3.3.1. Concrete Strength

The bond strength is basically a linear relationship with the tensile strength of RAC, and the bond strength increases with the increase of tensile strength, as shown in Figure 8a. Equation (2) [34] was adopted in this study, which reflects the relationship between the tensile strength and compressive strength of RAC.

$$f\_l = 0.18f\_{cu}^{\frac{2}{3}}\tag{2}$$

where *fcu* is the compressive strength of RAC; *ft* is the tensile strength of RAC.

**Figure 8.** Relationship of bond strength and various factors: (**a**) concrete strength; (**b**) embedded length; (**c**) cover thickness; (**d**) lateral stirrup ratio.

The relationship between the tensile strength ( *ft*) of RAC and the average bond strength (τ) is obtained by statistical regression, which is fit as Equations (3)–(5).

$$
\overline{\tau\_s} = 0.125f\_l + 0.431 \tag{3}
$$

$$
\overline{\tau\_u} = 0.511f\_t + 0.549 \tag{4}
$$

$$
\overline{\tau\_r} = 0.567f\_t - 0.097 \tag{5}
$$
