Experimental Studies On Prestress Losses In Concrete Members Under Ultra-low Temperatures

“Experimental studies on prestress losses in concrete members under ultra-low temperatures” is one of the sub-projects of Natural Science Foundation Project No.51478309.Previous results showed that the shrinkage and creep of concrete,the relaxation of steel strands and the coefficient of thermal expansion(CTE)of these two kinds of material behaved differently under cryogenic temperatures.Thus,the prestress losses under cryogenic temperatures differ from that under ambient temperature.Two series of tests,including 1)time-dependent prestress losses measurement under constant ultra-low temperatures and 2)prestress variation measurement in the process of cooling and reheating were carried out to investigate the prestress losses on member level.Twelve specimens were prepared for test 1).The investigated parameters were temperatures,initial stresses and moisture conditions of concrete.Test results show that with the decrease of temperature or initial stress,the time-dependent prestress losses decrease.In addition,the losses in water-saturated concrete members are less than that in natural-state members.Eight specimens were prepared for test 2).The cooling and reheating process can be divided into five stages,namely,initial cooling stage(I),stable cooling stage(II),low-rate cooling stage(III),low-rate reheating stage(IV)and stable reheating stage(V).The prestress variation in stage I and II depends on the coupling effects of investigated parameters.It is noteworthy that the stress starts to increase as the temperature of concrete drops to-30? and ends at-60?,which is caused by the “inverse expansion” of concrete.The increase of stress intensifies with the decrease of initial stress or the increase of water content.Two methods,namely separate estimation method and lump-sum method are proposed to estimate time-dependent prestress losses under ultra-low temperatures and predict the ultimate value.Based on the calculation and analysis,it is found that the loss caused by concrete shrinkage can be ignored under the temperature ranges from-40? to-100?.The ultimate value of prestress losses produced by concrete creep and tendon relaxation accounts for 1.2%~2.7% and 0.5%~1.4% of the initial stress,respectively.In addition,this paper also provides the range of prestress variation in the process of cooling and reheating,which accounts for-4.4% ~ +1.3% of initial stress.