Time-dependent evolution of strand transfer length in pretensioned prestressed concrete members

For design purposes, it is generally considered that prestressing strand transfer length does not change with time. However, some experimental studies on the effect of time on transfer lengths show contradictory results. In this paper, an experimental research to study transfer length changes over time is presented. A test procedure based on the ECADA testing technique to measure prestressing strand force variation over time in pretensioned prestressed concrete specimens has been set up. With this test method, an experimental program that varies concrete strength, specimen cross section, age of release, prestress transfer method, and embedment length has been carried out. Both the initial and long-term transfer lengths of 13-mm prestressing steel strands have been measured. The test results show that transfer length variation exists for some prestressing load conditions, resulting in increased transfer length over time. The applied test method based on prestressing strand force measurements has shown more reliable results than procedures based on measuring free end slips and longitudinal strains of concrete. An additional factor for transfer length models is proposed in order to include the time-dependent evolution of strand transfer length in pretensioned prestressed concrete members.     

多阶段预应力空间网架结构一次张拉计算法

该文提出多阶段预应力空间网架结构一次张拉计算法,将某一阶段第i次对i预应力杆增加的预应力值作为未知数,通过结构力学方法建立该阶段预应力准则方程式并求解之。然后可计算该阶段i次预应力各预应力杆的控制值(包括各预应力杆原有的内力值),后张预应力对先张预应力的影响值,以及网架结构杆件内力和节点变位,直至该阶段最后一次预应力时得到该阶段各预应力杆的设计内力值及网架结构内力和变位。重复这样的分析计算可得到多阶段预应力网架结构预应力杆的最终内力控制值及网架杆件内力和节点变位。采用该方法,预应力杆的内力值在预应力张拉过程中无需调整,可一次性完成预应力施加操作过程。该文中附有算例说明,计算结果表明:提出的多阶段预应力空间网架结构一次张拉计算法是正确、可靠和有效的。     

有粘结和无粘结相结合的预应力FRP筋混凝土梁抗弯承载力研究

纤维增强塑料筋(简称FRP 筋)是一种高强线弹性材料,非常适合用做侵蚀环境下的预应力筋,采用有粘结和无粘结相结合是提高预应力FRP 筋混凝土梁延性的一种新方法。对有粘结和无粘结相结合的预应力FRP 筋混凝土梁的抗弯承载力进行了理论分析和试验研究,基于平衡配筋率定义了有粘结和无粘结相结合的预应力FRP筋混凝土梁的破坏形态,推导了平衡配筋率和相应抗弯承载力的计算公式。为了验证公式的正确性,进行了9 根预应力FRP 筋混凝土梁的试验研究,计算结果与试验结果吻合良好。研究结果表明,在相同配筋的条件下,体内有粘结预应力FRP 筋混凝土梁的承载力最高,体内无粘结预应力FRP 筋混凝土梁的承载力其次,而无转向块的体外无粘结预应力FRP 筋混凝土梁的承载力最低。采用体内有粘结和无粘结预应力相结合,可以改善预应力FRP筋混凝土梁的延性。     

承托八层框架预应力转换梁设计研究

 介绍了某工程主入口处上托八层框架大跨度连续预应力转换梁的设计特点，包括受力分析、预应力筋的布置、截面设计、模拟实际施工的分析及张拉测试等，提出了应注意的问题及相应措施，取得了改善结构受力性能、提高抗裂性及满足正常使用的良好效果，为类似工程的设计提供借鉴。     

Simulating RC beams with unbonded FRP and steel prestressing tendons

A partial interaction based analysis to simulate the behaviour of RC beams with prestressed unbonded tendons is proposed. Unlike bonded reinforcement, the strain developed in unbonded reinforcing tendons under bending is uniform along the length of the member and is thus member dependant. Conventional analysis techniques incorporate correction factors and empirical components in defining the strain developed in both the unbonded and bonded reinforcement. Being semi-empirical, the post-cracking analysis cannot directly simulate the effects of tension-stiffening on the untensioned bonded reinforcement. Accordingly, this paper presents a segmental moment–rotation approach for simulating the behaviour of RC beams with unbonded prestressed reinforcement, such that the mechanics of the approach removes the reliance on empiricisms in defining the reinforcement and unbonded tendon behaviour. Validated against experimental results, the approach is shown to accommodate concrete creep, shrinkage and reinforcement relaxation, thus enabling prestressing losses to be quantified.     

A finite element assessment of flexural strength of prestressed concrete beams with fiber reinforcement

This paper presents an assessment of the flexural behavior of 15 fully/partially prestressed high strength concrete beams containing steel fibers investigated using three-dimensional nonlinear finite elemental analysis. The experimental results consisted of eight fully and seven partially prestressed beams, which were designed to be flexure dominant in the absence of fibers. The main parameters varied in the tests were: the levels of prestressing force (i.e, in partially prestressed beams 50% of the prestress was reduced with the introduction of two high strength deformed bars instead), fiber volume fractions (0%, 0.5%, 1.0% and 1.5%), fiber location (full depth and partial depth over full length and half the depth over the shear span only). A three-dimensional nonlinear finite element analysis was conducted using ANSYS 5.5 [Theory Reference Manual. In: Kohnke P, editor. Elements Reference Manual. 8th ed. September 1998] general purpose finite element software to study the flexural behavior of both fully and partially prestressed fiber reinforced concrete beams. Influence of fibers on the concrete failure surface and stress–strain response of high strength concrete and the nonlinear stress–strain curves of prestressing wire and deformed bar were considered in the present analysis. In the finite element model, tension stiffening and bond slip between concrete and reinforcement (fibers, prestressing wire, and conventional reinforcing steel bar) have also been considered explicitly. The fraction of the entire volume of the fiber present along the longitudinal axis of the prestressed beams alone has been modeled explicitly as it is expected that these fibers would contribute to the mobilization of forces required to sustain the applied loads across the crack interfaces through their bridging action. A comparison of results from both tests and analysis on all 15 specimens confirm that, inclusion of fibers over a partial depth in the tensile side of the prestressed flexural structural members was economical and led to considerable cost saving without sacrificing on the desired performance. However, beams having fibers over half the depth in only the shear span, did not show any increase in the ultimate load or deformational characteristics when compared to plain concrete beams.     

预应力钢绞线网加固混凝土圆柱的轴压性能

开发了一种预应力钢绞线(钢丝绳)网加固混凝土柱新技术, 对24根预应力加固混凝土圆柱和2根未加固对比柱进行了试验, 研究在单调轴向荷载作用下, 钢绞线间距、预应力水平对混凝土柱加固效果的影响。试验结果表明:较对比柱, 预应力加固柱的轴向峰值应力和应变最大提高幅度为83%和95%, 加固效果随预应力水平的提高而提高;随钢绞线间距的减小, 加固效果大幅度提高;钢绞线间距对加固效果的影响大于预应力水平的影响。建立了预应力钢绞线加固混凝土柱峰值应力和应变计算式, 提出了两种应力-应变全曲线关系模型, 计算结果与试验吻合良好, 可为相关研究提供理论支持。预应力钢绞线网加固混凝土柱技术是一种主动、高效、无损加固新技术, 具有推广和应用价值。     

锈蚀预应力混凝土梁承载力及破坏模式研究

随着服役时间增长,侵蚀环境下预应力混凝土梁因受力筋发生锈蚀而造成其延性与承载力降低,严重影响结构的安全使用。为分析侵蚀环境下预应力混凝土梁的承载性能,以集中荷载作用下锈蚀预应力混凝土梁为研究对象,分析混凝土梁锈蚀后各材料性能的劣化与预应力对其承载能力的影响,基于桁架-拱模型给出了桁架作用与拱作用的荷载分配系数,建立了预应力混凝土梁承载力计算模型及破坏模式判别方法,并通过76根预应力混凝土梁的试验数据对建议模型进行验证。研究结果表明:预应力混凝土梁承载力试验值与计算值之比的平均值为1.116,方差为0.033,吻合较好;基于建议分析模型对预应力混凝土梁破坏模式的预测判别与试验梁破坏模式符合程度较高,且该模型能反映预应力混凝土梁随着锈蚀程度增大其破坏模式发生演变这一特征。该文建议的理论模型可用于锈蚀预应力混凝土梁的承载力计算与破坏模式预测分析。     

Compressive-force path and behaviour of prestressed concrete beams

A study of the performance of prestressed concrete beams designed either by conventional design methods or to a physical model proposed in compliance with the ‘compressive-force path’ concept is presented. Results obtained from testing heavily prestressed beams are reported, which show that members designed to the latter concept may be safer than their Code counterparts. The behaviour of these beams has been compared with similar prestressed concrete beams (also designed either to current code provisions or to the proposed method)— but subjected to a smaller amount of prestressing—in an effort to monitor the effect of the level of prestressing on the behaviour of members designed by different methods. In contrast to the current way of thinking, it emerges that an additional amount of prestressing, instead of increasing the shear contribution of concrete, may, in fact, limit the load-bearing capacity of the member itself.     

无粘结预应力CFRP筋钢纤维轻骨料混凝土梁受弯性能试验研究

建立无粘结预应力FRP筋张拉锚固体系,对8根以CFRP筋为非预应力筋的无粘结预应力CFRP筋轻骨料混凝土梁与1根普通混凝土对比试件进行两点对称加载,观察其破坏过程与破坏形态,分析了混凝土种类、预应力度和净跨长度对开裂弯矩、弯矩-跨中挠度曲线、裂缝宽度等受弯性能的影响.从等效轴向刚度思想出发,修正了现有的以钢筋为非预应力...     

Constitutive modeling of reinforced concrete and prestressed concrete structures strengthened by fiber-reinforced plastics

A batch of constitutive models for steel reinforcing bar, prestressing tendon, concrete and fiber-reinforced plastic are proposed for the nonlinear finite element analysis of reinforced concrete structures, prestressed concrete structures, reinforced concrete structures strengthened by fiber-reinforced plastics and prestressed concrete structures strengthened by fiber-reinforced plastics. These material models have been tested against series of experimental data and good agreements have been obtained, which justifies the validity and the usefulness of the proposed nonlinear constitutive models.     

偏心受拉作用下预应力CFRP筋-型钢混凝土构件抗裂试验

为解决我国型钢混凝土桁架转换层拉杆及低层角柱在正常使用阶段易出现大面积拉裂缝的问题,以轻质高强、防腐的碳纤维增强树脂复合材料(CFRP)筋为预应力筋,提出可有效控制裂缝的预应力CFRP筋-型钢混凝土结构体系,并对其偏心受拉作用下的抗裂性能进行系统研究。以预应力水平、偏心距、纵筋直径及型钢翼缘厚度为主要参数制作11个构件,通过自行研发的拉-压转换桁架实现偏拉加载。结果表明:引入CFRP筋后CFRP筋-型钢混凝土构件抗裂度大幅提升,相较于普通偏拉构件,预应力大偏拉构件开裂荷载提高了64.8%~102.3%,预应力小偏拉构件提高了61.7%~117%,其抗裂性能与预应力水平、纵筋直径和型钢翼缘厚度正相关,与偏心距负相关。参照组合结构设计规范,提出构件开裂阶段中和轴的三种位置分布,并推导出开裂荷载公式,与试验值比较吻合度较高,可为其他复合材料筋在预应力偏拉体系的应用提供参考。      

40-Year-old full-scale concrete bridge girder strengthened with prestressed CFRP plates anchored using gradient method

A 17 m long internally prestressed concrete girder taken from a bridge in southern Switzerland was strengthened flexurally using prestressed CFRP plates and then tested to failure. The prestressing level in the plates was 32% of their nominal tensile strength. The load carrying behaviour of this test girder was compared to a reference girder and a girder strengthened using non-prestressed plates. For the anchorage of the prestressed CFRP plates, a method developed at Empa was used. In this method, the force in the CFRP plate is reduced to zero at the plate ends and permanent anchorage system is not required.

The experiments proved the feasibility of anchoring CFRP plates using the gradient method. Repeated loading showed stable tensile strains in the plates and shear stresses between the plates and concrete. Furthermore, prestressing resulted in decreased deflections and strains and a 45% increase in maximum recorded load.     

Experiments into impact behaviour of railway prestressed concrete sleepers

On railway track structures, dynamic impact loads with very high magnitude but short duration are often caused by wheel or rail abnormalities such as flat wheels and dipped rails. The possibility of the large impact loading to cause an extreme failure to an in situ concrete sleeper could be very low about once or twice in the design life cycle. However, to the current knowledge, the behaviour of the in situ prestressed concrete sleepers under the ultimate impact loading has not yet been comprehended, resulting in the design deficiency. A high-capacity drop-weight impact testing machine was thus constructed at the University of Wollongong, in order to evaluate impulsive resistance of in situ prestressed concrete sleepers under impact loads. This paper describes the detail of the high-capacity impact testing machine, as well as the instrumentation, the calibration, and the analysis of failure mode, crack propagation, flexural toughness, and energy absorption mechanisms with respect to railway prestressed concrete sleepers. The impact tests were carried out using the prestressed concrete sleepers manufactured in Australia. An in situ track test bed was simulated in laboratory and calibrated against the frequency response functions obtained from the experimental modal analysis. The experiments using the high-capacity impact testing machine to investigate the impact energy transfer mechanism of the prestressed concrete sleepers are highlighted.     

PBO纤维片材预应力外粘结加固集成新技术

近年采用外部粘结纤维增强复合材料(FRP)来加固结构越来越受到许多研究开发机构和生产单位的重视。结合作者在FRP方面的研究成果和最新开发出的高强高性能的PBO纤维材料，系统地提出了一种更为有效的预应力外粘结加固方法；通过与玻璃纤维、炭素纤维和芳纶纤维等的比较，认清了新型PBO纤维材料的特性；根据既有的试验和研究成果提出了一套完整的预应力加固方法的流程，包括概念、原理和几种降低端部界面应力和新型锚固的措施；室内小型试验和室外大型试验的结果表明，PBO片材预应力技术能够提供令人满意的加固性能。     

An experimental investigation on flexural behavior of RC beams strengthened with prestressed CFRP strips using a durable anchorage system

This paper investigates the effectiveness and feasibility of a prestressed carbon fiber-reinforced polymer (CFRP) system for strengthening reinforced concrete (RC) beams. The proposed prestressing system with a novel anchorage allows the utilization of full capacity of the CFRP strips. Eight small-scale and two large-scale concrete beams strengthened different configuration of prestressed CFRP strips are tested under static loading conditions up to failure. The main parameters considered include the level of prestressing applied, ranging from 20% to 70% of the tensile strength of the CFRP strips, and the use of mechanical anchorages at both ends of the CFRP strips. Thanks to the durable anchorage, the full range of flexural behavior was investigated including post-debonding. The results indicate that the beams strengthened using prestressed CFRP strips exhibited a higher first-cracking, steel-yielding, and experimental nominal moments as the level of prestressing force increased up to a certain point. After analyzing prestress effects in small scale tests, an optimum prestress level for strengthening concrete beams using CFRP strips is proposed and verified in large scale tests.     

离心成型预应力钢纤维混凝土电杆设计试验研究

对研制的呼称高27m离心成型预应力钢纤维混凝土电杆进行了受力性能试验研究。结果表明,在断线工况荷载作用下,电杆的受力表现出良好的整体性,各项性能均满足设计要求;杆身采用预应力和钢纤维双重增强作用,使电杆的抗裂能力提高,变形减小;一旦发生超载运行出现裂缝,裂缝细短且非贯通,卸载后变形回复能力强,残余变形很小;在极限承载时,整杆变形明显,具有良好的延性破坏预兆。同时,预应力钢纤维混凝土电杆在预应力筋放张和杆段对焊组装过程中,避免了普通预应力混凝土电杆在预应力筋挂板和杆段连接钢圈附近混凝土起皮爆裂现象。根据试验结果对比分析,建立了预应力钢纤维混凝土电杆抗裂度、裂缝宽度、挠度和正截面承载力设计计算方法。研究成果为河南省漯河~淮阳220kV线路工程建设提供了重要科研依据,为钢纤维混凝土在预应力电杆中的应用提供了重要参考。     

Feasibility of a novel system to prestress externally bonded reinforcement

The strengthening of concrete structures with externally bonded prestressed reinforcement has advantages compared to a passive system. The load bearing capacity is increased as well as the flexural stiffness. Prestressing allows to exploit the high tensile strength of the fibres of the external reinforcement. A number of systems are already commercially available, however their use is restricted as they are more expensive and show some practical inconveniences in use. In this paper, a novel system is presented and a feasibility study is carried out. The novel system aims to overcome the inconveniences of existing prestressing systems. The system has a reduced number of necessary operations and keeps a small distance between the concrete surface and the laminate. In the initial study presented, feasibility tests are carried out on simplified test systems and the design of the anchorage to the concrete and of the clamping system are discussed.     

Fatigue strengthening of damaged metallic beams using prestressed unbonded and bonded CFRP plates

Bonded fiber reinforced polymers (FRPs) reinforcement systems have traditionally been found to be an efficient method for improving the lifespan of fatigued metallic structures and have attracted much research attention. Nevertheless, the performance of a bonded FRP reinforcement system under fatigue loading is basically dependent on the FRP-to-metal bond behavior. In this paper, a prestressed unbonded reinforcement (PUR) system was developed. The proposed PUR system can be used as an alternative to bonded FRP reinforcement, particularly when there is concern about the effects of high ambient temperatures, moisture, water and fatigue loading on the FRP-to-metal bond behavior. The performance of cracked beams strengthened by the PUR system was compared with that of cracked beams strengthened by the prestressed bonded reinforcement (PBR) system. A theoretical method was developed to estimate the level of prestressing sufficient to arrest fatigue crack growth (FCG). Furthermore, the method was used to examine different passive, semi-active and active crack modes with a loaded, strengthened beam. The mechanism by which a prestressed FRP plate forms a compressive stress field at the vicinity of the crack tip was also examined. Finite element (FE) modeling was conducted and the results were compared with experimental results.     

Prestressed fiber reinforced concrete beams with reduced ratios of shear reinforcement

This paper presents an analysis of the influence of prestress and fibers on the shear behaviour of thin-walled I-section beams with reduced shear reinforcement ratio. Reduction of shear reinforcement in prestressed precast beams can make the reinforcement simpler and may increase the productivity in long line precasting beds. The use of short fibers can improve the shear strength and ductility. Nine concrete beams were built (six with prestressing forces) with three different mixtures: without fibers, with steel fibers, and with polypropylene fibers. Shear reinforcement ratios varied from 0 to 0.225% (geometric ratio). It was noted that prestressing increases cracking strength (both in bending and shear), extends the non-cracked area, and makes the compression struts less inclined. In the case of fiber reinforced concrete beams, control of cracking is more effective and consequently deflections are smaller. Ductility is also increased. Both fibers and prestressing reduce stresses in the stirrups and increase shear strength.