Flexural Behavior Of Concrete Beams Strengthened With Prestressed FRP Laminate

Healthy status of many building structures and bridges are becomingserious, which need to be rehabilitated and be strengthened according toso many reasons, such as physical factors, factitious factors and others.Strengthening technique of externally bonding fiber-reinforcedpolymer(FRP)sheet(which posses the qualities of highstrength-to-weight ratio and corrosion resistance, resulting in lowmaintenance costs)has been extensively employed, which substitutes themethod of externally bonding steel plate because of the shortcomings oferosion and operation complexity. However, this strengthening approachis almost useless to the flexural behavior of structures under normalservice stage, and at the same time, the strength usage ofhigh-performance laminate is low relative to its unidirectional tensilestrength.Under the Financial support of research subject of Ministry ofRailways and Doctoral Fund of Ministry of Education, an innovativesystem for prestressing FRP laminate is investigated. This is an idealtechnique because it combines the advantage of using noncorrosive andlightweight advanced composite materials in the form of bonded FRPlaminate with the high efficiency offered by external prestressing.Experimental study and analysis is carried out according to the idea. Maincontents are discussed as follows:1) Flexural behavior of concrete beams strengthened by bondingprestressed glass-fiber-reinforced-polymer (GFRP) platePrestressing levels and grades of concrete strength are varied in thetest in order to reveal their effect on the bending behavior of strengthenedbeams. The FRP longitudinal stress of and the shear stress of epoxy glueare analyzed after the prestress releasing on the basis of distortion ofcross-section of concrete beam. The expressions are derived for effectiveprestress, prestress loss and reverse deflection at the midspan of concretebeam.Based on the experimental observation, various types of failure are summarized for reinforced concrete members strengthened by externallybonding pre-tensioned FRP. Formulae are presented to calculate thebending moment under which the pre-compressive stress vanishes atextreme fiber of the critical section and the cracking moment for thestrengthened member. All possible strain combinations at ultimate limitstate of flexure are analyzed for the FRP-RC composite section.Expressions are proposed to distinguish between two modes of flexuralfailure respectively due to fracture of FRP and crushing of concrete. A setof equations are suggested for prediction of the corresponding strength,and meanwhile the applicability of the proposed formulas is validated bygood agreement of the analytical results with the measured values.2) Flexural strength of RC beams pre-stressed with end-anchoredCFRP plateA practice and innovative mechanical anchorage system isdeveloped to directly stress the CFRP laminates by jacking and reactingagainst anchors mounted on the soffit of the existing concrete beamsavoiding FRP debonding for externally-bonding prestressed FRP laminate.The anchorage/prestressing system is applied on 1/3 scale concrete bridgebeams. The effects of initial moment and bonded/unbonded prestensionedCFRP laminate are examined, with particular emphasis on flexuralbehavior.With layered finite-element method used and correspondingprogram written, numerical analyses are made for nonlinear overall loadand deflection process for RC beams strengthened by prestressed FRPLaminate. Comparison with experimental results and universalthree-dimensional analysis software show that the model and the programanalysis can predict the behavior of RC beams after strengthening. It alsocan analyze other strengthening method including FRP laminates with orwithout prestress, strengthening one under initial load or not by changingthe parameters combinations in the program, such as concrete strength,reinforcement ration, prestress level, cross-section area of FRP laminate,initial condition.Items of calculation of prestress loss are estimated for externalprestressing beams. And then formulae of flexural capacity for concrete beams strengthened by bonding prestressed FRP laminate undersustaining load are put forward according to different failure modes andstrain combination at the critical section.Stress increment of FRP of RC beams locally strengthened withexternal prestressed laminate is given based on the bond reductioncoefficient modifier and "equivalent deformation zone". Furthermore, theformula of ultimate flexural capacity and the design method of the localstrengthened beam are put forward.The calculation of deflection and crack width of RC beams locallystrengthened with external prestressed FRP laminate under service loadare discussed.Finally, the prestress strengthening system was used in Xiao-fengBridge, and the strengthened bridge shows better work property thanbefore.