<![CDATA[Crumb rubber concrete (CRC) improves sustainability and toughness but increases interfacial porosity and reduces rebar–concrete bond capacity; waste-tire rubber weakens the interfacial transition zone and reduces bond capacity, confining CRC largely to non-structural use. Steel fibers and nano-silica can each offset such losses at the macro- and micro-scale, yet their combined effect on the rebar–concrete bond, and its sensitivity to bar diameter and anchorage length, remains unquantified, and no calibrated predictive model exists for this material. This study addresses these gaps through monotonic pull-out tests on 24 deformed-bar specimens (14–22 mm) embedded in plain, steel-fiber-reinforced, and steel-fiber–nano-silica-modified CRC (5% rubber content), across two concrete grades and two anchorage lengths, supported by strain-gauge bond-stress measurements, a three-stage bond–slip model, and a validated three-dimensional finite element model. The hybrid modification raised ultimate bond strength by 18.6% over unmodified CRC and shifted the failure mode from brittle splitting toward ductile splitting–pullout. Bond strength increased with concrete grade and decreased with bar diameter and anchorage length, though the diameter-related loss was markedly milder than reported for unmodified matrices. Both models reproduced the measured bond–slip response within 1.3% error in ultimate strength (R² > 0.97). These results show that combined steel-fiber and nano-silica modification can offset the bond penalty typically associated with rubber substitution, providing calibrated design tools that support crumb rubber concrete as a viable, code-compliant option for load-bearing resistance.]]>
