Study On The Tension Softening Curve And Its Size Effects Of Concrete

Concrete is a kind of composite material,composed of water,cement,sand,stone and additive,etc.There are many randomly distributed cracks or damage points in concrete.When the external load is applied,the stress concentration occurs at the end of the crack,and the crack will also expand therefore.With the increasing of the load,the cracks continue to expand,and gradually form the macro cracks,which made the fracture system become unstable,and finally lead to the fracture failure.Crack propagation and the cracking of concrete directly affect the bearing capacity and stability of the structure,and even threaten people's life,such as the pier cracks,cracks in the dam and the nuclear power station prestressed concrete pressure vessel crack.The study on the fracture performance of concrete can provide a reference for judging the degree of damage of some serious cracks in concrete structure and the stability of engineering structure system.Therefore,the research on the fracture performance of concrete has important practical significance and engineering value.In this paper,the three point bending test method of RILEM is used to carry out the three point bending test of concrete specimens with different sizes and different strength grades.Electronic Speckle Pattern Interferometry?ESPI?technique with high accuracy,non-contact and real-time measurement is used to observing the surface displacement of the specimen beam.In order to making a comparative analysis,the Linear Variable Differential Transformers?LVDTs?and Clip Gauge were adopted to measured the displacement piece deflection and prefabricated incision ends.Through the collation and study of the test results,the P-? curves,P-CMOD curves and P-CTOD curves are obtained.It can be seen from the analysis that the P-? curves,P-CMOD curves and P-CTOD curves of the specimen beams are affected not only by the specimen size,but also by the concrete compress strength.The specimen of peak value,peak load deflection of the CMOD_c,the maximum load P_max that the specimen can withstand and the fracture energy increases because of the development about specimen size,also increases with the expanding of concrete strength.The CTOD_c and the concrete compress strength are less affected by the specimens' dimension.The measurement results of ESPI are in good agreement with the results measured by LVDTs and Clip Gauge,which proves the accuracy and feasibility of ESPI used in the study of concrete fracture performance.In addition,this paper uses Incremental Displacement Collocation Method?IDCM?to simulate the test data,inversing test tensile softening curve beam,and analyzing the effects of specimen size and the concrete compress strength of concrete on it.Besides,in order to verify the feasibility and accuracy of IDCM inverse concrete tensile softening curve,the tensile softening curve as the constitutive relationship,derived the specimen of P-CMOD curve,and ESPI,Clip and Gauge measured data are compared and found good agreement.Finally,experiments' conclusions of this paper are summarized and reviewed,expecting to provide references for the study of concrete fracture mechanics,and related with research.Moreover,suggestions and prospects for the future study are also putting forward.