Yun Lee

Fracture Characteristics of Concrete at Early Ages

The objective of this study is to examine fracture characteristics of concrete at early ages, i.g. critical stress intensity factor, critical crack-tip opening displacement, fracture energy, and bilinear softening curve based on the concepts of effective-elastic crack model and cohesive crack model. A wedge splitting test for Mode I was performed on cubic wedge specimens with a notch at the edge. By experimenting with various strengths and ages, load-crack mouth opening curves were obtained, and the results were analyzed by linear elastic fracture mechanics and FEM(finite element method). The results from the test and analysis showed that critical stress intensity factor and facture energy increased while critical crack-tip opening displacement decreased with concrete aging from 1 day to 28 days. Four parameters of bilinear softening curve from 1 day to 28 days were obtained from a numerical analysis. The obtained fracture parameters and bilinear softening curves at early ages from this study are to be used as a fracture criterion and an input data for the finite element analysis of concrete at early ages.

Development of Microplane Model Based on Degree of Hydration Concept

Recently, the microplane model has come to be regarded as a powerful constitutive model that simulates three-dimensional concrete behaviors. To depict the early age behavior of concrete material, nonlinear regression analyses were performed in this study with tensile and compressive test results for early age concrete to develop a microplane model labeled as M4. Material parameters in the developed microplane model are analyzed based on the degree of hydration concept. The analysis results show that an additional nonlinear function should be introduced in the existing negative deviatoric boundary of M4. In relation to the tensile behavior of the microplane model, the brittleness in the normal boundary in M4 increases as the degree of hydration increases. The proposed microplane model successfully describes the early age tensile and compressive behavior, and is consistent with an existing microplane model for hardened concrete.