Research On Flexural Behaviors Of UHPC Beams

Ultra-high performance concrete(UHPC)refers to a new type of cement-based composite material exhibiting ultra-high compressive strength,good tensile behavior,high toughness,and excellent durability.It has a broad application prospects in bridge engineering due to its different roles in reducing the structural self-weight,enhancing the bearing capacity,and improving the ductility.However,application of UHPC in bridges has not been fully promoted due to the lack of the specification on bridge design using UHPC in China.Therefore,in order to improve the relevant design/construction standards,a systemic research with an emphasis on experimental investigation and finite element analysis of UHPC beams is carried out in this thesis,focusing on the mechanical behavior,failure mode,crack resistance,and ultimate load carrying capacity.The main tasks in this thesis include the following:In order to understand the bending behavior,experimental investigations on 12 UHPC beams and 2 reinforced concrete(RC)beams are conducted,taking steel fiber volume fraction and reinforecement ratio as the key parameters.The result shows that the bending behavior and failure mode of UHPC beam has three different stages of elastic stage,crack development stage,and failure stage,which is similar to that of RC beam.The longitudinal strain across the cross section can satisfy a Plane Section Assumption.Failure of the beam is characterized by concrete crushing on the top flange.In addition,the critical loads of cracking,yielding,and failure,as well as structural stifftness of UHPC beam are higher than that of RC beam.Moreover,the yielding and failure loads,ductility,and crack resistance of UHPC beam are increased with the increase ofsteel fiber volume fraction(0%?3%)and reinforcement ratio.Utilizing the general purpose program ABAQUS,the finite element model(FEM)for analyzing UHPC beam is established.The simulated load-deflection curves,load-strain curves,and crack distribution are in agreement with the experimental results,which verify the effectiveness of the finite element analysis results.And then,the effect of different paratmeters on bending behaviors is investigated using FEMs,including steel fiber volume fraction,reinforcement ratio,Yield strength of rebar,and compressive and tensile strength of UHPC.The sesult indicates that the reinforcement ratio has great influence on the mechanical properties after cracking as well as yielding and failure loads of UHPC beams,while the steel fiber volume fraction has great influence on the mechanical properties in the elastic stage and its cracking load.The yielding and failure loads increase linearly with the yield strength of rebar increases,and the ductility of UHPC beam could increase effectively.Besides,compressive strength of UHPC has less effect on the cracking and yielding loads than it does on failure load slightly,while tensile strength of UHPC has better effect on the cracking load than the yield and failure loads.The reasonable calculation model is determined according to the mechanical behavior and failure mode of the UHPC beam.And then,the simplied calculation formula of cracking moment and flexural capacity is proposed based on the finite element anslysis results.It can be used as the reference in design and construction of similar structures in future.