Experimental Study And Numerical Simulation On The Size Effect Of Concrete Behavior In Triaxial Compression

In practical engineering structures,concrete is usually in the multi-axial tensile or compressive combined stress state.It is very important to study the mechanical properties of the concrete materials in the mult-iaxial combined stress state to ensure the safety and durability of engineering structures.At present,a large number of experimental studies have been conducted on the triaxial compressive properties of standard-sized concrete specimens both in China and oversea countries,but there are few studies on the size effect of triaxial compressive properties.In addition,due to the limited loading capacity of triaxial test equipment,only small-sized specimens can be used in triaxial compression test.The numerical simulation method which is not limited by test conditions can be used in a larger scale to study the mechanical properties and size effect of concrete under three-directional compressive stress.In this paper,experimental study,theoretical analysis and numerical simulation are combined to study the mechanical properties and size effect of concrete under three-directional compressive stress.The main contents are as follows.(1)Triaxial compression tests were carried out on 84 cubic specimens with three sizes of 100 mm,150mm and 200 mm respectively.The stress ratios of the test specimens was 0.00:0.00:1,0.05:0.05:1,0.10:0.10:1,0.15:0.15:1 and 0.20:0.20:1.Experimental results show that under each stress ratios,the specimens of the triaxial compressive strength,peak strain and ultimate strain had size effect phenomena.When the stress ratio was 0.05:0.05:1,size 200 mm specimen of the triaxial compressive strength,peak strain and ultimate strain were about 94.3%,104.4% and 112.8% of the results for the 100 mm and size 150 mm specimen were about 97.4%,102.2% and 106.0% respeectively.The size effect rate of triaxial compressive strength decreases with the increase of stress ratio.When the stress ratio was 0.20:0.20:1,the size effect of tri-axial compressive strength is about 32.2% when the stress ratio was 0.05:0.05:1.With thee increase of the stress ratio,the size effect of the triaxial compressive peak strain decreased first and increased then.From the uniaxial compressive state to the triaxial compressive state with the stress ratio of 0.20:0.20:1,the size effect of the triaxial compressive peak strain was 3.6%,2.2%,2.6%,8.7% and 16.8%,respectively.The size effect of triaxial compressive ultimate strain showed a falling trend with the increase of stress ratio.The initial elastic modulus of triaxial compression slightly increases with the increase of stress ratio,but its size effect phenomenon was so subtle that it could be neglected.There were two failure modes of the specimens under triaxial compression.Columnar splitting failure occurs when the stress ratio was between 0.00:0.00:1~ 0.15:0.15:1,and extrusion rheological failure occurs when the stress ratio was 0.20:0.20:1.(2)The theories of statistical size effect were introduced,including Weibull size effect law,Bazant-Xi size effect law and Planas size effect law.The Bazant size effect law of fracture mechanics and Carpinteri size effect law of multifractal size effect theory were deduced in detail.The values of the calculated parameters in the size effect law were determined based on the measured results of triaxial compressive strength.The critical size and the critical strength were defined and their recommended values were obtained.(3)The 3D random aggregate model was established with MATLAB,and then the three-phase mesoscopic numerical model of concrete test specimen was established with the APDL module of ANSYS,and the mechanical parameters of interfacial transition zone(ITZ)were determined.(4)The triaxial compressive strength of numerical specimens with different stress ratios was calculated by establishing the homogeneous elastic brittleness model,and the parameter analysis of its size effect was carried out,and the suggested values of critical size and critical strength of triaxial compressive strength of numerical specimens under different stress ratios were given.