Advanced Cement Based Materials最新文献

The purpose of this paper is to set a scenario for discussing and comparing different models of concrete fracture. Using as a reference the cohesive crack model, as conceived by Hillerborg, it was found that simpler models—the equivalent elastic crack, the R-Δa curve approach, or models based on two parameters such as Bazant's and Jeng-Shah's—fit inside this scheme and are hierarchically related. For each approach, the theoretical background is reviewed, the experimental aspects are discussed, and the relevance to size effect predictions is considered. This review supplies criteria for comparing the different models and provides interpretation for the measured parameters with a unified language.

Microscopical observations and macroscopical investigations of the crack propagation process have been carried out. They give a new insight into the fracture process zone, leading one to presume that during the crack propagation the crack opening is restrained. So, the crack opening cannot increase without an energy dissipation that has to be added to the one from the crack propagation. Generally, measurements of fracture energy per unit of crack surface give rise to size dependent values for mortar and concrete. We propose a simple model that takes into account the energy dissipated by restrained crack widening (RCW), with the aim to verify if it can explain the size dependence on the energy dissipated per unit of crack surface. The theoretical predictions from the proposed model and the experimental dissipated energy per unit of crack surface obtained by means of RILEM TC-50 procedure are in good agreement. So, size dependence could be mainly explained through the concept of RCW.

This paper, which represents the text of an introductory report presented at Cardiff workshop in 1995, reviews the size effect from two different viewpoints: (1) the need to eliminate or minimize the effect of specimen size on the measured material fracture characteristics, and (2) the exploitation of measured size effect on the nominal strength for determining the material fracture characteristics. Three methods of fracture testing are examined in the perspective of the size effect, and the merits and weaknesses of various methods in regard to the size effect are pointed out. The parameters of all three methods can be determined by simple formulas and linear regression from measurements allowing calibration of two parameters of the size effect law.

The present paper assesses the significance of shrinkage-induced clamping pressure in fiber-matrix bonding mechanisms in cementitious composite materials. The paper contains a description of an experimental setup that allows measurement of the clamping pressure, which develops on an elastic inhomogeneity embedded in a matrix consisting of a cementitious material undergoing shrinkage during hydration (autogenous shrinkage). Furthermore, the paper presents the analysis necessary to perform an interpretation of the experimental results and to determine the clamping pressure acting on any elastic inhomogeneity embedded in the same cementitious matrix material. Fiber-shaped inhomogeneities are of special interest in cementitious composite material systems, and results are presented for the development of clamping pressure on three typical fiber types in two typical cement pastes used in high performance cementitious composite materials. Assuming a Coulomb type of friction on the fiber-matrix interface and using typical values for the frictional coefficient, it is shown that the shrinkage-induced clamping pressure could be one of the most important factors determining the frictional stresses observed on debonded fiber-matrix interfaces during pullout.