Study On Reinforced Concrete Constitutive Model And Its Application In Blasting Demolition

Reinforced concrete structure is one of the most common structural forms in civil engineering.With the continuous development of society,some of the buildings are often demolished by controlled blasting technology after they lose their value.At present,the theory of blasting demolition lags behind the engineering practice.Engineering practice shows that the engineering experience is far from meeting the practical needs,so it is necessary to study the theory of blasting demolition.However,the demolition blasting projects are not repeatable,and influenced by the long test period,high cost,do not have the test conditions and other factors,using the finite element analysis method is very economical and effective which plays an important role in the design,analysis and evaluation of the blasting design.The correctness,rationality and reliability of numerical simulation results of blasting demolition of reinforced concrete structures are influenced by many factors.First of all,it is a prerequisite for the finite element analysis of reinforced concrete to select the appropriate material model of concrete.At present,most scholars adopt simple kinematic hardening model(MAT3)and brittle damage model(MAT96),however,these material models are not well reflect the mechanical properties of concrete materials under different loading conditions,although a few scholars use the concrete damage model(MAT72R3)and the continuous cap model(MAT159),but the parameter selection often rely on experience or reference,the research on concrete material model itself is less.Secondly,the reinforced concrete structure blasting demolition tends to be high in height,large in volume and complicated in reinforcement,the mechanical properties of two kinds of materials of steel and concrete can't be considered by using the integral model,However,the separation model and the combined model need to consume a lot of computing resources and time,and the interaction between steel and concrete is complex,the numerical simulation results is difficult to achieve the desired effect.Therefore,in this paper,concrete material model,and reinforced concrete constitutive model are studied by using finite element analysis method,finally,combined with the practical engineering case,the finite element analysis of the collapse internal force of the structure is carried out,the main research work and results are as follows:(1)Concrete damage plasticity model has been theoretically analyzed,combined with existing research results,aiming at the mechanical properties of the hardening ductility parameters(AH,BH,CH and DH)and softening ductility parameters(AS and BS)on concrete in damage plasticity model is not clear,the sensitivity of the model parameters under uniaxial tension,uniaxial compression and three axial compression is analyzed by means of ls-dyna single cube element test method,and finally the model parameters are revised and verified.The results found that AH,BH and CH have different effects on the peak strength strain of concrete under three different stress conditions,DH has no effect on the mechanical properties of concrete under three different stress conditions,AS is the parameter to control the concrete softening stage which has no effect on the tensile mechanical properties of concrete,the bigger the AS,the stronger the concrete ductility under uniaxial compression and three axial compression,however,BS has no effect on the mechanical properties of concrete under three different stress conditions.The concrete damage plasticity model can well describe the stress-strain relationship of concrete before peak strength by using all of the default parameters,the concrete damage plasticity model can accurately describe the different levels of concrete stress-strain relationship is only need to modify the softening ductility parameter AS=2.(2)The constitutive model of confined concrete is analyzed theoretically,and a confined concrete damage plasticity modle is established by modifying the parameters BH,AS and fc of concrete damage plasticity model.The test results show that using the confined concrete damage plasticity model to simplify the stirrups is feasible and reliable,the effect of stirrups on the concrete can be reflected by improve yield strength and deformation capacity of concrete;By using the method of combination modulus theory analysis of reinforced concrete tensile and compressive mechanical behavior,a reinforced concrete damage plasticity constitutive model is given by modifying the concrete damage plasticity model parameters E,FT,wf,FT1,BH,AS,fc of concrete damage plasticity model,this model combines the concrete tension and compression damage,strain rate effect,considering the stiffening effect,reinforced concrete compression effect and stirrup reinforcement ratio on the influence of stiffening.The test results show that reinforced concrete constitutive model is used to simulate the reinforced concrete,which can ensure the calculation precision and force characteristics,accelerate the calculation speed and simplify the modeling workload.(3)Combined with the actual engineering case,based on the model of reinforced concrete,on the premise of certain assumptions,the numerical analysis model of single frame and space frame tube structure is established and a simplified method for the numerical modeling is presented.Meanwhile,the redistribution of internal forces in the structure after the support column blasting is studied by means of the demolition component method and the differences between the two structures in the collapse process are compared,finally determination the blasting delay time of column.The study found that when remove the first row of struts in the incision range,single frame and frame tube structure can't occur collapse for the distribution of internal force balance,the reasonable initiation delay time of second row column can choose between 0.3s to 0.75s;On the basis of the removal of the first row of struts,then after 0.5s the second row of struts in the incision range were removed,single frame and frame tube structure collapse occurred due to the internal force distribution did not reach a new balance,the frame structure is to beam flexural failure caused instability and collapse,the reasonable initiation delay time of third row column can choose between 0.25 s to 0.75 s,however,the frame tube structure unstable collapse is caused by compression shear failure of the column,the reasonable initiation delay time of third row column can choose between 0.15 s to 0.5s;On the basis of the removal of the first and second row of struts,then after 0.33 s the third row of struts in the incision were removed,the frame structure is still to beam flexural failure caused instability and collapse,the reasonable initiation delay time of fourth row column can choose between 0.15 s to 0.4s,however,frame tube structure is combined beam flexural failure with column compression shear failure caused instability and collapse,the reasonable initiation delay time of fourth row column can choose between 0.10 s to 0.25 s.