Determining the coefficient of thermal expansion of FRP–concrete hybrid structural system constituents: Implementation and validation of a strain gauge–based method

The coefficient of thermal expansion (CTE) is a fundamental property governing the thermo-mechanical behaviour of materials and plays a critical role in the design, including serviceability assessment of fibre-reinforced polymer (FRP) composites and hybrid systems. The anisotropy of FRPs, the variability introduced by fibre type, matrix properties, fibre volume fraction, and manufacturing method, and the disparity in thermal response between polymers and cementitious materials make accurate characterisation of the CTE particularly challenging for hybrid assemblies subjected to thermal actions. This study investigates and implements a methodology for experimental CTE determination based on Wheatstone bridge strain gauge configurations combined with a reference material. Small-scale tests were conducted on the main constituents of a hybrid sandwich panel composed of pultruded GFRP box structure with polyurethane foam infill and a steel fibre–reinforced self-compacting concrete (SFRSCC) topping. Both half- and quarter-bridge arrangements were examined, and their performance was evaluated in terms of efficiency, consistency, and practicality. The methodology was assessed through full-scale experimental tests and numerical analyses. The quarter-bridge configuration, combined with a low-expansion titanium silicate reference material and dedicated post-processing, proved to be robust and economical for the intended structural application, whereas half-bridge configurations were unsuitable under variable temperature conditions. Full-scale assessment demonstrated that material-level CTEs obtained with the proposed method can adequately predict thermal strains at the structural level, accounting for the composite action between GFRP and concrete. Overall, the findings highlight the applicability of the proposed methodology as a cost-effective and complementary engineering-oriented procedure for CTE determination in structural applications.