Bridges Strengthening Methods Of Prestressed CFRP Plates And Distributed Evaluation Of Prestress Loss

Carbon fiber reinforced polymer（CFRP）composite materials has many advantages,such as high strength-to-weight ratio,non-corrosiveness,excellent fatigue characteristics,ease to transport and installation,which are applied to reinforcement of bridge and building members.Most of above research base on experiments of simply supported beam,and do not consider the mechanical characters of multi-span continuous beam bridges.prestress loss will inevitably occur in the reinforcement of prestressed CFRP plates.However,there is still a lack of medium of long-term and large-scale stress monitoring practical projects.In view of the above problems,this paper analyzes the longitudinal force mechanism and“inverted camber inhibiting effect”of prestressed CFRP slab reinforcement for continuous girder bridge.Secondly,based on the distributed monitoring characteristics of the long-gauge strain sensor,the static and dynamic prestress loss assessment method for prestress CFRP plates is proposed,which are based on integral value of long-gauge strain and value of long-gauge strain with respect to time t.A three-dimensional finite element model of strengthening small box girder bridges with prestressing CFRP plates was established to study the“inverted camber inhibiting effect”and static and dynamic prestress loss assessment method.Finally,the fatigue test of FBG sensor with long scale distance is carried out,which proves that it has good fatigue performance and can be used for long-term monitoring of Bridges.The main conclusions of this paper are as follows:（1）Because of the reinforcement of multi-span continuous beam bridges by prestressed CFRP plates,the deflection of its（i.e.,reinforced span）odd-numbered adjacent span will increase,and the concrete at the bottom of the beam appears tensile strain;the even-numbered adjacent spans appear inverted camber,and the concrete at the bottom of the beam appears compressive strain.the influence of reinforcement on its adjacent span is significantly reduced,when the number of adjacent spans exceeds two spans.Therefore,only the first adjacent spans should be considered the impact of the reinforced.（2）Because of the structural characteristics of multi-span continuous beam bridges,the reinforcement effect of prestressed CFRP plates is significantly lower than that of simply-supported beam bridges.If the reinforced span has adjacent spans on one side,the inverted camber of the reinforced span will decrease by about 28.7%;If the reinforced span has adjacent spans on both sides,the inverted camber of the reinforced span will decrease by about 46.5%.The bottom concrete strain amplitude（near the anchor points）of the reinforced span will decrease by 36%due to its adjacent span.（3）With the increase of l₀（i.e.,the distance between anchor point and support）,the area of negative bending moment（at supports of continuous beam bridge）becomes larger,and the negative bending moment and tensile strain decrease;the reinforcement area becomes smaller but the reinforcement effect is improved.（4）In order to reduce the impact of reinforced span on the safety of the adjacent span,the adjacent span can also be reinforced with prestressed CFRP plates;if the reinforced span is a side span,the ratio of the prestress applied to the reinforced span and its adjacent span is 1:0.4;if the reinforced span is middle span,the ratio of the prestress applied to the reinforced span and its adjacent span is1:0.3.If each span of continuous beam bridge needs to achieve similar reinforcement effect,the ratio of the tension prestress CFRP plates of the side span,its adjacent span and the remaining middle span should be 1:1.7:1.8.（5）Based on the distributed characteristics of long-gauge strain sensors,the static and dynamic prestress loss of bridges strengthened with stressed CFRP plates is proposed,which are based on integral value of long-gauge strain and value of long-gauge strain with respect to time t.The prestress loss of prestressed CFRP panels can be evaluated under the condition of no vehicle traffic and without traffic interruption respectively,and the error is within 3%.（6）The calibration coefficient of the long-gauge FBG strain sensor has no significant change after fatigue test,and there is a good linear relationship between the change value of the center wavelength and the strain.After fatigue test,the anchoring section and sensing section of the sensor were observed by electron microscope.The gap between the casing and the fiber in the anchoring section was filled with epoxy resin and solidified.There was no slip between the fiber and the casing in the anchoring section,and no fatigue microcracks on the fiber surface and cross section.It is proved that the test performance of the sensor has no obvious change after fatigue test,and it can be used for long-term monitoring of the bridge.