This paper presents experimental and analytical studies on the effect of the compressive strength of the concrete infill on the flexural behaviour of composite beams. Hollow pultruded Glass Fibre Reinforced Polymer (GFRP) square beams (125 mm x125 mm x 6.5 mm) filled with concrete having 10, 37 and 43.5 MPa compressive strength were tested under static four-point bending. The results indicate that filled GFRP beams failed at a load 100 to 141% higher than hollow beams and showed 25% increase in stiffness. However, the increase in concrete compressive strength from 10 to 43.5 MPa increased the ultimate load by only 19% but exhibited almost the same flexural stiffness indicating that a low strength concrete is a practical solution to fill the GFRP profiles to be used as beam applications. Moreover, the concrete infill prevented the premature buckling and web crushing of the GFRP tube. The maximum strain measured at failure is similar to the compressive strain determined from the coupon test indicating the effective utilisation of the GFRP material. Finally, Fibre Model Analysis which considered the partial confined stress -strain curve for the concrete infill gave an accurate prediction of the flexural behaviour of the concrete filled GFRP sections.

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Abstract:It is important to determine accurately the elastic properties of fibre reinforced polymer (FRP) composites material, considering that their member design is often governed by deflection rather than strength. In this study, the elastic properties of the pultruded glass FRP (GFRP) square sections were evaluated firstly using full-scale with different shear span to depth (a/d) ratios and tested under static four-point bending. Back calculation and simultaneous methods were then employed to evaluate the flexural modulus and shear stiffness and were compared with the results of the coupon tests. Secondly, the full-scale beams were tested up to failure to determine their capacity and failure mechanisms. Finally, prediction equations describing the behaviour of the pultruded GFRP square beams were proposed and compared with the experimental results. The results indicate that the back calculation method gives more reliable values of elastic properties of GFRP profiles. In addition, the behaviour of the beams is strongly affected by the a/d ratios. The shear was found to have a significant contribution on the behaviour of beams with lower a/d ratios while the flexural stress played a major part for higher a/d ratios. The proposed equation, which accounts for the combined effect of the shear and flexural stresses, reasonably predicted the failure load of pultruded GFRP square beams. It is important to determine accurately the elastic properties of fibre reinforced polymer (FRP) composites material, considering that their member design is often governed by deflection rather than strength. In this study, the elastic properties of the pultruded glass FRP