Variation of Concrete Strength, Permeability, and Porosity due to Specimen Type, Season, and Age

Abstract

Time and curing conditions may impact the strength and permeability of concrete. The strength and permeability of concrete with and without supplementary cementitious materials (SCMs) were evaluated as a function of specimen type, season during which construction occurred, and age. Three concrete mixtures in which the cementitious material consisted of 100% portland cement, 65% portland cement and 35% slag cement, or 60% portland cement, 25% slag cement, and 15% Class C fly ash were evaluated. Pavement slabs containing each mixture were cast in the summer, fall, and spring, along with companion 4 × 8 in. cylinders, to determine the effect of seasonal variations in environmental conditions on the strength and permeability of the concrete. Cylinders were cured in both the laboratory and the field, and cores were taken from each slab. Specimens were evaluated for compressive strength, ionic conductivity using the rapid chloride permeability (RCP) test, and void content using the boil test at ages of 28, 56, 90, 180, 360, and 720 days. Equations are presented that characterize the change in strength, ionic conductivity, and porosity over time, and relationships between lab-cured cylinder values and values from field-cured cylinders and cores for compressive strength, RCP, and boil test were established. The study demonstrates that concrete cast in moderate temperatures exhibited greater compressive strength, lower charge passed in the RCP test, and a lower percentage of voids in the boil test than concrete cast in high or low temperatures; the use of slag cement or slag cement and Class C fly ash as partial replacements for portland cement lessened the negative impact of high temperatures on these properties, but was detrimental to the early age properties of concrete cast in cold temperatures. Cores and field-cured cylinders exhibited lower compressive strength and greater ionic conductivity and voids than lab-cured cylinders. The equations developed in this report reasonably predict the change in strength, charge passed, and percentage of voids over time. No correlation was found between results from the boil test and results from the RCP test.