# Development of Analytical Model for Bonding of CFRP Rod in Concrete Subjected to Cyclic Loads
## 9 Discussion
corrected, with only 7.84% of error present, corresponding to a 43.39% improvement. This aspect of the model's improvement is particularly noteworthy, as bond strength directly influences the structural endurance and safety margins of built infrastructure. The average shear stress along the CFRP rod is of utmost importance, and yet the original FEM significantly overestimated the stress by an astonishing 51.27%. With the incorporation of the interface element, the error was remarkably reduced to a negligible 8.45%, representing a significant advancement in precision. The findings of this research underscore that interface elements are not mere computational artifacts but rather pivotal components, particularly for cyclic loading conditions. They hold the potential to transform finite element modelling from estimative guesswork into accurate, results-oriented predictive tools. Computational models equipped with these interface elements will provide critical insights into the future design, optimisation, and risk assessment protocols for concrete structures reinforced with CFRP rods. The results of the study indicate that the combination of CFRP rods with UHPFRC offers superior performance compared to conventional Grade 40 concrete. This is attributed to the enhanced bonding strength and the ductile behaviour of UHPFRC, which can better accommodate the cyclic loading conditions (Figures 27 and 28). The obtained results and comparisons of the interface elements demonstrate their efficacy, as they have been analytically formulated and validated based on experimental results. These results confirm that smooth CFRP has sufficient bonding properties to effectively reinforce concrete. However, when combined with high-performance concretes like UHPFRC, brittle CFRP exhibits exceptional performance due to the ductility of UHPFRC, which prevents abrupt damage to the CFRP composite. This is supported by the comparison presented in Table 16, which shows the data obtained by [PERSON] et al. and the results of the current research [35]. [PERSON] et al. investigated the behaviour of HSC specimens reinforced with ribbed CFRP rods under cyclic pull-out load testing. The comparison between the findings of the current research and [PERSON] et al. 's findings reveals a slight difference, with a roughly 50% discrepancy in carrying force. This discrepancy can likely be attributed to the type of CFRP bar utilised, as [PERSON] et al. [35] employed a ribbed CFRP bar, which enhances the bond between the CFRP rod and the cementitious material compared to the smooth CFRP bar used in the present study. Figure 27: Force Comparison of UHPFRC. Figure 29 compares the failure modes of [PERSON] et al. 's research and the current study's CFRP-UHPFRC specimens. However, the current study found a lower load carrying capacity. Despite this, CFRP bar rupture was the failure mode, which is a more favourable outcome than the concrete splitting observed in [PERSON] et al. 's study. This suggests that the combination of CFRP rods and UHPFRC provides a more favourable failure mode, contributing to the superior performance of this approach [35]. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & **Concrete** & \begin{tabular}{c} **Type of** \\ **CFRP** \\ \end{tabular} & \begin{tabular}{c} **CFRP Bar** \\ **Dia. (mm)** \\ \end{tabular} & **Length (mm)** & **Loading** & \begin{tabular}{c} **Bond Stress** \\ **Type** \\ \end{tabular} & \begin{tabular}{c} **Force** \\ **(MPa)** \\ \end{tabular} & \begin{tabular}{c} **Failure** \\ **Mode** \\ \end{tabular} \\ \hline [PERSON] & HSC & \begin{tabular}{c} Ribbed \\ Bar \\ \end{tabular} & 10 & Min. 25D = 250 & Cyclic & 8.93 & 70.1 & \begin{tabular}{c} Concrete \\ Splitting \\ \end{tabular} \\ Result of current & UHPFRC & \
\begin{tabular}{c} Smooth \\ Bar \\ \end{tabular} & 9.7 & Min. 25D \(\approx\) 300 & Cyclic & 4.53 & 41.5 &
\begin{tabular}{c} Rebar \\ Rupture \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 16: Result Comparison of [PERSON] et al. With Current Research [35].