Time-Dependent Degradation Mechanism of Basalt Fiber Reinforced Polymer (BFRP) in a Low-Alkalinity Environment

Basalt fiber reinforced polymer (BFRP) has been utilized as a corrosion-resistant substitute for steel rebar in concrete structures. However, embedded BFRP rebars may degrade over time within the alkaline concrete pore solution. While extensive literature has scrutinized BFRP degradation under highly alkaline conditions (e.g., pH~13 in normal concrete), comparatively few studies have addressed its behavior under lower alkalinity (e.g., pH~11–12 in carbonated/green concrete). To address this issue, this study systematically investigates the degradation mechanism of BFRP rebars under coupled factors of pH (7, 11, 12, and 13), temperature (23, 40, and 60 °C), and aging time (30, 60, and 90 days). Research outcomes indicate that a decrease in pH from 13 to 11 at 23 °C results in a reduction in diffusion coefficient from 7.071 × 10−7 mm2/s to 5.876 × 10−7 mm2/s. Moreover, lowering the temperature from 60 °C to 23 °C at pH 12 leads to a decline in the diffusion coefficient from 7.547 × 10−7 mm2/s to 6.758 × 10−7 mm2/s. Furthermore, following a 90-day immersion at 60 °C, decreasing the exposure pH from 13 to 11 can significantly improve tensile strength retention from 25.357% to 71.933%. In the same scenario, flexural strength retention (or interlaminar shear strength retention) increases from 20.930% to 87.638% (or 23.464% to 76.592%) in such a mildly alkaline environment. A comprehensive degradation mechanism is uncovered, linking macroscopic mechanical properties to microscopic characteristics (encompassing fiber corrosion, matrix cracking, and interfacial debonding). This degradation process can be expedited by alkali attack and thermal activation. These findings contribute valuable insights into the alkali-induced degradation process and furnish a comprehensive dataset regarding the durability performance of BFRP rebars.