Local Bond–Slip Behavior of Steel Bars Embedded in CaO-Based Alkali-Activated High-Strength Cementless Concrete for Sustainable Structures

CaO-based alkali-activated high-strength cementless concrete is considered an environmentally friendly and cost-effective material for sustainable structures; however, to enable its structural application, the bond behavior between steel bars and cementless concrete must be properly assessed. This study experimentally investigates the local bond performance and bond–slip behavior of cementless concrete in comparison with existing design guidelines. For this purpose, steel bars were embedded in cube specimens with dimensions of 150 × 150 × 150 mm3, and direct pull-out tests were conducted. The test variables included embedded length (2d, 4d, and 6d), bar diameter (D10 and D13), and concrete cover thickness (1.5d, 2.5d, and 7d). The results indicate that increasing the bar diameter did not lead to a significant change in bond strength; instead, the bond strength increased by approximately 4.8%, and yielding of the steel bar at the loading end was observed for specimens with an embedded length of 6d. When the cover thickness exceeded 2.5d, its influence on bond strength enhancement became negligible, with similar bond strengths ranging from 23 to 25 MPa. A comparison of the ultimate bond strength revealed that the predicted values based on ACI 318-25 showed the best agreement with the experimental results, with the lowest mean absolute relative deviation (MARD) of 22.14%. Furthermore, in terms of bond–slip behavior, the CMR model demonstrated better agreement with the experimental results than the modified BPE model, exhibiting a coefficient of determination (R2) of 97%.