Experimental Study On Linear Expansion Coefficient And Stress Relaxation Of Steel Strand At Cryogenic Temperatures

In order to understand steel strand’s properties,which can affect the pre-stressing force of pre-stressed concrete structure under cryogenic temperature,then provide calculation references for the loss of pre-stress force,the linear expansion coefficient and stress relaxation of low relaxation steel strand,strength grade of 1860 MPa and nominal diameter of 15.2mm used commonly in engineering,at cryogenic temperatures were studied in this paper.The test equipment has been developed to measure the thermal strain of the steel strand from room temperature to-165℃.The instantaneous linear expansion coefficients of steel strands at different cryogenic temperatures were determined and the mean coefficients of linear thermal expansion of steel strands at different cryogenic temperature range were also determined.The stress relaxation performance of the steel strand at different cryogenic temperatures(room temperature、-40℃、-100℃、-165℃)and different initial stresses have been tested.The main parts of the study are as follows:1)A cooling-thermal insulation device and test methods is developed to measure the thermal strain of the steel strand from room temperature to-165℃;A set of test devices have been developed to measure the stress relaxation of the steel strand at different cryogenic temperatures and different initial stresses.A complete set of standard process was formed to measure the linear expansion coefficient and stress relaxation performance of steel strand at cryogenic temperature.2)The instantaneous linear expansion coefficients and the mean coefficients of linear thermal expansion of steel strand at different cryogenic temperatures and different initial stresses were determined.3)The stress relaxation performance of the steel strand at different cryogenic temperatures and different initial stresses have been studied.Conclusions can be summarized as follow:1)The test device and the test methods are stable and reliable.2)The instantaneous linear expansion coefficient of steel strand increases with increasing initial stress and decreases with decreasing temperature.3)The mean coefficient of linear thermal expansion of steel strand increases with increasing initial stress and decreases with decreasing temperature.4)At cryogenic temperatures,the fitting curve over 10 h test result and the fitting curve over 100 h test result are similar.Thus the 10 h test result can be used to calculate the 1000 h fitting curve.5)The stress relaxation of steel strand decreases with decreasing temperature while under the same initial stress condition.6)The stress relaxation of steel strand increases with increasing initial stress under the same temperature.