Biaxiales Bruchverhalten von zementgebundenen Werkstoffen : Experimente, Bruchmechanismen und Modellierung

Abstract

Initiation and propagation of cracks lead to a multi-axial loading condition of concrete and other cement bonded materials. A new testing equipment has been developed in order to characterize the fracture mechanical behavior of non-reinforced and fiber reinforced cement bonded materials. A defined biaxial stress state is generated in the specimen with a hydraulic compression equipment, and specimens are splitted under stable crack propagation with a wedge splitting device. The complete load- displacement diagram is recorded and the specific fracture energy and the maximum splitting force are obtained from these measurements. The strain softening behavior is determined with numerical methods. The shape of the single edge notched specimens is cubic and therefore a large ligament cross section in comparison with the specimen volume is obtained. Cardboards and teflon slip layers serve to reduce the transverse strain between compression plates and specimen. The simple specimen shape is the reason that this new testing equipment saves much testing time and is inexpensive. Several influences like maximum coarse grain size, strength, grain shape, dry and wet storage, on the biaxial fracture behavior have been investigated. The experimental results could be explained quantitatively with a phenomenological fracture model. In addition, the influence of fiber type (polypropylen, steel and glass fibers), different fiber cross sections and volume content on the biaxial fracture behavior have been studied. The mechanisms of crack propagation are characterized and discussed on the basis of simple models. The numerically calculated bilinear 'strain softening' diagrams for not reinforced concrete show a logarithmic increase of the 'microcracking' part of the specific fracture energy during uni- and biaxial loading with increasing strength and maximum coarse grain size and a linear increase of the 'bridging' part. A model to calculate the fracture energy, which becomes free by the extension and fracture of statistically distributed fibers in the concrete matrix has been extended from the uniaxial to the biaxial loading case. The simulations coincide quite well with the experimental results and allow a more profound understanding of the mechanisms, of fiber reinforcement during fracturing.