Finite-Element Modeling of Early-Age Concrete Behavior

Abstract

Early-age cracking of concrete may influence the long-term durability of a structure. Cracking occurs when the tensile stress in concrete exceeds its tensile strength. Early-age stress development in concrete is influenced by temperature changes, modulus of elasticity, creep or stress relaxation, shrinkage, coefficient of thermal expansion, and the degree of restraint. In this dissertation, three-dimensional, finite-element analysis was used to model the early-age stress development of concrete. Four creep compliance models including the B3 Model, Modified B3 Model, B3 Model with RT, and B4 Model were incorporated in the finite-element model. Experimental results from restraint to volume change tests with rigid cracking frame were used to assess the accuracy of the finite-element analysis. The results show that the Modified B3 Model provides the most accurate prediction of the measured early-age concrete stresses. Extensive cracking was found in several cast-in-place concrete culverts in Alabama. A parametric study was performed by finite-element analysis of culverts and results revealed that the following measures will reduce the risk of early-age cracking in cast-in-place concrete culverts: lower coefficient of thermal expansion concrete, contraction joints, sand-lightweight concrete or all-lightweight concrete, scheduling the casting of the culvert wall to minimize the difference in its placement time relative to its previously cast base, and scheduling construction to avoid concrete placement during hot weather conditions. The high-stress nonlinearity coupled with creep was considered in this study by correcting the model with a reduced effective modulus when the tensile stress is above 70% of its tensile strength. The experimental results of concrete mixtures were used to verify the accuracy of the proposed finite-element model from initial setting to the age of cracking. The coefficient of determination for all stress data points above a concrete tensile strength of 70% increased from 0.39 to 0.81 when using the predictions from the proposed model compared to the original linear-elastic model. The proposed model that accounts for creep and high-stress nonlinearity has a coefficient of determination of 0.97 for all the data points from 22 concretes tested, and provides an accurate prediction of early-age concrete stresses from setting to cracking.