Heat Damage Effects Of Steam Curing On Concrete And Corresponding Improvement Measures

In recent years, the pre-stressed precast concrete elements are widely used in high-speed railways being built on a large scale in China. However, according to the survey of previous cases it was found that many key issues, such as surface cracks and brittleness of pre-stressed precast concrete, were still not full understood. In the present thesis, firstly, a state-of-the-art review was carried out and some issues that needed to be further studied were proposed, on the other hand, the common characteristic of steam-cured concrete elements widely used in the high-speed line was analyzed. Secondly, based on the above mentioned discussion, the influences of steam curing on microstructure deterioration and property degradation of concrete and corresponding mechanisms are systematically studied. Finally, according to the experimental results combining with the theoretical analysis of concrete material and physical chemistry principles, following creative achievements have been made:(1) The main influencing factors and development of deformation of concrete in the process of preheating, treatment and cooling duration were systematically studied, and the relationship between microstructure and macroscopic properties was also analyzed. According to the results, three heat effects causing degradation of microstructure and properties in the process of steam curing could be induced as follows:expansion effect, surface damage effect and embrittlement effect.(2) The irreversible residual size increment caused by steam curing is usually named as expansive deformation. Based on the anaysis on the mechanism of expansive deformation and the key factors affecting such deformation, a model was proposed to predict the expansive deformation of concrete occurring in steam curing process, and the validity of the model was verified by the experimental results. It was found that during the three stages of steam curing, i.e control heating, heating at constant temperature and control cooling, expansive deformation characteristics of concrete differs from each other for each stage. The quantity of gas and liquid phase in mixture before steam curing is the key factor influnecing expansive deformation of concrete.(3) The adverse effects of steam curing on hydration products and microstructure of cement paste were studied by means of micro and macro test methods. The experimental results showed that the adverse effects on surface concrete in contact with steam could cause a gradient distribution of micro and macro properties of concrete from exterior to interior of elements, such as capillary sorptivity, chloride transport and carbonation of concrete. Due to the surface damage effect, an surface layer with the depth of about10mm, showing larger porosity than inner concrete or standard-curing concrete, was formed. The mechanism of surface damage effect was disclosed by means of capillary sorptivity test, and the water absorption coefficient in the capillary sorptivity test was proposed to evaluate the development of surface damage effect.(4) In order to study the formation of gradient distribution of micro and macro properties of concrete, two groups of concrete specimens were cured in the molds with and without covers respectively. The temperature distributions in concrete were measured and the temperature stresses in different sections were calculated respectively. The results showed that the surface layer of specimen was severely damaged due to large temperature difference, however, concrete looks sound in the inner zone of specimen with little temperature difference. On the other hand, according to the experimental results of heat-mass transfer, it was found that a large amount of interconnected pores were formed in the surface layer, resulting in larger thermal expansion and contraction stress than the inner zone of concrete specimen. The deformation difference between the exterior and interior of concrete specimens resulted in the gradient distribution of micro and macro properties of concrete.(5) In order to study the influence of steam curing on the development of embrittlement effect on concrete, the impact toughness, tensile compressive ratio and flexure compression ratio of concrete were studied, the results showed that the embrittlement effect of steam curing should be the main cause for crack and peeling. On the other hand, in order to discuss the mechanism of embrittlement effect, the micro-morphologies and phase structures of C-S-H was observed, the experimental results showed that there were two main aspects. Firstly, the formation of coarse cement hydration product with low specific surface area due to steam curing caused the decrease of cohesive force, secondly, the stress concentration in the interfacial transition zone leaded to micro-crack formation.(6) Based on the abovementioned research results, specific experiments were carried out to study how to improve the steam cured concrete performance. The results showed that the total porosity of concrete and the size of pore with highest proportion in concrete could be significantly decreased by using double-mixture with ground slag and fly ash or multi-mixture with ground slag, fly ash and silica fume. On the other hand, the subsequent water curing after steam curing finished and steam curing on concrete with cover could significantly alleviate the surface damage and embrittlement effects.