Creep Behaviour Of Cylinders Made Of Normal-strength DCLs And High-strength FC And Stub Steel Columns Filled With Normal-strength DCLs And High-strength FC

It is indicated that structural members cast with demolished concrete lumps(DCLs)and fresh concrete(FC)are effective approaches of recycling demolished concrete.Some preliminary mechanical and creep behaviour and seismic properties of specimens made of DCLs and FC have been conducted by our research group,and test on the mechanical properties of compound concrete made of normal-strength DCLs and high-strength FC has also been conducted.To further improve the theoretical system of compound concrete made of DCLs and FC,the author carried out research on the creep behaviour of cylinders made of normal-strength DCLs and high-strength FC and stub steel columns filled with normal-strength DCLs and high-strength FC.The main research works as follows:1.Experimental study on basic creep behavior of three groups of cylindrical specimens made of C30 normal-strength demolished concrete lumps(DCLs)and C70 high-strength fresh concrete(FC),one group of cylindrical specimens made of the C70 FC alone,and one group of cylindrical specimens drilled from the C30 waste concrete has been conducted.Influences of the stress level and the replacement ratio of DCLs on the specimens’ basic creep have been discussed.It is found that:(1)the modulus of elasticity of the compound concrete made of normal-strength DCLs and high-strength FC is lower than that of the FC and significantly higher than that of the demolished concrete(DC),and decreases with the increasing of the replacement ratio of DCLs;(2)both the specific creep and the creep function of the compound concrete made of normal-strength DCLs and high-strength FC are remarkably less than those of the DC and relatively close to those of the FC,and increase with the increasing of the replacement ratio of DCLs;(3)the creep coefficient of compound concrete made of normal-strength DCLs and high-strength FC is lower than that of the fresh concrete and the demolished concrete;(4)when the stress level is lower than 0.4,the basic creep of the compound concrete made of normal-strength DCLs and high-strength FC is still approximately proportional to the stress level.2.Based on the two-phase material model,a prediction method for elastic modulus and basic creep of the compound concrete made of normal-strength DCLs and high-strength FC is discussed.The validity of the method is preliminarily verified by comparing the calculated results with the test results.It is found that:(1)experimental data of modulus of elasticity of the compound concrete made of normal-strength DCLs and high-strength FC is mostly lower than those evaluated using the Voigt model and Reus model;(2)formula based on the modified Voigt model can be used to predict elastic modulus of the compound concrete made of normal-strength DCLs and high-strength FC with a good accuracy,and the method based on the modified Voigt model can be used to predict basic creep of the compound concrete made of normal-strength DCLs and high-strength FC with a good accuracy.3.Two groups of stub steel columns filled with C30 normal-strength DCLs and C70 high-strength FC,one group of stub steel columns filled with C70 FC were subjected to long term sustained axial loads.Influence of the replacement ratio of DCLs on the specimens’ creep has been discussed.Based on the two-phase material model,the validity of the method for creep of stub steel columns filled with normal-strength DCLs and high-strength FC is discussed.It can be seen that:(1)the creep coefficient of stub steel columns filled with normal-strength DCLs and high-strength FC is higher than that of stub steel columns filled with FC;(2)when the replacement ratio of DCLs is between 20% and 33%,the influence of the replacement ratio of DCLs on the creep coefficient of stub steel columns filled with normal-strength DCLs and high-strength FC is small;(3)creep of the stub steel columns filled with normal-strength DCLs and high-strength FC is lower than that of the compound concrete made of normal-strength DCLs and high-strength FC,and the stabilization of the former is earlier;(4)the calculated results obtained using the presented method are generally in good agreement with the test results.