Study On Brittle-ductile Transition Condition And Testing Techniques Of Concrete Under Triaxial Pressure

Two totally different failure forms are showed in concrete under compressive loading.One is brittle fracture and the other is ductile deformation. The form of failure is characterizedby brittle fracture when concrete is objected to uniaxial compressive loading, the mechanismof which is the growth of micro cracks that will eventually lead to the macro cracks. However,if concrete is under triaxial pressure, the failure form of it is ductile deformation, themechanism of which is the volts within concrete crushed. In the paper, the brittle-ductiletransition condition of concrete under triaxial pressure and the resistivity-damage effect ofconductive concrete are mainly studied. Confined concrete is object of the former study.Firstly, the condition that a single volt in infinite large matrix is crushed is derived in theory,namely the brittle-ductile transition condition of confined concrete is obtained. Theoreticalresearch indicates that the ratio of (confined stress)/(axial stress) of concrete should besatisfied cr(cr0.22~0.25if the Poisson’s ratio of concrete is0.15~0.2).Secondly, an experiment is designed to test the brittle-ductile transition condition of concreteunder triaxial pressure. Experimental study demonstrates that brittle-ductile transition ofconcrete under triaxial pressure can be still taken place even if the ratio of (confinedstress)/(axial stress) of concrete is a little bit smaller than cr. Accordingly, the transitioncondition cracquired in theory is merely a sufficient condition of brittle-ductile transitionof concrete under triaxial pressure. The latter aims at studying the resistivity-damage effect ofconductive concrete. The resistivity-damage effect is very important for damage detecting inconcrete. In this study, such an effect is experimentally investigated. The material is formedby filling a small volume of carbon fibers into concrete. Then, the variation of resistivity of the concrete under static unixial loading and circle unixial loading is detected.Synchronously，the damage evolution is measured by ultrasonic method. Combining thosetwo methods, the relation between resistivity and damage is established. The experimentalresults indicate,1) exist a critical value (fcr0.7vol.%) of volume fraction of carbon fibers,when the volume fraction of the fibers exceeds such a critical value, the resistivity of concretedecreases rapidly;2) polarization effect is found in the conductive concrete in the case of thevolume fraction of carbon fibers being much less than1.3%;3) the resistivity of concrete isno longer a single-value function, but a multiple-value function of damage. The resultsobtained in this study will be valid for detecting the damage evolution of concrete, as well aspredicting the service life of concrete structures.