高温后圆钢管高强混凝土界面黏结性能试验研究

为揭示历经高温作用后圆钢管与高强混凝土界面间的黏结滑移性能,设计17个试件进行推出试验,试验考虑了高强混凝土强度等级、历经最高温度和界面黏结长度3个变化参数,通过试验,观察了试件的破坏过程及形态,得到试件加载端及自由端的荷载-滑移曲线,基于试验实测数据深入分析各变化参数对黏结强度、界面耗能及损伤的影响规律,提出了高温后圆钢管高强混凝土界面黏结强度的计算式以及黏结滑移本构方程。结果表明:随历经温度的升高,圆钢管与高强混凝土界面黏结强度呈现先增大后减小的变化规律;随着历经温度的升高,黏结强度随混凝土强度等级的提高而增大的影响程度逐渐减弱;界面黏结损伤随历经温度的升高有所推迟;界面黏结耗能能力随历经温度与混凝土强度等级的提高而下降。     

Fracturing behaviors of FRP-strengthened concrete structures

In this paper, we focus on the study of concrete cracking behavior and interfacial debonding fracture in fiber reinforced polymer (FRP)-strengthened concrete beams. An experimental program is systematically reviewed according to the observed failure modes, in which it is found that the interfacial debonding may propagate either within the adhesive layer or through concrete layer in the vicinity of bond interface. A finite element analysis is performed to investigate the different types of debonding propagation along FRP-concrete interface and crack distribution in concrete. For the numerical fracture models, interfacial debonding that initiates and propagates in adhesive layer is modeled by fictitious interfacial crack model. And concrete cracking, including the debonding fracture through interfacial concrete, is modeled by smeared crack model. Properties of the interfacial adhesive layer and concrete are considered to significantly influence the debonding propagation types and crack distribution. The interactions between interfacial bond strength, interfacial fracture energy of bond adhesive layer and tensile strength, fracture energy of concrete are discussed in detail through a parametric study. According to the results, the effects of these properties on different types of interfacial debonding, concrete cracking behavior and structural load-carrying capacity are clearly understood.     

Parameters affecting the debonding risk of bonded overlays used on reinforced concrete slab subjected to flexural loading

This research aims at better understanding the mechanisms involved in the cracking behaviour of bonded overlays used on reinforced concrete beams and slabs. The project involves the testing of reinforced concrete beams (1.8 m × 0.2 m × 0.2 m) repaired with concrete overlays using four different types of surface preparation and subjected to cyclic flexural loading. The concrete beam specimens were simply supported with a point load at midspan. Structural capacity (evolution of the apparent rigidity, maximum deflection, failure mode) and cracking behaviour (flexural cracking and interface debonding) were monitored. Technical data indicate that good adhesion alone is insufficient to guarantee bond durability. A total of 20 beam interfaces were prepared by scarification, sandblasting, chipping with a light jackhammer, and water jetting. A number of cores were tested to evaluate interface strength by direct shear and direct tension. Roughness was characterized quantitatively using a newly developed optometric method. The results indicate that cracking behaviour depends on the bond strength and on the surface roughness produced by a specific surface treatment. The relationships between adhesion, structural behaviour, and roughness were evaluated, and an updated debonding mechanism is proposed to take into account the influence of roughness. To achieve a monolithic behaviour, the surface treatment must generate critical adhesion and roughness levels. In addition to this roughness, the debonding risk decreases rapidly and monolithic behaviour is reached.     

FRP加固混凝土构件中裂纹扩展规律的数值模拟

用材料破坏过程分析系统MFPA(MaterialFailureProcessAnalysis)对FRP(FiberReinforcedPolymer)加固混凝土梁在外荷载作用下沿FRP板和混凝土之间界面剥落的不同破坏模式以及混凝土中的裂纹分布进行了数值模拟。通过分析,发现剥落破坏可能产生在FRP板与混凝土之间的粘结层内或其上方附近的混凝土中。模拟结果与实验结果较吻合。粘结层和混凝土的特性对于剥落传播的类型及裂纹的分布有很大的影响。着重探讨了混凝土的强度参数以及粘结层的强度参数对于整个构件承载能力及最终破坏模式的影响。     

有缺陷的装配式混凝土梁柱节点抗震性能试验研究

为了明确装配式混凝土框架结构节点缺陷对于节点抗震性能的影响,该文考虑了三种典型装配式梁柱节点核心区缺陷,对5个装配式混凝土梁柱节点和1个全现浇混凝土梁柱节点进行了拟静力试验,分析了其对破坏形态、滞回性能、骨架曲线、刚度退化、耗能能力等性能的影响。采用OpenSees非线性有限元分析程序模拟钢筋粘结滑移关系和节点区域剪切性能,讨论了装配式混凝土梁柱节点钢筋粘结削弱的影响。结果表明:核心区内部混凝土浇筑缺陷将使得钢筋过早出现滑移现象,对节点的强度及耗能能力产生影响;粗糙面的缺陷及柱底接缝灌浆层缺陷对于节点抗震性能的影响较小;有限元模型通过考虑节点域的钢筋粘结滑移关系,可以有效模拟装配式节点钢筋粘结削弱效应,从而为进一步研究装配式节点抗震性能不确定性,并进行地震易损性分析奠定基础。     

型钢高性能纤维混凝土粘结滑移性能试验研究

为研究型钢与高性能纤维混凝土之间的粘结滑移性能,以高性能纤维混凝土强度、保护层厚度以及型钢锚固长度为变化参数,设计10个型钢高性能纤维混凝土试件,进而对其进行推出试验,观察试件的破坏过程和裂缝发展形态,获取试件的荷载-滑移曲线。基于试验结果,探讨了各设计参数对特征粘结强度的影响规律,并建立了特征粘结强度的计算公式。对有效粘结应力计算公式进行推导,得到有效粘结应力-滑移分布曲线,并对其进行全过程分析。结果表明:适当增加型钢保护层厚度和混凝土强度,能有效提高型钢高性能纤维混凝土的特征粘结强度;特征粘结强度计算值与试验值吻合较好,表明该文提出的计算公式具有较高的精度;有效粘结应力能够反映型钢与高性能纤维混凝土界面间粘结应力的发展变化过程,并得到粘结应力各组份之间的比例关系。研究成果可为型钢高性能纤维混凝土相关计算理论提供试验依据。     

Deformation,yield and fracture of unidirectional composites in transverse loading: 2. Influence of fibre–matrix adhesion

The influence of the adhesion between fibre and matrix on the transverse properties of unidirectional composites was studied using a combination of experimental and numerical analyses. The interface is modelled on a nano(metre)-scale and the aim is to investigate its local influence on the ultimate macroscopic transverse properties. Fibre-to-matrix stress transfer (i.e. fibre-to-matrix surface interaction) is simulated by introducing elastic interface springs. Since these elastic springs represent the chemical (covalent) bonds formed at the interface as a result of oxidative chemical surface treatment, the micromechanical model can be directly related to the effects of this treatment. For the verification of the numerical analyses, the influence of the interface is determined experimentally by transverse testing of carbon fibre reinforced composites, using fibres that were subjected to different levels of surface treatment. A direct relation between the oxygen concentration on the surface of the fibres, the interfacial bond strength and the resulting transverse strength was found. The interface strength required to obtain perfect bonding was found to be dependent on the fibre volume fraction and at increased fibre volume fractions a higher level of adhesion is required.     

Numerical modelling of the effects of elevated service temperatures on the debonding process of FRP-to-concrete bonded joints

There are many issues concerning the performance behaviour of FRP-to-concrete interfaces at elevated service temperatures (EST). At EST, i.e. slightly above the glass transition temperature (T_g), some properties associated with the FRP composites, such as the stiffness, strength or the bond characteristics, degrade. This is a crucial issue and there are only a few studies that take into account such effects on FRP-to-concrete interfaces at EST. This paper examines, through a numerical analysis, the performance of FRP-to-concrete bonded joints at EST using a new discrete model based on truss elements and shear springs. The External Bonded Reinforcement (EBR) systems subjected to EST are analyzed. The numerical discrete model was implemented in a MATLAB routine and the bond–slip curves of the interfaces at EST were obtained from a model found in literature. The numerical results revealed that the interface at EST behaves similarly to one with two equal mechanical loads applied at both ends of the FRP plate. The load–slip curves or bond stresses, strains or slippages along the bonded length obtained from several bond–slip curves at different temperatures were obtained. Two different single-lap shear tests were simulated at steady-state (steady temperature followed by load increase) and transient state (steady load followed by temperature increase). Regarding the influence of the temperature on the adhesion between the FRP and concrete, the results showed that an increase in the temperature at an earlier situation, i.e. during a period where temperature had no influence in the concrete deformations, leads to an increase in the effective bond length of the interface affecting the initial strength of the interface.     

Bond behavior between steel reinforcement and recycled concrete

In this paper the bond behavior of recycled aggregate concrete was characterized by replacing different percentages of natural coarse aggregate with recycled coarse aggregate (20, 50 and 100 %). The results made it possible to establish the differences between the conventional concrete bond strength and the recycled concrete bond strength depending on the replacement percentage. It was thus found that bond stress decreases with the increase of the percentage of recycled coarse aggregate used. In order to define the influence of recycled aggregate content on bond behavior, normalized bond strength was calculated taking into account the reduced compressive strength of the recycled concretes. Finally, using the experimental results, a modified expression for maximum bond stress (bond strength) prediction was developed, taking into account replacement percentage and compressive strength. The obtained results show that the equation proposed provides an experimental value to theoretical prediction ratio similar to that of conventional concrete.     

Effect of elevated service temperature on bond between FRP EBR systems and concrete

The method to increase the ultimate capacity of reinforced concrete elements, by means of Fiber Reinforced Polymers bonded on their substrate with thermosetting resins, represents a technique useable worldwide. The bonding materials used, generally epoxy resins, are sensitive to a temperature increase. In fact, the curing process of epoxies leads to low glass transition temperature (T_g) values, that may cause a relevant decay of the mechanical properties of the adhesive, even under service thermal conditions. The reduction can influence the bond performance, compromising the effectiveness of the reinforcing technique. In this work the interface behaviour FRP–concrete at elevated service temperatures is analyzed. The experimental results show a relevant influence of the temperature on bond performance, in terms of type of failure of the interface, effective bond length and bond strength.     

Bond failure performances between near-surface mounted FRP bars and concrete for flexural strengthening concrete structures

Near-surface mounted (NSM) fiber reinforced polymer (FRP) has been established as an effective technique for strengthening concrete member. In preview literatures, bond failure was observed usually in the strengthened beam test for increasing flexural capacity. Bond behavior is of primary importance for the transfer of stress between the concrete and the FRP reinforcement to develop composite action. In this paper, a total of 22 tests were conducted to study the bond failure performance between NSM FRP bars and concrete besides only one test as a comparison. Failure modes, load–deflection curves, strain distribution of FRP bars, and local bond stresses at the FRP-epoxy adhesive interface from the tests were analyzed in detail. Some of the factors expected to affect bond performance were presented, namely: diameter of FRP bars, type to FRP material, concrete compressive strength and bonded length. The test results reported in this paper should be useful for further establishing local bond–slip constitute relationship and further verification of numerical simulation models, in addition to gaining a better understanding of bond failures for flexural strengthening concrete structures with NSM FRP bars.     

Durability of CFRP-concrete joints under freeze-thaw cycling

The long-term durability of fiber reinforced polymer (FRP) strengthening systems under freeze-thaw cycling is crucial to the safety of structures in cold climates. The durability of the FRP-concrete bond interface under freeze-thaw cycling was investigated in the study reported here, with exposure condition, concrete grade, and number of freeze-thaw cycles as the parameters considered. The behavior of the carbon FRP (CFRP)-concrete bond interface was investigated with single-face shear tests. The results indicate that the bond strength, bond stiffness, interfacial fracture energy, and maximum slip of the joints decrease with increases in the number of freeze-thaw cycles, and they are also affected by the exposure environment. The depth of cracking and effective bond length increase with increases in cycle number, thus affecting bond stiffness and strength. The deterioration of bond strength can be attributed to the damage caused to the concrete by the freeze-thaw cycling.     

Evaluation of bond quality at the interface between steel bar and concrete using the small-dimension break-off test

This paper investigates the feasibility of using the small-dimension break-off test for evaluation of the bond quality at the interface between steel bar and concrete. Experimental studies were performed on bar-type concrete specimens and reinforced concrete beams. Twelve bar-type concrete specimens containing plain and deformed steel bars with different diameters were used to develop the relationship between the break-off moment and the adhesive strength at the steel bar/concrete interface. Subsequently, three reinforced concrete beams containing normal reinforcing bars, epoxy-coated reinforcing bars, and bars smeared with oil to simulate various adhesive conditions at the bar/concrete interface were used to study how the break-off moment and the bond strength were affected by the different adhesive conditions. In addition, two beam specimens containing normal reinforcing bars were vibrated severely on a self-made shaking table shortly after initial setting of concrete to simulate the bond damage in fresh reinforced concrete beams due to unexpected vibration or impact. Experimental results show that the effective break-off moment has a good correlation with the adhesive strength at the interface between steel bar and concrete. The break-off moment increases with an increase in bond strength. It is demonstrated that the small-dimension break-off test is capable of evaluating damage at the steel bar/concrete interface.     

低取代度羧甲基淀粉对纤维粘附性能

通过改变一氯乙酸对淀粉的投料比，制备了一系列具有不同变性程度的低取代度羧甲基淀粉．然后以比粘附强度和断裂伸长为量化指标，通过轻浆粗纱法研究了淀粉羧甲基化变性、变性程度及纤堆类型对纤堆粘附性能的影响．实验结果表明，上速三种因素都对其比粘附强度有显著影响，当取代度为0．017时，比粘附强度最大。此外，还研究了CMS-PVA共混物对涤／棉纤堆的粘附性能。     

Bond of steel reinforcement with microwave cured concrete repair mortars

This paper investigates the effect of microwave curing on the bond strength of steel reinforcement in concrete repair. Pull-out tests on plain mild steel reinforcement bars embedded in four repair materials in 100 mm cube specimens were performed to determine the interfacial bond strength. The porosity and pore structure of the matrix at the steel interface, which influence the bond strength, were also determined. Test results show that microwave curing significantly reduces the bond strength of plain steel reinforcement. The reduction relative to normally cured (20 °C, 60% RH) specimens is between 21 and 40% with low density repair materials and about 10% for normal density cementitious mortars. The corresponding compressive strength of the matrix also recorded similar reduction and microwave curing resulted in increased porosity at the interface transition zone of the steel reinforcement. A unique relationship exists between bond strength and both compressive strength and porosity of all matrix materials. Microwave curing reduced shrinkage but despite the wide variation in the shrinkage of the repair mortars, its effect on the bond strength was small. The paper provides clear correlations between the three parameters (compressive strength, bond strength and porosity), which are common to both the microwave and conventionally cured mortars. Therefore, bond-compressive strength relationships used in the design of reinforced concrete structures will be also valid for microwave cured elements.     

型钢混凝土结构粘结滑移性能试验研究

进行了20榀试件的实腹式型钢混凝土粘结滑移性能的拉拔试验,分析了型钢与混凝土的粘结滑移机理、影响粘结滑移性能的主要因素以及沿锚固长度粘结应力和滑移量的分布规律,建立了型钢混凝土粘结强度和滑移量的计算公式,提出了型钢混凝土局部粘结破坏和整体粘结破坏极限荷载的计算原理与方法,根据混凝土板的冲切破坏理论,提出了确定临界保护层厚度的方法。研究成果为改进型钢混凝土结构的强度、刚度、变形、裂缝宽度计算理论以及进行这种结构的有限元分析提供了试验依据。     

Experimental study on the bond behavior of deformed bars embedded in concrete subjected to lateral tension

The bond behavior of steel bars embedded in concrete, including the anchorage capacity and bond stress–slip relationship, is highly concerned owing to its importance in the design and checking calculation of reinforced concrete structures. The purpose of this paper is to present an experimental study on the bond behavior of deformed bars embedded in concrete subjected to lateral tension. In the experiment, a total of 237 pull-out specimens are tested. It is shown that the failure mode is influenced by the strength of concrete, ratio of the cover depth to bar diameter, and unfavorable lateral tension. The bond strength and the slip at the peak stress decreases with the increase of the unfavorable lateral tension for specimens in pull-out and splitting failures, but the ratio of the residual to ultimate bond strength almost keeps constant for specimens in pull-out failure. Based on the experimental results, an empirical formula for the bond stress–slip relationship is proposed. The empirical formula is in good agreement with the experimental results for specimens with different strengths of concrete, bar diameters, and combinations of lateral tensions.     

高温后圆钢管再生混凝土界面黏结滑移性能及本构方程研究

为了揭示高温作用后钢管再生混凝土的界面黏结性能,以试件的最高经历温度(T)、再生粗骨料取代率(γ)为变化参数,设计并完成了20个高温后试件的静力推出试验。通过试验观察了试件的受力破坏过程及形态,获取了试件加载端和自由端的荷载滑移曲线,分析了各变化参数对界面黏结性能及黏结损伤演变过程的影响规律,并提出了高温后钢管再生混凝土的界面黏结强度计算方法及黏结滑移本构方程。结果表明:荷载-滑移曲线的形态可以分为T≤400 ℃和T=600 ℃两类,加载端和自由端的曲线形态相似但加载端的初始滑移发生得相对较早;界面黏结性能整体上与钢管普通混凝土相差不大(各取代率下性能差值的均值在11%以内);界面黏结强度随经历温度和再生粗骨料取代率的升高呈现先减小后增大的变化趋势;随着经历温度的升高,界面黏结抗剪刚度先减小后增大再减小,界面黏结耗能能力则逐渐增大;随着再生粗骨料取代率的升高,界面黏结抗剪刚度和耗能能力均呈现先增大后减小再增大的变化趋势;界面黏结损伤发生的早晚随经历温度的升高呈现先提早而后变晚的变化规律,而再生粗骨料取代率对其影响不大;界面黏结损伤发展速度随经历温度和再生粗骨料取代率的升高呈现先增大后减小的变化趋势。     

Illustration of stress/strain behavior and yarn fragmentation in a pseudo-hybrid composite system

To study the stress/strain behavior and the fragmentation of fibers in a hybrid composite in tensile loading, a model strand was constructed with cotton yarns as the low-elongation component and polyester yarns as the high-elongation component. The effect of interfacial shear strength on the performance of a hybrid composite was simulated by varying the twist multiplier of the strands. It was observed that a strand with a lower twist level was more likely to show a typical hybrid stress/strain behavior and a multi crack fragmentation pattern. The same was found for strands with the highest twist levels but with cotton contents between 22 and 33%.     

钢纤维掺量和轴压比对预应力扁梁框架力学性能的影响

利用ABAQUS非线性有限元分析研究钢钎维掺量和轴压比对无粘结预应力混凝土扁梁框架受力性能的影响。有限元模型中混凝土采用塑性损伤模型,考虑了钢纤维对混凝土本构关系的影响,同时采用弹簧单元模拟钢筋与混凝土间的粘结滑移作用。算例表明,采用本文的方法进行钢纤维预应力扁梁框架力学性能研究是可行的。在此基础上,分析不同钢纤维掺量和柱轴压比对无粘结预应力扁梁框架力学性能的影响。研究表明,较大的轴压比可以提高结构极限承载力,但降低结构延性和耗能,对于无粘结预应力扁梁框架,建议在抗震地区其轴压比限制不宜大于0.6。钢纤维可以有效提高预应力扁梁框架的极限承载力、延性和耗能,从而提高其抗震性能,其影响与轴压比的大小有关。