Research On Crack Resistance Of Concrete Thin-walled High Pier In High Alpine And High Altitude Areas

At present,thin-walled high piers are the main form of long-span bridge piers in high alpine and high altitude areas.They are widely used for they are lightweight,material-saving,simple in construction and stable in force.However,due to the special environment of low temperature and dryness,intense sunshine radiation,large temperature difference between day and night,frequent freeze-thaw alternation in alpine and high altitude areas,the cracking of thin-walled high pier structures is common and serious,which has a great impact on the durability and even safety of structures.Therefore,it has great practical significance and practical value to study the improvement of crack resistance and durability of Thin-walled High Pier Concrete in high alpine and high altitude areas.The paper explores mainly from two aspects: first,the improvement of material properties of high pier high performance concrete.Based on the investigation of plateau climate environment,pier diseases,quality of concrete raw materials and mix design,the paper puts forward the requirements of concrete performance and the suggestions of mix proportion optimization in high alpine and high altitude areas.Also,the paper explores the adjustment of water-binder ratio,fly ash content,air entrainment technology and internal curing technology through experiments to improve the frost resistance,the ion penetration resistance and shrinkage crack resistance of thin-walled high pier structure.Secondly,the temperature effect of thin-walled high pier structure under large temperature difference and sunshine radiation is analyzed by finite element method through ANSYS,and then analyzing the causes of cracking by considering the joint action of internal and external factors.Correspondingly,effective measures to prevent cracking of thin-walled high piers are put forward according to reinforcement design and construction maintenance.In the experimental part,based on the optimum mix ratio of aggregate gradation design,the effects of water-binder ratio,fly ash content,air entraining agent and internal curing agent on the strength and durability of high pier concrete in high altitude and cold regions are simulated and evaluated by mechanical properties test,rapid freeze-thaw cycle test,RCM chloride ion migration test,shrinkage performance test and plate restraint crack resistance test.The results show that within a certain range,water-binder ratio has a strong linear negative correlation with chloride ion migration coefficient and autogenous shrinkage,but the drying shrinkage decreases with the decrease of water-binder ratio.The pozzolanic effect,morphological effect and micro-aggregate effect of fly ash have a positive impact on the development of later strength and elastic modulus,and can improve shrinkage and crack resistance.Air entraining agent can introduce independent closed micro-pore.Mercury intrusion method test found that after air drawn into,the pore size distribution are improved to small areas,and the pore structure is more reasonable,so the frost resistance and crack resistance are obviously improved.The release mode of SAP and the micro-pump effect of ceramics sand make the hydration complete and maintain the relative humidity inside the concrete,so as to increase the strength of concrete and improve the shrinkage and crack resistance.Comprehensive experimental study and mechanism analysis show that the HPC of C35 high pier with fly ash,ceramics sand and air entraining agent can meet the requirements of strength and durability in high altitude areas.At the same time,the internal curing concrete has relatively good thermodynamic performance.Under the action of diurnal temperature difference and sunshine radiation,the extreme temperature difference and temperature stress of thin-walled high piers are reduced,which has a positive effect on the control of crack generation and development.The causes of cracking are analyzed by finite element simulation that the change of ambient temperature and humidity,the shrinkage difference between old and new sections,the thermal effect of hydration,and the restraint of steel bar on shrinkage will lead to the development of temperature cracks and shrinkage deformation cracks in thin-walled high piers.Reasonable allocation of horizontal stirrups and anti-cracking reinforcing bar net,warming mixing water and aggregate,control of interval time between section pouring and demoulding,heat-storing or steam maintenance can effectively prevent or alleviate the development of cracks in thin-walled high pier.