Technical Questions

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How is the required strength selected, measured, and obtained?

Q. How is the required strength selected, measured, and obtained?


A. The structural designer selects the specified compressive strength of concrete, fc, of the concrete. The building code (ACI 318) requires that the average strength be higher than the fc by an amount that varies with the variability (standard deviation) of the strength of concrete produced at the plant where the concrete will be obtained for the project (ACI 214). Compressive strength is determined by subjecting standard-cured cylindrical specimens to compression in accordance with ASTM C39. In the event test cylinders show unacceptably low strength, the building code (ACI 318) describes the action that needs to be taken. In pavements, the required strength is frequently expressed as flexural strength, which is determined using standard-cured beams following ASTM C78. Because of the difficulty of making, transporting, and testing beams and the high variability in the beam test, compressive strengths are preferred for acceptance after a relationship between the flexural and compressive strengths is established for the mixture to be used or for the one that is being used (ACI PRC 330).


References: SP-1(02); ACI 318-19; ACI 214R-11; ACI 301-20; ACI PRC 330-21; ASTM C39; ASTM C78

Topics in Concrete: 318 Building Code; Codes; Design of Concrete; Specifications for Concrete; Testing of Concrete

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