Effect of Core Geometry and Size on Concrete Compressive Strength
Type of Degreethesis
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To evaluate the in-place concrete strength for acceptance for a structural member with potentially substandard strength, the compressive strength of cores may be required for assessment. Depending on the geometry and size of the core specimen, the compressive test results may need adjustment to characterize the in-place concrete strength. The comprehensive experimental study performed examines core test results for factors including: three targeted strengths (6,000, 8,000, and 10,000 psi), two coarse aggregate sizes (No. 67 and 57), five core length-to-diameters (1.0, 1.25, 1.5, 1.75, and 2.0), two core diameter sizes (3 and 4 in.), and two coring directions (parallel and perpendicular). The data represent 390 core specimens obtained from 12 separate casts of concrete. The data indicate that the core diameter significantly affects the strength correction factors for various length-to-diameters (l/d). For 4 in. diameter cores, only a slight difference from the currently recommended core l/d strength correction factors in AASHTO T 24 (2007) is observed. However, 3 in. diameter cores do not behave the same as 4 in. diameter cores when evaluated for core l/d effects on compressive strength. The analyzed data indicate that as the volume of the core specimen decreases, the compressive strength results become less reliable. Additionally, AASHTO T 24 (2007) recommends a core diameter of at least 3.75 inches. Based on the analysis, cores having a l/d less than 2.0, should not have a core diameter less than 3.75 inches. For cores having l/d of 2.0, the data indicate that the average strength of 3 in. diameter cores is 94 percent of the average strength of a 4 in. diameter core. Results also indicate the average strength of cores drilled perpendicular to the placement direction is 96 percent of the average strength of cores drilled parallel to the placement direction.