Title: Effect of Moisture Condition on Concrete Core Strengths
Author(s): F. Michael Bartlett and James G. MacGregor
Publication: Materials Journal
Appears on pages(s): 227-236
Keywords: compressive strength; cores; curing; drying; valuation; high-strength concrete; moisture content; tests; wetting; Materials Research
In accordance with the provisions of ASTM C 42-90 and 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 hr 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 analysis of data from tests of 727 core specimens. It is shown that the compressive strength of a concrete specimen is influenced both by moisture content changes that are uniform throughout the specimen volume, and by moisture content gradients between the surface of the specimen and the 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 2200 to 13,400 psi. However, the strength ratios for any particular mix may differ appreciably from these general average values.