冻融环境下CFRP-粘土砖抗剪性能试验研究

为研究冻融循环对CFRP与烧结粘土砖界面的影响,通过一种简单易行的单面剪切试验方法测定了冻融作用下粘贴了碳纤维布的粘土砖试件的应变值,得到荷载-滑移曲线,分析了冻融循环作用下CFRP与烧结粘土砖粘结性能的变化规律,建立了冻融循环作用下CFRP-粘土砖界面荷载滑移退化模型.通过对比分析,模型预测值与试验值吻合较好.     

氯盐干湿循环作用下嵌入式CFRP筋与混凝土界面粘结性能研究

海岛工程建设中,潮汐作用下的海水干湿循环是影响纤维增强复合材料(FRP)加固混凝土结构耐久性能的主要因素之一。针对碳纤维增强复合材料(CFRP)筋嵌入法加固混凝土结构,通过CFRP筋从混凝土块中的拔出试验研究高性能纤维增强水泥基复合材料(HPFRC)和环氧树脂胶作为加固粘结材料时,海水干湿交替循环作用下嵌入式CFRP筋与混凝土界面的粘结性能,并对其破坏模式、极限粘结承载力进行分析。结果表明:环氧树脂胶作为粘结材料时,随干湿交替循环次数的增加,极限粘结承载力略有下降;而HPFRC作为粘结材料时,干湿循环90 d内的极限粘结承载力得到持续增长,可以达到作为粘结材料时极限承载力的70.3%,因此HPFRC可以作为粘结材料应用于潮汐环境下的FRP筋嵌入法加固海工混凝土结构中。     

干湿循环下硫酸盐对水泥基孔隙材料的侵蚀破坏综述

硫酸盐对水泥基孔隙材料的侵蚀破坏是影响其耐久性的主要因素之一.相较于完全浸泡在硫酸盐溶液中,水泥基孔隙材料在干湿循环作用下其破坏更严重且破坏机理更复杂.过去的几十年内,学者们主要将研究重点放在硫酸盐化学侵蚀上,对干湿循环作用下水泥基孔隙材料的硫酸盐物理侵蚀破坏研究较少.处于硫酸盐环境中的水泥基孔隙材料主要遭受硫酸盐结晶物理侵蚀、化学侵蚀和干湿循环三重作用,而水泥基孔隙材料遭受硫酸盐结晶破坏的机理研究一直是个难点.综合分析了国内外学者对干湿循环下,水泥基孔隙材料硫酸盐结晶侵蚀破坏的机理和微-宏观试验现象,以及影响侵蚀的主要因素:硫酸根离子浓度、温度和相对湿度的等的影响;并从微观机理方面对水泥基材料力学性能和力学行为方面进行总结分析.最后,对干湿循环作用下水泥基孔隙材料遭受硫酸盐结晶侵蚀破坏的研究进行若干展望.     

干湿交替硫酸盐环境中混凝土耐久性研究进展

阐述了干湿交替硫酸盐环境中混凝土硫酸盐侵蚀类型、机理模型、损伤评判准则和试验制度方法等,剖析了混凝土硫酸盐侵蚀机理与耐久性退化特征.分析结果表明既有研究存在许多不足,主要体现为研究成果与实测结果偏离较大、模拟试验侵蚀机理失真、试验制度参数选取缺乏依据,既有侵蚀模型多基于经验获得,尚未提出合理的混凝土硫酸盐侵蚀评判准则和量化方法.此外,未探讨模拟试验和真实服役环境下混凝土硫酸盐侵蚀间的相关性,试验结果的合理性和真实性值得商榷.     

CFRP与混凝土界面黏结滑移性能试验研究

对5根钢筋混凝土梁拉区粘贴CFRP,压区粘贴角钢,分析了不同受荷条件及损伤程度对CFRP和混凝土界面黏结滑移性能的影响;并绘制了CFRP荷载-应变曲线及应力-滑移曲线.结果表明:无损加固梁的极限黏结应力较卸荷加固梁分别提高了10.7％、29.3％,较持荷加固梁分别提高了36.0％、42.5％.相同损伤条件下,持荷加固梁较卸荷加固梁的极限黏结应力最大提高22.9％.说明试验梁受损情况及受荷条件对CFRP-混凝土界面黏结滑移性能影响较大,同时结合试验数据提出了基于界面黏结强度和刚度的损伤滑移本构关系,最后与试验值对比分析,二者吻合较好.     

干湿循环-硫酸盐侵蚀下矿物掺和料对混凝土耐久性的影响

研究了干湿循环-硫酸盐侵蚀耦合作用下,纯水泥混凝土、粉煤灰掺量10%和20%的粉煤灰混凝土及矿粉掺量15%和30%的矿粉混凝土的质量损失率、抗压强度和氯离子扩散系数的演变规律.采用X射线衍射、扫描电子显微镜、能谱分析研究了侵蚀后试件的微观形貌与物质元素组成.结果表明干湿循环-硫酸盐侵蚀耦合作用下混凝土性能发展分为2个阶段:即侵蚀初期性能的提高段与随后性能的劣化段:矿物掺和料的掺入不能改善混凝土的抗干湿循环-硫酸盐侵蚀性能.     

低温干湿循环条件下砂浆抗硫酸盐侵蚀性能试验研究

研究了不同水胶比水泥砂浆试件在低温干湿循环条件作用下的抗硫酸盐侵蚀性能.试验制作了0.36与0.5两种水胶比的普通硅酸盐水泥、中抗硫水泥以及矿粉-硅灰复掺的水泥砂浆试件,检测了试件标养28 d后的孔结构及各试件在(5±1)℃的3％Na2 SO4溶液中干湿循环后的强度、动弹性模量变化情况,对砂浆在低温干湿循环条件下的抗硫酸盐侵蚀性能进行了评价,并分析了检测指标间的相关性.结果表明:低温干湿循环条件下,0.5水胶比砂浆抗硫酸盐侵蚀性能低于0.36水胶比砂浆,抗硫酸盐侵蚀性能随着水胶比的降低而提高;低水胶比砂浆复掺矿粉-硅灰后抗低温干湿循环条件下的硫酸盐侵蚀性能提升明显;两个水胶比砂浆的相对动弹模均与抗压强度高度相关.     

再生混凝土抗硫酸盐侵蚀机理及可靠性分析

通过不同质量掺量的再生粗骨料(0%、30%和50%)和再生细骨料(0%和15%)来制备再生骨料混凝土(RAC),研究硫酸盐干湿循环试验中RAC质量和相对动弹性模量的劣化规律,并评判不同掺量下RAC抗硫酸盐侵蚀的能力。结果表明:随硫酸盐干湿循环次数的增加,相对动弹性模量先增大后减小,质量损失率则先下降后上升;干湿循环80 次时,再生粗骨料混凝土的相对动弹性模量最大损失达60%,质量最多增长0.43%。此外,利用一元Winner分布建立可靠度模型,对硫酸盐干湿循环下RAC的演变规律进行评判,并结合扫描电镜(SEM)试验对侵蚀后的RAC微观形貌进行分析。     

外贴式CFRP加固钢筋混凝土梁耐火性能研究综述

CFRP作为一种新型加固材料已有很长一段历史。外贴式CFRP加固技术(Externally Bonded Reinforced,简称"EBR-CFRP")是最常用的CFRP加固技术之一。但由于CFRP对温度较为敏感,基体在温度超过其玻璃化温度(Tg)后发生分解,导致CFRP各项力学性能急剧退化并丧失,这严重影响了该技术在建筑工程领域里的推广和发展。结合近十年关于EBRCFRP加固钢筋混凝土梁耐火性能的相关研究,就CFRP和CFRP-混凝土接触面的高温特性,EBR-CFRP加固梁耐火性能的试验研究、数值分析和预测方法等多个方面的研究成果进行了系统总结,并提出了进一步的研究方向。     

冻融循环作用下嵌入式BFRP筋与混凝土粘结性能研究

为探究高寒地区冻融循环作用对嵌入式FRP加固混凝土界面粘结耐久性能的影响,针对玄武岩纤维增强复合材料(BFRP)筋嵌入式加固混凝土结构,通过采用环氧树脂胶(EP)和高韧性纤维增强水泥基复合材料(ECC)作为粘结材料的嵌入式FRP筋-从混凝土中拉拔试验,分析冻融循环作用下嵌入式FRP-混凝土相邻层之间的破坏模式和粘结退化机理,研究冻融循环次数、粘结材料、混凝土基体性能对粘结性能的影响。结果表明:不同侵蚀条件下试件破坏模式基本表现为胶基体与混凝土剪切破坏、胶基体劈裂破坏、毗邻混凝土-胶界面混凝土薄层剪切破坏、ECC基体与混凝土剪切破坏、FRP与ECC界面粘结破坏五种破坏模式,试件的极限粘结承载力随着循环次数的增加而呈不同程度的下降,50次循环内对试件粘结性能的影响较小,ECC试件相比环氧树脂胶试件,粘结承载力低,但表现出良好的延性。     

Wetting behavior and mechanism between hot metal and carbon brick

Wetting behavior between hot metal and carbon brick is the first step to explore the erosion of carbon brick. In order to investigate the wetting behavior, wetting experiments between hot metal (with carbon content 2.87, 3.47, 4.12, 4.51, and 4.96 mass%) and carbon brick were carried out. The wetting angle under different conditions was measured, the reaction interface morphology and carbon structure were analyzed, the mechanism of hot metal wetting carbon brick was clarified based on interfacial energy and scaling law. The results show that: The wetting angle decreased gradually with the increase of wetting time for all initial carbon content experiments, the wetting angle increased with the increase of initial carbon content in hot metal. The dissolution of carbon occurred during the wetting process, which left a concave on carbon brick. The graphitization degree of carbon in carbon brick increased after the wetting behavior. In the initial stage of the wetting reaction, the change of interfacial energy caused by carbon dissolution reaction was the essential reason for the change of wetting angle. In the final stage of the wetting reaction, the wetting angle mainly depended on the concave shape after the concave was formed. The wetting model was established to explore the wetting behavior with dissolution reaction, the scaling relationship between the spreading radius of iron drop and time was obtained, which was convenient to evaluate the wetting behavior between hot metal and carbon brick.     

Using Twist-off Method for Measuring CFRP/Concrete Adhesion when Exposed to Cyclic Temperature Changes,Wet-Dry and Freeze-Thaw

Materials such as fiber-reinforced plastics (FRPs) are lightweight, strong, and for the most part, non-corrodible. Hence, the use of FRPs as retrofitting materials for concrete represents a viable, durable alternative to conventional practice using reinforcing steel. This paper addresses the magnitude of the residual shear bond strengths between rigid and flexible carbon fiber reinforced plastic (CFRP) sheets and substrate concrete, after being subjected to cyclic temperature changes, wet-dry and freeze-thaw, by means of a Twist-off method. In the experimental investigation, the shear bond strength of a number of CFRP/concrete specimens was monitored. These results were then used to make comparisons between the behaviors of the two types of CFRPs. Analysis of the results indicated that while 180 freeze-thaw cycles destroy the shear bond strength of rigid and flexible CFRP completely, the same number of wetting and drying and temperature change cycles can reduce these bonds by up to about 30 and 20%, respectively. The results also showed that despite the insignificant flexible CFRP failure, the failure in the rigid CFRP material was noticeable.     

Evolution and mechanism of dissolutive wetting between hot metal and carbon brick

The dissolutive wetting between hot metal and carbon brick is a complex problem from the viewpoint of carbon brick erosion. The current work aims to clarify the mechanism of dissolutive wetting at the interface between hot metal and carbon brick. The interfacial morphology is analyzed using SEM and EDS, and the change in wetting angle is studied using CCD camera and ImageJ software. Also, the interface shape model is established, the coupling control mechanism of dissolution and wetting is explored. The results reveal that the concave-like interface is formed after dissolutive wetting and the concave depth decreases with the increase of carbon content in hot metal. Moreover, a large amount of graphite flakes appear at the interface between hot metal and carbon brick after dissolutive wetting. The upper wetting angle initially exhibits a rapid decrease with time, followed by a gradual decrease. The lower wetting angle increases with time. The overall wetting angle initially decreases with time, followed by a stable trend. Hence, the dissolutive wetting can be divided into two stages.Furthermore, the interface shape model is established based on the scaling law and Noyes-Whitney equation to describe the dissolutive wetting. The variation of interface shape can be obtained by the interface shape model. The governing equation is obtained through the Onsager principle to clarify the coupling control mechanism of dissolutive wetting. Finally, wetting is determined by the relative situation of interfacial energy and dissolution free energy. The liquid structure of hot metal is changed due to the migration of carbon atoms from carbon brick to hot metal during dissolutive wetting, altering the free energy and facilitating the transition from the first stage to the second stage.     

Experimental and numerical analysis on fatigue durability of single-lap joints under vibration loads

Fatigue is one of the most common yet complicated failures that can cause damage to mechanical structures. Structural adhesively bonded joints are not exempt from this deleterious phenomenon and have to be assessed under vibration loads. In this work, fatigue characteristics of single-lap joints (SLJ) made of steel and carbon fibre reinforced plastic (CFRP) laminates under vibration loads are primarily investigated by experiments. The aim of this work is to analyze the changes in the ultimate load of the SLJ under vibration loads. The experimental results showed that SLJ will face cohesive failure after the uniaxial tensile loading test. In addition to the increase of vibration cycles, the ultimate load and failure displacement gradually decrease. In order to model the adhesive between joint components and simulate the damage propagation, a new traction–separation law called the embedded process zone (EPZ) and a damage factor are introduced and developed within the framework of cohesive zone Modeling (CZM) techniques. Meanwhile, the stress variations in the adhesive layer of SLJ in different vibration cycles are researched using the finite element method in ABAQUS.     

严苛侵蚀作用对优化导电混凝土性能的影响

干湿交替和硫酸盐腐蚀引起的损伤严重影响导电混凝土服役时的长期稳定性。本研究以碳纤维、石墨作为导电相材料,掺入粉煤灰和硅灰制备导电混凝土,在干湿交替和硫酸盐腐蚀耦合作用下,讨论粉煤灰、硅灰掺量对导电混凝土力学性能与电学性能的影响。综合导电混凝土的力学性能与电学性能衰变定义了服役性能劣化指标。结果表明：掺入粉煤灰和硅灰后提升了导电混凝土在干湿交替和硫酸盐腐蚀下的耐久性和导电稳定性;当粉煤灰和硅灰的总掺量一定时,提高粉煤灰占比能够有效降低干湿交替和硫酸盐腐蚀造成的强度损失,并提高导电混凝土的导电稳定性。     

Macroscopic Behavior of Carbon Nanotube (CNT)-Reinforced Composite Accounting for Interface Cohesive Force

The large aspect ratio of interfacial area per unit volume at CNT/matrix interfaces may significantly influence the macroscopic behavior of CNT-reinforced composites. The property of interfaces is governed by the cohesive law, which is determined from both van der Waals forces and chemical covalent bonds. This nonlinear cohesive law is incorporated in the micromechanics model in the current work to study the mechanical behavior of CNT-reinforced composites. It is found that carbon nanotubes can improve the macroscopic behavior of the composite at small strain, but tend to weaken the composite at relatively large strain because of the interface softening behavior or debonding. The increase of interface adhesion caused by the creation of chemical covalent bonds may significantly improve the composite behavior at large strain.     

A modified fiber‐reinforced plastics concrete interface bond‐slip law for shear‐strengthened RC elements under cyclic loading

The objective of this article is to realistically analyze fiber‐reinforced plastics (FRP) retrofitted reinforced concrete structures under cyclic loading taking into account FRP–concrete bond‐slip law with cyclic bond degradation. In literature, even though numerous studies have been conducted in FRP–concrete interface bond‐slip modeling under cyclic loads, a small number of them consider the influence of cyclic degradation on FRP–concrete interface bond behavior. Within this framework, the bond‐slip law for carbon fiber‐reinforced plastics–concrete interface is revised by utilizing Harajli's and Ko‐Sato's approaches. The procedure is distinct from others because it develops existing deficiencies of these approaches, whereas a more reliable modeling process is proposed for use in practice. Conventional bond‐slip law of Lu et al. is compared with this interface relationship stated in this investigation and the difference is clearly shown in terms of structural parameters. Experimental tests are conducted at the same time for verification. It is proved that cyclic bond degradation affects the interface behavior; thus, the structural response cannot be omitted in structural evaluations. Structural performance measures are obtained in good agreement for each level of cycles. The technique proposed clearly exhibits structural response difference between monotonic and cyclic loadings while good agreement is reached with experimental results. POLYM. COMPOS., 37:3373–3383, 2016. © 2015 Society of Plastics Engineers     

水泥改性膨胀土在侵蚀环境下的干湿循环效应研究

为了研究水泥改性膨胀土在复杂环境条件下的稳定性及耐久性,设计了室内模拟试验,探讨了纯净水、不同浓度及不同种类的化学溶液在干湿循环作用下对水泥改性膨胀土的侵蚀效应.从初始至破坏状态,记录了试样的宏观裂隙开展过程及体积的变化情况,对比分析了相应的荷载位移曲线及峰值强度,得到了水泥改性膨胀土在不同侵蚀环境下的力学特性随干湿循环次数的变化规律,并对相应的作用机理进行了分析.试验结果表明,侵蚀环境的干湿循环作用对水泥改性膨胀土的力学特性有明显的影响,且随着侵蚀溶液的浓度和干湿循环次数的递增而变得愈加显著.     

Influence of elevated temperatures on epoxy adhesive used in CFRP strengthening systems for civil engineering applications

This paper presents experimental investigations about the influence of elevated temperatures on the mechanical behaviour of an epoxy adhesive typically used in carbon fibre reinforced polymer (CFRP) strengthening systems and numerical investigations about the influence of changes underwent by the adhesive on the response of bonded joints between CFRP strips and concrete. The experiments included shear and tensile tests at elevated temperatures (up to 120 °C) on a commercial epoxy adhesive. In both types of tests, the mechanical response of the adhesive at different temperatures was assessed, namely in terms of stress vs. strain curves, stiffness, strength and failure modes. The results obtained highlighted the considerable reduction of both shear and tensile properties with increasing temperatures: at 70 °C the shear and tensile strengths are both reduced to around 15% of the corresponding ambient temperature strengths, while the tensile and shear moduli can be considered negligible. Analytical formulae were fit to the test data, describing the reduction with temperature of the adhesive's tensile and shear properties. In the numerical investigations, three-dimensional finite element models were developed to simulate previous double-lap shear tests performed on concrete blocks strengthened with CFRP strips according to either the externally bonded reinforcement (EBR) or the near surface mounted (NSM) techniques, using the epoxy adhesive characterized in the present study. Two distinct modelling strategies were adopted for the concrete-CFRP bond in order to assess the relative importance of the adhesive distortion and interfacial slippage at the concrete-adhesive-CFRP interfaces in the overall slip between concrete and CFRP: (i) to explicitly simulate the adhesive, considering the mechanical properties determined in the tests and assuming a perfect bond at all interfaces; and, alternatively, (ii) to simulate the CFRP-concrete interaction by means of global bilinear bond-slip laws for different temperatures. Comparison between numerical results and test data allowed quantifying the relative importance of the adhesive distortion and of the interfacial slippage at the bonded interfaces as a function of temperature, providing a better understanding of the contribution of these two mechanisms to the CFRP-concrete bond at elevated temperature. While the former effect is the most relevant at ambient temperature, with elevated temperature the interfacial slippage at the bonded interfaces becomes the most relevant mechanism.     

黏性颗粒团聚机理及流化特性研究进展

黏性力的存在导致黏性颗粒在流化过程中易发生团聚,干扰正常的流态化。近年来,黏性颗粒流的研究重心逐渐转移到本征的团聚机理和流化特性。本工作综述了4种主要黏性力的力学模型、团聚判据及流态化实验与模拟研究进展,从力、运动及动力学的角度阐述了黏性力作用机制和黏性颗粒流化特性。分析表明,在颗粒尺度上,4种黏性力发展程度差异较大,黏性力动力学模型和团聚过程机理将成为未来研究的主要方向。在反应器尺度上,耦合黏性力模型的离散单元法模拟将继续作为重要的研究方法,其中,机理模型和计算能力是后续模拟中需要突破的重点。