Experimental And Theoretical Research On Mechanical Properties And Bearing Capacity Of UHPC Members

Ultra High Performance Concrete(UHPC)is a new type of cement-based composite material mixed with different reactive powders and fiber materials,high compressive strength,good toughness and durability,and excellent properties such as tensile strain hardening.In order to further study the mechanics and durability of UHPC and promote the wide application of UHPC members,scholars at home and abroad have done a lot of theoretical and experimental research on UHPC,and have obtained rich research results.However,UHPC is often regarded as an overall homogeneous material in the research,and traditional methods are used to study the mechanical properties of macro-scale UHPC.UHPC is mainly composed of conventional concrete base material and fiber,and its properties are more similar to reinforced concrete structures.Therefore,it is necessary to analyze the strengthening effect of fibers on the internal mechanism of UHPC from a mesoscopic perspective,and then to study the influence of fiber type and content on the mechanical properties and bearing capacity of UHPC members.This paper analyzes the basic mechanical properties of UHPC through model tests,and proposes a theoretical calculation method of UHPC beam bearing capacity considering the contribution of fiber mechanics based on micromechanics.The main research results are as follows:(1)The influence of fiber type,fiber content,silica fume content,water-binder ratio and their interaction on the mechanical properties of UHPC is analyzed by orthogonal experiment.The results show that under the same water-binder ratio,steel fiber has the greatest impact on the mechanical properties of UHPC,with the increase of the total fiber content,the UHPC compressive strength and tensile strength increase firstly and then decrease,blending with lower fiber Compared with the amount of fiber,high fiber content is more conducive to the improvement of UHPC tensile strength.Combining the results of compressive and tensile tests,it can be seen that the total content of two or more fibers should not exceed 3.0% in UHPC formulation,and the content of single fiber should not exceed 2.0%,in addition,the content of silica fume should be 20%.The water-binder ratio should be 0.2.(2)Through uniaxial compressive tests,the UHPC specimens with PVA fibers,steel fibers and hybrid fibers are analyzed for the differences in the failure morphology,stress-strain curve,and compression toughness of the UHPC specimens.On this basis,in this paper,the crack deformation calculation formula is constructed based on the relationship between the area under the corresponding curve of peak load and the area under the corresponding curve of crack load,which can effectively solve the influence of human factors in determining the cracking point by the traditional drawing method,and a UHPC bilinear compressive constitutive model that can consider the influence of fiber type and content is proposed.(3)Through uniaxial tensile test,the influence of PVA fiber,steel fiber and hybrid fiber on the failure morphology and tensile stress-strain curve of UHPC specimens is analyzed.On this basis,the contribution mechanism of fibers to the residual tensile properties of UHPC specimens after cracking is discussed from the perspective of micro-mechanics,and a UHPC tensile constitutive model that can consider the influence of different factors such as fiber type and content is further proposed.In addition,a semi-empirical and semi-theoretical method is used to propose a calculation model for the bonding stress of PVA fibers and hybrid fibers.(4)In order to study the mechanical properties of ultra-high performance concrete-concrete(UHPC-NC)composite beams,five composite beam specimens under different UHPC high proportions were tested in bending,and the failure modes of different composite beams and the changes in bearing capacity were compared and analyzed.Combined with the UHPC tension constitutive model proposed in this paper,a formula for calculating the flexural bearing capacity of composite beams that can consider the high proportion of UHPC and fiber content is established,and the calculation accuracy of the formula is verified by combining the experimental results.(5)In order to study the shear performance and failure mechanism of UHPC beams,shear tests of UHPC beams were carried out with different shear span ratio,steel fiber content,UHPC strength and stirrup ratio as variables.The differences of failure modes,bearing capacity,ductility and bending toughness of the test beams under different design parameters were compared.The effects of UHPC compressive strength,stirrup ratio and fiber content on the shear capacity of UHPC beams are analyzed.From a mesoscopic point of view,a calculation method of fiber shear capacity that can reflect the fiber shear mechanism is established,and a formula for calculating the shear capacity of UHPC beams that considers the contribution of fiber shear alone is deduced,finally,the test results are combined to calculate the shear capacity of reinforced UHPC beams.The calculation accuracy of the formula was verified.