Micromechanical Observation of Fracture Process in Mortars

Abstract

Cement-based materials like concrete and mortars are quasi-brittle materials. These materials show a softening behaviour when tested in uni-axial tension. An example of such a behaviour is shown in Fig. 1, which is a result obtained from a tensile test on mortar in the special micro-tensile testing machine explained in this paper. The shape of the curve and the area under the curve which is a measure for the fracture energy, is strongly related to the heterogeneity of the material, see for instance Schlangen and van Mier [1]. At the weak spots in the material (generally the interface at grain boundaries) first micro-cracks develop in the material when a load is applied. Subsequent loading will result in a localized crack that will propagate through the material. This mechanism is believed to take place in all cement based materials, but is hard to observe. Stable fracture in a tensile test can only be realised with a closed loop hydraulic testing machine. Observations of micro-cracking and localization of these cracks is a difficult task. However to be able to improve materials a complete understanding of the fracture process is necessary. Furthermore durability is a hot issue in cement based materials. Ingress of water and ions result in internal reactions of concrete and embedded steel reinforcement which can result in fast degradation of concrete structures [2]. Understanding the fracture process, the way micro-cracks and localized cracks propagate through the material, is thus important in order to control transport through the material and design the material in such a way that a long service life of structures is guaranteed.