Experimental And Theoretical Study On Shear Performance Of UHPC Beams

Ultra high performance concrete(UHPC)is a new type of concrete with considerable compressive strength and tensile strength by adding superfine active admixture and steel fibers,which has superior mechanical properties.Existing research shows that adding steel fiber is able to enhance the bond force of cement matrix and improve the shear performance of structure,even can partially or completely replace stirrup to provide shear capacity.However,engineers have some concerns about the mechanical performance of UHPC beams without shear reinforcement,due to the lack of systematic research on the fiber orientation,fiber distribution and structural design method.To solve the above problems,this dissertation presents an experimental investigation on shear performance of UHPC beams under different ratio of stirrups and steel fibers.The shear strength model of UHPC derived from plastic theory is established,and verified by the experiment results along with existing codes.The calculation formula of the minimum stirrup ratio apply to UHPC beams is recommended.Details of the study are summarized as follows:(1)Experimental study on shear behavior of T-type UHPC beamsThe experimental research of six T-type UHPC beams,with different stirrup ratio and fiber ratio as parameters,are carried out to study the effects of parameter on the failure mode,shear resistance and post-cracking performance of the UHPC beams.The form of dividing ultimate deflection by shear cracking deflection is expressed as the ductility index.The transformation of failure mode and the change of critical inclined crack width under the influence of parameters are analyzed.The optimal ratio of stirrup ratio and fiber ratio,and the characteristic value of the maximum width of critical inclined cracking are put forward to prevent shear failure.(2)Calculation model of shear capacity/y of UHPC beam based on plastic theory To solve the tricky problem of determining the shear contribution of fibers,an enhanced calculation model,which consider fiber ratio,fiber orientation and bond strength between the fibers and matrix,is proposed.And a simplified shear capacity model of UHPC beam is obtained based on the upper bound theorem of plastic theory.The assessment of the proposed model,Chinese code JGJ/T 465-2019 and French code AFGC-Sétra 2002 is carried out to verify predictability of the shear resistance.(3)Shear behavior analysis of UHPC beams based on nonlinear finite element model Since it is difficult to observe the failure stages of the test beams during the test process,the plastic damage model(CDP model)of ABAQUS was used to analyze the whole process of loading.The CDP model is also used to the break through the limits of the test analysis.The influence of shear parameters on the shear performance of UHPC beams,such as spacing and diameter of stirrups,yield strength of reinforcement and compressive strength of UHPC,are studied.(4)Evaluation of calculation method of shear capacity of UHPC beams based on Database The shear database for UHPC beams is established.Based on the statistical analysis of the samples,the influence of main parameters on shear behavior is investigated,the existing shear strength models are evaluated.The formula of the minimum stirrup ratio of UHPC beams is proposed.