Mechanical Performance of Timber Beams Strengthened with Glass Fibre Reinforced Polymer Sheets

Cost is one of the decisive factors when selecting a fibre type for structural strengthening. This study therefore tested the hypothesis that a low-cost fibre can still provide a substantial improvement in the mechanical performance of strengthened timber beams. Four-point bending tests were carried out on 25 pine beams, comprising an unstrengthened reference series and four series strengthened with glass fibre reinforced polymer (GFRP) sheets, each series consisting of five specimens. The beams had nominal dimensions of 80 mm × 80 mm × 1600 mm. The reinforcement was bonded exclusively to the external surfaces, with a focus on the tension zone. Two variables were examined: the number of sheet layers and the extent of coverage of the timber surface. The reinforcement ratio ranged from 0.38% to 1.15%. The response of the beams was assessed in terms of load-bearing capacity, stiffness, ductility, and failure mode. Bonding three layers of sheet to the bottom face of the beams increased the load-bearing capacity by up to 58.20%. The effect on stiffness was less pronounced, with a maximum increase of 20%, which is attributable to the relatively low elastic modulus of the sheets. However, the ductility of the beams increased significantly (up to 126.14%, based on energy considerations), a result that can be directly attributed to the composite's high adhesion and high elongation at rupture. The transformed cross-section method and finite element simulations were used to predict the behaviour of the strengthened elements, and both showed good agreement with the test results within the elastic range. It is concluded that GFRP sheets are a rational strengthening solution, although satisfactory effectiveness was obtained only at higher reinforcement ratios; at least two layers are recommended for the configurations tested. The effect of the strengthening configuration on the load-bearing capacity, on the deflection at maximum load and on the ductility was statistically significant, whereas its effect on the bending stiffness was not.