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。     

反分析法确定钢纤维水泥砂浆拉应力与裂缝张开位移关系

在数值模拟钢纤维混凝土结构或构件和用基于断裂力学理论的设计方法设计钢纤维混凝土结构或构件时,钢纤维混凝土材料σ-w关系是一个重要的材料参数。该文根据三点弯曲缺口梁的荷载与裂缝张开位移(CMOD)曲线用反分析法确定了钢纤维水泥砂浆的σ-w关系。试验制作了五种不同体积含量的钢纤维水泥砂浆单轴拉伸试件、圆柱体压缩试件和梁试件,钢纤维体积含量分别为0.5%、1.0%、1.5%、2.0%和2.5%。试验发现,当钢纤维体积含量较小时,三点弯曲缺口梁的荷载与裂缝张开位移曲线呈现CMOD软化特性;而当钢纤维体积含量较大时,三点弯曲缺口梁的荷载与裂缝张开位移曲线呈现CMOD强化特性。对有CMOD软化特性的钢纤维水泥砂浆,可用三折线软化模型来模拟钢纤维水泥砂浆的σ-w关系;对有CMOD强化特性的钢纤维水泥砂浆,可用应力跌落-常残余强度模型来拟合。     

Analysis on Flexural Behavior of UHPFRC Beams based on Tensile Stress-Crack Opening Relationship

The objective of this study is to investigate the differences between the tensile stress-crack opening relationships of the small size notched beam and the real size beam which were made of two ultra-high performance fiber reinforced concretes (UHPFRCs) having different volume fractions and lengths of fibers. The stress-crack opening relationships of two UHPFRCs were first obtained from the inverse analysis for the small size notched beam tests. In addition, the three types of real size beams were manufactured for each mix: (1) plain beam, (2) beam with tensile reinforcement, and (3) beam with both tensile and compressive reinforcements. The flexural tests of the plain and reinforced beams were conducted up to a failure state. The load-deflection curves of the plain and reinforced UHPFRC beams calculated based on the tensile stress-crack opening relationship of the notched beams did not give an accurate prediction on the measured load-deflection curves of the real size beams. The tensile stress-crack relationships accurately fitting the measured load-deflection curves were additionally found, and the difference in the tensile stress-crack opening relationships of the small size notched beams and the real size beams was analyzed in this study.     

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.     

Adaptive inverse analysis (AIA) applied and verified on various fiber reinforced concrete composites

During the past decades several inverse approaches have been developed to identify the stress-crack opening (\({\sigma }-w\)) by means of indirect test methods, such as the notched three point bending-, wedge splitting-, and round panel testing. The aim is to establish reliable constitutive models for the tensile behavior of fiber reinforced concrete materials, suitable for structural design. Within this context, the adaptive inverse analysis (AIA) was recently developed to facilitate a fully general and automatized inverse analysis scheme, which is applicable in conjunction with analytical or finite element simulation of the experimental response. This paper presents a new formulation of the adaptive refinement criterion of the AIA method. The paper demonstrates that the refinement criterion of the nonlinear least square curve fitting process, is significantly improved by coupling the model error to the crack mouth opening and the crack opening displacement relationship (\(w_{\mathrm{cmod}}-w_{\mathrm{cod}}\)). This enables an adaptive refinement of the \({\sigma }-w\) model in the line segment with maximum model error, which entails significant improvement of the numerical efficiency of the AIA method without any loss of robustness. The improved method is applied on various fiber reinforced concrete composites and the results are benchmarked with the inverse analysis method suggested by the Japanese Concrete Institute (Method of test for fracture energy of concrete by use of notched beam, Japanese Concrete Institute Standard, Tokyo, 2003) and recently adopted in ISO 19044 (Test methods for fibre-reinforced cementitious composites—load-displacement curve using notched specimen, 2015). The benchmarking demonstrates that the AIA method, in contradiction to the JCI/ISO method, facilitates direct determination of the tensile strength and operational multi-linear \({\sigma }-w\) models.     

Modeling of concrete cracking—A hybrid technique of using displacement discontinuity element method and direct boundary element method

This paper presents a new approach by making use of a hybrid method of using the displacement discontinuity element method and direct boundary element method to model concrete cracking by incorporating fictitious crack model. Fracture mechanics approach is followed using the Hillerborg's fictitious crack model. A boundary element based substructure method and a hybrid technique of using displacement discontinuity element method and direct boundary element method are compared in this paper. In order to represent the process zone ahead of the crack, closing forces are assumed to act in such a way that they obey a linear normal stress-crack opening displacement law. Plain concrete beams with and without initial crack under three-point loading were analyzed by both the methods. The numerical results obtained were shown to agree well with the results from existing finite element method. The model is capable of reproducing the whole range of load–deflection response including strain-softening and snap-back behavior as illustrated in the numerical examples.     

钢-混凝土组合梁收缩徐变分析的有限元方法

基于按龄期调整的有效模量法,提出了部分剪力连接钢-混凝土组合梁在长期荷载作用下收缩徐变分析的简化有限元模型,并通过建立特殊的剪力连接件单元刚度矩阵和利用Newton-Raphson迭代方法考虑滑移效应,同时考虑了负弯矩区混凝土板开裂对组合梁刚度和强度的影响。利用该模型计算了连续组合梁在长期荷载作用下的挠度、应力、滑移量,计算结果与已有的理论计算结果和实验结果吻合,证明本模型用于分析钢-混凝土组合梁收缩徐变是可靠的。     

Fracture mechanical analysis of strengthened concrete tension members with one crack

A concrete tension member strengthened with fiber reinforced polymer plates on two sides is analyzed with non-linear fracture mechanics. The analysis of the strengthened tension member incorporates cohesive properties for both concrete and interface between concrete and strengthening medium, and results in closed form solutions for the load-crack opening relationship. To distinguish between single and multiple cracking in a design process, structural classification parameters are derived. The minimum reinforcement ratio for obtaining multiple cracking derived from the structural classification parameters, is investigated in a non-dimensional analysis, and found to depend strongly on the ratio between interfacial and concrete fracture energies.     

The interfacial fracture characteristics of bimaterial and sandwich blister specimens

The interfacial crack initiation characteristics of bimaterial and sandwich strip blister specimens were compared. Interface cracks were grown along glass/epoxy interfaces and the corresponding loads, normal crack opening displacements (NCOD) and crack front geometries were measured. Finite element analyses were used to compare NCOD, extract fracture parameters and examine near front stress fields. On the small scale of crack extension that could be measured, the bimaterial specimens exhibited resistance curve behavior whereas none could be resolved in the sandwich specimens. The phase angle dependence of toughness in the bimaterial specimens was similar to that obtained in previous blister tests but was much steeper than has otherwise been measured. The toughness of the glass/epoxy interface in the sandwich specimen decreased with decreasing epoxy thickness in spite of the fact that it was never completely spanned by plastic zones.     

高强钢绞线网增强ECC与混凝土界面黏结-滑移关系研究EI北大核心CSCD

良好的界面黏结是保证高强钢绞线网增强工程用水泥基复合材料(HSSWM-ECC)与混凝土协同工作的前提,其界面黏结的有效程度决定着HSSWM-ECC材料性能的发挥。为研究HSSWM-ECC与混凝土界面黏结性能,以混凝土抗压强度、界面黏结长度、黏结宽度和界面处理方式为参数,对设计制作的9组27个梁铰式试件进行了界面黏结性能试验。试验结果表明:界面黏结-滑移受力过程呈现明显的两阶段特征:非线性上升段和下降段。基于试验结果,探究了HSSWM-ECC与混凝土间界面黏结破坏特征和受力机理,构建了考虑各参数影响的界面黏结-滑移关系模型;采用微段分析法对模型特征参数进行了分析,结果表明:所建模型及特征参数计算与试验结果吻合良好,可较好表征HSSWM-ECC与混凝土界面黏结-滑移关系力学行为。     

Dynamic response of bimaterial and graded interface cracks under impact loading

The interfacial fracture in bimaterial and functionally graded material (FGM) under impact loading conditions is investigated using experimental and numerical techniques that are valid for both type of interfaces. Experiments are conducted on epoxy based specimens in three point bend configuration and the complex SIF is measured using an electrical strain gage mounted close to the crack-tip. A complementary two-dimensional finite element simulation is performed using tup force and support reactions as input tractions, and the SIF-time history is determined using a displacement extrapolation technique. The experimentally determined SIF-histories match closely with numerical simulation up to the time of fracture initiation. The test results show that the mode-mixity remains nearly constant through out the test in both the materials, and the mixity values correspond to their respective static counterparts. The general dynamic response of the bimaterial and FGM specimens in terms of impact load, support reaction and the magnitude of complex SIF are comparable, and the mode-mixity is the parameter that distinguishes the graded interface from the bimaterial case.     

Mechanical and fracture properties of a self-compacting version of CARDIFRC Mix II

CARDIFRC is the trade name of two main groups of ultra-high performance fibre-reinforced concrete mixes – Mixes I and II – differing primarily in the maximum size of quartz sand used (0.6 mm in Mix I, and 2 mm in Mix II). In this paper, the conversion of CARDIFRC Mix II to a self-compacting and industrially competitive ultra-high performance fibre-reinforced concrete (UHPFRC) is described. A full mechanical and fracture characterisation (i.e. size-independent fracture energy and the corresponding bi-linear stress-crack opening relationship) of this UHPFRC is provided.     

Mechanism for salt scaling of a cementitious surface

Freezing and thawing of concrete in the presence of deicer salts results in superficial damage known as salt scaling. Scaling damage consists of the removal of small flakes from the surface, leaving the body susceptible to water and ion ingress, thus posing a significant threat to the durability of the body. None of the proposed mechanisms for salt scaling account for all of the phenomenology observed during previous studies. We report a novel experimental method designed to measure the stress that arises when a solution is frozen on a cementitious plate. These experiments reveal a thermal expansion mismatch (or, bimaterial) mechanism that accounts for all of the observed salt scaling phenomenology. According to the bimaterial mechanism, scaling occurs when the stress in the freezing layer rises above the tensile strength of the brine-containing ice, resulting in cracking. A viscoelastic analysis of the stresses in the brine/ice layer shows that pure ice would not crack, but a layer containing >1% NaCl would. The damage from cracking of the ice is exacerbated by weakening of the cement paste by exposure to concentrated brine.     

具有理想与非理想界面的双材料四点弯曲试件的界面断裂韧性

本文作者用边界元法研究了具有理想与非理想界面的双材料四点弯曲试件的界面断裂韧性.分析了理想界面裂纹尖端的复应力强度因子以及非理想界面裂纹前沿的应变能密度与试件的几何尺寸、双材料的性质等的关系.计算结果表明,当裂纹的长度小于试件内支点的跨度时,上述两个物理量在很大范围内不随裂纹长度而变化.这种稳态的特性为两者的临界值的实验测定提供了方便.      

不同荷载条件下型钢-钢纤维混凝土组合结构的界面失效机理研究

为了解决型钢混凝土结构中型钢与钢筋相互干扰、混凝土浇筑困难等施工难题,将型钢混凝土结构中的钢筋笼完全或部分替换成钢纤维,形成了型钢-钢纤维混凝土组合结构。完成了36个试件的推出试验和13个试件的四点弯试验,分别研究了型钢-钢纤维混凝土组合结构在轴心力与弯矩作用下的界面失效,分析了不同荷载条件下的内力传递与破坏机理。钢纤维在混凝土裂缝处的拉拔行为形成了“桥接”效应,约束了裂缝扩展,改善混凝土裂后的抗拉性能,可以解决因保护层减小而导致的型钢与钢纤维混凝土界面黏结性能变差的问题,延缓甚至避免界面黏结失效的发生。轴心力作用下,因泊松比产生了型钢与钢纤维混凝土之间的界面挤压,钢纤维混凝土在两个正交水平方向承受拉力,这是钢纤维混凝土损伤与开裂的主要原因。钢纤维混凝土的损伤程度与界面的黏结性能有直接的关系,同时受到钢材泊松比的影响。在四点弯试验中,黏结裂缝集中出现在仅承受弯矩的跨中区域,型钢与钢纤维混凝土之间的内力传递以及由此产生的界面拉应力是导致黏结裂缝大量出现的根本原因。黏结裂缝首先出现在钢翼缘的肢尖位置,并随着荷载的增大由内向外发展,直至发展到表面,形成可视裂缝。跨中区域最终形成了钢纤维混凝土保护层梯形破坏面。     

Analysis technique for restrained shrinkage of concrete containing chlorides

Cracks in concrete containing chlorides easily occur due to restraint conditions and they can be the main reasons of durability and safety issues. In this paper, analysis technique which can handle mixed chloride and its effect on restrained drying shrinkage is proposed. For the evaluation of stress development and cracking time due to restrained drying shrinkage, free and restrained drying shrinkage test are carried out for concrete specimens containing different sodium chloride (NaCl) content. The results show that mixed chloride content increases restraint stress but does not increase strength. Considering the effect of chloride on shrinkage based on the test results, effective restraint stress development and cracking of concrete specimens containing different level of chloride are evaluated through utilizing previously developed models for behaviors in early-age concrete like hydration and moisture transport. The results from this proposed technique are verified by comparison with test results.     

New testing method for assessing the cracking sensibility of stressed tendon rods in aggressive environments

The paper presents a new testing method for assessing the cracking sensibility in aggressive environments of tendon rods for prestressed concrete structures, on the basis of Fracture Mechanics concepts. First, it approaches the fundamentals of the designed test in the context of existing fracture specimens, regarding the geometrical limitations introduced by the environmental assisted cracking of the rods, perpendicularly to the load direction. The analysis showed that fatigue precracked chevron-notched short bar specimen (SBS) is providing the largest measurement range of stress intensity factor to be explored in stress-corrosion cracking (SCC) tests. Then, the equation relating the elastic stiffness of SBSs to crack size was experimentally validated for specimens with true cracks produced by fatigue. SCC verification tests were made with an innovative horizontal loading device and the crack mouth opening displacement (CMOD) was acquired and numerically analyzed with a video digital image correlation system. The tests showed that SCC is fully governed by the small scale-yielding regime at the crack tip. Hence, the main process parameters as crack extension and environment-assisted stress intensity factor were empirically obtained for each time sequence of the SCC tests. To simplify further SCC tests instrumentation, an empirical correlation was stated between CMOD and crack opening displacement (COD) as measured by a conventional extensometer mounted on the loading grips of the specimen. This correlation was also validated in the SCC tests.     

高性能混凝土抗裂性能研究

该文对高性能混凝土的抗裂性能进行了系统的研究,主要包括:高性能混凝土收缩与抗裂性能的试验方法,研制出一种测试混凝土抗裂性能的诱导开裂试验方法,该方法避免了混凝土裂缝出现位置的随机性以及混凝土塑性沉降引起的材料不均匀等带来的测试结果不准确,同时研制出一种多点、实时、自动监测混凝土试件开裂时间的系统,可应用于测试混凝土早期的裂缝出现时间;进行了大量自生收缩试验和干燥收缩试验,对影响高性能混凝土自生收缩和干燥收缩的因素进行了深入的研究,提出了减少收缩变形的混凝土原材料选用和粉煤灰及矿渣单掺与多掺的最佳掺量,提出了高性能混凝土自生收缩和干燥收缩计算模型;采用平板诱导开裂法和圆环测试系统进行了大量的抗裂性能试验,深入研究了混凝土原材料特别是粉煤灰及矿渣影响高性能混凝土抗裂性能的规律,提出了实现高抗裂性能的混凝土原材料选用和粉煤灰及矿渣单掺与多掺的最佳掺量;研究成果成功地应用于实际工程中,取得了显著的技术和经济效益。     

Shear capacity of steel and polymer fibre reinforced concrete beams

This paper deals with the application of a plasticity model for shear strength estimation of fibre reinforced concrete beams without stirrups. When using plastic theory to shear problems in structural concrete, the so-called effective strengths are introduced, usually determined by calibrating the plastic solutions with tests. This approach is, however, problematic when dealing with fibre reinforced concrete (FRC), as the effective strengths depend also on the type and the amount of fibres. In this paper, it is suggested that the effective tensile strength of FRC can be determined on the basis of the tensile stress-crack opening relationship found from wedge splitting tests. To determine the effective compressive strength of FRC, it is proposed to adopt the formula used for conventional concrete and modify it by introducing a fibre enhancement factor to describe the effect of fibres on the compressive softening behaviour of FRC. The enhancement factor is determined as the ratio of the areas below the stress–strain curves for FRC and for conventional concrete. The outlined approach has been verified by shear testing of beams containing no fibres, 0.5% steel fibre volume and 0.5% polymer fibre volume. The tests results are compared with estimations and show satisfactory agreements, indicating that the proposed approach can be used.