结合压缩薄膜效应对BFRP筋自密实混凝土桥面板工作性能有限元分析

采用非线性有限元方法对英国北爱尔兰Thompson桥中的BFRP筋增强自密实混凝土桥面板进行数值分析,通过对比数值模拟结果和试验结果表明所建立的有限元模型能够较准确地反映BFRP筋增强自密实混凝土桥面板在轮压负载下的工作性能。基于有限元模型的有效性,采用非线性数值模拟对该新型桥面板进行极限承载力和破坏模式进行预测及参数化分析。结果表明,BFRP筋自密实桥面板极限承载力大约为欧洲轮压负载150 k N的6倍(100 t),由于现行桥面板设计规范忽略了压缩薄膜效应的存在而导致桥面板极限承载力设计过于保守,参数化分析发现配筋率和筋材类型对桥面板极限承载力影响不大,而混凝土强度和跨高比是影响桥面板极限承载力的主要因素。     

Behavior of high strength concrete with and without steel fiber reinforcement in triaxial compression

Based on an extensive experimental program, this paper studies the behavior of high strength concrete and steel fiber reinforced high strength concrete under uniaxial and triaxial compression. Triaxial stress-strain relations and failure criteria are used to evaluate the effect of steel fiber reinforcement on the mechanical properties of high strength concrete in triaxial compression, which is found to be insignificant.     

碱式硫酸镁水泥钢筋混凝土梁静力荷载和疲劳性能试验研究

为了研究碱式硫酸镁水泥钢筋混凝土梁的静力和疲劳性能,分别制备了C30、C40和C50三种强度等级的碱式硫酸镁水泥(BMSC)和普通硅酸盐水泥(PO·C)钢筋混凝土梁试件.试验结果表明,钢筋混凝土梁试件跨中挠度随着静力荷载的增加而增加.C40(BMSC)和C50(BMSC)的极限承载力分别为33 kN和38 kN,相比C40(PO·C)、C50(PO·C)的19 kN和21 kN分别提高了73.7％和81％.说明,高强度等级的BMSC试件具有较高的承载能力.在疲劳荷载的作用下,试件挠度值随着疲劳次数的增加呈下降趋势.高强度等级的BMSC试件疲劳循环寿命要多于PO·C试件,且强度等级越高,越为明显.此外,BMSC试件跨中挠度小于PO·C试件.说明,BMSC试件的刚度较大,抵抗变形的能力好.BMSC试件裂缝宽度发展包括三个阶段:快速发展阶段、稳定阶段和破坏阶段.碱式硫酸镁水泥完全能够用于制作钢筋混凝土构件,且性能更加优异,本文为碱式硫酸镁水泥混凝土能够大规模应用于结构工程提供了重要理论依据.     

多因素影响下FRP约束混凝土柱FRP有效极限拉应变研究进展

FRP侧向约束作用使混凝土柱处于三向受压状态,可提高混凝土柱的抗压强度和延性。FRP有效极限拉应变是确定FRP侧向约束力的关键参数,对FRP约束混凝土轴压峰值应力和峰值应变计算模型、应力-应变模型的建立至关重要,其影响因素众多。目前国内外研究者在FRP有效极限拉应变取值方面取得了一些有价值的研究成果,但得到的FRP有效应变系数离散性较大,考虑的影响因素不全面。基于现有研究成果,对FRP有效极限拉应变的影响因素进行了系统分析,指出了当前FRP有效极限拉应变研究存在的不足和未来需深入研究的方向。     

Fiber-reinforced one-part alkali-activated slag/ceramic binders

In the present experimental/numerical study, a combination of ceramic waste and ground-granulated blast furnace slag were used in the preparation of one-part alkali-activated binders. Moreover, the effect of fiber type and content on hardened-state properties and shrinkage was studied under two different curing conditions. In the first stage of this study water absorption, compressive strength, and flexural strength were assessed. Subsequently, the flexural performance of fiber-reinforced binders was simulated and predicted using finite element models under concentrated and distributed flexural loading, respectively. The experimental results showed that fibers improved mechanical properties, and enhancement was governed by fiber type and curing conditions. Moreover, the numerical results indicated that the developed fiber-reinforced binders offer a flexural load-carrying capacity in the range of 10–40?kN/m² and permissible service loads were well below the ultimate capacity.     

冻融环境下钢筋与粉煤灰混凝土的粘结承载力

通过对粉煤灰混凝土试件的直接拔出试验,分析粉煤灰掺量与冻融作用对粘结试件的初始滑移粘结强度、劈裂强度、极限粘结强度及粘结强度-滑移曲线下的面积作为粘结试件韧性评价指标的变化规律.试验结果表明:钢筋与粉煤灰混凝土的粘结强度随粉煤灰掺量的增加而降低;试件粘结韧性随粉煤灰掺量的增加先降低后增强,粉煤灰掺量较大的试件在加载过程中表现出良好的韧性;冻融作用对试件极限粘结强度的影响程度随粉煤灰掺量增大而减小,40%粉煤灰试件在冻融环境下表现出较好的粘结韧性.掺入较多粉煤灰可以在一定程度上减缓粘结试件的冻融损伤.     

冻融环境下GFRP管混凝土柱轴压性能试验研究

通过对21个玻璃纤维增强聚合物（GFRP）管混凝土柱和21个素混凝土柱分别在水溶液和质量分数为3.5 %的氯化钠（NaCl）溶液中进行冻融循环试验及轴压试验，且对冻融后的GFRP管混凝土柱进行了超声波检测，研究冻融环境下不同冻融介质和冻融循环次数对GFRP管混凝土柱轴压性能的影响。结果表明，冻融介质相同的情况下，随着冻融循环次数的增加，质量损失率增大，相对动弹性模量降低，GFRP管混凝土柱内异常点增多，承载力下降，极限应变下降；冻融次数相同的情况下，经盐冻作用的GFRP管混凝土柱轴压极限承载力降低更为明显。GFRP管混凝土柱盐冻循环150次后极限承载力下降了29.45 %，下降量是相同条件下水冻结果的2.19倍。盐冻循环后的GFRP管混凝土柱极限应变小于相同条件下的水冻循环极限应变。     

Effects of stress during heating on strength and stiffness of concrete at elevated temperature

Concrete in structures exposed to high temperatures is practically always heated under stress. Yet, there are few experimental studies in which the concrete was heated under stress and then loaded to the peak, and most of these were performed under uniaxial compression. This paper reports on an experimental study of the effects of different heat–load regimes on the stress–strain behaviour of partially sealed concrete under multiaxial compression, at elevated temperature. The specimens were first heated (stressed/unstressed), then loaded to the peak in multiaxial compression. In contrast with previous experimental research, the results show that concrete heated under relatively low compressive stress has lower strength and stiffness than concrete heated without load. The results suggest that the presence of stress during first heating produces a specific damage, which could be the cause for a major component of the load induced thermal strain (LITS) in concrete.     

Behavior of sisal fiber concrete cylinders externally wrapped with jute FRP

The use of environmentally friendly natural fibers as building materials is benefit to achieve a sustainable construction. This article performs a study on the use of natural jute fibers as reinforcement of concrete and natural sisal fibers in fiber reinforced polymer (FRP) composites as concrete confinement, i.e., sisal fiber reinforced concrete (SFRC) composite column wrapped by jute FRP (JFRP) (SFRC‐JFRP). Uniaxial compression test was conducted to assess the compression performance of the composite columns as axial structural member. A total of 24 specimens were tested. The effects of JFRP wrapping thickness and sisal fiber inclusion on the compressive performance of the composite columns were investigated. Results indicate that JFRP confinement significantly increases the compressive strength and ductility of both PC and SFRC with an increase in JFRP thickness. Besides, the inclusion of sisal fiber further enhances the strength as well as the efficiency of confinement under uniaxial compression. Also, the models for ultimate strength and ultimate strain of PC‐JFRP and SFRC‐JFRP are proposed. POLYM. COMPOS., 38:1910–1917, 2017. © 2015 Society of Plastics Engineers     

Triaxial behaviour of concrete under high stresses: Influence of the loading path on compaction and limit states

The aim of this study is to characterize the behaviour of concrete under high triaxial loading at levels of confinement and axial stress of the order of the GigaPascal. This study is carried out within a more general scope of understanding concrete behaviour under impact. The studied concrete has properties as close as possible to those used in current construction projects. A triaxial press of high capacity is used to characterize the triaxial behaviour of concrete according to various loading paths. Hydrostatic, triaxial, proportional and oedometric tests are performed and show the influence of the loading path on the compaction process. The triaxial and proportional tests show the existence of strain limit states, defining a limit states threshold independent from the loading path.     

Microstudy on creep of concrete at early age under biaxial compression

An interesting phenomena of crack restoration and increasing strength of concrete under biaxial compression creep were described in this paper. A small loading apparatus was prepared and a long work distance optical microscope with variable focus was used for studying the cracks. It was found from the micrographs that these cracks diminished under biaxial compression creep. There were increases in the strength of the creep specimens under the sustained biaxial compression load compared with the free companion ones. Multiaxial compression caused by the early temperature rise inside the mass concrete may strengthen the concrete and reduce the tensile cracks during and after temperature drop.     

Experimental and analytical investigations of creep of epoxy adhesive at the concrete–FRP interfaces

This paper presents the results of experimental and analytical investigations on the long-term behavior of epoxy at the interface between the concrete and the fiber-reinforced-polymer (FRP). Double shear experiments under sustained service load were performed on nine specimens composed of two concrete blocks connected by FRP sheets bonded to concrete using epoxy. The primary investigation parameters included the ratio of shear stress to ultimate shear strength, the epoxy thickness and the epoxy time-before-loading. Loading was sustained for periods up to nine months. We show that the magnitude of shear stress to ultimate shear strength and the epoxy time-before-loading could be the most critical parameters affecting creep of epoxy at the concrete–FRP interfaces. It was also found that the creep of epoxy can result in failure at the interfaces due to the combined effect of relatively high shear stress to ultimate shear strength and thick epoxy adhesive. This can have an adverse effect on the designed performance of reinforced concrete (RC) structures strengthened with FRP. Based on the experimental observations, rheological models were developed to simulate the long-term behavior of epoxy at the concrete–FRP interfaces. It is shown that the long-term behavior of epoxy at the interfaces can be properly modeled by analytically for both loading and unloading stages.     

Micromechanics of ITZ‐Aggregate Interaction in Concrete Part II: Strength Upscaling

Cracking in concrete typically starts in the immediate vicinity around the aggregates, i.e., in the region of the so‐called interfacial transition zones (ITZs), but the process is still not fully understood. Notably, crushing of concrete in compression results in fragments with interesting aggregate surface textures. Part of the aggregate surfaces is cleanly separated from the ITZ, while another part of the aggregate surfaces remains covered with a thin layer of cement paste. This suggests two different types of failure: ITZ‐aggregate separation and ITZ failure; which we here study based on the continuum micromechanics approach of the companion paper (part I). It provides access to both traction vectors acting on aggregate surfaces and three‐dimensional stress states within representative ITZ volumes for loading states below the elastic limit of concrete. When inserting these microtractions and microstresses into Rankine‐type strength criteria for the aggregate‐ITZ interface and for the ITZ, respectively, the micromechanics model allows for upscaling this microscopic failure behavior to concrete‐level criteria for crack onset. Comparing the latter to corresponding experimental results, reveals that under tension‐dominated loading both ITZ failure and ITZ‐aggregate separation appear to be realistic, while under compression‐dominated loading ITZ failure appears as the more likely mechanism. Also, comparing model and experiments shows that the ITZ‐aggregate separation strength amounts to at least half of the internal ITZ cohesion strength, but may be much larger than the latter.     

预应力钢带加固混凝土圆柱轴压性能试验

通过32个加固柱及4个对比柱试件的轴压性能试验,研究了钢带间距、层数对承载力及变形能力的影响。试验结果表明,混凝土圆柱试件经预应力钢带加固后,不仅可以提高柱的轴压承载力,还能使加固柱具有良好的变形能力,可有效改善其轴压性能。在试验研究的基础上,分析了预应力钢带加固混凝土柱轴压受力机理,提出了预应力钢带加固混凝土圆柱极限强度模型,计算值与试验值吻合程度较好。     

BFRP筋轴心受压力学性能试验研究

为详细考察玄武岩纤维增强复合筋(BFRP)的轴心受压力学性能,设计制作了36个受压试件,测试BFRP筋的受压破坏模式、抗压强度、压缩弹性模量、压缩变形率,研究直径、长细比、破坏模式对BFRP筋受压力学性能的影响.试验结果表明,BFRP筋的受压破坏模式分为剪切、胀裂、失稳三种,以剪切破坏为主;受压应力—应变关系为线弹性,...     

不同应变率下橡胶混凝土抗压性能及能量特性研究

对四种橡胶体积掺量(0%、5%、10%、20%)的级配良好橡胶混凝土开展单轴抗压试验,对力学性能和破坏形态方面进行分析,得到了橡胶混凝土综合性能最优时的橡胶掺量,进而对最优掺量组进行不同应变率下的单轴压缩试验,并分析了不同应变率下橡胶混凝土的能量特性。试验结果表明,橡胶混凝土表现为裂而不散的类延性破坏,而非普通混凝土的脆性破坏。随着橡胶掺量的增加,抗压强度大幅降低,但变形能力得到增强,在掺量为10%时,橡胶混凝土的抗压强度达标,变形能力最好。橡胶混凝土受压时能量演化和转化过程是输入能先大量转化为弹性能并储存;接着耗散能转化率开始增加,使试件表面产生大量微裂纹;最后弹性能快速释放,耗散能转化率占比明显提高,从而导致试件整体破坏。另外随着应变率增大,橡胶混凝土的抗压强度和初始弹性模量明显提高,而峰值应变降低,同时输入总能量、弹性能与耗散能均呈现上升趋势,其中弹性能增加更明显。     

持压荷载下剑麻纤维-ECC的毛细吸水性能

为解决地下结构侧墙等混凝土构件开裂渗漏问题,选取了绿色经济型剑麻纤维-工程水泥基复合材料(ECC)取代混凝土提高构件的抗渗性能。根据正交试验进行方案设计得到剑麻纤维-ECC的最优配合比;改进了实时吸水试验装置,实现了持压荷载与水分传输的同步耦合过程;对剑麻纤维-ECC开展了持压荷载下的毛细吸水性能试验,分析了压应力水平(10%~40%)对剑麻纤维-ECC试件的破坏形态、累计吸水量及毛细吸水率的影响规律,并与普通混凝土试件进行对比。结果表明:在10%~40%压应力水平下,随着压应力水平的提高,剑麻纤维-ECC的毛细累计吸水量和平均吸水率均先减小后增大,发生变化的压应力水平阈值为20%;在10%~30%压应力水平下,剑麻纤维-ECC的毛细累计吸水量及吸水率要明显小于同条件时的普通混凝土,剑麻纤维-ECC能够更好地阻碍水分传输,表明对压应力水平为10%~30%的结构而言,剑麻纤维-ECC可明显改善抗渗效果。研究成果可为剑麻纤维-ECC在地下结构侧墙抗渗中的应用提供理论支持。     

再生混凝土多孔砖砌体轴心抗压强度试验

制作再生混凝土骨料替代率为75%、抗压强度达到MU10的再生混凝土多孔砖。对18个再生混凝土多孔砖砌体试件在轴心荷载作用下的试验进行研究。考察再生混凝土多孔砖砌体在不同砂浆强度下的受压性能和极限承载力,分析砌体的受压变形过程和破坏特征。结果表明:再生混凝土多孔砖砌体的受压性能和普通混凝土多孔砖砌体的受压性能基本相似,极限状态时多孔砖砌体被贯通的竖向裂缝分隔成若干独立小立柱,小立柱被压坏,再生混凝土多孔砖砌体轴心抗压强度略小于普通混凝土多孔砖砌体。通过对试验数据的分析,参考现行《砌体结构设计规范》建议的公式,提出了再生混凝土多孔砖砌体的轴心抗压强度计算公式。     

粘性土应力路径试验

利用GDS多功能三轴仪,对南京河西地区原状粘性土进行了常规三轴压缩、减压三轴压缩和等p应力路径的固结不排水三轴试验,探讨不同应力路径下粘性土的变形和强度特性。实验表明:不同应力路径下土的应力应变关系都呈曲线形态相似的非线性应变硬化型,而土的峰值强度和土中孔隙水压力差异明显;相同应力路径试验得到的有效应力路径形态一致,常规三轴压缩试验中有效应力路径呈S形。     

Strength of Elastomeric Adhesive Films in the Triaxial Stress State

Abstract

In model experiments, the triaxial stress state is realized in thin polymer films placed between two solid surfaces and subjected to extension under the action of a force oriented perpendicular to the interface. In this case, the ultimate (fracture) stresses in adhesive joints of uncured flexible-chain polymers of narrow and wide molecular-mass distributions with solids of various nature have been studied as a function of the rate of loading. The results are in good agreement with the data obtained by investigating the durability of the same materials under the conditions of the triaxial stress state. In the region of cohesive fracture the strength of adhesives of narrow MMD is substantially dependent on molecular mass and temperature. In adhesive tear-off, the strength is noticeably affected by the nature of the support, but it is not practically influenced by the molecular mass of the adhesive. The results of the investigation of the strength properties of thin polymer films in the triaxial stress state have been compared with the data obtained for the same polymers subjected to uniaxial extension and shear flow.