Concrete suppliers and contractors are sometimes accused of producing or placing low-strength concrete. While low-strength concrete does occasionally occur, many times reported cylinder strengths are evaluated improperly creating a low-strength false alarm. Unnecessary low-strength concerns, rejected concrete, construction delays and expensive investigations can be avoided by properly evaluating concrete cylinder strengths.
Unless otherwise specified, concrete strength acceptance is based on the average strength of at least two 6 by 12 inch or at least three 4 by 8 inch “standard” cylinders made from the same sample of concrete and tested at 28 days. Standard cylinders are test cylinders made and cured in accordance with ASTM C31 and tested in accordance with ASTM C39. To ensure a proper strength evaluation, each individual class of concrete should be represented by at least five strength tests.
Strength of an individual class of concrete is acceptable if both of the following are met:
Requirement #1 – No individual strength test (average of two or three cylinders) falls below fc’ by more than 500 psi when fc’ is 5,000 psi or less, or falls below fc’ by more than 10 percent (0.10 x fc’) when fc’ is more than 5,000 psi.
Requirement #2 – Every average of three consecutive strength tests equals or exceeds the specified compressive strength (fc’).
The specified strength (fc’) is the concrete compressive strength used by engineers in the design calculations to determine member sizes and reinforcement details of concrete structures. Since concrete strength is a function of time and temperature, fc’ is based on test cylinders made and cured in accordance with standardized time and temperatures as specified by ASTM C31.
Due to material and statistical variability, expect about one in 100 tests to fail the strength requirements even though the concrete strength and uniformity are acceptable. Concrete strength acceptance is based on probabilistic concepts that account for the strength variations that naturally occur with concrete. Also, the design calculations used by engineers take into account concrete strength variations. In combination, the concrete strength acceptance requirements and the safety factors built into the design process ensure the load-carry capacity of concrete structures is adequate.
Acceptable strength example for 4,000 psi concrete
As shown in Table 1 (page 28), column #1 shows the strength test number (1-5) and column #2 shows the individual 6 by 12 inch cylinder compressive strengths A and B. By averaging the A and B single-cylinder strengths, strength tests shown in column #3 were computed. For test 1, the average strength of 4,110 and 4,260 is 4,185 psi [(4,110 + 4,260) 2 = 4,185 psi]. A strength test is the average strength of two or more single-cylinder strengths, not the strength of an individual or single cylinder.
Column #4 shows the average of three consecutive strength tests. For test 3, the average of three consecutive strength tests is the average of the strengths for tests 1, 2 and 3 or (4,185 + 3,960 + 4,435) 3 = 4,193 psi. For test 4, the average of three consecutive strength tests was computed by averaging the strengths for tests 2, 3 and 4 (3,960 + 4,435 + 3,745) 3 = 4,047 psi. As shown, there are no values for tests 1 and 2.
Strengths for tests 1, 3 and 5 (column #3) exceed the 4,000 psi specified strength. However, strengths for tests 2 and 4 (3,960 and 3,745 psi) fall below the specified strength, but these strengths are acceptable because they do not fall below 4,000 psi by more than 500 psi or both strengths exceed 3,500 psi. Therefore, Requirement #1 has been satisfied for all tests and the strengths 3,960 psi and 3,745 psi should not set off the low-strength concrete alarm.
Column #4 shows the average three consecutive strengths exceed the 4,000 psi specified strength; therefore Requirement #2 has been satisfied. Columns #5 and #6 reflect that both of the strength acceptance requirements have been satisfied; therefore, concrete strengths in this example are acceptable, even though two strengths fell below the specified strength of 4,000 psi.