Influence of Composition Modification of Basalt Fiber-Reinforced Polymer Bars on Alkali Resistance

The application of fiber-reinforced polymer bars has been considered an alternative for the non-metallic reinforcement of concrete structures. Basalt fiber-reinforced polymer (BFRP) is a new composite used to reinforce concrete structures. However, the main drawback of BFRP is its low modulus of elasticity. Therefore, hybrid reinforced fiber polymers, in which carbon fibers replace part of the basalt fibers, might be considered as a relatively "simple" modification that can increase the modulus of elasticity. The literature data suggest that modification of the epoxy matrix with nanosilica particles can positively influence resistance to high temperatures. Besides the mechanical characteristics of FRPs, the evaluation of alkali resistance is necessary for technical approval for construction applications. This paper focuses on testing the alkali resistance of basalt fiber-reinforced polymer (BFRP) bars and its modification through the partial substitution of basalt fibers with carbon fibers (HFRP) and the addition of nanosilica to the epoxy binder (nHFRP). The alkali resistance was tested based on the most common method described in ACI report 440.3R-04-part B6. This method consists of three procedures carried out at 60 °C on the specimens immersed in an alkaline solution, both with and without load. The changes in the mass and tensile strength of the bars are examined after 1, 2, 3, 4, and 6 months. The test procedures are time-consuming and expensive, particularly Procedures B (in alkaline solution) and C (in concrete cover), in which longitudinal tested specimens must be immersed in alkaline solution and subjected to constant strain at an elevated temperature for a 6-month period. Therefore, this study proposes a test setup to achieve a less time-consuming and cheaper assessment of the alkali resistance of FRP bars. Additionally, the usefulness of the shear strength test for the evaluation of alkali resistance of FRP bars is also discussed. The results (Procedure A) indicate that modification of the composition of BFRP did not decrease the resistance to the alkaline environment in the case of HFRP (5% lower than in the case of BFRP). Under the same conditions, the decrease in the tensile strength of nHFRP was 40% higher than in the case of BFRP. This indicates that additional modification of the composition by adding nanosilica to the epoxy binder did not provide the expected stability of tensile properties at elevated temperatures. The results of the evaluation of alkali resistance according to Procedure B show that the device proposed for maintaining constant strain during the seasoning is promising. At this stage, the device makes it possible to conduct the tests at ambient temperature and yields a significantly lower decrease in tensile strength (10-14%) after 6 months, demonstrating a significant effect of temperature on the results of the FRP alkali resistance test.