Studies On Fracture And Crack Resistance Of Concrete

Concrete is a composite material,which can be properly represented by three phases in microstrueture:cement paste,aggregate as well as interfacial transition zone between them, the bearing capacity and property of which depends on the property of each component and interaction between them.As the matrix compositions of concrete,fracture properties of cement paste and mortar have great influence on researching and improving the fracture performance of concrete.Due to heterogeneous and multiphase of concrete,there inevitably exits gaps and micro-cracks within it,and there are some inherent characteristics of concrete, such as low tensile strength,shrinkage and creep,etc.These are the main reasons for concrete cracks.Concrete cracking caused by various factors makes many structures in actual project work with cracks.A large number of recent studies and engineering practices show that cracking of concrete structures is almost inevitable.In the existing level of.economic and technical,requirement of science is to control concrete cracks within the harmful degree. Therefore,the fracture properties of cement paste and mortar are studied firstly,and then resistance to crack propagation of concrete are researched by placing reinforcing bar at different positions in concrete and pouring Ultra High Toughness Cementitious Composite (UHTCC) on tension face of concrete.Details of the present study are introduced as follows:(1) Fracture tests are performed by 156 three-point bending beams of cement paste and mortar with different sizes,different strengths and different ratios of the notch length to beam depth.Complete load versus crack mouth opening displacement(P-CMOD) curves are directly obtained;the initial cracking loads are determined by using resistant strain gauges which make up a full-bridge circuit;the measured values of initial fracture toughness are calculated,and the influences of test parameters on fracture toughness are analysed.The results show that cement paste is not an ideal brittle material,and there also exist a nonlinear fracture process and a steady crack propagation stage before unstable fracture.Double-K fracture model is also suitable for cement paste and mortar,and there are no size effects on the double-K fracture paramenters.Therefore,the Double-K fracture model can be used to predict crack initiation,steady crack propagation and unstable fracture of cement paste and mortar.(2) A fit is made for the tail of the P-δcurves using power and exponential function, respectively,so complete fracture energy of cement paste and mortar are obtained,and the fracture energy are size-independent.(3) Fracture tests of concrete three-point bending notched beams without any crack control measures are carried out.The full-bridge strain gauges and the half-bridge strain gauges that arranged on specimence are used to monitor the length of crack propagation and measure the cracking strain of each measuring point.According to the actual characteristics of concrete used in tests,the paramenters in the nonlinear softening stress deformation relation are redetermined,and the influence of the parameters on theoretical values of initial fracture toughness is analysed.(4) Concrete three point bending notched beams with reinforcing bar placed at three different positions are used to study the effect of reinforcing bar on crack propagation of concrete.Through the analysis of crack propagation process,it is found that fracture process of reinforced concrete can be divided into three stages:crack initiation,stable extension,and unstable failure.Therefore,the initial fracture toughness K_Icⁱⁿⁱ and the unstable toughness K_Ic^un of reinforced concrete are introduced.Double-K fracture criteria are used for describing the whole process of crack propagation of reinforced concrete three-point bending notched beams, then applicability of the criteria is verified by tests.Formulas for calculating crack length of reinforced concrete three-point bending notched beams are reestablished based on the functional relation between nondimensional parameters BE CMOD/P and a/D and the accuracy of the calculation formulas is proved.The effect of different formulas for calculating crack length on the critical crack tip opening displacement is discussed.(5) UHTCC is poured on tension face of concrete three point bending notched beams as crack control strip to study the control action of UHTCC on crack propagation of concrete. The influence of strain hardening properties of UHTCC on crack propagation process of concrete is analyzed.Basing on the assumption that interface deformation is consistent and neglecting the elastic deformation of non-cracking regions of UHTCC,a method for determining total crack width of UHTCC is proposed and a simplified model ofσ-w_u is obtained subsequently.According to the simplified model,stress value of UHTCC at arbitrary hour can be determined.Through the analysis of whole process of crack propagation of concrete three-point bending notched beams retrofired with UHTCC,the initial fracture toughness K_Icⁱⁿⁱ and the unstable toughness K_Ic^un of concrete three-point bending beams retrofitted with UHTCC were introduced,and the double-K fracture criteria are used for describing the whole process of crack propagation of it,then applicability of the criteria is verified by tests.Formula for calculating crack length of each specimen is established based on practical situation of crack expansion,and the accuracy of the formulas is verified.