UHPC三轴受压力学性能研究

为了研究三轴受压下UHPC的受力性能,以围压大小和钢纤维掺量为试验参数,进行20组UHPC试件常规三轴试验,分析UHPC的破坏形态、应力-应变曲线、峰值应力和应变等力学性能。结果表明：围压和钢纤维掺量均为零的试件破坏时呈劈裂破坏,其他试件则呈剪切破坏;围压和钢纤维掺量对应力-应变曲线弹性模量和弹性段曲线形状影响较小;随着围压增大,峰值应力和应变呈不断增大趋势;随着钢纤维掺量增大,峰值应力和轴向峰值应变呈先增大后不变和先增大后减小趋势,环向峰值应变则呈增大趋势。通过对UHPC八面体正应力-体积应变和剪应力-剪应变关系进行分析,基于Drucker-Prager二参数准则,建立了UHPC八面体破坏准则计算方法。     

钢纤维对超高性能混凝土抗弯力学性能的影响

为研究长、短钢纤维对超高性能混凝土（UHPC）受弯力学性能的影响,设计并制作了13组标准养护条件下的UHPC试件,其中3组为掺单一型短钢纤维,其他组均为掺混杂型钢纤维,对其进行立方体抗压及四点抗折试验。结果表明：对于掺加单一型短钢纤维的钢纤维/UHPC,钢纤维体积掺量为5vol%时,抗折强度最大,为19.98 MPa,继续增加钢纤维掺量,抗折强度反而降低;掺混杂型钢纤维的UHPC比单一型的抗折强度高,并且当长、短钢纤维体积掺量分别为2vol%和1vol%时,抗折强度达到最大,为23.55 MPa;钢纤维/UHPC的抗弯力学性能主要受长纤维的影响,短纤维影响较小;长纤维掺量对钢纤维/UHPC的抗折强度、延性以及抗弯韧性有一定影响,但是主要取决于长、短纤维的搭配,长、短纤维体积掺量最优搭配为2vol%和1vol%。     

变形钢筋/超高性能混凝土搭接黏结性能

超高性能混凝土(UHPC)是一种高强度、高韧性和高耐久性的水泥基复合材料。为了研究钢筋/UHPC的搭接黏结性能,进行了21组考虑搭接长度、纤维掺量和配箍率影响的钢筋搭接对拉拔出试验,3组考虑锚固长度影响的钢筋直接拔出锚固试验;试验出现了劈裂拔出破坏和钢筋拉断破坏2种破坏模式;钢筋/UHPC平均黏结强度随钢筋埋置长度的增大而减小,随配箍率的增大而增大;钢纤维掺量的增大,有利于增大对UHPC的约束作用,增加配箍率和适当增大纤维掺量均能减小钢筋/UHPC的临界搭接长度;结合前人的试验结果,拟合得到平均锚固和搭接黏结强度计算公式及临界锚固和搭接长度计算公式,根据混凝土结构设计规范,建立了钢筋/UHPC锚固和搭接长度简化算法,计算结果较为准确。      

钢纤维形状与掺量对UHPC施工及力学特性的影响

选用四种平直及两种端钩钢纤维,研究钢纤维体积掺量、长径比、形状、同形及异形纤维混掺对超高性能混凝土(UHPC)施工及力学性能的影响.通过相关实验得到了UHPC扩展度、抗压强度、抗折强度、能量吸收、断裂能及弯曲应力-挠度曲线;基于弯曲应力-挠度曲线及改进后的规范方法计算了UHPC的弯曲韧性指标;最后,开展了最佳纤维混掺比例的研究.结果表明:纤维掺量每增加0.5％,UHPC扩展度平均降幅为2.72％,抗压强度平均增幅为5.79％.抗折强度、弯曲韧性指数和能量吸收则先增后减(临界掺量为3.5％),断裂能呈上下波动(在3％时达最低值).随着纤维长径比的增大,UHPC扩展度降低,抗压强度、抗折强度、弯曲韧性指数、能量吸收值和断裂能基本呈递增趋势.相同长径比时,端钩形纤维UHPC扩展度、弯曲韧性指数优于平直形纤维,抗压强度、抗折强度、能量吸收、断裂能低于平直形纤维.同形纤维混掺UHPC扩展度、抗压强度稍低于对应的单掺纤维,弯曲韧性、能量吸收、断裂能总体上优于单掺试件;异形纤维混掺UHPC扩展度、抗压强度稍低于单掺试件,抗折强度与单掺试件各有所长,弯曲韧性、能量吸收及断裂能绝大多数优于单掺纤维.UHPC抗折强度变异性高于其抗压强度.单掺和混掺纤维时,UHPC试件的抗压强度、抗折强度综合最优分别为173.53 MPa、44.9 MPa和160.9 MPa、55.72 MPa;纤维混掺最佳组合为18 mm平直形、16 mm端钩形,且两者混掺比例为1:1时,UHPC的综合力学性能较优.     

箍筋约束超高性能混凝土短柱轴压承载力试验研究

通过对10组配置菱形、十字形复合箍筋的超高性能混凝土(UHPC)短柱和1组未配置钢筋的UHPC短柱进行轴压承载力试验,研究了其破坏过程和破坏形态,分析了箍筋间距、纤维掺量和箍筋形式对其轴向应变-轴向荷载曲线和应力-应变曲线的影响。结果表明,箍筋形式的闭合环数和纤维掺量对UHPC短柱的变形能力有一定程度的改善作用。箍筋间距和纤维掺量对试件轴压承载力及相应轴向峰值应变有显著的影响,箍筋间距对轴向峰值应变的影响更大。相同箍筋间距的菱形复合箍筋(DC)较十字形复合箍筋(CC)试件的峰值荷载有所提高。随着箍筋间距的减小,各试件归一化应力-应变曲线上升段斜率增大,但其下降段却表现出更大差异。纤维掺量和箍筋类形对UHPC试件的应力-应变归一化曲线影响较小。考虑箍筋约束效应及纤维约束效应,建立了复合箍筋约束UHPC短柱轴心受压承载力计算公式;计算结果与试验结果比较,吻合较好。     

钢纤维增强粉煤灰地质聚合物单轴受压过程的声发射特性

为探索纤维增强地质聚合物宏观力学行为与细观损伤演化特征之间的关联,对不同纤维体积掺量(纤维与拌合物的体积比)的钢纤维增强粉煤灰地质聚合物复合材料进行了单轴压缩试验.基于声发射技术,对试样压缩过程的声发射行为进行监测,研究了纤维体积掺量对地质聚合物单轴受压破坏行为及声发射特性的影响.结果表明:地质聚合物的强度、延性、声发射波形的上升斜率及平均频率均随纤维掺量的提高而增大,试件破坏形态由脆性灾变逐渐向延性破坏转变;在破坏前期,纤维体积掺量为0及0.5%的试件的声发射撞击率及能量释放率(简称能量率)都保持在较高水平,最终导致试件出现灾变破坏;而2.0%的纤维体积掺量使得声发射撞击率及能量率在应力-时间曲线的拐点处达到峰值,随后缓慢下降,最终导致试件呈现延性破坏;因此,仅依据声发射撞击率及能量率的快速上升来预测灾变破坏的发生,有时可能会出现谎报的情况.     

圆高强钢管UHPC梁抗弯性能研究

以含钢率和套箍系数为参数,开展5根圆高强钢管超高性能混凝土(UHPC)梁和2根圆普通钢管UHPC对比梁的纯弯试验;而后,采用已验证的有限元模型,对套箍系数进行了参数分析。试验研究表明,高强钢管UHPC梁发生延性破坏,组合截面满足平截面假定,受压区高强钢管对核心UHPC的套箍作用应被考虑。较普通强度的钢管,高强钢管能更及时地约束UHPC的横向膨胀。随着套箍系数的增加,钢管混凝土梁达抗弯承载力时,中性轴趋近截面中线,受拉区钢管应力减小,受压区钢管和混凝土的应力则增大。承载力计算分析发现,对于高强钢管UHPC梁而言,现有中国规范GB50936?2014的实用计算方法存在不准确且离散性较大等问题,为此该文提出了新实用计算方法。     

圆钢管玄武岩纤维再生混凝土短柱轴压力学性能

为研究圆钢管玄武岩纤维再生混凝土(BFRRC)短柱的轴压力学性能,以再生粗骨料取代率和玄武岩纤维掺量为变化参数,设计并完成了15根圆钢管BFRRC短柱试件的轴压试验。观察了试件的受力全过程及破坏形态,获取了试件的荷载-位移曲线及荷载-应变曲线,分析了变化参数对圆钢管BFRRC短柱轴压性能的影响,建立了可行的组合截面应力-应变全过程曲线方程。研究表明:试件均发生鼓曲破坏,但核心混凝土在钢管约束下处于碎而不散状态;随着再生粗骨料取代率的增大,试件的耗能性能、延性系数逐渐增大,耗能因子、延性系数提升幅度最高可达1.84%和10.36%,承载力逐渐降低,降低幅度最大达5.03%;随着玄武岩纤维掺量的增大,试件的耗能性能、延性系数逐渐增大,增加幅度最高可达2.97%和4.93%,承载力提高幅度不大;不同的玄武岩纤维掺量下,试件实测的荷载-位移曲线饱满,且具有较长的变形流幅,延性较好。      

铝合金板厚度对硼纤维/环氧复合材料单面修复效果的影响

采用单向硼纤维/环氧复合材料补片真空袋压工艺单面修复不同厚度含中心裂纹铝合金板,测试了修复试件的热学及准静态力学性能,并采用三维有限元模型分析了修复试件的残余热应变和应力强度因子。结果表明：修复试件的弯曲挠度随铝合金板厚度增大而减小;修复试件铝合金板下表面裂纹尖端附近的残余热应变随铝合金板厚度增大而增大,补片上表面的残余热应变则随铝合金板厚度增大而减小,这与有限元分析结果吻合较好。含中心裂纹铝合金板的应力强度因子随铝合金板厚度增大而减小,而单面修复试件的应力强度因子随铝合金板厚度增大而增大。采用相同长度和宽度的单向硼纤维/环氧复合材料补片单面修复后,铝合金板厚度为1. 76 mm修复试件的承载能力保留率为 93. 85 %,而厚度为 10. 20 mm修复试件的只有 84. 01 %;修复试件的刚度得到了完全恢复,等效刚度均大于完好试件的刚度。     

玄武岩纤维布约束高温损伤混凝土方柱轴压力学性能试验

对36个玄武岩纤维布增强聚合物基复合材料（BFRP）约束的高温损伤混凝土方柱和15个不同高温损伤的对比试件进行了轴压试验。试验表明,玄武岩纤维布横向约束能改变高温损伤后混凝土方柱的破坏形态,显著提高混凝土方柱的轴压强度和变形能力。其中三层玄武岩纤维布包裹的200℃、400℃、600℃和800℃高温损伤混凝土方柱轴压强度分别提高了48%、130%、206%和389%,轴向变形分别提高了433%、344%、319%和251%。采用典型的纤维增强聚合物基复合材料（FRP）约束常温未损伤混凝土轴压力学性能的设计模型预测FRP约束高温损伤混凝土的轴压强度和变形时存在较大的偏差。通过构建柱状膜结构静水压力平衡模型和约束混凝土方柱与FRP体积应变能平衡模型,分别改进了FRP约束混凝土方柱轴压极限应力和极限应变计算模型的基本形式。基于该基本形式和试验数据,分别确定了BFRP约束高温损伤混凝土方柱轴压极限应力和极限应变计算中与温度相关的参量,提出了适用于高温损伤混凝土方柱的轴压极限应力和极限应变的设计模型。      

Strength and durability performance of concrete axially loaded members confined with AFRP composite sheets

The performance of concrete columns externally wrapped with aramid fiber reinforced polymer composite sheets is presented in this paper. The confined and unconfined (control) specimens were loaded in uniaxial compression. Axial load and axial and hoop strains were measured in order to evaluate stress–strain behavior, ultimate strength, stiffness, and ductility of the wrapped specimens. Results show that external confinement of concrete by fiber reinforced polymer (FRP) composite sheets can significantly enhance strength, ductility and energy absorption capacity. An analytical model developed earlier by the author to predict the entire stress–strain response of concrete specimens wrapped with FRP composite sheets was applied. Comparison between the experimental and analytical results indicates that the model provides satisfactory predictions of the stress–strain response. The paper also presents the performance of the wrapped concrete specimens subjected to severe environmental conditions such as wet–dry and freeze–thaw cycles. The specimens were exposed to 300 cycles of wetting and drying using salt water. Results show that specimens wrapped with aramid fibers experienced no reduction in strength due to wet/dry exposure, but some reduction was observed due to freeze/thaw exposure.     

Substandard reinforced concrete members subjected to compression: FRP confining effects

The investigation focuses on the effectiveness of fiber-reinforced polymer (FRP) confinement in upgrading ductility and strength of reinforced concrete members under axial monotonic compression. An experimental program is presented that extends available database to address the behavior of old type members with square section, having extremely low concrete strength and potential longitudinal bars’ premature buckling. Reinforced concrete specimens were strengthened by carbon or glass FRP wraps while plain FRP confined concrete specimens were also constructed and tested to evaluate comparatively the confining effects of steel stirrups, FRP wraps, or of dual confinement. The achieved strength, ductility and energy absorption levels of the specimens were quantified to assess the effect of the longitudinal bars. Finally, a handy design-oriented empirical strength model is proposed. According to the proposed approach, no estimation of effective stress or strain at failure of FRP jacket is necessary. The satisfactory accuracy of the predictions of the proposed model is demonstrated through comparison against existing models and over a large database of results on uniform confinement as well as over presented specimens.     

Effect of confinement using steel, FRC, or FRP on the bond stress-slip response of steel bars under cyclic loading

The results of experimental investigation of the local bond stress-slip response of steel bars embedded in confined concrete and subjected to cyclic loading are presented. Different types of confinement and their effect on the bond stress-slip response were evaluated and compared. These included internal confinement by ordinary transverse steel ties or steel fiber reinforcement, and external confinement by fiber reinforced polymer (FRP) composites. Beam specimens with spliced reinforcement at midspan were tested. The test parameters included the size of the steel bars, the ratio of concrete cover to bar diameter, and the amount of confinement. Without confinement, the specimens suffered significant bond deterioration and loss in load resistance in the first or second load cycle after bond splitting. Confining the concrete with transverse steel, steel fiber reinforcement or FRP composites within the splice region increased the bond strength and reduced the bond degradation with the number of loading cycles, leading to significant improvement in seismic performance. The envelope curve of the cyclic bond stress-slip response showed very good agreement with earlier test results obtained under static load conditions and the results predicted using an analytical model of the local bond stress-slip response of steel bars embedded in confined concrete.     

Concrete columns confined with CFRP wraps

Today, there is an increasing use of fiber reinforced polymer (FRP) composites in civil engineering as reinforcement of existing structures. In particular, FRP composites offer higher strength and Young’s modulus than traditional steel devices, also easy handling and installation, excellent resistance to corrosion and they can be plastered after application. When FRP wraps are used to confine compressed pillars, the effective contribution of the composite cannot be accurately evaluated because it begins to work as the stress state of the member grows. In the present paper, some theoretical relationships available in literature and in current international design guidelines, regarding the evaluation of confinement effect on wrapped concrete columns, are analyzed and compared with experimental test results. Based on these considerations, we propose a new formula that is in good agreement with the experimental test results that were produced for the purpose and reported both in the present paper and in a previously published research study. Although empiric, the proposed formula is able to reproduce the structural response of wrapped specimens and the failure mode as observed in the experimental tests.     

高强箍筋约束超高性能混凝土柱轴压性能

通过5根高强箍筋约束超高性能混凝土(Ultra high performance concrete,UHPC)柱及4根普通箍筋约束UHPC柱的轴心受压试验,对其承载力、破坏形态、钢筋应变及应力-应变曲线进行了研究,并结合延性、韧性指数分析了体积配箍率、箍筋强度、箍筋间距及形式对约束UHPC轴压性能的影响。结果表明:所有约束柱均表现为延性破坏,高强箍筋可减轻约束UHPC的破坏程度;高体积率、小间距、形式复杂的高强箍筋约束UHPC,约束效率高,承载力及变形能力提高显著,轴压性能较理想;体积配箍率对轴压性能的影响程度大于箍筋强度;影响体积配箍率变化的因素中,箍筋间距对改善约束性能的贡献最大,依次是箍筋形式和直径;高强箍筋可有效约束UHPC,在提高约束UHPC强度、变形性能及残余承载力方面明显优于普通箍筋;纵筋微曲会加速保护层剥离,密配高强箍筋能有效延迟纵筋屈曲,显著提高约束性能;纵筋微曲会削弱高强箍筋对核心UHPC的约束效果,建议采用高强纵筋与高强箍筋组合。在试验的基础上给出了能较准确预测箍筋约束UHPC柱承载力的计算式。      

Effect of fiber orientation on the structural behavior of FRP wrapped concrete cylinders

In this study, 27 concrete cylinders with a diameter of 152.4 and a height of 304.8 mm were prepared. Among them, 18 cylinders were wrapped using two layers of fiber reinforced polymer (FRP) with six fiber orientations; six cylinders were wrapped using four layers of FRP with fibers in axial or hoop direction only; the remaining three cylinders were used as control. The FRP used was E-glass fiber reinforced ultraviolet (UV) curing vinyl ester. Fifteen coupon specimens were prepared to experimentally determine the tensile strength of the FRP with fibers oriented at 0°, 45°, and 90° from the loading direction. Co-axial compression tests were conducted on the wrapped cylinders and control cylinders. The test results were compared with existing confinement models. It is found that the strength, ductility, and failure mode of FRP wrapped concrete cylinders depend on the fiber orientation and wall thickness. Fibers oriented at a certain angle in between the hoop direction and axial direction may result in strength lower than fibers along hoop or axial direction. A larger database is desired in order to refine the existing design-oriented confinement models.     

非蒸养UHPC-钢板结构界面的受拉和剪切性能试验研究

超高性能混凝土（UHPC）在组合结构和钢结构加固领域具有很大的应用潜力,其中UHPC-钢板界面的力学行为对整体结构组合性能的发挥起到关键作用。该文通过一系列受拉试验和推出试验,对五种常见的非蒸养UHPC-钢板界面形式进行了研究,包括光滑钢板界面、花纹钢板界面、撒入骨料的环氧胶粘剂界面、焊接弯起钢筋界面和栓钉连接界面。试验结果表明UHPC直接粘结强度较低且并不可靠;花纹钢板界面和环氧胶粘剂界面的粘结强度较高,但在受拉或无约束受剪时发生脆性破坏;焊接弯起钢筋界面和栓钉连接界面具有明显的规律性和延性,然而施加约束带来的承载力提升并不明显。该文重点研究了栓钉连接界面的力学性质,发现经典理论和现有规范严重低估了UHPC锥形破坏的承载力,提出更符合实际情况的变角度破坏面假设;此外,该文通过试验结果标定和验证了UHPC中栓钉连接件荷载-滑移关系的指数函数模型。     

Influence of the properties of externally bonded CFRP on the shear behavior of concrete/composite adhesive joints

This paper concerns the strengthening of concrete structures with externally bonded composite reinforcement, and focuses mainly on the influence of the FRP characteristics on the mechanical behavior of the composite to concrete interface. An experimental investigation was conducted, based on the characterization of such bonded assemblies and using a double lap joint shear test. Twelve different series of specimens were studied in order to evaluate the influence of various parameters related to the FRP material (i.e. the use of carbon or aramid and of fiber reinforced systems, the type of manufacturing process, the values of the Young modulus, the thickness of the FRP and the bonded length) as well as several parameters related to the adhesive joint (i.e. the lap joint thickness, the curing conditions and the elastic modulus of the epoxy adhesive). Analyses of the strain and shear stress distributions along the lap joints emphasized significant effects of the FRP properties and epoxy curing conditions on the interfacial strength. In addition, a bond strength model is proposed in the last part of the study.     

Testing and modeling of a novel FRP-encased steel–concrete composite column

A composite column consisting of steel, concrete and fiber reinforced polymer (FRP) is presented and assessed through experimental testing and analytical modeling. The composite column utilizes a glass FRP (GFRP) composite tube that surrounds a steel I-section, which is subsequently filled with concrete. The GFRP tube acts as a stay-in-place form in addition to providing confinement to the concrete. This study investigates the behavior of the proposed composite columns under axial loading. A total of seven specimens were tested. The influence of concrete shrinkage on the compressive behavior of the composite columns was also investigated. Significant confinement and composite action resulted in enhanced compressive behavior. The addition of a shrinkage reducing agent was found to further improve the compressive behavior of the composite columns. An analytical model was developed to predict the behavior of the composite columns under axial loading.