Ductility and deformability of FRP strengthened reinforced concrete structures

  • P. Davies

    Student thesis: Doctoral Thesis


    There are many structures in need of strengthening and upgrading and the need for this is due to a number of factors:

    Structural deterioration over time

    Traffic volume increases

    Increases in allowable maximum vehicle weights

    Poor maintenance

    Poor construction quality

    Errors in initial design

    Steel plate bonding has historically been used to enhance load-carrying capacity but there are inherent problems with such systems (e.g. section weight, corrosion susceptibility). Consequently, the use of advanced composites for strengthening has become more widespread in recent years, as such materials have high strength and stiffness but without the disadvantages encountered with steel plates. However, FRP composites do not possess the desired ductile characteristics and, hence, there is hesitance in their use, due to concerns of potentially sudden collapse.

    In this research the investigation is based on Fibre Reinforced Polymer (FRP) strengthened reinforced concrete beams and slabs, with particular attention to the ductility of strengthened elements and their failure modes. A total of twenty beams and nine slabs were tested. The beams were 2.6m long, with a cros-ssection of 100mm x 200mm, and the slabs were 3m long, with a cross-section of 500mm x 100mm. All sets of elements consisted of a control member and strengthened members with varying amounts of CFRP sheet or plate.

    The main conclusions drawn are:

    The inherent ductility of CFRP strengthened elements is not as low as has been previously thought. Some of the tested elements with lower amounts of strengthening displayed acceptably ductile failure modes; i.e. the elements demonstrated levels of deflection/deformation prior to ultimate loads to ensure sufficient forewarning of impending failure.

    The crack patterns of strengthened elements vary considerably from unstrengthened ones, particularly with respect to beams. Cracks in strengthened members tend to be smaller and narrower, with an increased crack density. Additionally, cracks in strengthened elements tend to increase in number up to failure, unlike their un-strengthened counterparts.

    FRP strengthening is not suitable for elements already heavily strengthened in flexure. Experiments have shown that there is insignificant improvement in performance, as the elements all fail due to crushing of the concrete in the compression zone, making any strengthening redundant.

    A non-linear numerical model has been created and verified using the data acquired from the experimental programme. This will enable the prediction of element deflection up to ultimate load and, hence, allow the ductility to be quantified during the design stage.

    A method of predicting the maximum deformation of fibre composites strengthened RC beam elements has been developed, enabling designers to effectively determine the deflection of such elements.

    A revised method has been developed for determining average crack widths under load, which builds on existing theory to include the effect of the CFRP strengthening on the element behaviour.

    Date of AwardJun 2010
    Original languageEnglish
    SupervisorDavid Tann (Supervisor)


    • Fibre Reinforced Polymers (FRP)
    • Carbon composites
    • Reinforced Concrete
    • Flexure
    • Strengthening
    • Retrofitting
    • Ductility
    • Deformability
    • Failure Modes
    • Over/Under Strengthening
    • Optimisation
    • Practical considerations
    • Non-linear modelling

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