Technical Questions

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Using core strength to calculate compressive strength of an existing structure

Q. We're renovating an existing reinforced concrete structure and need to evaluate the structural capacity. We have core test results. Should we divide the core strength by 0.85 to obtain an estimate of f’c for use in equations used to calculate the structural capacity?


A. According to ACI 214.4R-10, “Guide for Obtaining Cores and Interpreting Compressive Strength Results” the in-place strength of the concrete at the location from which a core test specimen was extracted can be computed using the equation:

fc = Fl/dFdiaFmcFdfcore

where fc is the equivalent in-place strength; fcore is the core strength; and strength correction factors Fl/d, Fdia, and Fmc account for effects of l/d ratio, diameter, and moisture condition of the core, respectively. Factor Fd accounts for the effect of damage sustained during drilling, including microcracking and undulations at the drilled surface and cutting through coarse-aggregate particles that may subsequently pop out during testing. The values of the factors can be obtained using the following table:


Core Strength Correction Factors


References: ACI 318-14; ACI 214.4R-10; ASTM C42

Topics in Concrete: 318 Building Code; Quality Assurance; Repair; Testing

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