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 strength is discussed. The estimation of the self stress is based on measured 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 investigated. It was found that the following equation is valid to predict flexural strength of GPC (б_b) in terms of the self stress as a variable: б_b = б_gb + V_p (б_p − б_sp), where б_gb = flexural strength of gypsum base, V_p = specific volume of polymer, б_p = tensile strength of polymer, and б_sp = self stress generated in polymer phase. If extremely low water-gypsum ratio is adopted to prepare gypsum base, cracking is observed just after polymerization preceding any flexural loading. 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 impregnation. Since prediction of б_sp in the equation is based on tri-axial compressive strain of gypsum base that is within its elastic region, б_sp in polymer phase should positively exist. Even for this condition, the validity of the equation seems to be maintained, although the value in the parenthesis of the equation becomes negative. Based on this fact, an unstable physical state where one phase of a composite material is stressed beyond its macroscopic strength as an individual material owing to the crack arresting effect of the other phase (gypsum in this case) has been postulated. This state is designated as a “superstressed” state, taking its resemblance with supercooling or supersaturation into consideration.