ELECTRICALLY-INDUCED SHAPE CHANGES IN HARDENED CEMENT PASTES AND POROUS SILICA-GELS - THE DYNAMIC NATURE OF GEL PORE STRUCTURES DURING WATER TRANSPORT

Electrically induced strain studies of hardened cement pastes mere per formed using an inductance technique. Investigations were done as a fu nction of electrical history, measurement frequency, moisture content, and initial mater to cement ratio (w/c) during hydration. Expansive s trains as large as 4.5 x 10(-4) mere observed under held strengths of similar to 0.5 kV/cm in specimens which had been saturated with mater. Both switchable and nonswitchable strains mere observed under cycling with an ac electrical field. Evidence mas observed that the switchabl e strains are due to redistribution of water within the specimen, wher eas the nonswitchable strains are related to long-range mater transpor t toward the surface of the specimen. Investigations as a function of measurement frequency revealed a strong relaxation of the induced stra in in the frequency range of 6.67 x 10(-3) to 1 Hz. It is believed tha t ions in the diffuse double layer move under an electrical field and that water is transported by electro-osmosis. Electrically induced str ains then develop because of local swelling effects associated with a saturation gradient. Investigations as a function of w/c ratio reveale d that the induced strains are related to the volume fraction of gel p orosity. Electrically induced strain measurements of porous silica gel s with average pore sizes of 25 and 50 Angstrom mere then performed. T he characteristics of the electrically induced strains mere found to b e nearly identical to those for portland cement. It is believed that t he characteristics of the electrically induced strains are controlled by the entry and removal of physically absorbed mater within the gel p ore structures.