Numerical study of FRP-reinforced timber members subject to variable climates

The use of FRP reinforcement in small percentages has been shown to improve the short-term flexural behaviour of timber members. This technology has been successfully used in new construction and in the repair and renovation of existing buildings across Europe. The long-term behaviour of FRP reinforced timber has received less attention including the enhancement in the serviceability performance due to the FRP reinforcement. In this study, a coupled finite element numerical model was developed to examine the influence of a variable climate on the long-term deflection of FRP reinforced members. The time-dependent coupled hygro-mechanical model utilises a thermo-hygro analogy to define the movement of moisture through the member depending on the relative humidity of the surrounding environment. The model considers the elastic and viscoelastic behaviour of timber, in addition to the moisture dependent, mechano-sorptive creep and swelling shrinkage behaviour. The model has been validated against experimental results from long-term tests on unreinforced and reinforced timber beams under four-point bending. A parametric study was carried out to examine the influence of reinforcement material on the long-term behaviour of reinforced timber members over a ten-year period under a sinusoidal relative humidity cycle. The materials considered were glass fibre reinforced polymer (GFRP), basalt fibre reinforced polymer (BFRP), aramid fibre reinforced polymer (AFRP) and carbon fibre reinforced polymer (CFRP). Results have shown that unreinforced members experience the largest deflection over the ten-year period, as expected. The deflection behaviour of the FRP reinforced beams was found to be dependent on the stiffness of the FRP material with the least stiff GFRP reinforcement experiencing a greater deflection than the stiffer BFRP, AFRP and CFRP materials. By considering the relative creep deflection results, it has been shown that a single creep design factor kdef may be used to predict the long-term performance of reinforced beams regardless of FRP type.