The Research On Effect Of Plateau Climatic Conditions On Concrete Perfromance And Cracking Mechanism

Low air pressure,low humidity and large temperature differences are the typical climate characteristics of plateau regions.Through the on-situ investigation,it was found that the crackings of concrete were more serious than those in plain regions,especially 1~2 years after casting.It is significant to research the effect of plateau climate on material performances and structure stress of concrete,which were valuable to seek the cracking mechanism of concrete structure.Based on the homemade simulation chamber,the effect of low air pressure and humidity on mechanical properties,durabilities,shrinkage of concrete were researched.In order to seek the mechanism of macro-properties changes,the microstructure of cement paste and concrete was studied.Based on the temperature-humidity-air pressure coupling model,the temperature field,humidity field and coupling stress field of concrete pillar under normal weather and abnormal weather in plateau regions were calculated to analyze the cracking mechanism.Two designed strength C30 and C50 were used to study the compressive strength,flexural strength and splitting tensile strength of concrete under different relative humidity(20%,60% and 100%)and different air pressure(51kPa,61 kPa and 101kPa)conditions.The test results showed that the compressive strength,flexural strength,splitting tensile strength of concrete decreased under low relative humidity and low air pressure conditions,especially after 28 days.Compared with standard condition(100% RH and 101 kPa),compressive strength,flexural strength and splitting tensile strength decreased 18%~23.7%,20%~33% and 24%~33% under the harshest condition(20% RH and 51 kPa)respectively.It was found that the low air pressure had more impact on mechanical strength than relative humidity based on grey correlation analysis method.The effect of low air pressure(51,61,71,81,91,101 kPa)on durability and microstructure of concrete was researched by using C30 concrete.It was found that the durability of concrete became degradation under low air pressure conditions.Water evaporation from concrete was accelerated under low air pressure conditions.The water loss percentage under 51 kPa was about double of that under 101 kPa at the age of 90 days.The water absorption of concrete increased because of low air pressure,and the effect of low air pressure on slide concrete was greater than that on cubic concrete.The anti-permeability of concrete decreased under low air pressure conditions.Compared with 101 kPa condition,the chloride flux under 51 kPa at 28,45 and 90 days increased 25%,42% and 49%,respectively.The mass of scaled-off particles in deicer salt scaling test increased with decrease of air pressure.When the freeze-thaw cycles was 80 times,the mass of scaled-off particles under 51 kPa condition was about 4 times of that under 101 kPa condition.The total porosity and pore amount in the range of 500-1000 nm increased with decrease of air pressure.It was obvious that wider gap in the interfaces of concrete under low air pressure,which caused the reduction of bonding degree between cement paste and aggregate.The shrinkage of concrete under different relative humidity(20%,60%)and air pressure(51 kPa,76 kPa,101 kPa)was measured until 360 days.The shrinkage of concrete increased with the reduction of relative humidity and air pressure.After analyzing the test results,the impact factor of air pressure on shrinkage of concrete was built up,and it could be used to predict the shrinkage of concrete under low air pressure environment.Meanwhile,based on grey correlation analysis method,it could be found that the low air pressure had more impact on shrinkage of concrete than relative humidity.The concrete pillar model was built up to calculate the temperature field,humidity field and coupling stress field.After measuring temperature field,humidity field,stress field of the concrete pillar with the size of ?1m×2m and specimens with the size of 100×100×200mm,it was found that the numerical simulation results could match the measurement results well,which confirmed the availability of model.Based on the model above,the concrete pillar stress field under the normal weather and abnormal weather in plateau regions were analyzed.Under the daily temperature changes of concrete surface,the temperature and the thermal stress at all positions in concrete pillar showed to be periodical changes under the simulation of daily temperature changes.The stress on the surfa ce of concrete pillar could achieve at 1.3MPa and 5.1MPa when the temperature changes was 20? and 80?,respectively.The relative humidity at all positions of concrete pillar decreased under low relative humidity and low air pressure.The humidity stress increased with the decrease of depth,environmental relative humidity and air pressure.It was found that humidity stress could achieve at 2.01 Mpa under the condition of 10% relative humidity and 51 kPa air pressure.The thermal-humidity-air pressure coupling stress of concrete pillar was calculated out based on feature data of Harbin city and Lasa city.It was found that high stress fatigue was the reason for concrete cracking at later ages.The fatigue life cycles for Lasa City were much less than those for Harbin City,only 32 times.The “sudden drop of temperature”,“rainfall” and “blocked by could” as three kinds of abnormal weather in plateau regions were analyzed by using the model above.Generally,the tensile stress caused by “rainfall” weather was the most,and that caused by “blocked by could” weather was the least.For both “sudden drop of temperature” and “rainfall”,the tensile stress on the surface of concrete pillar had exceeded ultimate tensile stress,the cracking risks of these two kinds of weather were more than “blocked by could” weather.In conclusions,the study on the mechanical properties,durabilities,shrinkage,microstructures,temperature field,humidity field,coupling stress field and cracking mechanism of concrete under plateau conditons has been researched in this paper.The experimental results could provide the theoretical support for concrete design and pratical engineering,and the model built in this paper could be used to predict the concrete cracking in plateau regions.