The fracture characteristics of concrete significantly influence its cracking resistance and ductility. Although the brittleness of concrete can be mitigated by replacing natural aggregates (NA) with alkali-activated cold-bonded artificial aggregates (ACA), inherent mechanical weaknesses still limit its overall strength. This study investigates the use of high-strength, surface-treated ACA (SACA) to improve the fracture properties and strength of concrete. The mechanical and fracture properties of concrete containing SACA were evaluated. The interfacial transition zone (ITZ) was characterized by combining cement mortar–aggregate bond strength tests with backscattered electron microscopy analysis. The results showed that SACA substantially enhanced the ITZ. Compared with NA concrete, SACA concrete exhibited a 38.8% increase in bond strength and an 11.8% increase in critical crack propagation length. Compared with ACA concrete, it showed a 27.9% increase in compressive strength and a 1.3% increase in critical crack propagation length. These findings indicate that concrete prepared with SACA possesses improved fracture toughness and strength. Specifically, the SACA concrete achieved a compressive strength of 43.1 MPa, an effective crack length of 52.859 mm, initial and unstable fracture toughness values of 0.238 MPa·m1/2 and 0.690 MPa·m1/2, and an ITZ thickness of 7.86 μm.
To address the prolonged construction duration and low efficiency of conventional incremental launching methods, this paper proposes an intelligent self-rotating launching system with dynamic deviation correction for steel-concrete composite girder bridges. The system integrates an intelligent closed-loop control algorithm and real-time monitoring to enable automatic trajectory adjustment of the girder during launching. High-strength rubber pads are used to minimize abrasion, thereby improving launching precision and structural quality. The effectiveness of the proposed system was validated through a case study of a long-span steel-concrete composite girder bridge in northern China. Numerical simulations were conducted to optimize construction parameters, and field monitoring data confirmed the reliability of the model. Furthermore, using the validated simulation results and grey relational analysis, the key factors influencing girder stability during launching were examined. The results show that both the maximum compressive stress in the girder and the vertical displacement of the launching nose satisfy the design code requirements. The grey relational analysis ranks the influence of parameters on stability as follows: transverse stiffener spacing>distance of longitudinal stiffeners from the bottom plate> thickness of longitudinal stiffeners > thickness of transverse stiffeners > web thickness. The proposed intelligent launching system serves as a valuable reference for constructing long-span composite girders under complex conditions.
Urban rail transit (URT) and underground public space (UPS) are closely interacting infrastructure subsystems in high-density urban central districts, but strong interaction does not necessarily imply coordinated development. This study evaluates district-scale URT–UPS coordination in ten coastal urban central districts in China using a multidimensional indicator framework covering scale, morphology, intensity, and coordination support. An entropy-weighted coupling coordination model is applied within consistent central-district boundaries, with sensitivity analysis, equal-weight validation, AHP-based cross-validation involving 20 experts, and leave-one-case-out analysis used to test robustness. Results show consistently high coupling degrees (C = 0.8874–0.9993) but markedly differentiated coordination degrees (D = 0.4858–0.8805). Hangzhou, Shenzhen, and Guangzhou achieve favorable coordination through different subsystem configurations, while Xiamen demonstrates the compensatory role of subsystem balance. Metropolitan development level provides an enabling context but does not determine local coordination. Robustness tests confirm the stability of the principal cross-case structure. The findings support bottleneck-oriented planning that distinguishes capacity deficiency, subsystem mismatch, and spatial discontinuity.
Large quantities of gold tailings are generated and stockpiled worldwide, creating significant environmental and land-use challenges. However, the application of gold tailings powder (GTP) in high-performance engineered cementitious composites (HP-ECC) remains limited, and its effects on mechanical performance, fiber–matrix interfacial behavior, and microstructural evolution require further investigation. This study investigated the feasibility of using activated gold tailings powder (GTP) as a partial cement replacement (0–30%) in HP-ECC. The effects of GTP content on flowability, setting time, mechanical properties, pore structure, and fiber–matrix interfacial behavior were systematically evaluated. GTP reduced the flowability of fresh mixtures, but an appropriate replacement level improved the overall performance of HP-ECC. The optimum performance was achieved at a replacement level of 20%, where the compressive strength, flexural strength, and ultimate tensile strength reached 97.4 MPa, 19.4 MPa, and 8.16 MPa, respectively. The tensile strain capacity ranged from 3.93% to 5.55%, and all mixtures exhibited strain-hardening behavior with multiple cracking. Single-fiber pullout tests showed that the interfacial frictional bond stress of the GTP-20 mixture reached 1.79 MPa. Microstructural analysis suggested that the filler effect and potential pozzolanic activity of GTP contributed to pore structure refinement, porosity reduction, and enhanced fiber–matrix interfacial properties. These findings demonstrate that activated GTP is a viable supplementary cementitious material for HP-ECC and a promising option for the value-added utilization of gold tailings.
