PSI - Issue 64

Reza M. Fioruz et al. / Procedia Structural Integrity 64 (2024) 1142–1151 Firouz R. M. et. al./ Structural Integrity Procedia 00 (2019) 000 – 000

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b

Furnace

Furnace thermocouple

Test sample

Loading actuator

End-slip LVDT

Slip LVDT

End-slip LVDT

Slip LVDT

Thermocouples wires

Fig. 2. a) Direct pull-out test setup on the NSM CFRP system in the lab temperature, b) Thermo-mechanical pull-out test setup: a special furnace was attached to the surface of the concrete surface for imposing heat flux to the surface of NSM CFRP system [Images adapted from (Mohammadi-Firouz and Barros 2023)]. 3.2. Thermo-mechanical tests The thermo-mechanical setup is shown in Fig. 2b. The basic configuration of thermo-mechanical tests was similar to the pullout tests in ambient condition, with an additional special electric furnace, attached to the exposed surface of the bonded zone. The furnace is used to impose a heat flux on the surface of the NSM system, while a constant tensile load is applied to the CFRP. This resembles a structural element under service load at temperature conditions representative of a fire scenario. The constant load was 30% of the average maximum pullout force registered in the tests at ambient conditions. While the loading was kept constant at this level, the heating started to increase at the rate of 25°C/min, until the failure of the bond was reached. The heating rate was controlled by a thermocouple (TC) installed in the furnace and near the heating resistors (at about 110 mm distance from the surface of the concrete). In addition to the furnace temperature, three extra TCs registered the temperature at different locations within the mid-length of the bond zone: 1) surface temperature: a TC at the surface of the concrete and NSM system, exposed to the heating; 2) CFRP top: TC placed inside the groove at the top of the CFRP strip, close to the surface of the concrete; 3) CFRP mid: TC placed at the mid-height of the CFRP strip. The latter two TCs were attached to the CFRP before inserting them into the grooves, immersed in the adhesive. The experimentally adopted and the ISO 834 (ISO 834-1 2020) heating regime scenarios are also compared in Fig. 3. The ISO 834 curve is of interest to reproduce the real structural effects produced by rapid heating, while in the case of lab testing on small samples, thickness and boundary conditions are not representative of a real situation. Therefore, in this study smoother heating rate was adopted due to the smaller scale of the tested samples. The temperature of the furnace was programmed to reach up to 1000°C and then remain constant until the failure of the sample. 4. Analysis of experimental results The analysis of the failure modes of NSM sand-coated CFRP strips in pullout bond tests in previous tests showed a governing mechanism where the sand-coated CFRP was slipping out of the CBA medium, and no trace of the sand particles observed (Mohammadi-Firouz et al. 2023). This shows a good bonding performance between the sand coating layer and the CBA adhesive, indicating the weak interface at the bond of sand particles and the CFRP surface. Therefore, substituting the CBA adhesive with a material with some similar characteristics, such as GPA, sounds feasible in this context. The observations after performing the pullout tests on samples with GPA matrix showed a similar failure mode has occurred in specimens with CBA (Mohammadi-Firouz et al. 2023).