EFFECT OF SELF STRESS ON FLEXURAL STRENGTH OF GYPSUM-POLYMER COMPOSITES

Self stress generated in polymer impregnated gypsum (referred as GPC) when it is composed is estimated, and its influence on flexural streng th is discussed. The estimation of the self stress is based on measure d values of shrinkage caused by polymerization of impregnated monomer and elastic modulus of dried gypsum base just before impregnation. The effect of this self stress on flexural strength of GPC is investigate d. It was found that the following equation is valid to predict flexur al strength of GPC (6(b)) in terms of the self stress as a variable: 6 (b) = 6(gb) + V-p (6(p) - 6(sp)), where 6(gb) = flexural strength of g ypsum base, V-p = specific volume of polymer, 6(p) = tensile strength of polymer, and 6(sp) = self stress generated in polymer phase. If ext remely low water-gypsum ratio is adopted to prepare gypsum base, crack ing is observed just after polymerization preceding any flexural loadi ng. For somewhat higher water-gypsum ratio, specimens are not cracked, but their flexural strength is decreased after polymer impregnation. The self stress corresponding to this case turns out to be higher than the tensile strength of polymethyl methacryrate used for the impregna tion. Since prediction of 6(sp) in the equation is based on tri-axial compressive strain of gypsum base that is within its elastic region, 6 (sp) in polymer phase should positively exist. Even for this condition , the validity of the equation seems to be maintained, although the va lue in the parenthesis of the equation becomes negative. Based on this fact, an unstable physical state where one phase of a composite mater ial is stressed beyond its macroscopic strength as an individual mater ial owing to the crack arresting effect of the other phase (gypsum in this case) has been postulated. This state is designated as a ''super- stressed'' state, taking its resemblance with supercooling or super-sa turated into consideration. (C) 1998 Elsevier Science Ltd.