Study On The Influence Of External Stress To The Water Permeability And Carbonation Character Of Cement-based Materials

The durability of concrete is one of the key problems in cement and concrete field that many scholars are concerned with . In recent years, the focus of durability research is transferred from single degradation factor to multi ones. As we know, concrete structures in reality are always under various stresses or with microcracks of different width. So in this paper, compressive stress tensile stress and microcracks are introduced into water permeability and carbonation experiment, with the object to study the relationship between those factors and durability indexes. SEM XRD and MIP methods were also used to study the relevant mechanisms.The results from water permeability test indicated that the permeability coefficient of concrete was significantly different with or without compressive stress. The permeability coefficient(K) decrease with the increase of stress when the stress is below 60% of the ultimate compressive strength, and the decreasing trend can be expressed by the function K=Ko ean. When the stress acted on concrete specimens exceed 60% of ultimate compressive strength, the coefficient(K) will increase obviously but the largest change is still within the order of 10-9 cm/s.The time-dependant characteristic can be expressed by the function K=(a+ct)/(l+bt). The coefficient K decreased significantly in the first 40~60 hours of the water permeability test, and the decreasing rate became slow as the time goes on.The permeability behaviour of specimens(concrete, mortar and paste) with microcrack(0.1~0.35mm width) is different from those without microcracks. Water volume flowing across microcrack per hour will reduce with the increase of permeating time and the reducing is most significant in the first three weeks. In addition, water gradients, crack width, permeability time, specimen's kinds can also influence the self-sealing degree. XRD and SEM analysis showed that the main composition of the white substance which formed in cracks is CaCO3, which can block cracks and is probably the main cause of self-sealing. Two processes which called surface reaction process and diffuse reaction process are defined in the formation process of CaCO3.The phenomenons in self-sealing experiment can be explained by those two processes.Tensile stress will accelerate the carbonation of paste and mortar, which can be explained by the reason that shrinkage of carbonation is confined by tensile stress, andthe function x = C0eantb can describe this process. Carbonation can lower the totalporosity and lessen the aperture in the cement paste. It may be the deposition of CaCO3 in capillary that divide the large capillary into small one. Carbonation can also lower the water permeability of samples. The bigger the W/C ratio, the large the decrease rate of permeability.