To study the high-temperature mechanical properties of potassium magnesium phosphate cement mortar and the high-temperature resistance of its laminates. Potassium magnesium phosphate cement (MKPC) was prepared by using heavy-burning magnesium oxide and potassium dihydrogen phosphate as the main raw materials, borax as the retarder, and compounded with a certain amount of fly ash and silica fume. The effect of the mass ratio of magnesium to phosphorus (M:P), compounded fly ash and silica fume on the setting time and mechanical properties of MKPC was investigated. Furthermore, based on the better M:P, the compressive strength of MKPC mortar was studied after 3 h of constant temperature at 400 °C, 600 °C, and 800 °C, and the effect of fly ash and silica fume on the high-temperature resistance of MKPC was analyzed. The high-temperature resistance of MKPC was further evaluated by analyzing the temperature variation of potassium magnesium phosphate cement laminate during a constant temperature of 650 °C for 3 h. The results showed that the mechanical properties of potassium magnesium phosphate cement were influenced by different raw material ratios, and the mechanical properties of potassium magnesium phosphate cement were optimal when M:P was 2:1, fly ash was 5% and silica fume was 15%. The internal temperature of MKPC laminate increased slowly with time, and its high-temperature resistance was better.

Diaphragm cutouts are set to release redundant constraints and hence reduce weld fatigue at the connection of U-ribs to diaphragms in orthotropic steel decks. However, most fatigue cracks which originate from the edge of cutouts are in fact detected in the diaphragms. Therefore, a retrofit technology on cracked cutouts at the diaphragm is proposed and applied to the orthotropic steel box girder of a suspension bridge. Firstly, the stress concentration on the cutout is analyzed through refined finite element analyses. Furthermore, the fatigue cracked cutouts are retrofitted by changing their geometrical parameters. Thereafter, an optimized geometry and the size of diaphragm cutouts were confirmed and applied in the rehabilitation of a suspension bridge. On-site wheel load tests were carried out before and after retrofitting of the diaphragm cutout. The stress distributions along the edges of the cutouts and at the side of a diaphragm were measured under a moving vehicle. The stress spectra at two critical locations on the edge of a cutout was obtained under longitudinally and laterally moving vehicles. Finally, the fatigue life of the cutouts is assessed by the modified nominal stress method. The analytical and test results indicate that the wheel loads on the deck transmit stress to the diaphragms through the U-ribs, during the load transmission process, the stress flow is obstructed by diaphragm cutouts, resulting in local stress concentrations around the cutouts. In addition, the overall size of the cutouts should be small, but the radius of the transition arc should be large, thus the stress flow will not be obviously obstructed. After the retrofitting of the cutouts by improved geometry, the maximum stress decreases by 87.6 MPa, which is about 40% of the original stress. The equivalent constant amplitude stress is reduced by 55.2% when the lateral position of the wheel loads is taken into consideration. Based on the stresses obtained by finite element analysis (FEA) and experimental tests, the fatigue lives of the original cutouts are 1.7 and 4.9 years, respectively, which increase to 78.1 and 155.5 years, respectively, after the cutouts were retrofitted, which indicates that the improved geometry and retrofit technology can enhance the fatigue performance and extend the fatigue life of diaphragm cutouts with fatigue cracks.

Residual stresses change the stress ratio of fluctuating stresses, hence seriously affect the fatigue life of orthotropic steel decks (OSDs) under traffic loading. Residual stress distributions near the U rib-diaphragm joints are very complicated and need to be investigated further. In this paper, a systematic method has been proposed for calculating the residual stress field in the joint of U rib and diaphragm due to thermal cutting and welding. Firstly, a mathematical model of cutting heat sources was established to predict the temperature field. Then, a numerical elastoplastic thermomechanical model was built to predict the residual stress evolutions in a diaphragm-rib joint through the whole fabrication process involving flame cutting and welding. Moreover, the simulated temperature contours at the fusion zone and the residual stress distributions in the rib-diaphragm joint were compared and verified against the experimental ones. The numerical results showed a great agreement with the experimental ones, indicating that the heat source model can be used to accurately predict the temperature field during flame cutting. Finally, the validated numerical model was utilized to conduct parametrical analyses on the effects of thermal processing rates, e.g., the cutting and welding speeds and on the residual stress distribution in the rib-diaphragm joint. The results indicate that a faster cutting speed and a slower welding speed can decrease the residual stress magnitude at the rib-diaphragm joints and reduce the high-stress zone near the diaphragm cutouts.

In order to obtain the design method of hybrid fiber reinforced concrete with better mechanical properties, the hybrid effect of fiber and the optimal fiber dosage are studied. In this paper, basalt fiber (B fiber) and polyacrylonitrile fiber (P fiber) were adopted. The mechanical properties such as compressive strength, splitting tensile strength, and bending toughness of concrete specimens with different fiber volumes were tested. The compressive strength, compressive modulus of elasticity, tensile strength, flexural capacity, and bending stress-strain curves of concrete with different fiber volumes were obtained, and the coefficient of hybrid effect and the ratio of toughness were calculated accordingly. The results show that the addition of fiber has little effect on the compressive modulus of elasticity of concrete, and the tensile strength, flexural strength, and toughness ratio are significantly increased, but the compressive strength is slightly decreased. The mechanical properties of hybrid fiber reinforced concrete, such as compressive strength, tensile strength, flexural strength, and toughness ratio, are better than those of single fiber reinforced concrete. The analysis of the fiber hybrid effect coefficient shows that there is a good hybrid effect between the two types of fibers. When the volume contents of B fiber and P fiber are, respectively, 0.15% and 0.11%, the comprehensive mechanical property of fiber reinforced concrete is the best. Meanwhile, the mechanical properties of the early-age concrete (3 d and 7 d) with this admixture are better than those of the plain concrete.

In order to investigate the effect of optimized reinforcement of cross-section of steel box girders with fatigue cracks on other vulnerable parts (cross-section, U-rib and deck slab, etc.) under wheel load, and to reveal the stress distribution law of critical parts after the optimization of steel plate reinforcement or arc notch. In this work, a suspension bridge with fatigue cracks in the curved notch of the cross-sectional plate is considered as the research object, two types of curved notch optimization reinforcement solutions are considered and finite element analysis is performed. Longitudinal and transverse moving loading tests were conducted with a test vehicle to test the stresses in the critical parts of the curved cutout of the cross-section. Furthermore, the effects of the two optimized strengthening solutions on the stresses in the curved cutout, the sides of the diaphragm, the sides of the U-rib and the deck slab were analyzed, and the effects of the changes in the diaphragm stiffness on other critical vulnerable structures were analyzed. The study shows that when the “arc notch optimization” strengthening scheme is adopted for crack-free or short cracks, although it can effectively improve the stress transfer path of the arc notch of the diaphragm, it also weakens the cross-sectional area of the diaphragm and has little effect on the diaphragm side, U-rib and deck plate. When the long crack is reinforced by “arc notch optimization + steel plate reinforcement”, it is easy to cause a reaction to the diaphragm sides arranged at the junction area of the diaphragm arc notch and U-rib without steel plate coverage, and the stress will be slightly higher than that when the diaphragm is not optimized. The steel plate reinforcement hurts the lateral stress of the U-rib, but the reinforcement effect will not have any effect on the deck plate.