Study On Fracture Properties Of Concrete After Different Compression Stress Levels And Loading Time

Concrete structures are usually subjected to long-term loads such as self-weight,temperature stress and water pressure during the service stage.Previous studies have shown that continuous complex stress conditions and environmental coupling can induce the expansion of initial micro-cracks in concrete,resulting in the change of macroscopic mechanical properties of materials.With the increase of service life,the internal deformation of the structure is developing,and the crack resistance of concrete is also changing.In this case,the material performance parameters obtained based on short-term static experiments are no longer suitable for accurately evaluating the resistance of the structure under long-term loads.Therefore,the study on the fracture characteristics of concrete under long-term load and the analysis of the influence of compression creep on the fracture parameters,cohesion relationship and crack propagation process of concrete can play a guiding role in the evaluation of the safety of concrete structures during service.Based on this,the following work is carried out in this paper:（1）Axial compression creep experimentBasic creep experiments were carried out on prismatic specimens,and the size of all specimens were 100mm×100mm×330mm.In order to study the effect of different stress levels on fracture performance,the specimens with four stress levels of 0.2f_c,0.4f_c,0.6f_c,and0.8f_c were loaded for three months,and the specimens with stress levels of 0.4f_c and 0.6f_cwere loaded for one month,three months and six months,respectively,to explore the influence of loading time on fracture properties.The axial compression creep experiment shows that the creep strain mostly occurs in the early stage of creep,and the later creep strain develops slowly.At low stress levels（0.2f_c and 0.4f_c）,the creep coefficients of concrete are relatively close,and it can be considered that the occurrence of concrete is linear creep;when the stress level exceeds 0.4f_c,the creep coefficient increases with the increase of stress level,which is no longer a linear creep range.（2）Three-point bending beam experimentAt the end of the axial compression creep experiment,the I-type fracture experiment of the three-point bending beam was carried out immediately,and then the concrete fracture parameters were obtained.After the fracture test,the concrete in the non-fracture area was cut into standard specimens and the basic mechanical properties were tested to analyze the influence of compression creep on the basic mechanical indexes of concrete.The analysis of the fracture test data shows that the initiation toughness and fracture energy of concrete after continuous compression are higher than those of the accompanying specimens,but the instability toughness and critical crack propagation length are significantly reduced,which indicates that the specimen shows the performance of brittleness enhancement after continuous compression.From the ratio of crack initiation load to ultimate load,it can also be seen that the concrete cracking is close to failure after long-term axial compression load,and the safety reserve of the specimen is reduced.（3）Tensile softening curve of concrete after compression creepThe I-type fracture experimental data were obtained from the concrete three-point bending beam experiment,and the tensile-softening constitutive relation of the specimen under different loads was deduced by using the improved J integral method proposed by Niwa.Then,the changing law of the turning point in the bilinear softening constitutive model was]fitted,and the expression ofβwith the change of stress level and loading time is given.The results show that the w₀ of the specimens after compressive stress is lower than that of the control specimens,and the maximum decrease is 57%.However,the change of the shape parameters of the softening curve under the stress level of 0.8f_c is different from that of 0.4f_c and 0.6f_c.（4）Numerical simulation of crack propagation process of concrete after compression creepANSYS finite element analysis software was used to numerically simulate the crack propagation process of concrete three-point bending beam after compression and creep,and the calculated load-crack opening displacement curve was compared with the curve obtained by experiment.The results show that the numerical results of the load-crack opening displacement curve are consistent with the experimental results,and the peak load error of each specimen is within 10%.This conclusion verifies the correctness of the tensile softening constitutive relation deduced in this paper and the rationality of the simulation method for crack propagation process based on the initiation toughness criterion.