Effect of Moisture Condition on Concrete Core Strengths

In accordance with the provisions of American Society for Testing and Materials (ASTM) C 42-90 and American Concrete Institute (ACI) 318-89, it is current practice to either dry concrete core specimens in air for 7 days or soak them in lime-saturated water for at least 40 hours before they are tested. In this paper, the effect of moisture condition on the strengths of mature cores obtained from well-cured elements is investigated by reviewing available literature and performing regression analyses of data from tests of 727 core specimens. Tests show that the compressive strength of a concrete specimen is influenced both by moisture content changes that are uniform throughout the specimen volume and moisture content gradients between the surface of the specimen and interior. The air-drying and soaking periods specified in ASTM C 42-90 and ACI 318-89 are too short to cause a uniform change of moisture content throughout the volume of the core. The effect of these treatments is to create a moisture gradient that artificially biases the test result. The strength of air-dried cores is on average 14 percent larger than the strength of soaked cores. The strength of cores with a negligible moisture gradient is on average 9 percent larger than the strength of soaked cores. These general average values are constant for concretes with strengths ranging from 2,200 to 13,400 psi. However, the strength ratios for any particular mix may differ appreciably from these general average values.