Abstract
Determining early age concrete strength can facilitate the construction process and prevent catastrophic failures. The strength gain of a concrete mixture is a function of curing temperature and age. Results of two nondestructive methods of assessing early age concrete strength are reported: the maturity method and pullout testing. The concrete specimens consisted of 12 variations of two cement types, two aggregate types and three water to cement ratios; they were cured at 37, 55, and 73 degrees-F, and the outdoor environment. Cylinder compression and pullout tests were performed from 12 hours to 7 days. Plots of the average cylinder compressive strength and pullout force for each curing temperature versus maturity showed four nonlinear curves. When the maturity data was transformed by the natural log function, linear relationships were formed for the different curing temperatures with the exception of the cylinder strength/maturity data curve at 37 degrees-F. Regression equations were developed for cylinder strength and maturity, and for pullout force and maturity, with the datum temperature modified from the usually accepted minus 10 degrees-C to maturity by increasing the datum temperature changed the relative of the data lines plotted for each temperature with the data line for 37 degrees-F changing from the lower bound to the upper bound of the data set. A prediction model of the cylinder strength in terms of maturity was developed and a model for the prediction of strength by the pullout force was also obtained by combining cylinder strength and pullout regression equations. However, in the pullout model the concrete strength gain in the cylinders differed from the strength gain in slab specimens, and therefore the strength should be closely monitored when using this prediction equation.
