2.3. Time-Independent Static Flexural Test

The flexural behavior of the HFRP bars was tested under three-point bending to identify ultimate strength and deflection values at room temperature. These tests were carried out for both the pre-conditioned freeze–thaw samples soaked in water above 0 °C with 0% load (FT samples) and the untreated samples (RT samples). A total of five HFRP bar specimens were tested for both FT samples and RT samples to obtain average time-independent static flexural parameters. The test was conducted at room temperature (20 °C) with relative humidity at approximately 66%. The fixture utilized for time-independent testing was custom designed and manufactured by the Laboratory of FRP Composites and Structures at the Harbin Institute of Technology, Harbin, China. The diagrams and photographs of the static bending test fixture are presented in Figure 2. The steel bars on each side of the fixture can support the ends of HFRP bars. The distance between the supporting steel bars is the test span of the HFRP bars. The plates that hold the supporting steel bars are connected to the base legs, which are connected to a fixed base beam. A hydraulic jack (RSC 10150, Zupper, Zhejiang, China) with a stroke of 150 mm and maximum capacity of 10 T is installed on the base beam. The upstroke of the piston can induce mid-span bending of the HFRP bar test specimens and loading is performed manually, resulting in an average load control rate of approximately 0.02 kN/s. Strain gauges are installed along the axial surface of the bar to measure average axial strain during bending. A linear variable differential transducer (LVDT, SOP, Jiangxi, China) is utilized to measure the bending deflection corresponding to the applied load. Figure 2c presents a schematic of a sample cross section of an HFRP bar and the corresponding arrangement of strain gauges, as well as the LVDT utilized for monitoring bending strain and deflection. Time-independent flexural testing was performed to determine the ultimate stress σxl, ultimate strain εx0, ultimate bending load fl, and ultimate deflection δfl of the HFRP bars.

Short-term test fixture setup. (a) Diagram showing HFRP bars before bending; (b) Diagram showing HFRP bars on bending with an upstroke of jack shaft; (c) Location of strain gauges (t₁ and t₂), linear variable differential transducer (LVDT) (t₃), supports at both ends (L₁ and L₂), and applied load on mid-span (L); (d) and (e) photographs of the test setup.