Alkali–Silica Reactivity of Sand Used in Sand-Coating Fiber-Reinforced Polymer Bars as Internal Reinforcement for Concrete

New guidelines require that sand particles used to provide a bond between the FRP reinforcing bars and concrete to be free of any expansion due to alkali–aggregate reactions. The current study aims at establishing the preliminary groundwork for assessing the potential alkali–silica reactivity (ASR) of sand particles used as sand coating for FRP bars. Two sand types widely used as sand coating were examined with three testing methods to determine their ASR. Mortar-bar samples prepared with the two sands were monitored for length change for 14 days and compared to reference bars produced with innocuous sand. Analysis under a stereomicroscope and scanning electron microscope (SEM) was performed to detect any signs of silica gel or microcracks in the mortar bars. The long-term reactivity of the sands was assessed by immersing glass-FRP (GFRP) bars in concrete cylinders exposed to an aggressive environment of 1 N NaOH solution at 80°C. The cylinders were then cut into small slices and examined under a stereomicroscope to detect any formation of silica gels between the sand particles and the surrounding concrete. Petrographic examination of the conditioning cylinders after 6 months of exposure revealed no gaps at the GFRP–concrete interface. The three testing methods were found to be suitable for examining the sand-coated bars to ensure fine aggregates free of expansive reactions.