THERMODYNAMICAL APPROACH TO THE BRITTLE-FRACTURE OF DRY PLASTERS

The evolution of the fracture toughness, K-lc and fracture energy, G(l c), of set plasters was determined on notched beams as a function of s ample porosity, P, and characteristic size, W. Toughness was found to decrease with decreasing crack width. For set plasters of 57.7% porosi ty, the lowest toughness measured was K-lc = 0.13 MPa m(1/2) for a cra ck width of 0.2 mm. For this crack width, fracture toughness and fract ure energy linearly changed with porosity: K-lc = 0.5 (1-1.3 P) MPa m( 1/2) and G(lc) = 13.47 (1-1.12 P)J m(-2). Dense plasters were more dif ficult to break than porous ones. The fracture energies were affected by the velocity of the fracture propagation, which induces damaging an d multicracking of the material, so that the roughly calculated chemic al surface energy of set plaster was too high. After correction it was estimated to be 0.4 J m(-2). Finally, because toughness increased wit h increasing sample size, it was concluded that fracture toughness and energy were not intrinsic parameters of the material. On the other ha nd, for our sample porosities and sizes, the reduced rupture force, F- rupt W--0.65 is a constant and seems to be a characteristic parameter of the mechanical resistance of set plaster beams.