Basic Mechanics And Fracture Behavior Of Steel Fiber Lightweight Aggregate Concrete Before And After High Temperature

As a new building material,lightweight aggregate concrete not only has good properties such as light weight,high strength,thermal insulation,fire resistance and impermeability,but also can make use of industrial waste to turn waste into treasure,which conforms to the development principle of national green building.In this paper,192 specimens of lightweight aggregate concrete are made to carry out cubic compression,splitting tension,axial compression and three-point bending fracture tests.The effects of three variables(0%,1%),different temperature levels(normal temperature,200 C,400 C,600 C)and aggregate types(all-lightweight aggregate,sub-lightweight aggregate)on lightweight aggregate mixing are studied.The main contents of this paper are as follows:Firstly,96 cubic specimens(100mm*100mm*100mm)were tested for cubic compressive strength and splitting tensile strength respectively,and the failure modes and strength variation laws of the specimens under compression and tension were obtained.The expressions of compressive and tensile strength varying with temperature were established respectively.Finally,the conversion expressions of compressive and tensile strength were established to provide both for structural designers and finite element analysts.Reference to the relationship.The results show that the compressive strength and tensile strength decrease with the increase of temperature,and the decrease of tensile strength is greater.The addition of steel fibers has little effect on the improvement of compressive strength at room temperature,but it has a great effect on the improvement of compressive strength after high temperature,especially above 600 C,and obviously improves the tensile strength.Then,48 prism specimens(100mm*100mm*300mm)were tested for axial compressive strength,and complete stress-strain curves were obtained.The change rules of mechanical properties such as axial compressive failure mode,stress-strain curve,axial compressive strength,modulus of elasticity,peak stress and Poisson’s ratio were analyzed and studied,and their expressions with respect to temperature change were established.Axial compression performance involves less energy consumption,brittleness index,high temperature damage evolution curve and so on.Finally,two uniaxial compression constitutive models for non-fiber lightweight aggregate concrete and steel fiber lightweight aggregate concrete before and after high temperature are established respectively,which can provide reference for structural fire protection design,post-disaster safety assessment and repair and reinforcement.The results show that with the increase of temperature,the curve becomes more smooth,the peak stress,modulus of elasticity,energy dissipation and brittleness index decrease,while Poisson’s ratio and peak strain increase significantly,indicating that the damage effect of high temperature is bidirectional.The addition of steel fibers can improve the damage degradation of concrete at high temperature,increase its peak stress-strain,elastic modulus,Poisson’s ratio and other mechanical properties.The energy dissipation ability is greatly improved,and the brittleness index is obviously reduced,which has obvious toughening effect.The mechanical properties of all-lightweight aggregate concrete are better than that of sub-lightweight aggregate concrete because of its aggregate types and shape characteristics.At the last time,48 specimens of prefabricated notched beams(100mm*100mm*400mm)were tested for three-point bending fracture performance,and complete load-midspan deflection curves and load-crack opening displacement curves were obtained.The fracture morphology,load-displacement curves,fracture energy,fracture toughness,characteristic length,initiation toughness and instability toughness were analyzed and studied.Finally,a double K fracture criterion is established to quantitatively describe the whole process of crack development,so as to promote the application of new lightweight aggregate concrete materials in practical engineering,especially in special structural engineering.The results show that fracture energy and fracture toughness are the two main indexes to judge the fracture performance,and the trend of change keeps high consistency and has strong correlation.Fracture energy,fracture toughness,characteristic length and instability toughness all increase first and then decrease with the increase of temperature.400 C is a turning point,while initiation toughness decreases with the increase of temperature.The addition of steel fibers can obviously improve the toughness and brittleness of lightweight aggregate concrete,improve the fracture performance of concrete,and enhance the fracture resistance of lightweight aggregate concrete.