Comparative Assessment of Carbon Nanotube and Steel Fiber Reinforcement in Recycled Aggregate Shotcrete

Recycled coarse aggregate shotcrete (RCAS) offers a resource-efficient route for underground support, but adhered mortar, heterogeneous interfaces and brittle crack propagation limit its mechanical reliability. This study critically integrates two independent experimental investigations of multi-walled carbon nanotube (MWCNT) and steel-fiber (SF) modification to evaluate the complementary roles of matrix-scale densification and crack-scale bridging. The original experimental conditions are preserved while comparable strength, deformation, pore-structure and tensile-energy indicators are examined. In the MWCNT-modified mixtures, a dosage of 0.06 wt% yielded the highest reported 28 d compressive strength (38.25 MPa), whereas 0.09 wt% maximized the splitting-tensile (3.95 MPa) and flexural (4.88 MPa) strengths. Higher dosages reduced performance as dispersion deteriorated. In the separate SF-reinforced series, increasing recycled aggregate replacement from 0% to 100% reduced direct-tensile strength from 2.6 to 2.2 MPa, whereas increasing fiber content from 0% to 2% at 50% replacement increased tensile strength from 1.5 to 3.3 MPa. Together, these observations distinguish nano-scale hydration and pore refinement from fiber-mediated stress transfer after cracking. A mechanism-based reinforcement framework is proposed to balance constructability, strength, ductility and energy absorption. Because the two experimental programs differ in aggregate quality, mixture design and test conditions, combined MWCNT–SF effects remain a hypothesis requiring dedicated experimental validation.