Experimental Study on the Damage Mechanism of Hybrid-Fiber-Reinforced Desert Sand Recycled Concrete Under Freeze–Thaw Cycles

With the continuous growth of the demand for concrete in infrastructure construction, natural aggregate resources have become increasingly scarce. The preparation of concrete using desert sand and recycled aggregates has emerged as an effective approach to achieving the sustainable development of building materials. However, desert sand recycled concrete still confronts critical durability-related challenges when exposed to freeze–thaw conditions. We examined how hybrid fibers (steel fibers and hybrid PP fibers) affect the mechanical performance and freeze–thaw durability of desert sand recycled aggregate concrete, along with the underlying mechanisms. Mechanical properties (compressive, splitting tensile, flexural strength) and freeze–thaw damage indicators (mass loss, dynamic elastic modulus) were tested. The findings indicated that at a 30% desert sand replacement ratio, the concrete achieved optimal initial mechanical properties. For the hybrid fibers group (F0.15-S0.5) with 0.15% hybrid PP fibers and 0.5% steel fibers incorporated, relative to the control group, its compressive strength rose by 31.6%, while mechanical property loss was notably mitigated after 125 freeze–thaw cycles. Freeze–thaw damage models based on the exponential function and the Aas-Jakobsen function were established. Microscopic analysis indicated that the fibers effectively suppressed crack propagation and interfacial transition zone (ITZ) damage. This research offers critical experimental evidence and theoretical frameworks for the application of fiber-reinforced desert sand recycled concrete in cold-climate regions.