Micromechanics-based analysis for predicting asphalt concrete modulus

The elastic modulus of asphalt concrete （AC） is an important material parameter for pavement design. The prediction and determination of elastic modulus, however, largely depends on laboratory tests which cannot reflect explicitly the influence of the microstructure of AC. To this end, a micromechanical model based on stepping scheme is adopted. Consideration is given to the influence of interracial debonding and interlocking effect between the aggregates and asphalt mastic using the concept of effective bonding. Tests on asphalt mixture with various microstructures are conducted to verify the proposed approach. It is shown that the prediction is generally in agreement with experimental results. Parameters affecting the elastic modulus of AC are also discussed in light of the proposed method.     

Seismic Analysis for Rigid-Framed Prestressed Reinforced Concrete Bridge in Tianjin Light Railway

The seismic analysis of a rigid-framed prestressed concrete bridge in Tianjin Light Railway is performed. A 3-D dynamic finite element model of the bridge is established considering the weakening effect caused by the soft soil foundation. After the dynamic characteristics are calculated in terms of natural frequencies and modes, the seismic analysis is carried out using the modal response spectrum method and the time-history method, respectively. Based on the calculated results, the reasonable design values are finally suggested as the basis of the seismic design of the bridge, and meanwhile the problems encountered were also analyzed.Finally, some conclusions are drawn as ：1 ) Despite the superiority of rigid-framed prestressed concrete bridge,the upper and lower ends of the piers of the bridge are proved to be the crucial parts of the bridge, which are easily destroyed under designed earthquake excitations and should be carefully analyzed and designed; 2) The soft soil foundation can possibly result in rather weakening of the lateral rigidity of the rigid-framed bridge, and should be paid considerable attention; 3) The modal response spectrum method, combined with time-history method, is suggested for the seismic analysis in engineering design of the rigid-framed prestressed concrete bridge.     

Experimental study on complete stress-deformation curves of larger-size concrete specimens subjected to uniaxial tension

INTRODUCTION Failure of reinforced and pre-stressed concrete structures is initiated in many instances by cracking of plain concrete. The post-cracking softening behavior of concrete subjected to tension has been realized for many years (Li et al., 1993). However, the resistance of cracked concrete is generally ignored in conven-tional design practice. Until recently it has become increasingly evident that it is necessary to account for the post-cracking resistance of concrete when per-f…     

Analysis and behaviour of FRP-confined short concrete columns subjected to eccentric loading

Fibre-reinforced polymer （FRP） composites were widely utilized in civil engineering structures as the retrofit of reinforced concrete （RC） columns. To design FRP jackets safely and economically, the behaviour of such columns should be predicted first. This paper is concerned with the analysis and behaviour of FRP-confined RC circular and rectangular short columns subjected to eccentric loading. A simple design-oriented stress-strain model for FRP-confined concrete in a section analysis was first proposed. The accuracy was then proved by two test data. Following that, a parametric study including amount of FRP confinement, FRP strain capacity, unconfined concrete strength and shape of column section is provided. Some conclusions were obtained at the end of the paper. The work here will provide a comprehensive understanding of the behaviour of FRP-confined concrete columns. The simplicity of the model also enables a simple equivalent stress block to be developed for direct use in practical design.     

Analytical modeling for corrosion-induced cover cracking of corrosive reinforced concrete structures

An analytical model for predicting the corrosion-induced cracking of concrete cover of reinforced concrete(RC) structures was developed.The effects of influence factors such as practical initial defects,corrosion rate,strength and elastic modulus of concrete on the corrosion-induced cracking of concrete cover were investigated.It was found that the size of practical initial defects was the most effective factor.Therefore,improving the compactness of concrete is an effective way to improve the durability of RC structures.It was also demonstrated that the accelerated corrosion tests may be unfavorable in the study of the relationship between cracking time and crack width.     

Strengthening an in-service reinforcement concrete bridge with prestressed CFRP bar

Carbon fiber reinforced polymer （CFRP） bars were prestressed for the structural strengthening of 8 T-shaped reinforced concrete （RC） beams of a 21-year-old bridge in China. The ultimate bearing capacity of the existing bridge after retrofit was discussed on the basis of concrete structures theory. The flexural strengths of RC beams strengthened with CFRP bars were controlled by the failure of concrete in compression and a prestressing method was applied in the retrofit. The field construction processes of strengthening with CFRP bars including grouting cracks, cutting groove, grouting epoxy and embedding CFRP bars, surface treating, banding with the U-type CFRP sheets, releasing external prestressed steel tendons-were introduced in detail. In order to evaluate the effectiveness of this strengthening method, field tests using vehicles as live load were applied before and after the retrofit. The test results of deflection and concrete strain of the T-shaped beams with and without strengthening show that the capacity of the repaired bridge, including the bending strength and stiffness, is enhanced. The measurements of crack width also indicate that this strengthening method can enhance the durability of bridges. Therefore, the proposed strengthening technology is feasible and effective.     

Simulation of top-down crack propagation in asphalt pavements

Top-down crack in asphalt pavements has been reported as a widespread mode of failure.A solid understanding of the mechanisms of crack growth is essential to predict pavement performance in the context of thickness design,as well as in the design and optimization of mixtures.Using the coupled element free Galerkin (EFG) and finite element (FE) method,top-down crack propagation in asphalt pavements is numerically simulated on the basis of fracture mechanics.A parametric study is conducted to isolate the effects of overlay thickness and stiffness,base thickness and stiffness on top-down crack propagation in asphalt pavements.The results show that longitudinal wheel loads are disadvantageous to top-down crack because it increases the compound stress intensity factor (SIF) at the tip of top-down crack and shortens the crack path,and thus the fatigue life descends.The SIF experiences a process "sharply ascending—slowly descending—slowly ascending—sharply ascending again" with the crack propagating.The thicker the overlay or the base,the lower the SIF; the greater the overlay stiffness,the higher the SIF.The crack path is hardly affected by stiffness of the overlay and base.     

Natural Frequency of Full-Prestressed Concrete Beam

Dynamic tests of three bonded and two unbonded full-prestressed concrete beams were carried out.The purpose was to seek the relation between prestressing force and natural frequency.Test results indicate that the frequency of prestressed concrete(PSC)beam increases with the increase in prestressing force approximately.The results are different from the dynamic characteristics of isotropic material beam subjected to compressive axial force which were put forward by Clough et al.The reason is that the beams were considered as isotropic,homogeneous,linear elastic material in the traditional analysis method.However,more accurate results are required in the analysis of frequency of PSC beam.The constitutive mode of PSC member is analyzed based on microstructure of concrete in this paper.The orthotropic linear elastic mode is used to analyze the relation between dynamic frequency and prestressing force of concrete beam,at the same time the equivalent stiffness of prestressed tendon relating to the prestressing force is added to the bending deformation stiffness of the beam.The analytical value agrees well with the test result,indicating that the current analysis method in this paper is feasible to full-prestressed concrete beam.     

Strain Monitoring and Stress Analysis of a Post-Prestressed Tunnel Liner

The strain monitoring and stress analysis of a new type of post-prestressed tunnel liner were carried out. The instrumentation block of the tunnel liner, with the dimensions of 12.06 m in length, 6 500 mm in diameter, and 650 mm in thickness, was post-prestressed with the unbonded tendons, each of which consists of 8 pieces of double-looped strands and the axial spacing of the tendons is 500 mm. Concrete strain meters, rebar meters, load cell and zero-stress meters were installed for the strain monitoring. The tensioning loads were applied incrementally in three cycles (50%, 77% and 100%) at the concrete age of 28 d and the tensioning work lasted for 187.1 h. Strain readings were taken before and after each cycle during tensioning period and at the specified time interval after tensioning period. It is found that concrete creep developed over tensioning period is 30% of total strain and 41.5% of elastic strain respectively. Prestress force in the unbonded tendon and concrete stress in the liner were evaluated according to the observed strain variations. Both of them are time-dependent, and about 5.3%, 8.3% and 9.0% of the prestress losses are observed at the age of 1 d, 30 d and 60 d respectively after stressing. The distribution of prestress in the liner is relatively uniform and meets the design requirement.     

Numerical Calculation of Concrete Slab Response to Blast Loading

In the present paper, a dynamic plastic damage model for concrete has been employed to estimate responses of a reinforced concrete slab subjected to blast loading. The interaction between the blast wave and the concrete slab is considered in 3D simulation. In the first stage, the initial detonation and blast wave propagation is modelled in 2D simulation before the blast wave reaches the concrete slab, then the results obtained from 2D calculation are remapped to a 3D model. The calculated blast load is compared with that obtained from TM5-1300. Numerical results of the concrete slab response are compared with the explosive test carried out- in the Weapons System Division, Defence Science and Technology Organisation, Department of Defence, Australia.     

小板块水泥混凝土路面温度应力影响因素分析

针对近年来农村公路水泥混凝土路面结构中出现的主要病害问题,介绍了小板块水泥混凝土路面温度应力的计算原理,以通用有限元软件ANSYS为基础,采用三维有限元法,对小板块水泥混凝土路面的温度应力进行分析,探讨了小板块水泥混凝土路面结构层参数对其温度应力的影响规律,为目前农村公路小板块水泥混凝土路面结构的设计应用及其后续理论研究提供了有益的参考.     

水泥混凝土路面断板的原因及其防治

水泥混凝土路面承重强度高,稳定性好,适用于重载交通道路,在有重载要求和短线路段有较好的发展前景。但断板病害对道路的正常使用和使用寿命十分不利,也给路面养护、修复带来极大的困难,必须采取方法加以防治。     

南方高温地区水泥混凝土路面早期裂缝防治研究

研究了三种外掺料对路面混凝土早期开裂性能的影响。通过理论分析, 将水泥路面的早期开裂机理归结为混凝土具有塑性收缩、自收缩和干缩等早期收缩性能; 采用室内试验分别研究了聚丙烯纤维、钢纤维和粉煤灰对混凝土早期收缩性能的改善效果。研究结果表明, 聚丙烯纤维和粉煤灰能够大幅度降低混凝土的早期收缩值, 钢纤维的改性作用则不明显。建议为全面评价不同混凝土的长期开裂性能, 应增加开裂后的疲劳和冲击试验研究。     

FAILURE SURFACE AND CAPACITY OF REINFORCED CONCRETE MEMBERS SUBJECTED TO BIAXIAL TENSION

A computer program is developed for the nonlincar analysis of prestressedconcrete and nonprestressed concrete members subjected to combined biaxial bending andaxial load.The strength-interaction diagrams and failure surface are obtained.Thestrength design formulae are proposed.     

公路弯道处桥头跳车的路面力学计算分析(英文)

为了分析汽车行驶在公路弯道桥头过渡段处时发生的桥头跳车现象对路面受力的影响,建立了四轮双轴载重车辆动力运动模型. 忽略汽车行驶过程中的一切阻尼及减振作用的影响,得出公路桥头跳车处的车辆动力运动方程. 根据车轮在桥头位置阶段的变化,充分考虑其边界受力条件,按位置阶段变化总结得出车辆动力运动方程的解析解. 结合工程实例计算,分析和研究得出汽车在高等级公路弯道路桥过渡段处经历桥头跳车过程中后轮对路面产生的荷载压力方程,此方程可推广应用到复杂应力对路面的影响. 结果显示桥头搭板处容易出现应力过大、集中现象,需要进行补强设计.     

关于农村公路建设问题的思考

结合某地农村公路的实地调查,主要是通过水泥混凝土路面的使用实例,浅谈农村公路建设中存在的问题。分析公路线形设计、路面结构选择及公路养护问题中需考虑的因素及其建议。根据水泥路面投入使用获得的经济效益及社会效益,推荐在农村公路建设中使用水泥混凝土路面。     

Evaluation of accelerated deterioration in NAPTF flexible test pavements

Previous research studies have successfully demonstrated the use of artificial neural network （ANN） models for predicting critical structural responses and layer moduli of highway flexible pavements. The primary objective of this study was to develop an ANN-based approach for backcalculation of pavement moduli based on heavy weight deflectometer （HWD） test data, especially in the analysis of airport flexible pavements subjected to new generation aircraft （NGA）. Two medium-strength subgrade flexible test sections, at the National Airport Pavement Test Facility （NAPTF）, were modeled using a finite element （FE） based pavement analysis program, which can consider the non-linear stress-dependent behavior of pavement geomaterials. A multi-layer, feed-forward network which uses an error-backpropagation algorithm was trained to approximate the HWD back- calculation function using the FE program generated synthetic database. At the NAPTF, test sections were subjected to Boeing 777 （B777） trafficking on one lane and Boeing 747 （B747） trafficking on the other lane using a test machine. To monitor the effect of traffic and climatic variations on pavement structural responses, HWD tests were conducted on the trafficked lanes and on the untrafficked centerline of test sections as trafficking progressed. The trained ANN models were successfully applied on the actual HWD test data acquired at the NAPTF to predict the asphalt concrete moduli and non-linear subgrade moduli of the medium-strength subgrade flexible test sections.     

减水剂对混凝土早期抗裂性的影响(英文)

为了提高混凝土的早期抗裂性能,使用了不同种类的减水剂.采用平板法和温度应力试验法,研究了2种不同分子结构的聚羧酸减水剂和1种缓凝型萘系减水剂对混凝土早期抗裂性能的影响.结果表明,与萘系相比,聚羧酸减水剂并非一定能提高混凝土的早期抗裂性能,这与聚羧酸的分子结构有关.早期塑性抗拉强度和动弹性模量较高,将有利于抗裂,在此前提下,较低的水化热也有利于避免裂缝的产生.温度应力试验法与平板法结合可综合反映混凝土的干缩应力和水化热温度应力随时间的演化.     

预应力混凝土柱下条形基础受力分析探讨

以一柱网尺寸(9m×9m)较大的建筑物为例,说明预应力混凝土柱下条形基础的受力分析,并与相同条件下的钢筋混凝土柱下条形基础进行技术、经济比较,得出预应力混凝土柱下条形基础较钢筋混凝土柱下条形基础的优缺点。     

预应力混凝土转换梁设计与施工

通过工程实例,介绍运用中国建筑科学研究院编制的结构设计软件PKPM计算预应力混凝土转换梁的方法,绘制了施工图,并讨论了预应力混凝土转换梁施工中应注意的关键问题.