Influence Of Surface Energy On Fracture Energy And Strength Of Cement Based Materials

The deterioration, short service life and failure of cement based materials have become worldwide problems, which attract close attention from science and engineering fields of concrete both at home and abroad. The impregnating of water, seawater or other salt solutions is one of the important reasons to reduce durability of concrete among numerous deterioration factors. Cement based material structures are always in permanent or temporary contacted with water, seawater and other salt aolutions, however, moisture content and surface tension of immersion liquid affect the surface energy of pore materials. Surface energy is the energy needed in unit surface aera, which changes with moisture content and surface tension of immersion liquid. As a result, mechanics performance, such as fracture energy and strength, will be influenced by surface energy. Fracture energy is one important parameter of nonlinear fracture mechanics. Compressive strength has also been taken into consideration in concrete design beforehand since strength loss can be related with safety and performance of structures in service.Tests of compressive strength, wedge splitting, three point bending, four point bending (for SHCC) and free shrinkage were carried out in mortar, concrete and PVA-SHCC in this research. The influences of moisture content and surface tension of immersion liquid on cement based material were investigated experimentally. In addition, concrete was treated by silane water repellent agent, and then the fracture energy and the compressive strength of water or seawater saturated concrete after water repellent treatment was investigated.Results indicate that dried concrete and mortar have the highest surface energy and the fracture energy and the strength are maximal. The larger surface tension of immersion liquid and more moisture content exist, the lower fracture energy and strength are produced. The effect is notable when water is replaced by seawater with larger surface tension, strength and fracture energy is reduced 25% and 35% separately. The swelling strain of mortar also increases linearly with the increase of surface tension of immersion liquids. For SHCC, in dried condition, compressive strength is the highest, but the fracture energy becomes lower because of poor toughness and short strain hardening range, however the Fracture energy will increase if more moisture gets into the pores. The higher concentration of NaCl solution, the larger surface tension is. Correspondingly the fracture energy and strength is reduced by surface tension of NaCl solution. For concrete prepared with NaCl addition, strength increase when the amounts of NaCl added from 0% to 3% of water content, and decrease with the addition of NaCl from 3% to 23.4%. Fracture energy decreases with the addition of NaCl from 0% to 23.4% generally. Water repellent treatment can prevent the impregnation of water and seawater, and the fracture energy will increase, but this effect is not so noticeable in compressive strength.