Effect of Environmental Loads on Concrete Shrinkage and Strain in GFRP Bars in Slabs on Ground: Field Monitoring and Analysis

Glass fiber–reinforced polymer (GFRP) bars have been increasingly used in civil engineering over recent decades, with expected service lives exceeding 100 years, owing to their advantages such as a high strength-to-weight ratio and corrosion resistance compared to conventional steel reinforcement. A recent large-scale field application of GFRP bars is the construction of a 21.3-km-long flood mitigation channel in Jazan, Saudi Arabia, constituting the largest GFRP bar–reinforced structure in the world. According to the current design guide, the design of GFRP bar–reinforced slabs on ground is based on a modified version of the subgrade-drag equation originally developed for steel rebars due to insufficient field data. This study addressed this concern by reporting the outcomes of an experimental program involving field monitoring of large-scale slabs on ground cast during the peak summer month. Ten slabs (6 m × 1.1 m × 0.2 m) were constructed on a lean concrete subbase, exposed to ambient conditions for over 840 days, and monitored to investigate the effects of environmental loads on slab-on-grade systems under real-world conditions. The variables of the study included reinforcement types [ribbed-type GFRPs, two types of sand-coated GFRPs, conventional ribbed-steel rebars, the basalt fiber–reinforced polymer (BFRP) mesh, and a geogrid], spacings (200 and 300 mm), and weather conditions during casting. These factors were analyzed for their effects on crack widths, crack distribution, and the evolution of rebar and concrete strains at various critical specimen locations, such as midspan and quarter spans. The results revealed that the slab reinforced with the BFRP mesh exhibited the best crack and strain control, while geogrid-reinforced and unreinforced slabs exhibited the largest crack widths and higher strains. GFRP bar–reinforced slabs were characterized by a single prominent early-age crack, while steel-reinforced slabs developed multiple distributed cracks, which may promote steel corrosion. Specimens cast during summer exhibited larger crack widths and earlier cracking compared to those cast in winter.