Coupling Effects of Seawater Immersion and Prestressing on the Durability of BFRP Bars Embedded in Seawater-Sea Sand Geopolymer Mortars

Basalt fiber-reinforced polymer (BFRP), a composite material known for its cost-effectiveness and excellent resistance to chloride ion corrosion, combined with environmentally friendly geopolymer is expected to contribute to the development of sustainable marine engineering structures. However, concerns still remain regarding the durability of this structure in marine environments. This paper presents an experimental investigation of the durability of basalt fiber-reinforced polymer bars immersed in seawater under the combined effects of the seawater sea-sand geopolymer mortar (SSGM) covering and prestressing. The prestress level was maintained at 20% of the ultimate tensile strength of the BFRP at room temperature in the first 7 days; the alkaline content of the SSGM, represented by the mass ratio of Na2O in the alkaline activator to the precursor, was either 4% or 6%; and the seawater temperatures were room temperature, 40 degrees C, or 60 degrees C. The prestress losses of the BFRP bars were monitored over the entire period, and tensile tests were conducted at 30, 60, 120, and 240 days. The results show that the prestress losses of the BFRP were less than 40% after 240 days of immersion. Prestress accelerated the degradation of the BFRP performance due to fiber damage and microcracking. The alkalinity of the activator in the SSGM did not have a significant effect on the tensile strengths of the BFRP bars immersed in seawater. However, the higher the alkalinity, the higher was the compressive strength of the SSGM and the lower were the prestress losses of the BFRP, which was attributed to the increased compactness friction caused by shrinkage of the SSGM. Carbonates combined with hydrogen ions in the water to form carbonic acid, and hydroxyl groups combined with the cations in the sea sand and the SSGM to form a strongly alkaline solution; these solutions were harmful to the long-term performance of the BFRP bars.