Large Span Concrete Box Beam Construction Technology

Combined with the engineering practice, the method of large span post tensioned prestressed box beam construction technology was introduced, which can provide the reference for the similar project.     

A Modified Chloride Ion Diffusion Model in Concrete

Based on the analysis of influencing factors chloride ion diffusion in concrete and the simplified model induced by Fick's second law, a chloride ion diffusion model was established, in which , the influences of water-cement ratio, humidity, time, temperature, chloride ion binding capacity of concrete and freeze-thaw cycle are taken into consideration. The Validity of the new model is verified through the comparison of results between the proposed model and the existing analytic /numerical model. Compared with the analytic model, the proposed model can consider the seasonal change of humidity and temperature,and the time-variant effect of freeze-thaw cycle.Compared with the numerical model, the computational efficiency of the proposed model is higher.     

Boundary Element Method for Time-dependent Chloride Diffusion and the Chloride Distribution in Concrete

The process of chloride diffusion in concrete is time-dependent.The boundary element method (BEM) with a time-dependent diffusion coefficient is presented for chloride diffusion in concrete based on the suitable transformation of variables.The fundamental solution of the partial differential equation for time-dependent chloride diffusion in concrete is developed,and the compensation length of the diffusion field is defined as well as the compensation coefficient.The scheme of BEM with a time-dependent diffusion coefficient is developed.Two examples are given to demonstrate the accuracy and efficiency of the presented method and the rationality and the importance of the compensation length for the method.     

Microcracking and Chloride Penetration of Concrete under Uniaxial Compression

A non-destructive method of evaluation of specific crack area was used to characterise microcracking in concrete prisms during uniaxial compression loading and unloading. Chloride profiles were also measured after the same concretes were exposed to chloride environment. The relation among microcracking, stress-strength ratio and chloride penetration of concrete were analysed based on the experimental results. Under compressive load, the apparent chloride diffusion coefficient decreases until about one third of the ultimate load. Further increase of compressive load improves the chloride diffusion coefficient again. There is a close link between microcracking and stress-strength ratio. The index of specific crack area can be used to indicate the microcracking and study chloride transport behaviour into concrete under the influence of microcracking.     

Service Life Prediction of Concrete Structures under Chloride Environment

The problems rendered from the conventional time marching, referring to stepwise time marching scheme (STMS) adopted in boundary element method (BEM) for chloride diffusion in concrete structures, were investigated, and a new time marching, referring to initial time marching scheme (ITMS) in BEM, was developed for evaluation of service life of the concrete structures under chloride environment. Results of the numerical examples show that the ITMS-BEM proposed can eliminate domain integral and simplify the computational model, so that the stability in iteration process can be improved, resulting in better efficiency and accuracy, compared with the STMS-BEM. It can also be concluded that the dimensions of the diffusion of chloride can affect the service life of the concrete structure significantly, which should be taken into account in structural design.     

3-D Dynamic Response of Reinforced Embankment and Track System Subjected to Moving Traffic Load

An analytic method was used to study the dynamic response of track system on reinforced embankment subjected to moving traffic load. Based on Biot's dynamic poro-elastic theory, dynamic analytical model of track system and reinforced embankment was established. The rails were described as an infinitely long Euler beam subjected to moving axle loads and sleepers were represented by continuous mass. Reinforced embankment embedded between track system and poro-elastic subsoil was modeled as a transversely isotropic layer. Subsoil was fully saturated poroelastic medium governed by the Biot's theory. Using the Fourier transform, the governing equations were then solved analytically in the frequency-wave-number domain. The time domain responses were obtained by the inverse Fourier transform computation. The influences of the train velocity, thickness of reinforced layer, load amplitude and reinforcement's ratio on the displacement responses of rails were carefully investigated. Computed results show that the vertical displacement of rails increases with the increase of load speed initially and decreases later. The displacement of rails with reinforced embankment is less than the ones without reinforced embankment. The displacement of rails increases with the increase of load amplitude and decreases with the increase of reinforcement's ratio.     

Approach to Determine Value of Variable Vapor Permeability of Autoclaved Aerated Concrete

The sorption isotherms and vapor permeability of autoclaved aerated concrete are obtained with the static gravimetric method and the cup method at 15℃, 25℃ and 35℃, respectively. The approaoh to determine value of variable permeability has been discussed. Meanwhile, it has been compared with the fixed value in Thermal Design Code for Civil Building (GB 50176-93). The results show that temperature dependence of the sorption isotherms and vapor permeability of autoclaved aerated concrete is small within test temperature range. Moreover, for adsorption process in medium or whole RH range, the average permeability obtained from our method is almost the same as that in the Code. This method can be extended to other porous building materials to obtain accurate permeability throughout whole RH range for both adsorption and desorption processes.     

矿粉粒度及反应温度对高磷鲕状赤铁矿制备碳化铁的影响

基于对50根弯曲破坏钢筋混凝土圆柱低周反复试验结果的分析,建立了完整滞回环的数学表达式并推导得出等效阻尼比计算模型;以双柱墩桥梁为例,说明了建立桥梁整体结构等效阻尼比与墩柱端部塑性铰等效阻尼比关系的方法。研究表明,完整滞回环数学表达式较好地反映了弯曲破坏钢筋混凝土圆柱的滞回特性,得到的等效阻尼比模型计算结果与试验结果符合较好;采用建立的桥梁整体结构等效阻尼比与墩柱端部塑性铰等效阻尼比的关系进行pushover分析更能反映实际情况。采用等效阻尼比模型算得的目标位移与基于Rosenblueth模型和Kowalsky模型算得的位移之间存在较大差距。     

蠕变模型对约束钢梁抗火性能分析的影响

钢材在高温和荷载作用下产生明显蠕变变形,影响火灾中结构的变形和受力性能。现有的蠕变模型较多,但没有一个广泛适用的蠕变模型。不同的蠕变模型对钢结构抗火分析结果有很大影响。为了量化蠕变模型对约束钢梁抗火性能分析的影响,对5种常用的蠕变模型进行了对比分析。采用编写的约束钢梁计算程序,分别计算5种蠕变模型下约束钢梁的抗火性能并与试验数据进行对比。结果表明,采用Norton蠕变模型的计算结果与试验数据吻合最好。最后对影响约束钢梁抗火性能参数进行了研究,研究发现,Harmathy蠕变模型对约束梁抗火性能分析结果影响最大;不同蠕变模型对不同荷载比、约束刚度下的约束钢梁抗火性能影响程度均不同。