Emerging as a new technology, carbon fiber-reinforced polymer (CFRP) has been intro- duced to rehabilitate and strengthen steel structures using an adhesive agent. However, the outdoor service temperature is potentially degrading to the mechanical strength of the adhesive, as well as affecting the bonding of the strengthened steel structure. Therefore, this paper aims to investigate the bond relationship of CFRP-strengthened steel plates exposed to service temperatures. Two types of experiments were conducted to determine the tensile and flexural performance of CFRP-strengthened steel plates. The experiments were designed using a Box-Behnken design (BBD) and response surface methodology (RSM) by considering three parameters: service temperature (25 Celcius, 45 Celcius and 70 Celcius), number of CFRP layers (one, three and five layers) and bond length (40, 80 and 120 mm). The findings show the dominant failure mode transformed from adhesion failure between steel and adhesive interfaces to adhesion failure between CFRP and adhesive interfaces as the service temperature increased. The tensile strength improved by 25.62% when the service temperature increased. Field emission scanning electron microscope (FESEM) analysis proved that the strength enhancement is due to the densification and reduction of the adhesive particle microstructure gaps through the softening effect at service temperature. However, service temperature is found to have less impact on flexural strength. Incorporating the experimental results in RSM, two quadratic equations were developed to estimate the tensile and flexural strength of CFRP-strengthened steel plates. The high coefficient of determination, R2, yields at 0.9936 and 0.9846 indicate the high reliability of the models. Hence, it can be used as an estimation tool in the design stage

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Response surface methodology (RSM) is a tool for designing experiments in Design Expert software. It provides functions for mathematical modeling and statistical and optimization analysis. It is also used to design experiments, based on input variables, to reduce the number of experiments. In RSM, the relationship of independent and dependent factors can be established. The input variable is known as the independent factor, and the output variable is known as the dependent factor. The input variables are predetermined based on the collected data from literature reviews and randomly manipulated to design a complete set of experiments. The output variables are the results obtained from the hands-on experiment.

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Service Temperature Influence: The potential negative impact of outdoor service temperatures on the mechanical strength of the adhesive used in CFRP-strengthened steel structures. It notes a transformation in the dominant failure mode from adhesion failure between steel and adhesive interfaces to adhesion failure between CFRP and adhesive interfaces as the service temperature increases.

Tensile Strength Improvement: Despite the potential degradation, the findings indicate a positive impact on tensile strength. The tensile strength of CFRP-strengthened steel plates improved by 25.62% as the service temperature increased. This improvement is attributed to the densification and reduction of adhesive particle microstructure gaps due to the softening effect at higher service temperatures.

Quadratic Equations for Estimation: The text presents two quadratic equations derived from the experimental results and response surface methodology. These equations can be used as estimation tools in the design stage to predict the tensile and flexural strength of CFRP-strengthened steel plates. The high coefficient of determination (R2) values (0.9936 and 0.9846) indicate the reliability of these models.

Experimental Design and Analysis: The use of Box-Behnken design (BBD) and response surface methodology (RSM) in the experiments demonstrates a systematic and robust approach to studying the bond relationship of CFRP-strengthened steel plates under different conditions. This helps in understanding the complex interaction of parameters such as service temperature, number of CFRP layers, and bond length.

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Temperature Impact on Bonding Strength: Tensile bonding strength increases with temperature, reaching up to 60.71% improvement at 70 Celcius with five layers of CFRP.

Bond Length Influence: In tensile bonding, increasing the bond length enhances strength, with about 11.01% improvement. The strength capacity can increase by about three times when using five layers of CFRP with an appropriate bond length.

Adhesive Softening Effects:The softening effects of the adhesive at service temperatures contribute to the densification and homogenization of the interface, resulting in improved bonding strength.

Market Considerations:Potential customers could include construction companies, infrastructure developers, or any industry requiring enhanced structural strength in diverse environmental conditions.

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