# Technical Questions

## 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 PRC-214.4R-21, “Obtaining Cores and Interpreting Core 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 length-to-diameter 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:

References: ACI 318-19; ACI PRC-214.4-21; ASTM C42

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