Department of Architecture Building and Planning, Eindhoven University of Technology


When specimens are tested in compression the lateral expansion is usually confined by friction between the loading platens and the specimen. For design purposes, a value for the compressive strength of a material, without these confining effects is desired. The amount of confinement depends on the friction properties of the surfaces of the specimen and the steel loading platens and on the mechanical properties of the specimen. Consequently, different conversion factors for size and dimensions, so called shape factors, for different materials are required to obtain a design value for strength.

Compression tests on specimens of various sizes made of calcium silicate and aircrete were performed. Various dimensions of the loaded areas in combination with various height over thickness ratios were used. Slender specimens showed vertical splitting cracks, squat specimens showed the well known hour glass fracture model, indicating the different effects of confinement by the loading platens and the effects of the material structure. Multiple regression analysis with linear models, in which thickness, length, slenderness and moisture content were the main parameters, gave more insight in their effects on compressive strength. The research shows that shape factors must depend on the material used. For calcium silicate shape factors were found that are smaller than those given in EN 772-1. For AAC it is reasonable to use a shape factor equal to one as described in the Dutch code NEN 6790:2005.