1 Department of Computer Science & Engineering, University of Houston-Downtown, Houston, TX 77002, USA.
2 Michael Baker International, 101 South Spring Street, Suite 100, Little Rock, AR 72201, USA.
3 Faculty of Environment and Technology, The University of The West England, Bristol BS16 1QY, UK.
4 Department of Building, Civil & Environmental Engineering, Concordia University, Montreal, QC H3G 1M8, Canada.

Fiber-reinforced polymer (FRP)-reinforced concrete beams are increasingly used in infrastructure, yet their flexural behavior under fatigue and harsh environmental conditions remains insufficiently studied. This study investigates the fatigue response and structural behavior of 12 glass-FRP (GFRP)-reinforced concrete beams under four environmental regimes: indoor control, continuous alkaline immersion, cyclic wet-dry alkaline immersion, and outdoor exposure in Montreal. Four pre-cracked beams were subjected to up to one million load cycles, while deflection and crack mouth opening displacement (CMOD) were monitored. Structural behavior was evaluated in terms of flexural capacity, load-deflection response, crack development (CMOD), stiffness degradation, and serviceability limit state (SLS) performance before and after fatigue loading. Results show that W& D and Immersion beams exhibited the largest deflections (d_exp/dc_ode = 158% and 92%, respectively), whereas Outdoor and Control beams maintained robust load capacity with minimal fatigue effect. Flexural toughness indices varied from 8.61 to 18.45 across specimens, highlighting environmental influence on energy absorption. Serviceability limit state criteria were reached between 400,000 and 850,000 cycles, depending on conditioning. Overall, GFRP-RC beams demonstrated strong residual strength and predictable degradation patterns, providing quantitative insight into fatigue performance under combined environmental and cyclic loading.