Evaluation of In-Place Concrete Strength by Core Testing
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When the average compressive strength of molded cylinder specimens do not meet the minimum required strength for a batch of concrete, testing must be performed on the in-place concrete to evaluate its strength. There are a variety of in-place testing methods used throughout the concrete industry. Of these, core testing is the most popular. Other methods, such as cast-in-place cylinders and pullout inserts can be used to evaluate the in-place strength of the concrete in question. The purpose of this project was to evaluate several different factors, such as aggregate type, strength level, restraint, concrete age and type of cementitious material, to determine their influence on the in-place concrete strength. Another goal of the project was to determine the relationship between core strength and molded cylinder strength. To do this, field slabs were cast with different aggregate types, supplemental cementitious materials, and strength levels. Testing was conducted near the edges of the slab as well as in the interior of the slab to determine if restraint had an impact on microcracking. Cores were recovered and tested in compression at ages of 28, 42, 91, and 365 days. Cast-in-place cylinders and pullout tests were also conducted at the same ages. From the project, it was found that core strength is on average 87% of molded cylinder strength. Therefore, it is recommended that if the average core strength is 85% of the minimum design strength, the in-place concrete has satisfactory strength. This is accordance with ACI 318 (2011). It was also found that restraint and aggregate type had an impact on some test methods but not others.