Interface Fracture in Cement Based Materials

Macroscopic observed crack growth in heterogeneous materials like concrete is initiated by microcracking in the interfacial zone (IZ) between matrix and aggregate. Interface fracture has proven to be a dominant mechanism in concrete fracture. In a micromechanical lattice type model that has been developed in the Stevin Laboratory, cracking of the IZ is simulated by re- moving a beam from the finite element mesh. The main issue of the lattice of the numerical models available for heterogeneous mate- model (and most rials) is a sound explanation of the fracture parameters needed in the model. Therefore, an experimental program has started to study the fracture be- haviour of the IZ between matrix and aggregate, and to validate the lattice model in this respect. It will be shown that the density of the IZ has a major effect on the stress-crack opening behaviour measured in the experi- ments. A two dimensional splitting type experiment is designed, which is easy accessible to numerical simulations with the lattice model. Next to a conventional experimental measuring technique an optical microscope is applied to monitor crack growth at the surface of the specimen. Recently the microscope has been adapted for automatically scanning surface cracks, propagating through the specimen. 295 way .... ,,.., ... _...., ....... .., ...., . ._.. .. u_ .. .., ........ u"- the two U-JL._,,._,_._.__., _ _.__.._,..._._ ...... ~ ... ~ ..... ~ .. .., • ................ 'LIL.LL (1993). is UiJUl'-'"'-u ............ ..... removing a as soon as a fracture criterion is exceeded. Wang ( 1994) gives a representation of the IZ as an element without any thickness (singular seems to be more realistic since the interfacial (Bentz et 1992), is much less length. It is mentioned interfacial thickness depends strongly on type and nature of the aggregates used, as will be shown in this paper too.