Influences of Installment Tensioning Technology on Bearing Capacity of Rectangular Prestressed Concrete Beam

To obtain the relationship between use of installment tensioning technology and the bearing capacity of prestressed concrete beams, tensile and cyclic loading tests are conducted on five rectangular prestressed concreted specimens. Two types of prestressed tensioning methods are adopted in the tests, one uses installment tensioning technology to tension beams on days 4, 12, and 8, 12, respectively, and the other adopts the conventional method to tension the beam on day 28 only. The age of concrete, number of times tensioned, and the prestress value of early tensioning are considered in tests. Results show that use of installment tensioning technology has no effect on the bending failure pattern of prestressed concrete beams, but it reduces prestress loss and increases crack and yield loads. © 2017, Tianjin University and Springer-Verlag Berlin Heidelberg.

Experimental research on refractory performance of reinforced concrete beams strengthened with CFRP sheets bonded with an inorganic adhesive

To meet the requirement of fire endurance for concrete structures strengthened with CFRP sheets, this study develops an inorganic adhesive whose strength at 600 ℃ is not lower than that at normal room temperature. The inorganic adhesive is then used to bond CFRP sheets on reinforced concrete beams in order to strengthen them. The fire protection of the CFRP sheets is done using the thick-type fireproofing coatings for tunnel ( TFCT) and steel structure ( TFCSS) respectively. Four specimens are tested in the furnace together. Specimens are exposed to fire for 1. 5 h in according to the ISO834 standard fire curve,and then naturally cooled for 1 h. In the tests,the largest displacements at the mid-span positions of specimens are only from 1 /1400 to 1 /318 of actual span corresponding to the highest temperatures from 300 ℃ to 470 ℃. After the specimens are naturally cooled to the normal temperature and the fireproofing coatings are then removed,it can be seen that the CFRP sheets keep in a good state,which indicates that CFRP sheets can be tightly bonded on the concrete and work well together with the concrete beams during and after fire. Besides,the tests also verify that the fire performance of TFCT is superior to TFCSS for the strengthened beams.

An Rapid Assessment Method for Bearing Capacity of RC Girder Bridges Based on Residual Strain

In order to realize the in-situ evaluation of reinforced concrete bridges subjected to fatigue for a long time or after earthquake, an evaluation method for cumulative damage of concrete structures based on unloading elastic modulus was proposed. First, according to the concrete stress-strain curve and the statistical relationship between residual strain and cumulative strain, the calculation method of static equivalent strain and residual strain concrete based on unloading elastic modulus and the method for estimating the strength of concrete after damage were proposed. The detailed steps of field test and analysis and the practical damage indicators of residual strain were given. Then, the evaluation method of existing stress and strain of Reinforced Concrete Bridge under dead load and the concept of “equivalent dead load bending moment” were put forward. On this basis, the paper analyzed the root cause of the decrease of bearing capacity of Reinforced Concrete Bridge after fatigue damage, and pointed out that the equivalent strain or residual strain of reinforced concrete increases under the fatigue effect, which led to the decreasing of actual live moment and deformation performance while the ultimate load-carrying capacity remained constant or very little decrease. The evaluation method of structure residual capacity was given, and through comparative analysis of eight T reinforced concrete beams that had been in service for 35 years with the static failure tests, the effectiveness of the method was verified.

CFRP加固损伤钢筋混凝土梁抗冲击性能试验研究

对采用碳纤维增强复合材料(CFRP)加固的不同损伤程度的钢筋混凝土梁进行落锤冲击试验,以冲击高度、加固层数为试验参数分析CFRP加固损伤试件的裂缝发展、破坏形态及抗冲击性能,并研究试件在冲击荷载下的动力性能。结果表明:试件损伤程度越高,其斜向裂缝发展越密集,试件的破坏程度越严重;落锤冲击高度越高,裂缝发展越趋近于跨中局部化;当采用多层CFRP布加固损伤试件时,试件在落锤冲击作用下保持较好的完整性,仅外部混凝土粉碎掉落;随着CFRP布粘贴层数的增加,试件整体产生弯剪破坏,损伤试件加固修复后的抗冲击能力及刚度相较于完好试件得到显著提升,提升幅度为70%~72%,但增加CFRP布粘贴层数对试件的加固修复效果提升作用有限,且加固效果与抗冲击能力并非呈线性关系;基于有效应变计算的结果可知,采用CFRP布加固修复不仅仅具有加固补强的作用,同时能够在一定的程度上改善试件的整体抗冲击性能。

界面缺陷对CFRP加固钢筋混凝土梁抗弯性能的影响研究

为研究界面缺陷对CFRP加固钢筋混凝土梁抗弯性能的影响,制作了3根钢筋混凝土梁对比试件,并进行了四点弯曲试验。同时,采用ABAQUS分析软件建立了非线性有限元模型,并进行了数值模拟分析。结果表明:当存在多个不连续缺陷时,试件上的CFRP提前发生剥离,剥离荷载下降了20.77%,导致试件的破坏模式发生改变,虽然极限荷载仅降低2.46%,但挠度降幅高达15.37%,延性下降显著;界面缺陷附近易产生应力集中,促使多裂缝均衡发展,导致裂缝间距减小;界面缺陷影响CFRP剥离起始发生位置,更易引起CFRP沿试件宽度方向贯穿剥离。

CFRP-PCPs复合筋加固钢-混组合梁负弯矩区抗裂试验与理论计算

为解决钢-混组合梁负弯矩区混凝土板易开裂而造成刚度减弱的行业难题,以预制的CFRP-PCPs(carbon fibre reinforced polymer-prestressed concrete prisms)复合筋作为中支座的腹筋,并以复合筋的数量、预应力张拉水平和截面尺寸为主要参数设计制作了五根复合筋钢-混组合连续梁和一根普通组合连续梁,研究其在单跨单点荷载下负弯矩区的抗裂性能。结果表明:复合筋能显著增强中支座截面刚度,对负弯矩区的裂缝控制能力提升显著;与普通组合梁相比,复合筋梁的开裂荷载提高近30%,裂缝数量与裂缝宽度减少近50%;复合筋面积是提高裂缝控制能力的主要因素,预应力水平和截面尺寸对开裂荷载的影响较弱;复合筋的开裂荷载与预应力水平、面积正相关,其中预应力水平是影响复合筋效用时长的关键因素。基于现有理论和试验数据,本文提出CFRP-PCPs复合筋钢-混组合梁负弯矩区开裂荷载和复合筋的开裂荷载计算公式,计算结果与试验值较为吻合,可为复合筋在钢-混组合梁桥实际工程中应用提供参考。

低预应力CFRP板加固RC梁模型试验研究

为研究低预应力CFRP加固RC梁的力学性能,设计进行了CFRP加固方法的对比模型梁试验。通过对直接粘贴、机械张拉粘贴锚固成为低应力CFRP板及粘贴加热伸长后回缩成为低预应力CFRP板这3种方法,将同规格的CFRP板粘贴在混凝土试验梁表面,通过分级加载试验,获得不同施工方法的CFRP板加固后开裂荷载、极限荷载、裂缝发展及荷载-挠度曲线等。试验结果表明:3种施工方法下进行机械张拉低预应力CFRP板加固对原梁抗剪承载力性能提高较大。在正常使用阶段,机械张拉低预应力CFRP板加固梁和加热张拉低预应力CFRP板加固梁出现斜裂缝的荷载等级高于直接粘贴CFRP加固梁出现斜裂缝的荷载等级。机械张拉低预应力CFRP板加固梁和加热张拉低预应力CFRP板加固梁产生的裂缝宽度小于直接粘贴CFRP板加固梁产生的裂缝宽度。相对最明显的是机械张拉低预应力CFRP板,裂缝一直维持在较低水平。粘贴锚固的低预应力CFRP加固方式操作相对简单,可应用于桥梁维修或拆除前临时加固工程等。

体外预应力CFRP筋加固钢筋混凝土梁的理论与数值分析

随着服役时间的增长和车辆荷载的增加,老旧的钢筋混凝土桥梁面临承载力不足、变形超限等问题,采用体外预应力CFRP筋对其加固是一种有效的解决方法。采用有限元分析软件ABAQUS对某跨度24 m的铁路桥梁进行数值模拟与参数分析,其中,根据不同的CFRP预应力筋的直径(31 mm、43 mm、61 mm)和预应力大小(250 MPa、500 MPa、750 MPa、1000 MPa、1250 MPa),获得模型梁的开裂弯矩、梁底钢筋屈服弯矩以及梁开裂时的跨中变形。将《混凝土结构设计规范》(GB 50010—2010)等现行规范的理论计算结果与数值模拟结果进行对比,发现两者吻合良好,误差在15%以内,从而验证了规范中钢筋混凝土梁开裂弯矩计算公式、正截面承载力计算公式以及跨中挠度计算公式对于体外预应力CFRP筋加固钢筋混凝土梁的适用性与准确性,为实际工程加固设计提供参考。

CFRP-PVC管内置螺旋筋混凝土柱抗震性能试验研究

聚氯乙烯(PVC耐腐蚀)、碳纤维(CFRP轻质高强)和螺旋筋(强塑性)进行组合可以使材料特性充分互补从而为混凝土提供约束,有效改善钢筋混凝土的力学性能。为研究CFRP-PVC管内置螺旋筋混凝土柱的抗震性能,以螺旋筋直径、螺旋筋间距和剪跨比为变化参数,完成9个试件的低周反复加载试验。结果表明:绝大部分试件发生了CFRP-PVC复合管断裂的压弯破坏,螺旋筋在达到极限荷载后在受压区提供明显约束作用,从而提高抗震性能;所有试件破坏延性在2.76~4.35之间,极限层间位移角均大于1/30,满足规范要求;螺旋箍筋直径的减小、螺旋筋间距的增加和剪跨比的增大均使试件的抗剪承载力和延性降低,但对刚度退化影响不大。基于ABAQUS软件建立的精细有限元模型能较准确地预测CFRP-PVC螺旋筋复合约束混凝土柱在恒定轴力和反复水平力下的承载力和内部混凝土开裂行为,参数分析表明:轴压比、混凝土强度显著影响试件的承载力和变形能力,纵筋强度和直径对试件承载力和屈服后的刚度影响较大,并提出混凝土强度与轴压比限值的匹配关系。所提出的压弯承载力计算方法在较低轴压比和使用普通强度等级混凝土时具有良好的适用性。

Prediction of Load Carrying Capacity of Corroded Reinforced Concrete Beam

A novel method for prediction of the load carrying capacity of a corroded reinforced concrete beam (CRCB) is presented in the paper. Nine reinforced concrete beams, which had been working in an aggressive environment for more than 10 years, were tested in the laboratory. Comprehensive tests, including flexural test, strength test for corroded concrete and rusty rebar, and pullout test for bond strength between concrete and rebar, were conducted. ne flexural test results of CRCBs reveal that the distribution of surface cracks on the beams shows a fractal behavior. The relationship between the fractal dimensions and mechanical properties of CRCBs is then studied. A prediction model based on artificial neural network (ANN) is established by the use of the fractal dimension as the corrosion index, together with the basic information of the beam. The validity of the prediction model is demonstrated through the experimental data, and satisfactory results are achieved.

预应力CFRP绞线和钢筋混合配筋混凝土梁的抗弯性能

为了探索碳纤维复材(CFRP)绞线在混凝土结构中的应用,采用CFRP绞线作为预应力筋,开展了预应力CFRP绞线和钢筋混合配筋混凝土梁的抗弯性能试验研究。试验结果表明:平截面假定适用于混合配筋的混凝土梁;配置预应力CFRP绞线可以显著提高混凝土梁的抗弯承载力,相比于普通钢筋混凝土梁,混合配筋混凝土梁的开裂荷载、屈服荷载和极限荷载分别提高了74%~146%、27%~48%和29%~50%,但混合配筋混凝土梁的延性明显降低;在混凝土压碎失效模式下,预应力水平和预应力CFRP绞线数量对延性影响并不明显;GB 50608—2020《纤维增强复合材料工程应用技术标准》中预应力纤维复材(FRP)筋混凝土梁的抗弯承载力计算公式可以应用于预应力CFRP绞线和钢筋混合配筋混凝土梁的抗弯承载力计算。

Axial Bearing Capacity of Short FRP Confined Concrete-filled Steel Tubular Columns

The bearing capacity of FRP confined concrete-filled steel tubular （FRP-CFST） columns under axial compression was investigated. This new type of composite column is a concrete-filled steel tube （CFST） confined with fiber-reinforced polymer （FRP） wraps. Totally 11 short column specimens were tested to failure under axial compression. The influences of the type and quantity of FRP, the thickness of steel tube and the concrete strength were studied. It was found that the bearing capacity of short FRP-CFST column was much higher than that of comparable CFST column. Furthermore, the formulas for calculating the bearing capacity of the FRP-CFST columns are proposed. The analytical calculated results agree well with the experimental results.

CFRP布抗剪加固预加载RC梁的力学性能研究

【目的】为了研究端部锚固方法和预加载对碳纤维复材(CFRP)加固钢筋混凝土(RC)梁抗剪性能的影响,【方法】进行了4根矩形RC梁的剪切试验,分析了失效模式、荷载-挠度曲线、特征荷载、箍筋和CFRP应变的发展,最后,采用既有的CFRP加固RC梁抗剪承载力计算方法对文章试件的抗剪承载力进行了预测。【结果】试验结果表明,在可靠的端部锚固下,采用U形CFRP条带加固可以显著提升RC梁的抗剪性能;抗剪加固效果受CFRP条带的端部锚固方法影响很大,机械锚固显著优于CFRP布压条锚固,相比于未加固梁,采用机械锚固的加固梁抗剪承载力提高了约60%,而CFRP布压条锚固的抗剪承载力仅提高了9.8%;预加载虽然明显降低了加固梁的开裂荷载,但对抗剪承载力几乎没有影响。【结论】研究结果可为CFRP布抗剪加固RC梁的工程应用提供参考。

CFRP-PVC管约束珊瑚海水海砂混凝土柱轴压性能试验研究及有限元分析

为研究碳纤维增强聚合物-聚氯乙烯复合管(CFRP-PVC)约束珊瑚海水海砂混凝土(CSSC)柱的轴心受压性能,对6个试件进行轴心受压加载试验和有限元参数分析,重点研究了PVC管内、外表面CFRP层数和脱空缺陷对该组合柱轴压性能的影响。结果表明,CFRP-PVC管可以有效约束CSSC,使其处于三轴受压状态,当内、外壁均粘贴CFRP时可以有效减缓突然破坏的特征。相同情况下,管内壁粘贴1层CFRP时具有更好的延性和承载力,管外壁粘贴2层CFRP时约束应力提高系数高达121.5%,脱空缺陷仅对延性有显著影响。基于ABAQUS软件建立了CFRP-PVC约束CSSC柱有限元模型,准确还原了组合柱在轴向荷载作用下的破坏过程和CFRP损伤形态。当组合柱套箍系数小于0.3或长径比大于14.54时,荷载-位移曲线会失去强化段,提高外层CFRP层数会小幅提高强化段刚度。提出了适用于CFRP-PVC约束CSSC的承载力计算方法,误差为1.2%。

Shear capacity of reinforced concrete columns strengthened with CFRP sheet

This paper discusses the results of tests on the shear capacity of reinforced concrete columns strengthened with carbon fiber reinforced plastic (CFRP) sheet. The shear transfer mechanism of the specimens reinforced with CFRP sheet was studied. The factors affecting the shear capacity of reinforced concrete columns strengthened with CFRP sheet were analyzed. Several sug-gestions such as the number of layers, width and tensile strength of the CFRP sheet are proposed for this new strengthening technique. Finally, a simple and practical design method is presented in the paper. The calculated results of the suggested method are shown to be in good agreement with the test results. The suggested design method can be used in evaluating the shear capacity of reinforced concrete columns strengthened with CFRP sheet.

CFRP-不锈钢夹层管混凝土柱轴压试验及承载力计算方法

为了提升薄壁不锈钢结构的承载性能和耐久性,在不锈钢管内外壁粘贴碳纤维增强复合材料(Carbon Fiber-Reinforced Polymer,CFRP)复合约束海水海砂混凝土,制得一种CFRP-不锈钢夹层管海水海砂混凝土结构。以CFRP粘贴层数和方式为变化参数,对18个短柱试件进行了单调轴心受压试验,观察了试件的受力破坏过程及形态,获取了荷载-位移曲线和材料应变分布数据,分析了试件轴压力学性能的变化规律。结果表明:内外贴CFRP能有效地提高结构的承载能力和变形能力;无CFRP和内贴CFRP试件的破坏形态均为剪切破坏,但破坏形态会随着外贴CFRP层数的增加向腰鼓破坏转变;在相同粘贴方式下,试件的承载能力、变形能力和耗能能力随着CFRP层数的增加呈非线性提高;在相同粘贴层数下,内贴CFRP的力学性能提升效果优于外贴CFRP,粘贴1层和2层CFRP时,内贴试件的极限承载力相较于外贴试件能再提高5.6%和6.7%,同时内贴1层CFRP的试件的耗能能力与外贴2层CFRP的相当;应变分析显示,在试件破坏前,CFRP与不锈钢协同工作良好。文中最后基于极限平衡法,考虑不锈钢的应变硬化效应,提出了CFRP-不锈钢夹层管混凝土柱极限轴压承载力的计算公式,并收集了73个样本的数据进行验证,发现计算值与试验值较为吻合。

季节温度循环后CFRP板-混凝土界面承载力模型研究

通过双面剪切试验,研究了-20~60℃的温度循环作用后CFRP板-混凝土界面极限承载力的变化规律。对比分析了未经季节温度循环和经历50、100、150次季节温度循环后材料的宏观力学性能、界面破坏特征和界面极限承载力等变化规律。结果表明:随着季节温度循环次数的增加,环氧树脂胶由于后固化作用,其抗拉强度、抗剪强度及弹性模量升高,伸长率有所降低;季节温度循环次数的增加对混凝土和CFRP板的力学性能影响较小;C30、C50混凝土强度界面破坏形式不同;随着季节温度循环次数的增加,界面极限承载力增加。同时,在Naubauer&Rostasy模型、Chen&Teng模型和陆新征模型的基础上,提出了季节温度循环后界面极限承载力修正模型,结果发现模型计算值与试验值吻合较好。

Bearing capacity of concrete filled bidirectional FRP tube subjected to axial compression

External confinement by fiber reinforced polymer (FRP) is an efficient technique to increase the bearing capacity and ductility of concrete. To better study the mechanical behavior of bidirectional FRP confined concrete, the yield criterion of bidirectional FRP is presented based on the static equilibrium condition in this paper, and a model for calculating the bearing capacity of bidirectional FRP confined concrete is established. The model can capture the character of bidirectional FRP confined concrete. Effects of the confinement effect coefficient, the unconfined concrete strength and the material properties of FRP on bearing capacity are analyzed. Results show that each parameter has different effects on the bearing capacity of bidirectional FRP confined concrete.

Influence of steel corrosion to flexural behavior of coral aggregate concrete beam

To study the flexural behavior and calculation model,8 coral aggregate concrete(CAC)beams with different types of steel were designed.The flexural behavior of CAC beam was tested.The failure mode,bearing capacity,the maximum crack width(ws)and average crack spacing(lm)were studied.A calculation model for the bearing capacity of CAC beam was proposed.The results indicated that with the steel strength increased,the cracking moment(Mcr)and ultimate moment(Mu)of CAC beam increased,and the development of the ws gradually slowed,which effectively inhibited the formation of cracks and improved the flexural behavior of CAC beam.For CAC structures in the ocean engineering,it is recommended to use organic new coated steel to extend its effective service life.In addition,considering the influence of steel corrosion,a calculation model for the Mcr,Mu,lm and ws of CAC beam was established.

CFRP约束方钢管再生混凝土柱轴压性能研究

为了提高废弃混凝土的利用率,同时弥补再生混凝土导致的性能下降,提出了在传统的方钢管混凝土外包裹CFRP复合材料、同时采用再生粗骨料取代天然粗骨料的CFRP方钢管再生混凝土结构。为了分析该结构的受力与变形性能,进行了9根短柱、10根长柱的轴压试验。试验表明:CFRP可限制钢管与混凝土的变形,提高钢管再生混凝土柱的承载力。最后进行了承载力计算分析,在已有研究的基础上得到了该结构构件的承载力计算式,该计算式在本项目研究中已进行计算验证,结果吻合良好。