A micromechanical constitutive model for the behavior of concrete

Abstract

A two-dimensional constitutive law for the behavior of concrete under load has been developed in this paper. Concrete is considered to be a composite material made up of two components: cement mortar and aggregate. The cement mortar is defined as the cement paste (cement and water) plus the fine aggregate. The interaction between these two components, at the local level, governs the macroscopic behavior of concrete. The model proposed herein describes the microstructure of concrete by fabric of a representative sample using the notion of a class of contacts, each class having a common orientation of the contact normal to the aggregate-cement mortar interface. Each class is further broken down into cracked and bonded contacts. Cracked contacts allow slip along the interface and are modeled as granular material. Bonded contacts do not allow slip along the plane of contact. In developing the model, first the overall stresses are related to the interface forces at the microlevel. Simple constitutive relations are used to relate the rate of change of the local interface forces, for both cracked and bonded contacts, to the local velocity gradient. Finally, macroscopic constitutive equations are postulated using a Taylor-averaging method. An incremental procedure for numerical integration of the rate equations is described. Qualitative validation of the model is achieved through comparison with published test data. A limited parametric study is followed by a study of the microstructural changes as depicted by a numerical simulation of monotonic axial compression. The results of these simulations seem to be qualitatively consistent with the observed behavior of concrete.