On small projects, the results of tests on concrete after the conventional 28 days of curing may be valuable only as a record. In these cases, the evaluation is limited to three options: (1) accept results, (2) remove and replace faulty concrete, or (3) conduct further tests to confirm option (1) or (2) or for limited acceptance at a lower-quality rating. The same comment can be applied to a specific element of a large project. If the element supports 28 days’ additional construction above, the consequences of these decisions are expensive.
Samples sufficient for at least five strength tests of each class of concrete should be taken at least once each day, or once for each 150 yd3 of concrete or each 5000 ft2 of surface area placed. Each strength test should be the average for two cylinders from the same sample. The strength level of the concrete can be considered satisfactory if the averages of all sets of three consecutive strength-test results equal or exceed the specified strength and no individual strength-test result falls below ƒ’c by more than 500 psi. ƒ’c
If individual tests of laboratory-cured specimens produce strengths more than 500 psi below , steps should be taken to assure that the load-carrying capacity ƒ’c of the structure is not jeopardized. Three cores should be taken for each case of a cylinder test more than 500 psi below . If the concrete in the structure will be ƒ’c dry under service conditions, the cores should be air-dried (temperature 60 to 80F, relative humidity less than 60%) for 7 days before the tests and should be tested dry. If the concrete in the structure will be more than superficially wet under service conditions, the cores should be immersed in water for at least 48 h and tested wet.
Regardless of the age on which specified design strength is based, large ƒ’c projects of the long duration offer the opportunity for adjustment of mix proportions during the project. If a running average of test results and deviations from the average is maintained, then, with good control, the standard deviation achieved may be reduced significantly below the usually conservative, initially assumed standard deviation. In that case, a saving in cement may be realized from an adjustment corresponding to the improved standard deviation. If control is poor, the owner must be protected by an increase in cement. Project specifications that rule out either adjustment are likely to result in less attention to quality control.
For a recommended overall basis for project specifications and procedures, see ‘‘Guide to Formwork for Concrete,’’ ACI 347R. For materials, details, etc., for builders, see ‘‘Formwork for Concrete,’’ ACI SP-4. For requirements in project specifications, see ‘‘Standard Specifications for Structural Concrete, ACI 301.