Numerical Simulation Research On The Effect Of Material Performance Changes On The Shear Behavior Of Concrete Beams

The shear failure of reinforced concrete(RC)beams is a dangerous brittle mode that should be avoided,and how to reasonably evaluate its shear strength is still a research topic that has attracted continuous attention by researchers.The shear bearing mechanism of RC beams is complicated,and the current domestic and foreign codes use different semi-empirical equations to evaluate its shear strength.The assessment of performance deterioration of steel bars and concrete over time,and the development of new durable reinforcing materials for concrete are the two main topics of durability research topics of concrete structures.The deterioration of material properties adds more uncertainty to the shear problem of RC beams,and the different mechanical properties of new reinforcing materials compared with ordinary steel bars makes the shear performance more complicated.Alkali-aggregate reaction is one of the main deterioration phenomena of RC structures.Numerous studies have shown that in addition to causing damage to concrete,the expansion force generated by AAR also causes brittle rupture at the bending potion of the stirrup and local bond deterioration near the rupture position.This will inevitably reduce the anchoring performance of the stirrup and thus the cooperative work of the stirrups with concrete,and eventually result in the change of the load-bearing mechanism and shear strength.At present,the load-bearing mechanism of the stirrup rupture beams has not been fully revealed,and thus there is no reasonable method to quantitatively evaluate its shear strength.As a new type of reinforcing materials,FRP bars in lieu of steel bars will solve the problem of steel bar corrosion of traditional RC structures.Compared with the ordinary steel bars,FRP bars have different mechanical properties such as low elastic modulus,no obvious plasticity,weak transverse shear resistance,and low tensile strength at its bending portion.Therefore,the shear performance of FRP bar reinforced concrete beams must be significantly different from that of the ordinary RC beams.Although multinational codes have developed several different shear-strength evaluation formulas for FRP bar reinforced concrete beams,the evaluation results among each formula are rather discrete and all the results are too conservative.To this end,this study focused on the shear performance of AAR-induced stirrup rupture beams and FRP bar reinforced concrete beams.Based on the previous experimental study by our team,a FEM analysis was carried out to reveals its load-bearing mechanism.Finally,two reasonable and accurate evaluation formulas of shear strength were proposed through data regression and theoretical analysis.Within the scope of this study,the following conclusions are drawn:1.Shear performance of RC beams containing anchorage defect ’stirrups(1)Compared with the intact beams,the stiffness and shear strength Vu decreased with the rupture of stirrups and the increase of the local unbond length.The decrease in the shear contribution Vs by stirrups increased with the rupture of stirrups and the increase of the of the local unbond length,while the shear contribution Vc by concrete decreased with the rupture of stirrups,but showed little difference with the the increase of the local unbond length.(2)The reduction rate of Vu,which was caused by the rupture of stirrups and the increase of the local unbond length,declined with the increase of the beam size.From the perspective of load-bearing mechanism,the larger beam size increased the residual effective bonding length of the side-leg of the rupture stirrups,which enhanced the overall tensile stress of the stirrups at failure,and eventually led to the smaller decline rate of Vs.However,the decline rate of Vc was slightly affected by the beam size.(3)An evaluation formula of the shear strength for the stirrup-rupture RC beams was proposed,and the rationality and effectiveness of the formula was verified through the test result of published literature.2.Shear performance of FRP reinforced concrete beams(1)There are two main failure modes for FRP reinforced beams failed in shear compression:crushing of the concrete in the shear compression zone and rupture of the bending position of the stirrups.When the mechanical coefficient k=pv×fbend/(?)(ρv,fbend,fc’ are steel ratio of stirrups,tensile strength of bend portion of stirrups,and conrete compression strength,respectively)is small,the stirrup rupture failure will occur,and when the k is large,the concrete crushing failure will occur.(2)The Vu of FRP beams increased with the relative axial stiffness pl×El(pl、El are the reinforcement ratio and elastic modulus of the longitudinal bars,respectively).From the perspective of load-bearing mechanism,when concrete crushing failure occured,Vc increased with ρl×El,while Vs decreased with the increase of ρl,but increased with El;when stirrup rupture failure occured,Vc increased greatly with ρl×El,but Vs just showed slight increase.(3)For all simulated beams,the Vu of FRP beams increased with pv×Ev.From the perspective of load-bearing mechanism,with the increase of pv,Vs increased gradually,but Vc changed little.the increase of Vc in the failure mode of the bending part of the stirrup is higher than the failure mode of the concrete crushing,as El of the main bars increased,the increase rate of Vc with Ev declined.However,the increase rate of Vs with Ev had little correlation with the failure mode and El.Compared with the ordinary RC beams,the beams reinforced with BFRP main bars and steel bar stirrups had greater deformability,more ductility and toughness.(4)When concrete crushing failure occurred,Vu,Vc and Vs all increased with fc’.The increase of Vs was affected by the type of main bars and fbend,while the increase of Vc had little correlation with the both.When BFRP main bars were used,the increase rate of Vs was relatively smal and was not affected by fbend;while when ordinary steel main bars were used,the increase rate of Vs with fc’ became more pronounce for the beams with higher fbend of stirrups.(5)On the basis of experimental and analytical results,a shear-strength prediction formula for FRP bar reinforced concrete beams was proposed.By using the test results of 127 beams from published literature,the rationality,accuracy and applicability of the proposed formula were confirmed by the comparison of evaluation results with that of the other evaluation formulas.