average of 85 percent of the specified strength are entirely realistic. To expect core tests to be equal to f'c is not realistic, since differences in the size of specimens, conditions of obtaining samples, and procedures for curing do not permit equal values to be obtained." You could also check an article by Adam Neville in the November 2001 issue of Concrete International, "Core Tests: Easy to Perform, Not Easy to Interpret."
A very comprehensive document on this subject is the "British Concrete Society’s Tech. Report" No. 11, which provides a good summary of research on core-strength results. In general, it finds that cores can have a strength of 60% to 90% of the specified strength. The main issue is how many cores must be taken to represent a batch or a day’s work. One would normally apply statistical analysis to determine this.