Four-Stage Fracture Model Of Concrete And Its Application In Fracture Analysis Of Airfield Rigid Pavement

The fracture of concrete is a common failure in engineering,its' fracture process is a failure stage from micro scale to macro scale,experiencing from linear elasticity to micro-cracks initiation and accumulation,then micro-cracks aggregating to macro-crack,macro-crack propagation and finally complete failure.At present,the fracture process of concrete is divided into crack initiation,stable expansion and unstable expansion stage,the initiation and propagation of macro-cracks is neglected and uncertain,as a result,it's hard to fully excavate and to develop the residual bearing capacity,crack resistance,toughness and deformation capacity of materials and structures.It is of great value to determine the initiation time of the macro-crack for better evaluating the crack resistance and toughness of concrete,and it's more reasonable to use the macro-crack initiation point as the criterion for determining the failure of airfield rigid pavement with strong supporting.In view of this,by means of laboratory test,theoretical analysis and numerical simulation,the fracture characteristics of three point bending beam?TPB beams?are investigated,the four-stage fracture model and fracture criterion of concrete are proposed.The fracture control parameters of the four-stage fracture model of concrete are determined.Based on damaged plasticity model and cohesive crack model,numerical models for damage and fracture simulation of TPB beams are established,the rationality of the four-stage fracture model of concrete is verified by numerical simulation.Finally,the four-stage fracture model of concrete is applied to study the whole fracture process of airfield rigid pavement slab.The main research contents and conclusions are as follows:By means of digital speckle image correlation method,et.al,the distribution and evolution of the displacement field,strain field,fracture process zone during pure mode I and I-II mixed mode fractures of concrete are monitored and investigated.The results show that the evolutions of opening displacement and horizontal strain around the notch tip with the load percentage show obvious four-stage characteristics.Affected by front boundary effect,stress concentration effect,ligament effect and back boundary effect,fracture process zone begins to form and expand at 80% of peak load,and fully develops at 60% of post-peak load and then extends to the back boundary of beam until it disappears.Based on the observations from the laboratory tests,drawing on the experience from the double-K and double-G fracture theories of concrete,the four-stage fracture model and its fracture criterion of concrete are proposed.By introducing fracture toughness of micro-crack initiation,fracture toughness of macro-crack initiation and fracture toughness of structure when it is completely destroyed,the critical conditions of micro-crack initiation,macro-crack initiation and structure complete failure are determined.The four-stage fracture parameters of concrete are determined by means of resistance strain gauge method,integral analysis method and limit strain approximation method et al.It is concluded that the four-stage fracture parameters are independent on the specimen size without size effect,also,their values change with strength and softening curve parameters of concrete,which is proved to reflect the mechanical properties of concrete material.The simplified KR and GR resistance curves based on four-stage fracture parameters are established.The simplified resistance curves have clear functional expressions.Both of them can not only describe the four-stage fracture of concrete reasonably,but also adapt to the stability analysis of crack development.Based on the concrete damaged plasticity model,the numerical simulation of damage-fracture process of TPB beam is realized,the evolutions of the principal stress and damage of TPB beam further validate the rationality of the four-stage fracture model.The influence factors of damage distribution characteristics are studied,it is shown that concrete strength and softening curve parameters affect the damage distribution,the peak load and the shape of post-peak softening curve in vary degrees.V-shaped notch can accelerate damage and fracture of beams.Numerical methods to determine the double K,double G and the four-stage fracture parameters of concrete are proposed.The fracture parameters determined by the numerical method are in good agreement with those determined by the previous methods.The numerical method has high accuracy,which proves its validity in determining the above fracture control parameters.By adopting cohesive crack model in combination with the extended finite element method,the numerical simulations of dynamic crack propagations of mode I crack and I-II mixed mode crack are realized,the accuracy of the models are verified by comparing with the experimental results.Numerical results show that either the dynamic propagation of mode I crack or I-II mixed mode crack can be both described by the four-stage fracture model.By influence factor analysis and sensitivity analysis,it is concluded that the elastic modulus and ultimate tensile strength have the most significant influence on mode I crack propagation characteristics of concrete,while the Poisson's ratio has a relatively small influence,the main factors affecting the propagation of mode II cracks are initial crack length,elastic modulus,initial crack location,while,the influences of mode I fracture energy and Poisson's ratio are relatively small.The four-stage fracture model is applied to study the whole fracture process of airfield rigid pavement slab.Based on the snow-melting rigid pavement test platform constructed in Harbin Institute of Technology,a three-dimensional finite element model of full-scale rigid pavement structure is established,FWD test is carried out to validate the accuracy of the model.In view of the concrete damaged plasticity model,damage and fracture simulation of airfield rigid pavement structure is realized.By introducing P_micro?P_macro?P_failure,three critical loads corresponding to different critical fracture moments are determined,four-stage fracture criteria of airfield rigid pavement are put forward,and the critical load as the macro-crack iniatiated and induced at the cement concrete slab of airfield rigid pavement is recommended as thecriterion for judging failure or complete fracture of airfield rigid pavemet structures.The research results provide control criteria for reasonable control of fracture of airfield rigid pavement structures.