FRP-confined concrete under axial cyclic compression

One important application of fiber reinforced polymer (FRP) composites in construction is as FRP jackets to confine concrete in the seismic retrofit of reinforced concrete (RC) structures, as FRP confinement can enhance both the compressive strength and ultimate strain of concrete. For the safe and economic design of FRP jackets, the stress–strain behavior of FRP-confined concrete under cyclic compression needs to be properly understood and modeled. This paper presents the results of an experimental study on the behavior of FRP-confined concrete under cyclic compression. Test results obtained from CFRP-wrapped concrete cylinders are presented and examined, which allows a number of significant conclusions to be drawn, including the existence of an envelope curve and the cumulative effect of loading cycles. The results are also compared with two existing stress–strain models for FRP-confined concrete, one for monotonic loading and another one for cyclic loading. The monotonic stress–strain model of Lam and Teng is shown to be able to provide accurate predictions of the envelope curve, but the only existing cyclic stress–strain model is shown to require improvement.     

循环荷载作用下的混凝土弹塑性损伤本构模型及数值实现

该文建立了一种描述混凝土在循环荷载作用下复杂力学行为的弹塑性损伤模型。该模型借助四参数等效应变将复杂的多轴问题转换至单轴等效应变空间中求解,考虑了混凝土卸载过程中的刚度退化现象及不可逆变形。同时针对等效应变的非负特性,提出了拉压转换处理方法,从而在求解损伤变量时无需区分拉损伤和压损伤。该文提出的弹塑性损伤模型数学形式简洁,且实现过程不依赖于四参数模型,实现方法普遍适用于各类混凝土等效应变模型,通过模拟单轴循环荷载试验和Koyna大坝动态损伤过程,验证了该文模型的正确性与可靠性。     

Plastic strain and stress deterioration of FRP-confined concrete prisms under axial cyclic compression

This paper presents the investigation and modeling of plastic strain and stress deterioration of FRP-confined concrete prisms under axial cyclic compression. Also the effect of the cross-section shape of specimens on these parameters is investigated. For this purpose, the experimental results of cyclic stress–strain behavior of circular, square and rectangular concrete prisms confined by FRP-composites are presented and examined. Besides, the effect of repeated complete cycles on the plastic strain and stress deterioration is studied. The results show that the relation between plastic strain and first unloading strain is linear and depends only on the two parameters of concrete strength and the shape of cross-section, but the stress deterioration depends only on the first unloading strain. The predictive formulations for plastic strain and stress deterioration are presented and compared with the experimental results.     

Lateral behavior of concrete under uniaxial compressive cyclic loading

In reinforced concrete structures under seismic loading, concrete is subjected to compressive cyclic stress. Although cyclic stress–strain response has been described before, the cyclic behavior of strains in the direction orthogonal to loading has not been characterized yet. Such behavior can be of great importance for evaluating the efficiency of the confinement under cyclic loading. For this purpose an experimental program on cylindrical specimens of concrete strength from 35 to 80 MPa subjected to uniaxial cyclic compression was carried out. Stress versus longitudinal and lateral strains curves have been obtained both for the hardening and softening branches under monotonic and cyclic loading. Governing parameters of the lateral behavior are identified and correlated to describe the response of the lateral strain. Additionally, an analytical model to obtain the lateral deformations of concrete under cyclic uniaxial compression has been formulated and verified experimentally. Finally, some examples are presented in order to illustrate the applicability of the proposed model and its possible incorporation into a 3D constitutive cyclic model.     

FRP管约束混凝土的轴压应力-应变关系研究

目前FRP约束混凝土轴压应力-应变关系大都只考虑FRP管环向受拉。针对FRP管约束混凝土的轴心受压性能进行分析,考虑其承受压力造成约束模量降低的影响,在现有约束混凝土模型的基础上,提出一种考虑FRP管在双向受力情况下的应力-应变关系分析模型,并与试验结果进行了分析对比,分析结果与试验结果吻合较好。还依据这一模型进行了参数研究。     

Influence of concrete strength and confinement method on axial compressive behavior of FRP confined high- and ultra high-strength concrete

This paper presents an experimental investigation on the effect of concrete compressive strength and confinement method on confined high and ultra high-strength concrete (HSC and UHSC) specimens. A total of 55 fiber reinforced polymer (FRP) confined concrete specimens were tested under monotonic axial compression. All specimens were cylinders with 152 mm diameter and 305 mm height and confined by carbon FRP (CFRP). Three different concrete mixes were examined, with average compressive strengths of 35, 65 and 100 MPa. The effect of the confinement method was also examined with FRP-wrapped specimens compared to FRP tube-encased specimens. Axial and lateral behavior was recorded to observe the axial stress–strain relationship and lateral strain behavior for concentric compression. Ultimate axial and lateral conditions are tabulated and the complete stress–strain curves have been provided. The experimental results presented in this paper provide a performance comparison between FRP-confined conventional normal-strength concrete (NSC) and the lesser understood area of FRP-confined HSC and UHSC. The results of this experimental study clearly indicate that above a certain confinement threshold, FRP-confined HSC and UHSC exhibits highly ductile behavior, however for the same normalized confinement pressures, axial performance of FRP-confined concrete reduces as concrete strength increases. The results also indicate that ultimate conditions of FRP-wrapped specimens are similar to those confined by FRP tubes, however a performance difference is evident at the transition region. The performance of 10 existing stress–strain models were assessed against the experimental datasets and the performance of these models discussed. The results of this model assessment revealed the need for further development for stress–strain models developed specifically for FRP-confined HSC or UHSC.     

Effect of confinement level,aspect ratio and concrete strength on the cyclic stress–strain behavior of FRP-confined concrete prisms

Behavior of rectangular concrete columns confined with FRP composites depends on several parameters, including unconfined concrete strength, confinement level, aspect ratio of cross-section (defined as the depth/width of the cross-section), and the sharpness of the section corners. For modeling the cyclic stress–strain behavior of FRP-confined rectangular concrete columns, effect of column parameters on the cyclic behavior of these columns should be examined properly. In this paper, effects of unconfined concrete strength, confinement level and the aspect ratio of cross-section are studied. The test database includes 10 prisms from recent study of authors and 18 prisms from a new experiment. Results of tests show that some aspects of cyclic behavior of FRP-confined concrete prisms such as envelope curve and stress deterioration are unaffected by the considered parameters. Results also indicate that the plastic strain decreases as the unconfined concrete strength increases, but it is independent of the aspect ratio and the confinement level. While the reloading path in all specimens was almost linear, the unloading path was highly nonlinear and was affected by unconfined concrete strength.     

钢纤维高性能轻骨料混凝土多轴强度和变形特性研究

为了研究钢纤维掺量和三轴应力比对高性能轻骨料混凝土破坏准则和本构关系的影响规律,进行了钢纤维全轻混凝土和钢纤维次轻混凝土多轴强度和变形特性的试验研究,考虑到试验机加载能力和新拌高性能轻骨料混凝土的工作性能,选取的钢纤维体积掺量为0、0.5%、1.0%和1.5%,试验加载路径有单轴拉、压,双轴等压和真三轴压。结果发现在单轴应力和低应力比条件下,钢纤维能够明显地发挥增强阻裂作用,随着钢纤维掺量的增加,中间主应力对极限抗压强度和峰值应变的影响越来越大,且钢纤维体积掺量对两种轻骨料混凝土应力-应变曲线下降段有一定的影响;在高应力比条件下,钢纤维体积掺量对峰值强度、峰值应变和应力-应变曲线下降段无明显影响,但对高应力比下轻骨料混凝土应力-应变曲线上特有的应力平台区域有较大的影响。考虑钢纤维含量特征参数的影响,对普通骨料混凝土的Kotsovos破坏准则进行了相应的修正,得出了适合钢纤维增强轻骨料混凝土的破坏准则表达式。     

橡胶混凝土单轴受压疲劳性能研究

为掌握橡胶混凝土单轴受压疲劳性能,用粒径为30目的橡胶颗粒以不同掺量代砂制备橡胶混凝土,进行等幅循环荷载单轴受压疲劳试验研究。采用Miner累积损伤理论定义损伤量,并建立橡胶混凝土疲劳应变的损伤模型。使用概率统计方法对橡胶混凝土疲劳寿命的试验结果进行可靠性分析,得到等幅循环荷载作用下橡胶混凝土单轴受压疲劳寿命分布规律。结果表明:应力水平相同时,橡胶混凝土的疲劳寿命优于普通水泥混凝土,且随橡胶掺量的增加,混凝土的疲劳寿命随之提高。橡胶混凝土的疲劳应变变化符合普通混凝土疲劳应变发展的三阶段规律,橡胶混凝土疲劳寿命服从对数正态分布。采用双对数方程对橡胶混凝土的疲劳寿命进行线性回归分析,可得到P-S-N曲线及疲劳极限强度。     

Model to predict the viscoelastic response of a semi-crystalline polymer under complex cyclic mechanical loading and unloading conditions

To predict the behaviour of a semi-crystalline polymer under complex cyclic mechanical loading and unloading conditions, a non-linear rheological model, previously developed to predict the viscoelastic behaviour under uniaxial monotonous loading and unloading conditions, was used as a starting point and generalised in three-dimensions (3D). The initial model was created on the assumption that the amorphous phase of the semi-crystalline polymer follows the particular reversible mechanism of viscoelastic deformations. This assumption allowed to model the mechanical behaviour of the semi-crystalline polymer under loading and unloading conditions without changing the parameters at the point when the loading was reversed. The parameters of the 3D generalised model under tension, torsion (shearing) and compression conditions were identified from the fitting of the respective uniaxial mechanical tests. It was demonstrated that the 3D model agreed with the experimental data obtained under complex proportional and non-proportional mechanical loading. It was also demonstrated that the model is able to reproduce more then one loop of the cyclic loading.     

考虑压-弯-剪相互作用的纤维混凝土剪力墙变形能力分析

为了准确预测地震作用下高性能纤维增强混凝土剪力墙的受力性能,综合考虑压、弯、剪相互作用的破坏机理以及纤维增强混凝土超高受拉应变硬化性能,基于修正压力场理论和单向弯剪模型,提出了高性能纤维增强混凝土剪力墙变形能力分析模型。通过与4片高性能纤维增强混凝土剪力墙低周往复试验骨架曲线的对比研究,验证了剪力墙分析模型的准确性,讨论了影响其变形能力的的主要因素。研究结果表明:该文提出的考虑轴力-弯矩-剪力相互作用的高性能纤维增强混凝土剪力墙荷载-变形模型,可用于单调荷载作用下剪力墙的变形能力分析。     

An Assessment of Nonlinearly Viscoelastic Constitutive Models for Cyclic Loading: The Effect of a General Loading/Unloading Rule

Predicting the unloading and/or cyclic deformation behavior of polymers is a challenge for most nonlinear viscoelastic constitutive models. Experimental data of an epoxy polymer under uniaxial loading/unloading and two other types of cyclic loadings are used to assess the predictive capabilities of three types of nonlinear viscoelastic models. A general loading/unloading criterion and a switching rule, proposed recently by the authors, are further modified and incorporated into each of the three models. For each model, predictions by both the original formulations and that incorporating the proposed loading/unloading rule are compared with the test data. It is clearly shown that such a rule is essential to correctly simulate the unloading and cyclic loading behavior of polymers. By introducing such a rule to constitutive models, the quantitative predictions can be improved, to various degrees of success, with respect to cyclic deformation features such as ratcheting under cyclic loading with a mean stress and stress relaxation under cyclic straining with a mean strain.     

定侧压混凝土双轴压疲劳性能研究

利用大型混凝土静、动三轴试验机,完成了3种定侧压混凝土的双轴压疲劳试验。观察了试件破坏形态。将单轴压疲劳试验结果与其他试验者的结果作了比较,验证了结果的有效性。在试验基础上,分析了双轴压混凝土的疲劳特性和变形规律;建立了相应的S-N关系及考虑侧应力影响的统一疲劳方程。研究结果,为受复杂重复荷载作用的混凝土结构设计和分析提供了试验依据。     

双轴受压状态下的高延性纤维增强水泥基复合材料本构模型

基于Darwin和Pecknold考虑混凝土双轴力学行为的方法,建立一个同时考虑双轴受压状态下非线性力学行为和抗压强度变化的高延性纤维增强水泥基复合材料(ECC)二维正交各向异性本构模型。在因双轴加载而产生的正交各向异性的2个方向上引入等效单轴应变,建立非线性应力-等效单轴应变关系以考虑ECC的双轴非线性行为,并采用一条双轴强度包络线确定2个方向上的抗压强度。推导模型的显式数值算法,编写包含该算法的用户自定义材料子程序UMAT,并嵌于有限元计算程序ABAQUS v6.14中。通过对两组不同配合比的ECC试件在不同应力比下的双轴受压加载试验进行数值分析验证本模型的有效性。数值计算得到的主压应力方向上的应力-应变曲线及预测的抗压强度与试验结果吻合较好,表明该文提出的本构模型能够有效地预测ECC在双轴受压状态下的非线性力学行为和破坏强度。     

软化特征可控的双参指数型约束混凝土本构及其应用

约束混凝土本构模型的下降段影响着钢管混凝土的力学性能预测,结合6组方钢管混凝土短柱轴压破坏试验结果,提出一种软化特征可控的双参指数型约束混凝土本构。双参指数型本构考虑了方钢管宽厚比、混凝土强度、钢管截面含钢率等因素的影响,能够很好地模拟在不同约束条件下混凝土的软化行为。基于双参指数型本构,对6组方钢管混凝土试验结果进行了轴压破坏的受力全过程模拟,计算结果和试验结果吻合良好。     

岩石可释放应变能及耗散能的实验研究

地下岩石结构的变形破坏是能量耗散与能量释放的综合结果.岩石结构内部储藏的可释放应变能和己耗散能的计算,涉及到在当时工况下岩石的卸荷弹性模量和泊松比,并与加载速度与载荷水平有关.该文在不同加载速度及不同载荷水平下,对岩石试件进行了单压加卸载实验,得到了卸荷弹性模量与泊松比、可释放应变能与耗散能的变化规律;进行了SHPB动...     

Dynamic compressive behavior of unidirectional E-glass/vinylester composites

Results from an experimental investigation on the mechanical behavior of unidirectional fiber reinforced polymer composites (E-glass/vinylester) with 30%, 50% fiber volume fraction under dynamic uniaxial compression are presented. Specimens are loaded in the fiber direction using a servo-hydraulic material testing system for low strain rates and a Kolsky (split Hopkinson) pressure bar for high strain rates, up to 3000/s. The results indicate that the compressive strength of the composite increases with increasing strain rate. Post-test scanning electron microscopy is used to identify the failure modes. In uniaxial compression the specimens are split axially (followed by fiber kink band formation). Based on the experimental results and observations, an energy-based analytic model for studying axial splitting phenomenon in unidirectional fiber reinforced composites is extended to predict the compressive strength of these composites under dynamic uniaxial loading condition.     

Surface-sulfonated polystyrene microspheres improve crack resistance of carbon microfiber-reinforced Portland cement mortar

The formation of microcracks is a major concern for the integrity and durability of concrete and other cementitious composites, since such microcracks cannot be easily controlled and detected within the matrix of composite. This work explored the possibility of using surface-sulfonated polystyrene microspheres (SPSM) featuring an average diameter of 0.7 ± 0.5 μm to improve the crack resistance of carbon microfiber-reinforced mortar. The compressive strength and EIS data demonstrated that the incorporation of SPSM at 0.15% by weight of cement led to a denser and more refined microstructure of the cement mortar composite, likely attributable to the active interactions between SPSM and cement hydration products as well as carbon microfibers. The crack resistance of these cement-based composites was evaluated using a non-destructive test under uniaxial compression loading along with a fatigue test under uniaxial compression cyclic loading. The data, in the form of critical stress, specific crack area, and fatigue strain, revealed that even at a small dosage the incorporation of SPSM to carbon microfiber-reinforced mortar retarded the initiation of unrecoverable microcracks and slowed down the propagation of microcracks under uniaxial compression loading, and improved the crack resistance and toughness of the specimens under fatigue loading.     

钢-混凝土组合简支梁滞回性能非线性有限元分析

为实现钢-混凝土组合简支梁滞回性能三维非线性有限元分析,该文提出混凝土在单轴拉、压应力下的损伤变量计算方法,给定了混凝土的卸载规则,完善并提出混凝土应力-应变滞回本构关系和钢材循环本构关系,应用ABAQUS有限元软件建立混凝土棱柱体试件、钢材试件和钢-混凝土组合简支梁有限元三维非线性有限元模型,对混凝土棱柱体试件在单轴受压和受拉反复荷载下的试验结果进行分析,对钢材试件在单轴拉压循环荷载下的试验结果进行分析,对钢-混凝土组合简支梁在循环荷载下的荷载-挠度滞回关系、梁端荷载-滑移滞回关系以及栓钉的荷载-侧向变形滞回关系曲线等试验结果进行分析,计算结果与试验结果符合较好。     

缝合式三维编织C/SiC复合材料拉伸加卸载力学行为

本文通过开展缝合式三维编织C/SiC复合材料的准静态单轴拉伸及循环加卸载试验,研究加卸载行为对复合材料的损伤及材料内部能量耗散的影响。通过对材料的断口分析,探究加卸载对材料破坏强度的影响规律。结果表明,加卸载行为会消耗材料内部的能量,对纤维束与基体之间的界面造成损伤,进而降低材料的承载能力;材料滞回曲线的面积随着卸载点应力的增大而增大;材料的整体失效属于脆性破坏,复合材料断口表现出明显的分层现象,且单轴拉伸时断口相对于循环加卸载更整齐。