## 3 Bond Behavior and Bond Strength
### Influence of Bar Diameter, Modulus of Elasticity, and Surface of the Bar  
Several studies show that bond performance is highly influenced by the type of FRP bar, bar diameter, and the surface treatment and configurations. [PERSON] et al. [20], [PERSON] et al. [25] and [PERSON] et al. [23] reported that the average bond strength decreases when the diameter of the bar increases. During pullout testing, the bond stress is not uniformly distributed along the length of the bar; instead, it exhibits a gradual decrease from the loaded end to the free end. This stress distribution contributes to the lower bond strength observed in specimens with larger diameter bars. Additionally, the shear lag phenomenon further aggravates the reduction in bond strength as bar diameter increases. The differential movement between the core and the surface fibers under tensile loading leads to a non-uniform distribution of normal stress across the cross-section of the bar. As the diameter increases, this non-uniformity becomes more pronounced. Furthermore, the Poisson effect has also been identified as a contributing factor to the reduction in bond strength [26; 30]. However, [PERSON] et al. reported that the finite element analysis conducted for the thick-walled cylinder demonstrated that the influence of the Poisson effect on bond strength is minimal [20]. Figure 6 shows the effect of bar diameter on bond strength. However, these general trends are not observed exclusively in combination with FRCC but have also been reported in the bond behavior between conventional concrete and FRP. Figure 5: Bond stress–slip relationship [21]. Figure 6: Effect of FRP bar diameter on bond strength: (**a**) BFRP bar [25]; (**b**) GFRP bar in three different matrices [20]. [PERSON] et al. [23] performed a bond test using RILEM beam test method using two types of GFRP bars: low/standard modulus \"LM\" and high modulus \"HM\". The experiment results were compared against those obtained from established code-based design equations. It is reported that the bond strength of HM bars was lower than LM bars, which was primarily attributed to the premature detachment of sand coating from the rebar core. This issue is believed to be specific to the HM bar used for their study, prompting a recommendation for further investigations using commercially available HM bars. The bond strength measured from beam tests of both LM and HM bars embedded in ECC was found to exceed the prediction made by CSA S086-12 [31], CSA S6-06 [32], ACI440.1R-15 [33], and other existing predictions. Among these, ACI440.1R-15 provided a close value of bond strength prediction compared to the other equations, with a ratio of predicted to experimental values ranging from 1.18 to 1.49. Consequently, it is suggested that modifications to these codes are warranted to enhance the accuracy of bond strength predictions for ECC. Figure 7 shows some examples of types of rebars and surface configurations. [PERSON] et al. [20] developed a simplified bond-strength model based on an experimental result of a direct pullout test that considered the FRP bars' surface geometry, such as the rib spacing and rib height. The model is validated using a proposed equation for bond strength. The maximum bond strength value from the proposed equations demonstrated a strong correlation with the experimental results. It is important to note that this model specifically pertains to ribbed reinforcing bars, where bond failure is predominantly governed by shear failure of the matrix. For reinforcing bars with different surface treatments, a more comprehensive investigation is required to accurately assess the calculation of maximum bond strength. [PERSON] et al. [28] conducted an experimental investigation to study the bond-slip behavior of two types of GFRP bars, helically wrapped (HW-GFRP) and sand-coated (SC-GFRP), and a steel bar in PVA-ECC. From the experimental results, it is reported that the interfacial bond strength of HW-GFRP was much higher than that of SC-GFRP and similar to the steel bar.