Deterioration Mechanism Of Strain Hardening Cement-based Composite

Strain Hardening Cement—based Composite(SHCC) is featured with its tensile strain-hardening characteristic and multi-crack characteristic. It ultimate tensile strain can reach4%.So It can be applied into crack controlling structure, enhancing the seismic resistance of structures and so on. There are a lot of papers on design theory, basic mechanical performance and durability performance of SHCC up till now. But most of these results are based the undamaged SHCC. This is different for real saturation of SHCC in practice. Based on the above problem, this thesis will focus on the cracking mechanism and its influence on durability of SHCC. The detailed contend of this thesis are as follows:(1) Digital image process was used to investigate the crack pattern of SHCC both on finished surface and formed surface under direct tension load. Then statistical and potential methods were used to deal with the crack width distribution. Results indicate that the crack density of formed surface is lower than finished surface. But the average crack width of formed surface is bigger than finished surface. And the crack width parameters such as crack density, average crack width follows polynomial equation with the order of3. Also it was established that the Gamma distribution is the most accurate statistical distribution to describe the distribution of crack widths of SHCC at different tensile strains. Based on this, capillary absorption of and chloride penetration into SHCC under different strain level were performed. At the same time silane impregnation was used to protect the cracked SHCC. From the results, we can get that with the increase of strain level, water was absorbed quicker, more water was absorbed and the coefficient of capillary absorption will increase linearly with strain level. And for chloride penetration, the depth and amount of chloride increased with strain level too. But silane impregnation can reduce the penetration of water and chloride into SHCC. Finally, three evaluation methods of the influence of crack pattern on permeability were performed. This can be used to evaluate the durability of cracked SHCC.(2) Neutron radiography has been successfully applied to visualize the process of water penetration into cracked SHCC and to quantify the corresponding time-dependent moisture distributions in cracked SHCC. Results indicate that in un-cracked SHCC, less water can be found. But for cracked SHCC especially for the specimens with big crack, water will penetration into the specimens along the crack immediately and then water migrated further into the surrounding matrix from water filled cracks. Later water moved into the matrix adjacent to the cracks which was mechanically damaged by direct tension. But for integral water repellent SHCC, little water can penetrate into it.(3) Based on moisture diffusion, a method of inverse analysis has been proposed to calculate the moisture diffusion coefficient and shrinkage stress of SHCC. Evolutionary algorithm was selected during optimization process. A quite good fit of analysis results to the experimental data could be achieved. This indicates that the accuracy of the calculated results by the proposed inverse analysis method is very high. This method can solve the feedback problem of milti-parameter. From the experimental results, we can get that with the decrease of RH, the weight loss and shrinkage rate of SHCC and its matrix will increase. And at the beginning of exposure time, all of the specimens will loss water quick and also the speed of shrinkage is quick. But with the increase of exposure time, the speed of water losing and shrinkage will go down. At the same time, the vapor diffusion coefficient of SHCC is smaller than its matrix. Because of the moisture gradient, the surface layer of SHCC was tensioned. And the tension stress can reach the ultimate tension stress, and then crack will happen. But strain softening will not happen, the stress will remain the sameo With the increase of drying time, the drying front kept moving into deeper layer step by step. So from the surface layer to dapper layer of SHCC, the stress gradient exist, it will change from tensile stress to compressive stress. From the stress distribution, the whole deterioration process is very clear.(4) Accelerated freeze-thaw test on SHCC has been conducted, and change of relative dynamic elastic modulus, mass loss, bend property, water capillary absorption, chloride penetration, carbonation and bond behavior are gained. After300freeze-thaw cycles, the reduction of dynamic elastic modulus is just4%and the weight loss is just2%. So the adding of PVA fiber increases the frost property. With the increase of freeze-thaw cycles, the bond strength between PVA fiber and matrix will decrease, then more fiber will be pulled out and less fiber will be destroyed under tensile stress. In this way, the bending strength will go down for the prism of SHCC under4-point bending, but the deformation of the prism will increase. From the results of water capillary absorption test, chloride penetration test and carbonation test of SHCC before and after frost damage, the influence of frost damage is very little. From the results of pull out test, the bond strength between steel bar and normal concrete decreases linear with increase of freeze-thaw cycles. And all of concrete specimens fail in a split way. But for SHCC, the bond strength between steel bar and SHCC decreases with the increase of freeze-thaw cycles a little. Pull-out failure happened for all SHCC specimens.