Study On The Shear Performance Of T Concrete Beams Strengthened With Near Surface Mounted FRP Bars

With the rapid development of our country’s economy,a large number of bridge built in the 1960 s～70 s due to the low design load standard and the backward construction method,in today’s increasing vehicle load,has been unable to maintain the normal use,become a dangerous bridge,the bridge.At present our country quantity of concrete bridge accounted for about 14%of the total number of bridge in city,and the number continues to increase,so the repair and reinforcement of the bridge is imminent.Traditional strengthening methods have been unable to meet the needs of nowadays social function,in recent years,near surface monted FRP strengthed concrete structure technology obtained widespread attention and research of scholars both at home and abroad.Compared with the surface sticking FRP strengthed method,near surface monted FRP reinforcement can avoid abrasion,impact,as well as good fire resistance.Therefore,near surface monted FRP reinforcement technique has wide engineering application background.Concrete beam design in the practical engineering are to follow the strong shear weak flexure principle,to ensure that the ductility of the structure.Due to the changes of design or function and the influence of some external factors,it is easy to cause bending or shear bearing capacity shortage,which requires the bearing capacity of reinforcement.Caused by lack of shear bearing capacity of shear fracture belongs to brittle failure,damage is more serious,especially to be paid more attention.Therefore,the near surface monted FRP strengthed concrete t-shaped cross-section beams was experimental study on shear performance.The main research content are as follows:A total of 17 test beams was designed,including 14 near surface monted FRP strengthed concrete T beams and three standards of strengthened beams,and the experimental of shear performance was studied.Shear performance of before and after reinforcement concrete T beams was respectively analyzed,the influence of strength grade of concrete,ratio of longitudinal reinforcement and depth,surface features,the types of FRP bars,reinforcing methods,FRP rebar diameter,spacing and shear span ratio and other parameters on the shear performance was mainly considered.The results showed that the failure mode of the strengthened beams is more complex,including the flexural failure,shear failure and shear bending failure,no matter what kind of failure mode,the stiffness degradation of strengthened specimens have been postponed,displacement is also reduced.The pure bending section cracking load of strengthened specimens had not improved,but the bend shear cracking load increase significantly,and the number of diagonal crack decreased obviously,crack width reduced significantly,some even stop developing after encountering FRP bars.Ultimate load of strengthed specimens increased by 0.84%～15.1%in comparison with standard beam,the main influencing factors are the concrete strength grade,surface features of FRP bars,the types of FRP bars,reinforcement method and the shear span ratio.In order to study the influence of concrete strength grade,ratio of longitudinal reinforcement and depth,surface characteristics,type,diameter of FRP bars,reinforcing methods,FRP bars spacing and the shear span ratio on the ultimate bearing capacity,ultimate bearing capacity increase rate,stirrups strain and FRP bars strain of strengthened specimens,system comparison and analysis of test results is performed.The results showed that the grade of concrete strength,longitudinal reinforcement ratio,surface characteristics,type,diameter of FRP bars has little effect on the mid span displacement of strengthened specimens,while the reinforcing methods,FRP bars spacing,the shear span ratio effect the displacement significantly.The ultimate bearing capacity of the strengthed specimens,stirrups strain and FRP bars strain are decreased with the grade of concrete strength and the ratio of longitudinal reinforcement and depth,the ultimate bearing capacity increase rate increased with the increase of concrete strength grade,and decreased with the increase of ratio of longitudinal reinforcement and depth.The surface characteristics of FRP reinforcement effect the bearing capacity of strengthened specimens,the ultimate bearing capacity increase rate,stirrup strain and FRP bars strain significantly.The ultimate bearing capacity,ultimate bearing capacity increase rate,stirrups strain and FRP bars strain of CFRP strengthed specimens and the unbonded U hoop reinforcement beam is higher than the contrast of GFRP strengthed specimens.The reinforcement effect is not ideal no matter too big or too small of FRP bars which embedded in the specimens.With the decrease of the FRP bars spacing,the ultimate bearing capacity,ultimate bearing capacity increase rate,stirrups strain and FRP bars strain of strengthed specimens have been improved,but the bearing capacity decreased when the spacing is too small,also the utilization rate of FRP bars.Through analysising the shear failure mechanism of ordinary strengthed concrete beam and T concrete beams strengthened with near surface mounted FRP bars,It is concluded that the shear failure mechanism of near surface monted FRP strengthed concrete beam is similar to the ordinary strengthed concrete beam,the action mechanism is similar to stirrup,thus truss theory can be used to carry out overall analysis.Experimental data showed that with the method of similar stirrup to calculate the shear capacity which FRP bars contribute is also realistic.According to the statistical results of experiment of FRP bars strain,and combined with the result of data fitting,the utilization coefficient of FRP bars is determined.The parameters of formula in this paper including:FRP bars spacing,elastic modulus,FRP bars effective strain and angle between the main diagonal crack and the longitudinal axis of specimen.