Report rep039 : Experimental Determination of the Stress-Crack Opening Curve for Concrete in Tension - Final report of RILEM Technical Committee TC 187-SOC

When a crack grows in any material, a strong strain concentration appears ahead of the crack tip that leads to a more or less extended nonlinear zone. For plastic materials such as metals, the nonlinear zone is dominated mainly by plastic deformation until the final stages of failure. For quasibrittle materials such as concrete, which can endure very little plastic strain before starting to fail, the nonlinear zone consists of a narrow band of damaged material with micro cracks, partial overlapping cracks and zones bridged by aggregates. In other quasibrittle mate-rials fiber or grain bridging may be a contributing mechanism.

A simplified model for the quasibrittle form of fracture is the cohesive crack model. In this model that narrow band of partially broken material is idealized as a crack (a line, in two di-mensions, or surface in three dimensions), but a crack that is still able to transfer stress be-tween its two faces.

The stress-crack opening relationship is the basic ingredient of the standard cohesive crack model, and the task of the present Committee was, basically, to recommend one or various methods to determine it.
RILEM Technical Committees pioneered the development of testing method for fracture properties of plain concrete in tension, beginning with TC 50 ‘Fracture Mechanics of Con-crete’. Four recommendations were issued between 1985 and 1992 dealing with the determina-tion of fracture parameters, but these recommendations mostly address the determination of one or two fracture parameters, and, although two of them used concepts from the Hillerborg’s model, none addressed the issue of setting up a general methodology to determine the stress-crack opening curve.

Due to the multiplication of methods developed by different research groups along the years, it became apparent, at the end of the 1990s, that it was advisable to try to extract a common method from the disperse set of procedures one could find in the literature. Consequently, RILEM TC-187-SOC was launched, and proposals for tests methods were collected from the Committee members.

This report contains three contributions. The first one is the result of the debates in the Committee and in the Task Group on Uniaxial Tension Tests, and sets the minimum demands for such kind of tests. The second contribution deals with the indirect tests and contains a rela-tively detailed description of the experimental methodology of the splitting tests of cylinders and of stable three-point-bending tests, incorporating recent experimental improvements. It also contains, in a separate section, a detailed description of an explicit inverse analysis that makes use of the results of both the splitting and the bending tests. The third contribution deals with general inverse procedures based on optimization methods to extract the stress-crack opening curve from a single stable test on a notched specimens, either a three-point bend beam or a wedge-splitting specimen.

Contents

Chapter 1 - Introduction

Chapter 2 - Minimum demands for deformation-controlled uniaxial tensile tests

Chapter 3 - Indirect tests for stress-crack opening curve

Chapter 4 - Inverse analysis procedures for determining the tensile stress-crack opening curve of concrete