Influence of infill concrete strength on the flexural behaviour of pultruded GFRP square beams

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 × 125 mm × 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 100–141% 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.