Ultimate load behaviour of bamboo-reinforced lightweight concrete beams

The classification of seven bamboos studied in accordance to their physical and mechanical properties, the type and method of application of water repellent treatment of bamboo splints and the bond strength between bamboo and lightweight concrete are described in this paper. The results of the experimental analysis of two simplysupported bamboo-reinforced lightweight concrete beams with an overall length of 340 cm and a cross-section of 12 cm by 30 cm width and height, respectively, subjected to two point loads up to collapse are presented. One beam of the same dimensions and concrete mix reinforced with steel was also tested for comparison of the results. Finally a recommendation based on the analysis of the results for the design of such beams is proposed.     

Application of fiber reinforced high performance composites in spun-cast elements

Spin casting is an effective method to produce concrete pylons, masts or pipes. Through the centrifugation process the concrete is compacted and the desired shape, mostly round or ellipsoidal, is obtained. The pre-cast elements made using conventional concrete generally have to be reinforced with steel bars which are susceptible to corrosion. Furthermore, the placement of the steel reinforcement is time consuming and hence expensive and leads to rather thick and heavy structural elements. The application of short fiber reinforced cement (FRC) or mortar, as presented in this paper, is a suitable alternative for such weak-loaded bending elements. Special requirements regarding workability and strength have to be considered. Optimization of cement matrix was achieved with a blend of microfine cement and ordinary Portland cement, improving the rheological properties of the fresh mixture and resulting in a very dense cement matrix with excellent mechanical properties. Reinforcement with different kinds of short fibers of carbon and polyvinylalcohol was studied. Flow properties of the FRC were optimized with regard to the centrifugation process applying a new cone-consistency test. The mechanical properties of conventionally cast specimens and of centrifuged prototypes were investigated.     

Investigation of structural behaviours of laterally restrained GFRP reinforced concrete slabs

The corrosion of reinforcement in concrete bridge deck has been the cause of major deterioration and of high costs in repair and maintenance. Glass fibre reinforced polymer (GFRP) reinforcement is a more durable alternative to steel reinforcement and has higher strength to weight ratio. Due to the low value of elasticity and brittle behaviour of GFRP, the service behaviour of GFRP reinforced concrete structure is critical. However, laterally restrained slabs, such as those in bridge deck slabs, exhibit arching action or compressive membrane action (CMA) which has a beneficial influence on the service behaviour such as the deflection. This paper presents the results of experimental tests and numerical analysis of laterally restrained GFRP reinforced concrete slabs with varying some structural variables. The analysis results are discussed and conclusions on the compressive membrane action in GFRP reinforced concrete slabs are presented.     

钢管混凝土梁柱连接形式及性能分析

钢管混凝土结构是充分利用钢和混凝土各自优点组合而成的一种新型结构形式,近年来在高层建筑中得到越来越广泛的应用。作为一种应用前景广阔的新型结构形式,其梁柱连接直接影响这种新型结构的进一步推广。文章介绍了钢梁-钢管混凝土柱和RC梁-钢管混凝土柱两类连接的主要形式,分析了不同连接形式的受力性能和传力方式。     

FATIGUE OF STEEL REINFORCEMENT BARS IN CONCRETE: A REVIEW

Abstract— During the last few years there has been an intensification of interest in the fatigue performance of steel reinforcement bars in concrete structures. Although fatigue has not proved to be a problem to date, loading cycles are becoming increasingly severe so that the margin of reserve strength is progressively being reduced. In this paper the main parameters associated with fatigue of reinforcement are reviewed with particular attention given to conditions related to highway bridges. Fatigue testing can be either axial in air or by bending of reinforced concrete beams. The latter is experimentally less convenient but simulates more closely the service environment. Endurances can be influenced by type of steel, geometry and size of the bars, nature of the loading cycle, welding and presence of corrosion. The relative behaviour of butt welded joints is considered and it is shown that the reduction in fatigue strength commonly attributed to flaws and to the stress concentrations associated with welds does not always occur for bending fatigue of reinforced concrete beams.     

Hybrid fiber reinforced concrete (HyFRC): fiber synergy in high strength matrices

In most cases, fiber reinforced concrete (FRC) contains only one type of fiber. The use of two or more types of fibers in a suitable combination may potentially not only improve the overall properties of concrete, but may also result in performance synergy. The combining of fibers, often called hybridization, is investigated in this paper for a very high strength matrix of an average compressive strength of 85 MPa. Control, single, two-fiber and three-fiber hybrid composites were cast using different fiber types such as macro and micro-fibers of steel, polypropylene and carbon. Flexural toughness tests were performed and results were extensively analyzed to identify synergy, if any, associated with various fiber combinations. Based on various analysis schemes, the paper identifies fiber combinations that demonstrate maximum synergy in terms of flexural toughness.     

配筋重组竹受弯试件力学性能试验

针对竹材弹性模量低、受弯试件截面刚度小、易发生受拉区竹纤维拉断破坏等不足,提出配筋竹结构的概念,给出了配筋竹结构的关键制造工艺,通过4个重组竹受弯试件的四点弯曲试验,研究了配筋竹试件的破坏形态及承载特性。研究参数包括筋材类型和筋材面积。研究结果表明,配筋试件的承载力及截面刚度显著提高,配置钢筋试件刚度提高效果优于FRP筋试件,由于配筋的增强作用,竹材的受压塑性能够充分发展,配筋试件表现出更加明显的塑性发展过程,其上、下边缘的极限压应变和极限拉应变均显著大于对比试件,截面应变分布符合平截面假定,内部筋材与外部竹材的应变对比表明内部筋材与竹材能够很好地共同工作。配筋竹结构是有效提高竹结构工作性能的一种新型结构。     

Reliability of high performance fibre composites

Over the last decade, reinforcement made of fibre reinforced plastics has emerged as an alternative to steel reinforcement in concrete subject to aggressive environments. The high rupture strengths of these new materials allied with other attractive properties make them particularly interesting as prestressing tendons. In such applications the reliability has to be satisfactory not only in the prestressing operation but during actual use. This makes it necessary to have a material characterization where both the short term and the long term material responses are considered.     

Flexural strength of reinforced concrete T-beams with steel fibers

The ultimate strength of reinforced concrete T-beams reinforced with conventional steel bars and short discontinuous steel fibers are studied. It is found that the presence of steel fibers reduced effectively the deflection, width of cracks and also improved the ductility and flexural rigidity of the concrete beams. Hence, an appreciable increase to the ultimate compressive strain is observed as well as the increase in the ultimate compressive strength. These are reflected by an increase in the value of the compressive block parameters. In addition, an increase in tensile strength is achieved and a rectangular tensile stress distribution is proposed. It was found that a negligible difference in moment capacity between overreinforced and underreinforced concrete beams. Therefore, it may be economical to use more amount of tension reinforcement than that allowed by the codes. Theoretical equations are developed to calculate the ultimate strength of reinforced concrete T-beams taking into account the effect of amount of compression reinforcement and amount of steel fibers. Theoretical equations show good agreement when compared with experimental results.     

High performance composites in spun-cast elements

Spin-casting is an effective fabrication method to produce concrete poles, masts or pipes. Through the centrifugal process the concrete is compacted and the desired shape, mostly round or ellipsoidal, is obtained. The concrete generally has to be reinforced with steel bars which are susceptible to corrosion. Furthermore, the placement of the reinforcement is time consuming and hence expensive and leads to rather thick and heavy structural elements. The application of short-fiber reinforced cement pastes or mortar is a suitable alternative.

Special requirements regarding workability and strength have to be considered. An optimization of the cement matrix was achieved with a blend of microfine cement and ordinary Portland cement, improving the rheological properties of the fresh mixture and resulting in a dense cement matrix with excellent mechanical properties. Reinforcement with different kinds of short-fibers of carbon and polyvinylalcohol (PVA) was studied.

Flow properties of the fiber reinforced cement composite were optimized with regard to the centrifugal process by applying a newly developed cone-consistency test. The mechanical properties of conventionally cast specimens and of centrifuged prototypes were determined and their durability was verified. Workable, high strength, ductile and durable fiber reinforced composites were obtained in spin-cast elements.     

锈蚀钢筋混凝土柱的修正压-剪-弯分析模型研究

随着服役时间的增加,混凝土结构中钢筋易发生锈蚀,引起混凝土结构承载性能下降,严重影响工程结构的继续使用。该文在分析纵筋锈蚀后的屈曲效应、箍筋锈蚀后的约束效应、混凝土和钢筋材料性能劣化的基础上,建议了考虑锈蚀影响的钢筋、混凝土及锈蚀钢筋与混凝土界面粘结性能的本构模型,以锈蚀钢筋混凝土柱为研究对象,对反复荷载作用下钢筋混凝土柱的分析模型进行修正,建立了锈蚀钢筋混凝土柱压-剪-弯交互作用下极限承载力计算模型,并通过21根锈蚀混凝土柱的试验结果对建议分析模型进行了验证。研究结果表明：锈蚀钢筋混凝土柱极限承载力试验值与计算值之比的平均值为1.021,方差为0.014,建议模型极限承载力预测值与试验结果吻合较好,可用于低周反复荷载作用下锈蚀钢筋混凝土柱的承载力分析。     

An experimental study on self-centering and ductility of pseudo-elastic shape memory alloy (PESMA) fiber reinforced beam and beam-column joint specimens

The use of energy dissipating materials both as reinforcement and as external devices for passive control of reinforced concrete structures is a widely accepted method in the construction industry. The present technologies have some limitations related to ageing, durability, corrosion and geometry restoration after strong events (for example, yielding in steel). An alternative reinforcement material such as shape memory alloy (SMA) could offer scope for having large recoverable deformations (self-centering ability), thus improving performance especially after a severe loading has occurred. In addition to enhancing the ductility of reinforced concrete, they can provide a wide range of performances, from full self-centering to high energy dissipation capabilities, as well as high resistance to large strain cycle fatigue and better durability. This paper reports on several experiments and analyses for evaluating the self-centering ability, ductility of beams and beam-column joint achieved by randomly distributed pseudoelastic SMA (PESMA) fibers. Three point loading experiments on prisms and specially designed prototype beam-column joint specimens with steel and PESMA fibers show that the ductility and self-centering ability of the composite with PESMA fibers is far superior to that of the steel fibers.     

Fatigue behaviour of concrete bridge deck slabs reinforced with GFRP bars

GFRP bars are often used for the internal reinforcement of concrete bridge deck slabs as an alternative to traditional steel reinforcements with excellent results in terms of corrosion resistance. Several experiments on bridge decks were conducted to evaluate their structural behaviour but their fatigue performance still needs an adequate experimental investigation. This paper presents the results of an experimental campaign on four full scale concrete bridge deck specimens reinforced with GFRP bars that were designed, constructed and tested to resist cyclic moving loads. Two hydraulic jacks were used to simulate moving concentrated loads. After the cycles, the load was increased to the static failure. The slabs reinforced with GFRP bars showed a better fatigue performance compared to the requests of the European codes.     

Assessment of Eurocode-like design equations for the shear capacity of FRP RC members

Fibre reinforced polymer (FRP) bars represent an interesting alternative to conventional steel as internal reinforcement of reinforced concrete (RC) members where some properties such as durability, magnetic transparency, insulation, are of primary concern. The present paper focuses on the assessment of Eurocode-like design equations for the evaluation of the shear strength of FRP RC members, as proposed by the guidelines of the Italian Research Council CNR-DT 203 [CNR-DT 203/2006. Guide for the design and construction of concrete structures reinforced with fiber-reinforced polymer bars. National Research Council, Rome, Italy; 2006]. Both the concrete and the FRP stirrups contributions to shear are taken into account: the new equations derived with reference to Eurocode equations for shear of steel RC members are verified through comparison with the equations given by ACI, CSA and JSCE guidelines, considering a large database of members with and without shear reinforcement failed in shear.     

On the durability of composite rehabilitation schemes for concrete: use of a peel test

Composites present a potentially cost-efficient and more durable alternative to the use of externally bonded steel plates for the rehabilitation and strengthening of concrete members, due to their high stiffness-to-weight and strength-to-weight ratios, corrosion resistance and overall ease of application in the field. Although a number of demonstration projects have shown the initial viability of such schemes, a number of critical questions still remain as related to short- and long-term durability as well as damage and failure mechanisms. In this paper a modified peel test is used to investigate the durability of the bond between concrete and composites under five different environmental exposure regimes. Two different epoxies were used with E-glass and carbon fibre reinforced composites. Differences in peel force and interfacial fracture energies based on material and environmental influences are discussed and modes of failure are presented.     

Corrosion resistance of steel embedded in sulfoaluminate-based binders

The use of new binders for structural concrete raises questions about the durability of reinforced concrete structures. Calcium sulfoaluminate cement (CSA) has become in recent years an environmentally-friendly alternative to ordinary Portland cement (OPC), but the protection capacity of CSA concrete in relation to the corrosion of embedded steel reinforcement needs to be studied. This paper describes a study on the corrosion behaviour of carbon steel embedded in concrete made with CSA-based binders. It is shown that the pore solution of sulfoaluminate concrete was sufficiently alkaline to passivate embedded carbon steel rebars. Compared to traditional Portland and Portland-limestone cements, CSA cement led to higher carbonation rate of concrete, but to lower corrosion rate of steel in carbonated concrete, likely due to the higher electrical resistivity. Corrosion rate was negligible up to 95% RH at 20 °C. Blending of CSA and OPC cements improved steel passivity and concrete resistance to carbonation.     

轴压荷载下钢管钢筋混凝土损伤状态超声检测研究

在轴压荷载下分别对素混凝土、钢筋混凝土、钢管混凝土和钢管钢筋混凝土的损伤状态进行超声检测,得到不同荷载下超声通过柱的中截面的波形图,定义波形图均方根振幅和均方根振幅比,并分析混凝土裂隙的变化过程。通过对波形图进行快速傅里叶变换得到频谱图,基于固体介质裂化吸收高频声波能量,使超声主频向低频移动的原理,应用频谱图的第一主频及其幅值变化分析钢管内混凝土和钢筋混凝土的损伤演变过程。结果表明：声波波形均方根振幅、均方根振幅比和第一主频及其幅值可以有效反映钢管内混凝土和钢筋混凝土的裂隙变化情况。轴压荷载作用下钢管内钢筋混凝土的损伤呈五段式变化,分别为核心混凝土裂缝产生并发展、混凝土在钢管约束作用下逐步密实、混凝土裂缝迅速扩展、钢管套箍作用加强段和钢管屈曲后构件丧失承载力。通过钢管外壁的应变分析,表明钢管对核心混凝土的约束作用阶段与超声检测判定的混凝土损伤演化过程一致。超声波技术可以准确检测轴压荷载作用下钢管内钢筋混凝土的损伤状态并确定钢管对混凝土套箍作用的发挥阶段。     

不同荷载条件下型钢-钢纤维混凝土组合结构的界面失效机理研究

为了解决型钢混凝土结构中型钢与钢筋相互干扰、混凝土浇筑困难等施工难题,将型钢混凝土结构中的钢筋笼完全或部分替换成钢纤维,形成了型钢-钢纤维混凝土组合结构.完成了36个试件的推出试验和13个试件的四点弯试验,分别研究了型钢-钢纤维混凝土组合结构在轴心力与弯矩作用下的界面失效,分析了不同荷载条件下的内力传递与破坏机理.钢纤...     

Explicit finite element modeling of static crack propagation in reinforced concrete

We propose a methodology to model complex fracture processes in reinforced concrete beams subjected to static loading. The discrete cohesive approach, accompanied by an insertion algorithm, is adopted and a modified dynamic relaxation method is chosen as an alternative solver. The concrete matrix and steel re-bars are modeled explicitly; the connection in between is represented by means of interface elements. Such elements allow for slip of re-bars and transmit forces to the matrix that may generate secondary cracking around the reinforcement. The methodology is validated against three-point bending tests on lightly reinforced concrete (LRC) beams.