Wenjie Li

To investigate the posture control of super-long-span heavy steel box girders during synchronous lifting, this study takes the integral lifting project of the 82 m-span steel box girder of Xiaotun Bridge on the Fuyi Expressway as a case study. A fluid–solid–thermal three-field coupled numerical model was established using Midas NFX 2024 R1 (a general-purpose finite element analysis software for multi-physics and fluid–structure interaction simulations) to explore the alignment and end-displacement characteristics of the steel box girder throughout the lifting process. The results show that under combined thermal and wind loads, girder deflection presents a daily cyclic pattern: temperature rise induces upward arching, while wind-induced vibration generates a mid-span instantaneous amplitude of ±25.0 mm, with a maximum coupled deflection of 31.78 mm. Girder end-displacement increases significantly at lifting heights of 5–25 m and peaks at 25 m. With further height increase and shortened sling length, sway frequency rises while maximum displacement gradually declines. When the plane tilt ratio exceeds 0.17% or the overall unbalanced displacement at lifting points exceeds 12 mm, local stress exceeds 95% of the allowable value, implying potential instability risks. For construction safety, a synchronous intelligent hydraulic lifting system based on the “displacement synchronization and load balancing” strategy was applied. Supported by real-time sensor feedback and adjustment, the system achieves millimeter-level lifting precision and welding positioning accuracy. This study provides a reference for similar synchronous lifting practices of large-span steel box girders.

This paper aims to address the problems of aerial orientation control and safety in sudden extreme working conditions during the integral intelligent lifting of large-span heavy steel box girder. Based on the project of Xiaotun Bridge of Fuyi Expressway, a new integral lifting system and control method were proposed. To prevent extreme conditions during the lifting process, the mechanical properties of steel box girders in both synchronous and asynchronous integral lifting states were investigated using the finite element method. Moreover, the mechanical properties of the steel box girders during the lifting process were analyzed through on-site monitoring. The results show that an alignment device has been added to the existing synchronous hydraulic lifting system, which achieves precise control of the aerial orientation and stress of each component of the steel box girder through informal lifting, micromotion lifting and other methods. In the synchronous lifting condition, the failure of the lifting point will cause the redistribution of internal forces in the lifting sling, thereby endangering the lifting lugs, but the stress and deformation of the steel box girder change relatively little. In the asynchronized lifting condition, the use of double lifting points may potentially result in overturning and torsion of the girder. However, under the asynchronous lifting within the displacement error limits, the overall mechanical performance of the steel box girder meets the codes. The on-site monitoring results correlate closely with the simulation results for the normal lifting conditions of the steel box girder. The maximum stress and vertical displacement of the steel box girder are approximately 74 MPa and 70 mm, respectively, indicating that the overall girder structure is safe and reliable throughout the entire lifting process.

Homogenized micro-crack crushing is an optimal rehabilitation technology for concrete pavement; however, when there are weak road base issues, some measures need to be taken to treat the diseases. Grouting is a common technique for addressing weak road base issues. This study developed a new visual indoor grouting test system to analyze the diffusion and consolidation of slag-based geopolymer slurry. The reactants of the geopolymer and the consolidation state of the slurry and aggregate were observed. Moreover, the reinforcement effect of the slurry on a weak road base was studied through the on-site grouting and excavation of the test pit. The results show that, during indoor grouting tests, as the size of the aggregate decreases, the slurry diffusion depth gradually decreases: only 9.5–4.75 mm aggregate formed a complete cylindrical specimen. In the tests of unformed cylindrical specimens, the 9.5–4.75 mm aggregate will develop 20–50 mm splitting surfaces, while the 4.75–2.36 mm aggregate will develop slurry bulbs and veins of different sizes, but the development is not obvious in the 2.36–1.18 mm aggregate. Fine aggregate grouting will exhibit the pressure filtration effect—especially for the 2.36–1.18 mm aggregate, the pressure filtration effect is the most obvious. An SEM microstructural analysis demonstrated that the geopolymer with a water–slag ratio of 0.4 has a faster hydration and dissolution, which results in a decrease in the density of local reactants. However, the polymerization of geopolymers is more complete. The pores of the coarse aggregate are larger and the slurry filling is denser, while the pores of the fine aggregate are smaller and the consolidation is loose locally. The consolidation of aggregates has cracks at local locations, but the width of the cracks is relatively small. On-site grouting applications revealed that the geopolymer slurry filled the bottom voids of pavement slabs and deep gaps in the road base layers, and the average deflection of the driveway decreased from 104.8 (0.001 mm) to 48 (0.001 mm) after grouting. Weak road base conditions were successfully treated, leading to a significant improvement in bearing capacity.

Homogenized micro-crack crushing can fully retain the bearing capacity of concrete pavement, but local weak road base needs to be reinforced before being directly overlaid with hot-mixed asphalt. Therefore, indoor tests were conducted to study the mechanical properties of slag-based geopolymer as a grouting material for weak road base, and the morphology and influence of polymerization reactants were observed. Concurrently, on-site grouting tests were conducted to study the grouting effect. The results show that the compressive strength, flexural strength and bonding strength of slag-based geopolymer all increase with age. The maximum compressive strength and flexural strength of the geopolymer at 28 d were 18.88 MPa and 6.50 MPa, respectively. The maximum flexural bonding strength at 14 d was 4.58 MPa. As the ratio between water and slag powder increased, the compressive strength and flexural strength gradually decreased, while the bonding strength first increased and then decreased. In the range of ratios of water to slag powder from 0.26 to 0.28, the above three strengths were relatively high, and the compressive shear bonding strength was the highest when the ratio of water to slag powder was 0.28. The shrinkage of the slag-based geopolymer increases with the increase in ratio of water to slag powder, and the porosity also increases, resulting in a decrease in compactness after consolidation. When the ratio of water to slag powder was 0.28, the reactant was mainly a gel-phase material, and the shrinkage crack of the consolidated geopolymer was relatively small. After grouting the weak road base of the concrete pavement, the voids at the bottom of the concrete pavement slab were effectively filled, and the deflection of the pavement slab was significantly reduced. The average deflections of monitoring line I, monitoring line II and monitoring line III decreased by 49%, 41% and 54%, respectively, after grouting. After solidification, the slag-based geopolymer was distributed in layers, which further compacted the road structure layer and improved the bearing capacity.

Homogenized micro-crack crushing is a new method of concrete pavement rehabilitation that makes full use of the bearing capacity of the original concrete pavement, whereby the treated pavement can be directly overlaid with hot mixed asphalt concrete. In order to solve the problem of longitudinal crack extension of a cracked core in the weak position of local pavement, the crack propagation mode of concrete slabs under low-velocity impact from different numbers and distributions of impact heads was studied using numerical simulation. Moreover, field tests were also carried out to determine the optimal layout of drop hammer impact heads, and the numerical simulation results were compared with the experimental data. The results show that, when a concrete slab is impacted by a single impact head, the bottom of the slab first cracks and then the crack develops upward until the main crack runs through the concrete slab. However, when the concrete slab is impacted by multiple-impact heads, including three or four impact heads, the concrete slab forms a triangular or quadrilateral fracture core, respectively. The numerical simulation results are in good agreement with the laboratory experiments. Through the optimized arrangement of the hammer’s impact heads, the problem of longitudinal crack extension of a cracked core can be effectively solved, and the probability of reflection cracks can be reduced.