Mechanism-dependent post-fire bond behavior of reinforced concrete: Coupled effects of cement composition and aggregate type

Bond deterioration in reinforced concrete after fire exposure remains insufficiently understood under rapid heating conditions representative of standard fire scenarios, particularly with respect to the coupled effects of cement composition, aggregate type, and governing failure mechanisms. This study investigates post-fire bond behavior using pull-out specimens produced with CEM I and limestone-blended CEM II/A-LL cements combined with basalt and quartz aggregates. Specimens were exposed to temperatures up to 800 °C in accordance with the ISO 834 standard fire curve. Mechanical testing was complemented by scanning electron microscopy and thermogravimetric analyses to characterize microstructural degradation. Bond degradation follows a temperature-dependent transition, with splitting-controlled behavior becoming dominant in the range of 400–500 °C, followed by a gradual shift toward pull-out-controlled behavior at higher temperatures, becoming dominant at approximately 800 °C. Bond strength correlates strongly with residual tensile strength across the investigated temperature range, whereas compressive strength provides a weaker indicator. A combined model incorporating both residual tensile and compressive strengths was developed, achieving high predictive accuracy (R² = 0.97). At intermediate temperatures (150–400 °C), CEM II/A-LL mixtures exhibit enhanced bond retention due to improved interfacial stability and distributed microcracking. At higher temperatures, bond degradation becomes dominated by matrix decomposition, with aggregate effects becoming increasingly influential. The results indicate that the relative influence of the cement matrix and aggregate is governed by the prevailing failure mechanism. In the splitting-controlled regime, bond behavior is primarily controlled by matrix integrity, whereas at higher temperatures, where frictional pull-out dominates, aggregate characteristics become increasingly influential. This mechanism-based interpretation enables more reliable prediction and assessment of bond performance in fire-damaged reinforced concrete than conventional strength-based approaches.