Self-healing of Cracks in Strain Hardening Cementitious Composites Under Different Environmental Conditions

Abstract

Strain Hardening Cementitious Composites (SHCC) may be characterized by their high strain capacity, which is obtained by adding fibers to the cement-based fine mortar. PVA fibers are often used. When a crack is formed because of imposed strain, fibers bridge the new crack surfaces and hinder wide opening. In this way multiple crack formation is observed instead of widening of few existing cracks. Crack formation is one of the most frequent reasons for corrosion of steel reinforcement and of limited service life of reinforced concrete structures. Water and aggressive aqueous solutions can migrate deep into concrete via existing cracks. Initially it was believed that micro-cracks in SHCC are so fine that they will not transport liquids by capillary action. But by means of neutron radiography it could be shown that even very fine cracks are very quickly filled with water by capillary action. From the water filled cracks water migrates further into the porous structure of hardened cement paste. From these results it follows that formation of fine cracks as such is not sufficient to protect concrete from deep penetration of water and aggressive aqueous solutions. Surface treatment with a water repellent agent can solve the problem. But self-healing of fine cracks may turn out to be an inherent protection of concrete. The influence of self-healing on capillary absorption was studied experimentally. It can be shown that if pre-strained SHCC with micro-cracks is exposed to high relative humidity capillary absorption is gradually reduced. If pre-strained SHCC is stored in water or in saturated aqueous Ca(OH)2-solution, self-healing is accelerated and capillary absorption reaches finally values of the material without cracks. Hence, increased durability of SHCC is not primarily due to the fineness of cracks but rather due to accelerated self-healing.