Anti-frost Design Method And Preventive Measures For Concrete Structure In The Qinghai-Tibet Plateau

The Qinghai-Tibet Plateau has the typical environment characteristics of high altitudes and thin air, which make the number of days per year that has large temperature difference between day and night in the region is more than that of in the plain. Therefore, the early-age frozen damage and the freeze-thaw damage are the main failure mode of many concrete structures in the Qinghai-Tibet Plateau.As the backward economic development, the construction in the Qinghai-Tibet Plateau started very late, and is still in the beginning stages. At present, the design method of freeze-thaw durability of concrete and the corresponding preventive measures are still copy the mode of the construction of plain region without taking full account of the large difference in the environment characteristics between the two regions. Actually the frequent change between positive and negative temperature and low atmospheric pressure could have a massive impact on the design method of freeze-thaw durability of concrete and the corresponding preventive measures.In order to improve the freeze-thaw durability and anti-freezing ability in the early age of concrete in the Qinghai-Tibet Plateau, the following studies are carried on in this dissertation.Firstly, some impact factors which affect the extent of freezing-thawing damage of concrete are analyzed, and on this basis the typical environment characteristics which affect the design method of freeze-thaw durability of concrete in the Qinghai-Tibet Plateau are summarized. Then a calculation formula is created to calculate the number of freezing and thawing cycle of concrete a year by regression analysis on the basis of corresponding meteorological data from 20 weather stations of Qinghai-Tibet Plateau in nearly 40 years. With this formula the Qinghai-Tibet Plateau is divided into different regions where the concrete construction request for different requirements for its freezing and thawing resistance, and the recommended design grade of freezing resistance for concrete in the Qinghai-Tibet Plateau are proposed. These studies mentioned above lay the foundation for the design of anti-freezing concrete in the Qinghai-Tibet Plateau.Secondly, the critical compressive strength of concrete with different strength grades in the Qinghai-Tibet Plateau are proposed through analyzing the effect of early-freezing on the properties of concrete. Then a rational method for predicting the strength development of concrete is proposed through improving the current calculation of apparent activation energy Ea used in existing maturity method, Furthermore, an experiment were conducted where the concrete were cured under constant and varying temperature respectively. The experimental results show that the error of prediction in strength development using the improved method are fall within 10 percent, which is apparently higher than that using the method recommended in ASTM C457 standards. Thus this improved method can be used to predict the strength development of concrete in the Qinghai-Tibet Plateau to prevent early-freezing of the concrete.Thirdly, The effects of atmospheric pressure changes on the air-entraining capability of air-entraining admixtures, the air void parameters of hardened concrete and the stability of air bubbles in the concrete are studied using a low-pressure test chamber to simulate plateau environments with thin air and low atmospheric pressure. Results indicate that the atmospheric pressure of the environment significantly affects the performance of air-entraining admixtures, The air content of fresh concrete decreases linearly with the drop of air pressure. The air content of fresh concrete decreases by approximately 20-52% when the atmospheric pressure is 50 kPa with respect to when at normal atmospheric pressure (101 kPa). The air void parameters of concrete mixed when atmospheric pressure is low are worse than when it is mixed under normal atmospheric pressure and the bubble stability of fresh concrete mixed under low pressure is bad. Accordingly, several preventive measures to improve the frost resistance of concrete constructed in the Qinghai-Tibet Plateau are proposed. In addition, the results mentioned above also provide an experimental basis for the design of anti-freezing concrete in the Qinghai-Tibet Plateau.In the end, the relationship between the freeze-thaw durability index of concrete and the material parameters such as water cement ratio, spacing factor and air content are firstly established, and then based on research results, which is the divided region of the Qinghai-Tibet Plateau according to the request of concrete for different requirements for its freezing and thawing resistance and the influence of low atmospheric pressure on the properties of air-entraining concrete respectively, the current design method for anti-freezing concrete are improved.The research results in this paper have great theory significance and utility value for improving the freeze-thaw durability and anti-freezing ability in the early age of concrete in the Qinghai-Tibet Plateau.