The steel box girder bridge is a structure composed of mutually vertical stiffening ribs (longitudinal ribs and transverse ribs) that carry the loads of vehicles. Since the external loads are usually complex and variable, the rational design of the bridge components is a topic that deserves more attention. The purpose of this study is to explore the optimal range of some of the component design parameters, expecting to reduce costs while ensuring the stress-carrying capacity. A finite element model (FEM) based on ABAQUS was built and the results were verified by laboratory experiments. The varied thicknesses of the bridge deck, diaphragm, and U-rib were explored based on the validated FEM. The simulation results fit well with the experimental results, which proved that the FEM was quite reliable. The stress analysis results demonstrated an optimal range of 18–20 mm for bridge deck thickness, 14–16 mm for diaphragm thickness, and 8–10 mm for U-rib thickness. The present study holds significant reference value for the design and optimization of multiple steel box girder bridge components, which could further provide a theoretical foundation for related research in this field.

The early mechanical strength of high blending fly ash (FA) concrete in the hardening process is extremely low and greatly affected by curing temperatures, which seriously affects the construction speed and durability. To solve this problem, the influence of limestone powder (LS) as an activator on the enhancement of early mechanical strength and durability of high blending FA mortars cured under 5, 20, and 50 °C was investigated. The results reveal that LS can increase the early compressive strength of mortars with 30% FA cured under 5, 20, and 50 °C at 3 d by up to 21.0%, 18.2%, and 21.2%, respectively. Meanwhile, FA mortar with LS cured under 20 °C can achieve a maximum resistance to freeze–thaw cycles and chloride ion erosion, followed by under 50 °C and 5 °C. Under high curing temperatures of 20 °C and 50 °C, the nucleation effect of LS is more conducive to the cement hydration in FA mortar to produce more hydration products and the microchemical activity of LS can facilitate CaCO₃ from LS to react with calcium aluminate in cement to form calcium carboaluminate (Mc). However, a low curing temperature of 5 °C will significantly reduce the nucleation effect and microchemical activity of LS, causing LS to act more as a filler in FA mortar. LS as an activator can promote more hydration products in FA mortars and make the inner structure of FA mortar denser. LS can greatly reduce the total porosity of FA mortar cured under 20 °C and 50 °C.

Fly ash (FA) and limestone powder (LS) are both environmentally friendly substitutes for cement. The mechanical strength and durability of hydraulic concrete with FA and LS cured under low temperature deserve attention. In this study, the mechanical strength and durability properties of cement mortar based on FA and LS cured under a low temperature of 5 °C were investigated. The compressive strength and resistance to freeze–thaw cycles and chloride ions of mortar with FA and LS were evaluated. The results reveal that adding 5% LS can increase the compressive strength of mortar containing 30% FA by 14.31% at 3 d and 1.63% at 180 d. The addition of 5% LS can result in a 0.58% reduction in the mass loss rate and an increase of 2.98% in RDM compared to the mortar with 30% FA after 100 freeze–thaw cycles. Meanwhile, the addition of 5% LS results in a 5.68% reduction in the chloride diffusion coefficient compared to the mortar with 30% FA. However, for the mortar containing 60% FA, any proportion of LS cannot increase the compressive strength of FA mortar from 3 d to 180 d and will decrease the resistance to freeze–thaw cycles and chloride ions. For the mortar containing 30% FA, the addition of 5% LS can promote the hydration product of C-S-H and obtain the minimum total amount of large capillary pores and air pores. Adding 10% LS can result in the mortar with 30% FA obtaining the lowest unit CO₂ emission, and adding 5% LS can result in the mortar with 60% FA obtaining the lowest unit CO₂ emission.

The crack in the rock is a system that coexists with multiscale and interaction cracks, and it is necessary to evaluate the deformation, stability, and strength of rock mass in many engineering projects and the failure of rock are related to the distribution of cracks in the rock. Cracks of different lengths were set in rock-like materials, and uniaxial loading experiments were carried out under 30 test conditions by changing the length of the rock bridge and the short crack as well as the crack angle. During the experiment, a high-speed camera system was used to record the failure process of the specimens. The following conclusions were obtained: when the crack angle is in the range of 30°to 60° with the loading direction, the shorter cracks are more likely to propagate and coalesce. Most of the specimens initiate cracking from the outer tip of the longer crack and the key point of crack instability is the outer tip of the longer crack. The coalescence mode of the cracks is mainly related to the length of the rock bridge length and the crack angle.

Fiber reinforcement technology has become one of the effective ways to improve the mechanical properties and deformation capacity of concrete. This study investigated the effects of single-doped and blended-doped basalt fiber (BF) and polyethylene fiber (PEF) on the drying shrinkage and mechanical strength of mortars. Meanwhile, the microstructure and reinforcement mechanism of single-doped and blended-doped BF and PEF mortars were studied. The results show that the mortar with a single-doped 6 mm PEF has the strongest resistance to drying shrinkage, and that blended fibers also effectively enhance the resistance to drying shrinkage of mortars. The compressive strength and flexural strength of the blended-fiber mortars are both higher than those of the single-fiber mortar. When the fiber length was 12 mm and the BF/PEF was 1:1, the compressive strength and flexural strength of the mortar at 28 d were respectively 18.6% and 56.1% higher than those of the mortar without fiber. Furthermore, when the fiber lengths were both 12 mm and 18 mm, the splitting tensile strength of the blended-fiber mortar at 28 d was higher than that of the single-fiber mortar and the mortar without fiber. When the fiber length was 12 mm and the BF/PEF was 1:1, the splitting tensile strength of the blended-fiber mortar was 103.3% higher than that of the mortar without fiber. The BF is randomly distributed in the mortar in the form of single filaments, and it exhibits brittle fracture when the mortar fails. When the mortar is damaged, PEF exhibits the phenomenon that the fibers are pulled out, and its surface is covered with hydration products, demonstrating excellent interfacial bonding performance. BF and PEF can interlock and intertwin with each other, forming a three-dimensional network structure in mortar, and jointly exert a complementary reinforcing effect of rigidity and flexibility.

To explore the effects of freeze–thaw cycles on polyethylene fiber (PF)-reinforced concrete (PFRC), five sets of specimens with varying PF volume percentages (0%, 0.1%, 0.2%, 0.3%, and 0.4%) were tested. Results indicated that up to 0.3% PF volume, increasing PF content improved crack initiation toughness, unstable toughness, and fracture energy. Beyond 0.3%, these properties slightly decreased. Freeze–thaw damage reduced concrete toughness and energy, but PF mitigated this effect. Higher PF content led to smaller losses in fracture toughness and energy during freeze–thaw cycles. A calculation model for PFRC's freeze–thaw damage was developed from experimental data, and a bilinear softening constitutive relationship curve for PFRC under freeze–thaw conditions was derived based on Petersson's concrete model.