A two-step multiscale model to predict early age strength development of cementitious composites considering competing fracture mechanisms

A finite element-based multiscale model is employed to examine the early-age mechanical behavior of cementitious composites under a mode-I loading condition. The mechanical response of early-age mortar and concrete are influenced by the time-dependent development of mechanical properties of the cement paste as well as the mechanical and morphological properties of the aggregate phase. A competing mechanism exists in the fracture process (i.e., crack propagation) of the aggregate and cement paste phases. This paper investigates the distinct role played by the properties of each phase. This paper addresses the problem through a two-step homogenization approach that first starts at the meso-level of the mortar, where the fine aggregates are embedded within the cement paste. The second step considers the homogenization of concrete composed of the mortar matrix resulting from the first homogenization step, and coarse aggregate particles. For the purpose of computational homogenization, the continuous scheme for bulk modeling combined with the discontinuous scheme for modeling the transition of mesoscopic diffusive damage to macro-crack are employed. Inter-and intra-phase cohesive zone elements are utilized to represent the fracture process. The results indicate the relative significance of elastic and cohesive properties of composite constituents for the development of tensile stiffness and strength, damage evolution, and macro-crack patterns. A competing mechanism was observed for early-age cracking of the cementitious composites. When the tensile strength of the cement paste is lower than the tensile strength of aggregate phase, the crack propagates across the paste. When the tensile strength of the cement paste exceeds that of the aggregate, the cracks begin to deflect and propagate through the aggregates. As such, a critical degree of hydration (associated with a certain time) exists below which the cement paste phase is weaker than aggregate phase at the onset of hydration. This has implications on the inference of kinetic based parameters from mechanical testing (e.g., activation energy). Our results show a promising quantitative agreement with experimental observations. (C) 2019 Elsevier Ltd. All rights reserved.