Abstract:Passive zone improvement is one of the most effective measures to control the deformation of deep excavation in soft soil. However, the soil mechanical parameters of improvement zone often show strong spatial variability, leading to the uncertainty of overall deformation behavior of deep excavation. Therefore, a three-dimensional model simulating the process of excavation is built by FLAC^3D software, random field models for cement-soil parameters are established based on K-L transformation, then deterministic and probabilistic analyses are conducted. The maximum horizontal displacement of diagram wall and the maximum ground surface settlement in both deterministic and probabilistic analyses are compared. Distribution patterns for the two deformation indicators are fitted, and the influence of coefficient of variation in cement-soil properties on the effect for different passive zone improvement thicknesses is also investigated. The results indicate that:a critical improvement thickness exists while considering the spatial variability of cement-soil; the spatial variability of cement-soil has limited influence on the overall deformation behavior of deep excavation and needs no special consideration in holistic performance evaluation.

Abstract:The foundation pit excavation in urban city construction will form a sevious threat to the nearby existing pipeline. Present analytical methods based on FEM numerical simulation and classical elastic foundation beams theories can hardly consider the pipeline-soil separation phenomenon results from the differences of stiffness and deformation between pipeline and soil mass, which will lead to an unsafety prediction of pipeline displacement induced by foundation pit excavation. Based on Pasternak model of elastic foundation beams, an analytical solution of pipeline deformation induced by adjoining foundation pit excavation considering pipeline-soil separation is proposed. Parameter analysis indicates that the maximum displacement of pipeline increases with increase of the length of pipeline-soil separation portion. Moreover, vertical shear stiffness of the soil mass has a significant influence on pipeline deformation when the flexural rigidity of pipeline is small. The analytical model is validated by comparison among analytical calculation results.

Abstract:To investigate the impact of rock block proportion (RBP) on shear properties and meso-failure mechanism of mudstone soil-rock mixture (SRM). Four groups of sample with RBP ranging from 0% to 80% were prepared for large-scale triaxial tests under three different confining pressures (0.2、0.4、0.6、0.8 MPa). and the change rules of deviatoric stress、volumetric strain and shear strength of sample with different RBPS. Then, a series of numerical analysis were conducted based on particle flow numerical method PFC2D by digital image processing. It is shown that the shear strength increases with the increase of RBP. Unlike other types of soil-rock mixture, sample shows the law of shearing contraction and then dilatation under low confining pressure, and the stronger the final shear dilatation with the higher RBP. The shear contraction decreases under high confining pressure, and the stronger the final shear contraction with lower RBP. With the increase of SRM, the internal friction angle shows slow-fast-slow and "S" growth trend and growth faster between 20%~80% RBP. The cohesion decreases all the time, but the rate of decline is decreasing. Numerical tests indicate that the failure of SRM is tension-shear mixing failure during shear tests. The skeleton-effect of rock blocks becomes more and more obvious with the increase of SRM, the shear zone bypassing the rock blocks and multiple irregular and small shear zone is formed.

Abstract:Biaxial compressive tests of cuboid low liquid-limit clay specimens with holes under plane strain for different loading rates were conducted by use of designed loading and measuring system. The vertical pressures were applied by the testing machine, and the internal and lateral pressures were applied by gasbags. To obtain the full deformation process of clay specimens with holes during the displacement-controlled loading process, a digital camera is used to record speckle images of clay specimens with holes after internal and lateral pressures were applied and before the displacement-controlled loading was conducted. Displacements can be abfained by use of the digital image correlation method. Distributions and evolution of the maximum shear strains were measured by use of least-square fitting of local displacements that can better eliminate noises in displacements. To obtain quantitative maximum shear strains, curve monitored lines were arranged according to positions of clear shear bands, and two straight monitored lines were arranged at both sides of each curve monitored line. The following results were obtained:when the longitudinal strain reaches a certain value, high-angle strain localization bands from the top and bottom of the hole for lower loading rates developed, due to tensile failure. While there are no high-angle strain localization bands for higher loading rates, when the longitudinal strain shear strains of most monitored lines outside shear bands have decreasing tendencies as they approach the hole surface. When the longitudinal strain is higher and monitored lines are far away from the hole surface, the maximum shear strains of most monitored lines in shear bands have decreasing tendencies as they approach the hole surface. The change of the maximum shear strains of monitored lines outside shear bands are complex as they approach the hole surface.

Abstract:The construction scale of concrete face dams is breaking through 200~300 m span in China. It is vital for the seismic performance and safety control of super-high dams to investigate the seismic damage of face slabs. Quadtree cross-scale modelling and non-linear SBFEM-FEM coupling analysis method are introduced to study the fine damage evolution process of high concrete face slab dam under earthquake by combining generalized plastic model of soil, elastic-plastic contact model and plastic damage model of concrete. The results show that the damage zone mainly occurs near the elevation range of 0.6H~0.9H. With the refinement of the grid along the slope, the damage tends to be localized, which can more reasonably reflect the damage phenomenon at the top of the face slab. It is suggested that the size of concrete face along slope direction should be 0.5~1.0 m. Most areas of concrete face can be divided into two or one layers in normal direction, which can satisfy the calculation accuracy. However, for partial area of the slab top, three layers can be considered. The cross-scale analysis method based on Quadtree-SBFEM-FEM achieves the fine damage evolution research of concrete face, which can provide important reference and guidance for the precise location of the weak area of structures under earthquake and the effective analysis of the seismic safety control method.

Abstract:Aiming at the instability and slip in the deep soft foundation section of in-service highway, a quick treatment method of micropile with coupling beam at pile top is proposed. Combined with the actual project of soft foundation reinforcement in Kaiyang highway from mileage K3202+577 to K3202+640, the reinforcement techniques micropile and coupling beam are introduced. The measured deep horizontal displacement and settlement of soft subgrade are greatly reduced after reinforcement, indicating that this method can effectively mifigate sliding. The numerical analysis model for the deformation of deep soft foundation reinforced by coupling beams and micropiles is established by Plaxis 3D. The rationality of the model is verified by comparison with the field measured data, with influences of spacing, row spacing and form of coupling beam on treatment effect investigated. The results show that the lateral displacement increases with the increase of pile spacing, and when the pile spacing exceeds 10 times pile diameter, the soil lateral displacement increases obviously.The addition of coupling beam can reduce the lateral displacement of subgrade soil and optimize the stress of micropiles. The treatment effect of different types of coupling beams is ranked as follows:bidirectional cross coupling beam, followed by unidirectional cross coupling beam, then oblique coupling beam, while longitudinal coupling beam is the least. The conclusions can provide reference for the optimization design of similar projects.

Abstract:In order to investigate the influence of mechanical properties of geogrids on the deformation and long-term stability of reinforced tailings, a rheological model for tailings reinforced by geogrids has been proposed based on the viscoelasto-plastic model with four parameters for mechanical properties of geogrids under long-term low stress loading condition. In this model, the force analysis of reinforced complexus is divided into two stages, i.e., the elastic and plastic states. The time point when plastic deformation (plastic arrival time) occurs is taken as the dividing point of two stages, and then the constitutive relations of the two stages of reinforced complex are put forward. Results show that the viscoelasto-plastic model with four parameters can accurately reflect attenuated creep and stress relaxation characteristics of geogrids. In the first stage, the stress of geogrids decreases with time, leading to the redistribution of micro-stress in tailings. This continues until the tailing yields and moves into the second stage. In this stage, the overall strain of reinforced composite increases as a result of creep of geogrids. The force of reinforced composite soon changes from first stage to second stage and the overall strain is mainly caused by the second stage. The plastic arrival time is significantly influenced by the Kelvin coefficient of geogrids model parameters and the internal friction angle.

Abstract:There were many deficiencies in the accuracy of soil resistivity test technology, and electrode arrangement is an important reason, especially in two-electrode method. If the contact between electrode and soil and the distance of two electrodes can be controlled, electrode influences will be avoided to the greatest extent. Therefore, based on two-electrode test model, zinc-contaminated sand was selected as simulated soil to test its AC resistivity. Electrode arrangement conditions of contact degree and electrode distance were set up, and the influences of them on resistivity measurement were studied combining with COMSOL simulation. The influences of electrode arrangement on resistivity test were studied, and an analytical model and error calculation formulas were established. The results show that test and simulation can verify and supplement each other. The greater contact degree, the more favorable resistivity test, and the influence of contact on resistivity test gradually decreases as it increases. The greater electrode distance, the weaker effect on resistivity test and the slender sample is more beneficial for improving the test accuracy. Conductive model is divided into two transition segments and normal segments in series based on whether it is affected by polarization. The length and polarization of each segment are characterized by radial current density, which decreases with the increase of contact, but slightly affected by electrode distance. Normal resistivity can be obtained by fitting the testing results under different electrode distances using the electrode arrangement model.

Abstract:Based on the improved genetic algorithm and 8 neighborhood boundary tracking method, this paper adopts the real concrete section digital image to obtain binary image and extract the boundary coordinates of coarse aggregate. According to the extracted coordinates, a program was directly compiled to generate ITZ, and the real concrete mesoscopic model was obtained. Then, the obtained concrete mesoscopic model was imported into COMSOL software to simulate the process of chloride ions transporting the concrete in the underwater area of the ocean, and the cloud chart of chloride ions concentration in the concrete at different time was obtained. The results show that the modeling and simulation methods used in this paper are consistent with the experimental results of long-term exposed concrete in sea, which can be used to study and evaluate the durability of concrete under marine environment. The presence of the ITZ will accelerate the diffusion of chloride ions into the concrete. The greater the thickness is, the faster the diffusion process will be. The thickness of the interface area will double, and the apparent chloride ion diffusion coefficient of concrete will increase by 12.3%. By comparing the chloride ion transport simulation results of the real concrete mesoscopic model and the circular random aggregate model generated by parametric generation, it is found that the concentration of chloride ions in the circular random aggregate model is always smaller than that in the real microscopic model.

Abstract:Structural durability reliability assessment should be based on the system and should also reflect the characteristics of the structural properties and dynamic time-varying characteristics. In this paper, based on Bayesian dynamic updating, taking the depth of concrete carbonization as a random variable, the traditional static carbonization model is modified based on the real bridge detection information that reflects the structural personality characteristics. Taking the reliability index as the evaluation standard, a differential equivalent recursive algorithm that can be included in the correlation between components is introduced and embedded in the Bayesian dynamic update framework. The dynamic evaluation model and calculation process of carbonization durability of the system are proposed, and the dynamic reliability calculation method of carbonization durability of complex system is put forward. The calculation program is developed based on MATLAB platform. The dynamic durability evaluation of the reinforced concrete arch bridge was carried out by carbonation simulation tests. Results show that the reliability of carbonization durability of the arch ring and the column increased while the reliability of carbonization durability of bridge deck decreased via the modified model. The durability of the system is lower than the reliability of the component.

Abstract:Sea-crossing bridges in China are mainly built in nearshore island area where wave condition varies spatially. The accurate estimation of the wave height in the bridge site is of great significance for bridge design and construction organization. An artificial neural network (ANN) estimation model of wave height in nearshore island area was developed based on open ocean environmental forecasting data. Pingtan Strait sea-crossing bridge site was selected as the research object. The BP neural network commonly used in the ANN was adopted to train the data provided by the open ocean forecasting station and the measured wave height data in the bridge site area. In order to verify the feasibility of the model, the wave height in the bridge site for 80 consecutive days was estimated. By comparing the results of previous model and the measured data, it is found that the trend of the estimation and the measured value is generally consistent. The root mean square error satisfies the prediction requirements and the ideal prediction effect is obtained. The research showed that the proposed ANN estimation model can use the open ocean forecasting information to effectively estimate the wave height of the nearshore island area for sea-crossing bridge with a relatively simple modeling process.

Abstract:Based on the uniaxial tensile tests of 135 accelerated corrosion steel bars from 45 concrete specimens, quantitative relationships between uniform and non-uniform corrosion characteristic parameters of reinforcement bars and the mechanical and deformation parameters were established. Then the stress-strain model of corroded steel bars based on the above characteristic parameters was proposed. The results show that the correlations between the mechanical/deformation property parameters and the corrosion characteristic parameters of corroded steel bars are relatively high. The above mechanical/deformation property parameters of corroded steel bars decrease linearly with increase of the mass loss ratio, the maximum cross-sectional loss ratio and the relative maximum corrosion depth, while increase with the relative minimum residual cross-sectional area and the area pitting factor. Moreover, the yielding platform of the corroded steel bars deteriorates obviously with increase of degree of corrosion. When the mass loss ratio, the maximum cross-sectional loss ratio and the relative maximum corrosion depth is greater than 20%, 60% and 40%, respectively, or the relative minimum residual cross-sectional area and the area pitting factor is less than 40% and 60%, respectively, the yielding platform of corroded steel bars almost disappears. Both uniform and non-uniform corrosion characteristic parameters are suitable to describe the stress-strain curves of steel bars with slight corrosion, while the non-uniform corrosion characteristic parameters are more suitable for steel bars with severe corrosion.

Abstract:The usage of scrap tires as construction materials in civil engineering is one of the most promising ways in recycling this troublesome waste material. In order to investigate the effect of adding granulated rubber in improving the performance of sand, the compressibility and shear properties of granulated rubber-Fujian sand mixtures was studied by confined compression tests and direct shear tests. The influence of rubber content and water content on the mechanical characteristics of rubber-sand mixtures were studied, and a hyperbolic model was established to predict the shear stress-strain relationship of rubber sand. The research findings are as follows:The compression deformation of rubber-sand mixtures increases with the rubber content, and the compression deformation of saturated mixtures is greater than that of the dry mixtures. The shear stress-horizontal displacement curve of rubber-sand mixtures shows a strain-hardening characteristic. There is no obvious relationship between shear strength and rubber content, and the pore water has great influence on internal friction angle of rubber sand. The internal friction angles of dry samples decrease with the increase of rubber content. The internal friction angles of saturated samples increase with the rubber content. The hyperbolic constitutive model can be used to predict the shear stress-displacement relationship of rubber sand.

Abstract:Understanding of the tensile creep behavior of early-age low-calcium fly ash-based geopolymer concrete can provide reference for stress analysis and crack prediction of tensile concrete. A self-design testing set-up was used to obtain the creep behaviors of concrete such as the creep and creep growth rate under the constant tensile stress. The effects of different initial loading ages (2d, 3d and 4d) on the behavior of tensile creep of early-age low-calcium fly ash-based geopolymer concrete were investigated. The results show that the condition of high temperature sealing curing can make the low-calcium fly ash-based geopolymer concrete reach the strength stability in a short time. The creep characteristics of the low-calcium fly ash-based geopolymer concrete are similar to those of the ordinary cement concrete. The initial creep growth rate is faster, and it decreases rapidly with time. Under the same stress-strength ratio of applied load, the larger the initial loading ages, the smaller the specific creep, and the smaller the creep growth rate at the initial stage of loading. The initial loading age has greater effect on the initial stage of loading compared with the mid and final stages.

Abstract:A new type of through-node of rectangular tube concrete column-SRC beam steel suitable for rapid construction is proposed. With reference to two actual building specifications, four-node test pieces were designed. Based on the pseudo-static load tests, the seismic performance of the joint specimens was discussed, and the seismic behaviors, such as failure phenomena, hysteretic behavior, stiffness, ductility and energy dissipation capacity were analyzed. The test results show that the failure mode of the joint is the compression failure of the concrete at the beam end concrete, coupled with the buckling of steel flange and finally forming a plastic hinge. The hysteresis curve of the specimen is relatively full, and the stiffness is obviously degraded. The ductility coefficient is between 2.33 and 3.8. The equivalent viscous damping coefficient is between 0.283 and 0.4. Compared with the existing nodes, the new type of joint has high rigidity and avoids the problem of difficult core concrete pouring.

Abstract:Shrinkage and creep may increase the deflection of concrete composite beams and therefore affect the mechanical properties of components. In order to investigate the increase pattern of the deflection of concrete composite beams under action of shrinkage and creep, a calculation method for the deflection of concrete composite beams induced by shrinkage and creep was established. The comparison between the proposed method and numerical simulation indicates that the results obtained by the two methods are consistent. Based on this method, the sensitivity of loading age and stress level of concrete composite beams was analyzed. The results show that the deflection of concrete composite beams varies with the loading age when the stress level is different. When the stress level is low, the deflection increases with the loading age, whereas the deflection declines with the rising of loading age. Differential creep at the overlap surface of concrete composite beams is much smaller than the differential shrinkage, and the deflection caused by the difference of the precast concrete and cast-in-place concrete at the overlap surface is mainly caused by difference of shrinkage.

Abstract:The number of steel bars in reinforced concrete shear walls is generally large while the diameter is small, making the workload of connections heavy and the quality difficult to guarantee in assembling. In such a case, the longitudinal reinforcement combined connection is a possible solution. Based on the section design method, the feasibility of the longitudinal reinforcement combined connection is analyzed, which is verified through the quasi-static test of three shear wall specimens. The mechanical behavior of assembled reinforced concrete shear walls with different connecting rebars is analysed by finite element simulation method. The results show that the assembled reinforced concrete shear walls with connecting rebars equalizes to the original reinforcements in terms of area and strength and evenly distributed along the wall length still exhibit good seismic performance. It is indicated that the ductility of shear walls with connecting rebars in edge members may be reduced. Therefore, it is advised to adopt it with caution. It has little effect on the in-plane mechanical properties of shear walls to combine the distributed steel bars by connecting rebars, which is a more reasonable way of combining connections, but the location of the connecting rebars should be evenly distributed along the wall length rather than concentrated near the neutral axis. The out-of-plane mechanical behavior, crack resistance and impact resistance of the assembled reinforced concrete shear wall with longitudinal reinforcements combined by connecting rebars are not considered in this study, which should be taken into account in furthor application.

Abstract:In order to study the effect of the replacement ratio of recycled coarse aggregates and the volume of waste fibers on the creep failure time, creep deformation and creep of waste fiber recycled concrete, a group of ordinary concrete and 4 groups of waste fiber recycled concrete specimens creep tests at 85%, 90% and 95% stress levels were carried out under laboratory conditions. The test results showed that with the increase of the replacement rate of recycled coarse aggregate, the time of creep failure, the creep deformation and the creep degree increase; with the increase of the volume of waste fibers, the creep deformation and creep degree decrease, the time of creep failure increases. The addition of waste fibers can effectively alleviate the degree of compression and creep damage of recycled concrete. The ACI209R creep prediction model was modified based on the replacement rate of recycled coarse aggregate and the volume of waste fiber. The model prediction results were in good agreement with the experimental values.

Abstract:Waves and storm surges induced by strong typhoons pose a serious threat to the safety of sea-crossing bridges. In order to study the damage mechanism of the superstructure and the substructure of the small and medium-sized sea-crossing bridges under the combined action of storm surge and wave, and the affecting law of wave under different water levels during the storm surge on the substructure, the damage patterns of the small and medium-sized sea-crossing bridges induced by the combined action of typhoon storm surge and wave are investigated. Survey results show that the failure patterns of the superstructure of the small and medium-sized sea-crossing bridge under the action of storm surge and wave mainly include beam body dislocation, beam body shedding and beam body fracture. The main failure reasons are that the beam is forced to undergo unconstrained movement. The failure patterns of the substructure are mainly structural deviation, collapse and failure of the girder connection failure, and the main cause is large hydrodynamic effect and impact of superstructure. Subsequently, wave loads on bridge substructure are studied by CFD three-dimensional numerical simulation and an empirical formula of wave impact on pier under storm surge is obtained. Finally, based on the investigation and numerical simulation, some suggestions and measures are proposed for structural design and structural construction.

Abstract:The environmental control system in the subway stations accounts for a relatively high energy consumption. The installed capacity of environmental control system in the subway station facility room in the temperate zone is large, which is the key to achieve energy efficiency. The design of environmental control system in the subway station equipment and management room presents a modular feature, and the problems in its design and operation are still unclear. On-site measurement was carried out in the equipment and management rooms of three underground stations in Kunming in November 2018. Test parameters include the air supply and discharge volume, supply air temperature of air-conditioner, supply and exhaust air temperature of ventilation system in each room. The results show that the supply air volume, exhaust air volume and ventilation times differ greatly from the design value; the ventilation times of air-conditioning rooms is much higher than the limited value. The fresh air temperature at the fresh air shaft can not represent the supply air temperature of each room, and the temperature increase caused by the supply air fan and air duct heat transfer cannot be neglected. The indoor temperature is generally low, and the air temperature in the rooms with people at 20℃~24℃. The air temperature in the air-conditioning room without people is the lowest, and sometimes it is below the lower limit of the specification range. The air temperature in the rooms without air-conditioning is obviously affected by the heat generation, ventilation times and outdoor environment.

Abstract:Polyaluminum chloride (PAC), microbial flocculant (MBF) and granular activated carbon (GAC) could accelerate the formation of aerobic granular sludge and enhance structural stability. The result of scanning electron microscopy(SEM) showed that the external structure of aerobic granular sludge in control group and MBF group was reticulate and loose, while the PAC group and the GAC group had denser surface structure. And fluorescence in situ staining of extracellular polymers (EPS) showed that the distribution of protein and β-polysaccharide in the main structural components of EPS were the same between the control group and MBF group, which were evenly distributed. The formation mechanism of these two groups consisted to the "EPS hypothesis". PAC-enhanced particles formed a "double-layer structure of protein shell-beta-polysaccharide core". In GAC group, high-density proteins encapsulated granular activated carbon. The formation mechanism of these two groups was more in line with the "crystal nucleus hypothesis". Besides, PAC, MBF and GAC all significantly improved the hydraulic shear resistance of aerobic granular sludge. The capacity of anti-hydrolase resistance was PAC group > GAC group > MBF group > control group.

Abstract:For efficient reuse of municipal solid waste incinerator bottom ash (MSWIBA), the physical properties and chemical composition test of regenerated powder from MSWIBA, colloidal mortar strength test by substituting cement of different proportion of regenerated powder were carried out. It is shown that the specific density of regenerated powder from MSWIBA is lighter and the particle size is smaller and more uniform than that of cement. The main chemical composition of the regenerated powder are Si, Al and Ca in the form of oxides, which account for about 70% of the total mass, which is similar to the chemical composition of cement, while the content of SiO2 and Al2O3 is little higher, and CaO is little lower. The compressive and flexural mortar strength show a decreasing trend with the increase of the substituting proportion. The strength activity index still exceeds 55% when the replacement proportion reaches 30%, which indicated that recycled micro-powder had certain gel activity. In addition, replace cement with the regenerated powder has slight effect on the early mortar strength than long-term strength, but the early intensity increased slower with the increase of replacement proportion, and long-term strength decreased significantly.

Abstract:The catalytic mechanism is an important issue in catalytic reaction process. In order to enhance the photocatalytic efficiency of NO removal, we fabricate the Bi/BiOI nanos-phere photocatalyst via the in situ reduction of Bi³⁺ on the BiOI surface. Through a series of characterization analysis, we found the microstructure and optical properties of the catalyst changed significantly after the metal Bi was deposited on the surface of BiOI. Compared with pure BiOI, the prepared Bi/BiOI catalyst exhibited better photocatalytic activity for the removal of gaseous NO. Highly efficient visible light absorption and electron-hole pair separation are induced by surface plasmon resonance (SPR) of metal Bi. Meanwhile, by applying the in situ Diffuse Reflection Fourier Transform Infrared Spectrum (DRIFTS) and ESR spectra dynamically monitor the reaction process of Bi/BiOI photocatalytic oxidation of NO. Combining with the results of free radical capture, the reaction products were revealed from the molecular level. The mechanism of photocatalytic oxidation of NO by Bi/BiOI was proposed based on surface plasmon resonance (SPR).