Experimental Study On Effective Preloaded Performance Of Prestressed Concrete With Different Strength Grades Under Ultralow Temperature

Due to air pollution,global warming and other environmental problems,natural gas,with its characteristics of clean,efficient and high calorific value,occupies an increasing proportion in the global energy market.In engineering,natural gas is usually stored and transported after being liquefied,and prestressed concrete is used for the structure of large LNG storage tanks.At present,there are very few studies on the ultralow temperature preloaded performance of prestressed concrete,most of them are at the macroscopic qualitative level,and no universal conclusions have been drawn.In this paper,the effective preloaded performance of prestressed concrete with different strength grades under ultralow temperature is explored through experiments.The concrete strength grade is selected as the main influencing factor and the coupling effect of low temperature,concrete moisture content and precompression stress level is also considered.The low temperature action conditions include conventional cooling and freeze-thaw cycles.The concrete strength grades include C40,C50,C60 and C70.The low temperature range of the effect is from 10°C to-160°C and a low temperature point is set every 20°C from 0°C,and the temperature range of the freeze-thaw cycle is from10°C to-80°C,8 freeze-thaw cycles.Low,normal and high typical concrete moisture content are given by drying,untreated and soaking.Low(0.3)and high(0.6)initial precompression stress level are applied.The results show that the low temperature effective preloaded performance of prestressed concrete with different strength grades is extremely complicated under the coupling action of main influencing factors.With the decrease of the low temperature,the precompression stress loss rate of the prestressed concrete with normal moisture content of different strength grades shows the characteristics of rapid growth-slow growth or basically unchanged-again growth in stages,and the axial shrinkage strain of the members also increases linearly.The precompression stress loss rate of prestressed concrete with normal moisture content decreases linearly with the increase of concrete compressive strength,and the decreasing rate increases first and then tends to be stable with the decrease of low temperature.With the increase of concrete moisture content,the precompression stress loss rate of strength grade C40 and C50 prestressed concrete shows a downward convex change at most low temperature,while that of strength grade C60 prestressed concrete is on the contrary.Under the action of ultralow temperature freezethaw cycles,the precompression stress loss rate of prestressed concrete with various strength grades shows a relatively regular periodic fluctuation with time.When the initial prestressing level is low,the higher the strength grade of concrete,the smaller the upper limit value of the precompression stress loss rate,the lower limit value of the precompression stress increase rate and the amplitude.Based on the fitting of the experimental results,the calculation expression of the precompression stress loss rate of prestressed concrete with different strength grades under the action of conventional cooling and freeze-thaw cycles is given,and the simplified calculation expression of the precompression stress loss rate of prestressed concrete under low temperature is given by making necessary assumptions from the point of mechanical analysis.A large number of experimental results obtained fill in the lack of research on the effective preloaded performance of prestressed concrete at low temperature,and provide a basis for the design of concrete structures of large LNG storage tanks and the improvement of relevant specifications.