The following is our in-house guideline that (I believe) follows ACI 318 Chapter 5:
Acceptance of Concrete
Per Chapter 5 of the ACI Building Code Requirements for Structural Concrete
(ACI 318)
A strength test is defined as the average strengths of TWO cylinders tested at 28 days.
A strength test is acceptable if BOTH of the following are met:
1. Every arithmetic average of any three consecutive tests (6 cylinders) equals or exceeds f'c.
2. No individual strength test (average of two cylinders) falls more than 500 psi. below the required strength, f'c.
If either of the above two requirements are not met, contractor should take steps to increase the average of future concrete tests.
If Item 2 above is not met, then the following should also be followed:
1. With the lower f'c derived from the strength tests above, the structural engineer should review the design of the affected portion of the structure to determine if the lower f'c is acceptable. If the lower f'c is acceptable, then no further efforts are required and the concrete can be accepted. Steps still should be taken by the contractor to increase the compressive strength of the concrete for future mixes.
2. If the lower f'c is found to be critical to the performance of the structure, then further testing should be performed using drilled core samples.
Core samples should be obtained û three at a time, for each strength test that falls below 500 psi. If the concrete in the structure will be dry under service conditions, cores shall be air dried for 7 days before the test and tested dry. If the concrete in the structure will be more than superficially wet under service conditions, cores shall be immersed in water for at least 40 hours and be tested wet.
The concrete can be accepted if BOTH of the following are met:
1. The average of the three cores is equal to or greater than 85% of f'c.
2. No single core is less than 75% of f'c.
If the cores don't meet the required levels the following are all options to consider:
1. Perform a load test of the structure in the area under consideration (per ACI Chapter 20)
2. Provide additional structural framing to strengthen the portion of the affected structure to meet the required load carrying capacity.
3. Accept the concrete if acceptable to the owner.
4. Reject the concrete and remove and replace the portion that is considered below strength.
