Abstract: Concrete abrasion and erosion in navigation lock structures have become increasingly serious under the combined cyclic effects of prolonged hydraulic scouring and repeated vessel impact. This degradation commonly initiates with microcracking and spalling of the concrete protective layer, resulting in exposure and accelerated corrosion of embedded reinforcement—thus critically undermining structural integrity and long-term serviceability. As damage is overwhelmingly localized within the near-surface concrete zone, this study developed functionally graded concrete composites (FGCCs) with enhanced erosion- and wear resistance, based on the design philosophy of functionally graded materials (FGMs). The resulting FGCCs achieve seamless integration of structural load-bearing capacity and surface-level functional durability—directly improving protective performance and extending service life. First, an erosion- and wear-resistant concrete (EWRC) was formulated, and its workability, mechanical properties and deformation behavior were systematically compared against those of a reference concrete (RC). Importantly, interfacial bonding governs the overall mechanical robustness and durability of FGCCs; yet, while synchronous casting is known to enhance interfacial continuity, systematic research on FGCCs for lock engineering remains limited—particularly regarding how vertical synchronous casting influences interfacial bonding strength and permeability-related durability. To address this gap, the steel wire mesh was introduced as a standardized vertical interfacial separator, enabling direct comparative evaluation of interfacial bonding splitting tensile strength and impermeability under synchronous versus conventional staged casting. Phase composition and nanoscale interfacial structure were further characterized via X-ray diffraction (XRD) and Vickers microhardness (HV) test. Results showed that the erosion- and wear-resistant admixture significantly enhanced both mechanical performance and functional durability: relative to RC, EWRC exhibited 18.1% higher 28-day compressive strength and 16.4% higher tensile strength, along with a 48.8% improvement in erosion- and wear resistance. The relative shrinkage ratio of the erosion- and wear-resistant concrete to the reference concrete was between 108.1% and 113.6%, and the two concretes had good deformation compatibility, which can ensure high compatibility during the hydration process. Under synchronous casting, 28-day interfacial bonding reliability exceeded 80%; incorporation of 25-mm-diameter steel wire mesh increased this to >90%, reflecting substantial enhancement in interfacial tensile capacity. In contrast, under staged casting, bonding splitting tensile strength was markedly superior for roughened interfaces versus smooth ones; although interface agents further improved bond reliability, the 28-day values remained below 65%—significantly lower than the tensile strength of the bulk concrete matrix, indicating a persistent interfacial weakness. Regarding permeability, synchronous casting yielded an interfacial relative permeability coefficient statistically indistinguishable from that of the bulk concrete’s vertical section—confirming effective interfacial homogenization. Under staged casting, however, the new–old concrete interface exhibited a relative permeability coefficient 3.3–7.0 times higher, establishing it as a preferential ingress pathway for aggressive species. Interface roughening and interface agent application both significantly improved impermeability: the agent reduced the relative permeability coefficient by 25.0% for smooth interfaces and by 31.3% for roughened interfaces—demonstrating synergistic efficacy. The XRD analysis identified calcium silicate hydrate (C-S-H), quartz (SiO₂), calcium carbonate (CaCO₃), and calcium hydroxide (Ca(OH)₂) as the dominant hydration phases in both concretes; notably, ettringite (AFt) formation was intensified by the admixture—suggesting enhanced early-age microstructural densification. Microhardness mapping revealed that in synchronously cast specimens, the interface was morphologically indistinct, with fine aggregates interpenetrating across it; HV values remained consistently stable at 120–150 HV near the interface, with no measurable softening zone. Conversely, staged-cast specimens exhibited a sharp, planar interface in smooth specimens and a topographically irregular interface in roughened specimens. Critically, the microhardness profile across the new–old interface followed a pronounced “U-shaped” distribution: matrix hardness exceeded 100 HV on both sides, whereas values directly at the interface fell consistently below 100 HV—providing quantitative microstructural evidence of interfacial weakness. Overall, synchronous casting not only eliminated macroscopic interfacial discontinuity but also achieved nanoscale interfacial homogeneity and mechanical continuity, thereby delivering superior structural integrity and durability over staged construction.