Lijun Wan’s research while affiliated with Northeast Forestry University and other places

Recycled aggregate (RA) made from construction waste has limited application due to its high porosity, numerous cracks, and other disadvantages such as reduced concrete strength when using aggregate). In this paper, carbonated recycled aggregate (CRA) was obtained by carbonating recycled aggregate, and natural aggregate (NA) in polyurethane mortar (PUM) was replaced at different ratios. The physical changes and pore structure of RA before and after carbonation, the mechanical properties (compressive and flexural strength) of PUM, and the three-point bending bond performance were systematically investigated. The compactness of PUM was detected by ultrasonic velocity (UPV) test. The change mechanism of PUM microstructure was revealed by scanning electron microscopy (SEM). The results show that when the replacement rate is 50%, the compressive and flexural strength of carbonated PUM increased by 7.32% and 15.83%, respectively, and bond strength increased by 14.19%, all of which are greater than the increase at a replacement rate of 25%. The mechanical properties of P with RA as aggregate decreased, but the generated calcium carbonate after carbonation treatment can fill the surface pores and enhance the pore structure of CRA, and the mechanical properties of three-point bending bond performance of the test pieces with the same replacement ratio were improved compared with those replaced by RA.

In cold regions, the strength of soil–rock mixture (SRM) is gradually reduced by freeze–thaw (F–T) cycles, in order to study the deterioration mechanism of mechanical properties of SRM under F–T cycles, indoor triaxial tests were carried out by taking into account the effects of the number of F–T cycles, the rock content and the confining pressure, and combined with the numerical simulation of particle discrete elements to analyze the microscopic deformation of the failure characteristics of the SRM, as well as the development law of the shear surfaces and the contact force chain. The results showed that the F–T cycle did not have much effect on the stress-strain curve morphology, but the peak and initial slope of the stress-strain curve increased with the increase of the rock content. The F–T cycle has a more obvious deterioration effect on the failure strength, elastic modulus and cohesion of the samples, but has less effect on the internal friction angle. The failure strength and elastic modulus of the samples increased with the increase of the confining pressure. With the increase of rock content, the failure strength, elastic modulus and internal friction angle of the samples increased, but the cohesion decreased. Under the flexible loading condition, the SRM samples exhibit swell failure, and the shear zone formed after failure is roughly distributed in an “X” shape, and the failure morphology of the SRM samples, as well as the morphology and size of the shear zone, are all affected by the rock content and the confining pressure. The contact force between particles in the SRM samples increases with the increase of rock content, and the chain of coarse force is mainly distributed in the vicinity of rock particles, but the F–T cycle will weaken the contact force between particles, thus reducing the strength of the SRM.

Bridge grid support stability against typhoons is a key factor in ensuring the safety of bridge construction. This study employs wind tunnel force tests and Particle Image Velocimetry (PIV) experiments to investigate the typhoon stability of grid-type high supports. Based on wind tunnel force tests, the static force coefficients of grid-type high supports are measured under different flow conditions and wind angles. Utilizing PIV technology, a novel analysis is conducted on the flow field visualization of high-pier steel tube support models in horizontal and vertical plane flows. Quantitative analyses of the vortex core intensity and turbulence of single and double-pier supports are performed. The impact of wind direction on the aerodynamic characteristics of grid-type high supports is determined.The research indicates significant changes in drag, lift, and torque when calculating the wind resistance of grid-type high supports. Therefore, the influence of static wind loads in three directions needs to be fully considered in the calculations. Under a 45° wind deflection angle, vortex movement becomes more intense, reaching maximum vortex strength and turbulence intensity. This results in an increase in the average and fluctuating aerodynamic forces of the model. Compared to the four-legged grid-type high support model, the six-legged model exhibits relatively smaller vortex strength and turbulence intensity at the vortex core. Noticeable interference exists among the components of the grid-type high support, thus spatial three-dimensional characteristics should be considered in numerical simulations.By conducting wind tunnel experiments on grid-type high supports in typhoon-prone areas, the study ensures the safe use of these supports, enhances economic benefits, and broadens the application scope of grid-type high supports.

Rainwater inlets are an important part of the bridge drainage system, and this paper carries out modeling tests on the interception performance of bridge rainwater inlets in the Daxinganling region of China. The effects of slope, different inlet methods, and grate opening forms on the interception efficiency of rainwater inlets are mainly investigated. The results show: (1) Rainwater inlet interception efficiency increases with increasing cross slope and decreases with increasing longitudinal slope. (2) The innovatively proposed inlet method of spraying above the flume, which is less efficient than the rainwater inlet interception of the diffuse flow method at the front of the flume, the inlet method is more in line with the actual situation. (3) Combined with the weir flow formula to fit the interception efficiency formula, the relative error between the predicted and measured values of the interception efficiency of the formula is less than 5%, indicating the applicability of the formula. (4) Comparing the interception efficiency of grates with different opening forms, it is concluded that the longitudinal curved opening is the best, followed by the longitudinal linear opening, and the transverse opening is the worst. In the transverse opening grates, the fewer longitudinal bars, the better the interception efficiency.

To address the issue of excessive mid-span deflection in rigid frame-continuous girder bridges during the later stages of normal use and to provide a basis for setting the pre-camber of this bridge type. This study focused on a specific rigid frame-continuous girder bridge. Based on the outcomes of finite element analysis, a GM(1,1) model developed using grey theory was utilized to forecast the vertical deflection of the primary girder in the construction phase of a rigid frame-continuous girder bridge, resulting in notable outcomes. The results suggest that the GM(1,1) model grounded in grey theory offers precise estimations of the vertical deflection of the primary girder during the prestressing tendon tensioning stage, displaying a minimal deviation of merely 1.73 mm compared to actual measurements. Consequently, applying the GM(1,1) model based on grey theory for projecting the vertical deflection of the primary girder in the construction process of rigid frame-continuous girder bridges can advance the accuracy of construction management. Through comparative analysis of measured values estimated values and theoretical values at the construction site, it is evident that the predictive model effectively controls the displacement, thus serving as a reference for construction control in similar projects.

Purpose The application of reinforcing old bridges by adding external prestressed steel bundles is becoming more and more widespread. However, the long-term safety performance test of the strengthening method is rarely carried out. In this paper, the bearing capacity of a 420 m prestressed concrete (PC) continuous girder bridge after five years of strengthening is analyzed. Design/methodology/approach The bridge model of the bridge structure and strengthening scheme is established by the finite element software of the bridge. The theoretical load-bearing capacity of the bridge under the latest standard load grade is obtained by finite element analysis. The actual bearing capacity of the bridge is obtained by field test. Through the comparative analysis of theory and practice, the health state of the bridge after five years of reinforced operation is judged. The damage to the overall stiffness and external prestressing of the bridge is also analyzed. Findings The results of deflection and strain show that the stiffness and strength of the secondary side span and the middle span decrease slightly, and the maximum reduction of bearing capacity is 4.5%. The static stiffness of the whole bridge decreases as a result of cracks, and the maximum decrease is 21%. In the past five years, the relaxation loss of the external prestressing of the bridge is 3.31–3.97%, which is the main reason for the decrease in bearing capacity. Originality/value Through the joint analysis of the bridge stiffness and the loss of external prestressing, the strengthening condition of the bridge after five years of operation is effectively analyzed. The strengthening effect of the external prestressed steel beam strengthening method is analyzed, which can provide a reference for similar bridge strengthening.

To investigate the effects of nano-SiO2 (NS) and polyvinyl alcohol (PVA) fibers on the durability and mechanical properties of cementitious composites subjected to saline freeze–thaw cycling, a series of PVA fiber-reinforced cementitious composite (PFRCC) specimens were prepared using various fiber contents, and a series of NS and PVA fiber-reinforced cementitious composite (NPFRCC) specimens were prepared using various combinations of NS and fiber contents. Durability and fracture toughness tests were subsequently conducted on the specimens after different numbers of saline freeze–thaw cycles. The results indicate that the degradation of material properties can be divided into slow and accelerated damage stages before/after 50 freeze–thaw cycles. The durability and fracture toughness of the specimen series tended to increase, then decrease with increasing NS and PVA contents, suggesting optimum levels. When the PVA fiber content was 0.5%, PFRCC specimens had the best durability after saline freeze–thaw cycles; when the NS and PVA fiber contents were 1.0% and 0.5%, respectively, NPFRCC specimens had the best durability and fracture properties, and the initiation toughness, destabilization toughness, and fracture energy after 100 saline freeze–thaw cycles were 120.69%, 160.02%, and 451.31%, respectively. The results of this study may guide future exploration of the durability and mechanical properties of concrete subjected to freeze–thaw action.

The objective of this paper is to investigate the effect of calcium nitrite (CN) on improving the mechanical properties and microstructures of early-frozen cement paste. Cement pastes containing 1%, 1.5%, 2%, 2.5%, and 3% CN were prepared. One batch of samples was frozen at −6 °C for 7 days and then cured at 20 °C, and the other batch of samples was directly cured at 20 °C as a control. The compressive strength, ultrasonic pulse velocity, and resistivity of all specimens at different target ages were measured under these two curing conditions. The hydration products and microstructures of typical samples were observed using XRD and scanning SEM. The results showed that the addition of 1.5% CN could promote cement hydration and enhance slurry densification, thereby increasing the compressive strength, ultrasonic pulse velocity, and electrical resistivity of the slurry, and positively affecting the early freezing resistance of the slurry. However, when the CN dosage exceeded 1.5%, the internal structure of the slurry was loose and porous due to the generation of a large amount of nitrite–AFm, which negatively affects the properties of the cement paste. In addition, the effectiveness of CN is only limited to temperature environments above −6 °C. Concrete antifreeze suitable for lower temperatures still requires further research.