GFRP durability appraisal: mechanical testing of naturally aged composite panels

The findings of a durability study undertaken on pultruded glass fibre reinforced polymer (GFRP) cladding panels are presented. Sourced at demolition from the Second Severn Crossing Visitors' Centre building in the UK, the panels offered the rare opportunity to assess the characteristics of naturally aged composite material. Mechanical properties were determined and compared with the properties of new, equivalent material. The phenomenon of polymer hardening, typified by a reduction in the material strain limit over time, was investigated by further mechanical testing procedures. By contrasting the properties as found for panels taken from each of the four external walls of the building, factors concerning environmental exposure and factors relating to the original fabrication conditions were investigated. The results indicate that, regardless of the exposure conditions, in 17 years the mechanical material properties appear not to have significantly diminished, despite aesthetic quality suffering due to lack of maintenance. It has, however, been shown that ultraviolet exposure causes a hardening of the resin component of the composite, resulting in an increase in the compressive elastic modulus, but a reduction in the threshold of the brittle fracture of the matrix in tension. This final result has not been documented before and is significant in understanding the long-term performance of composites.