Effect of Porosities on Strength of Plaster of Paris

Abstract

Experiments are conducted to study the variation of cube compressive strength, prism flexural strength and cylinder split tensile strength of Plaster of Paris (PoP) specimens with size and with addition of Expanded Polystyrene(EPS) beads, which act as porosities. To study the effect of size, geometrically scaled homogeneous PoP specimens are prepared and the corresponding strengths are determined. To study the effect of porosity, for a fixed volume fraction of EPS beads(Vf), the strength is measured for geometrically scaled specimens of varying sizes. The drop in compressive strength from D=100mm to D=150mm cube specimens is more for Vf=20% as compared to when Vf=0%, 10%. This drop is attributed to change in mechanism of failure from tetrahedral cleavage fracture to distributed parallel cracking. This result progresses the idea of transition from brittle to quasi-brittle material behavior with increase in the porosity under uniaxial compressive loading. In the next study, for a varying volume fraction (Vf = 2%, 5%, 10%, 20%, 30%) of EPS beads, specimens were prepared for a constant cube size of 100 mm and cylinder size of 46 mm diameter. For these specimens, cube compression strength and split tensile strength showed a mild increase followed by monotonic decrease in strength with increasing Vf. The mild increase is attributed to the phenomenon of crack arresting caused by the defects. This result motivates the need for a rigorous theoretical and numerical study for a complete understanding of this phenomenon.