Material and bonding characteristics for dimensioning and modelling of textile reinforced concrete (TRC) elements

The collaborative research center “Textile Reinforced Concrete (TRC) – Development of a New Technology” (SFB 532) established at Aachen University (RWTH Aachen) is investigating the basic mechanisms of this new composite material. The use of technical textiles as reinforcement material in cementitious binder systems allows the production of thin-structured elements as will be dimensioned, modelled, and produced within the research project. For this reason the material properties of the single components have to be known and will be integrated in analytical and numerical simulations of textile reinforced structures. Thus key parameters on the meso-level are introduced. These are on the one hand the tensile strength and elastic modulus of filaments and rovings, on the other hand mechanical and fracture mechanical parameters of the matrix, and finally the bonding characteristics of filaments as well as rovings embedded in the cement based matrix.     

纤维编织网增强混凝土的拉伸力学模型

通过单束纤维与纤维编织网增强混凝土（TRC）薄板的单轴拉伸试验, 研究了环氧树脂浸渍后纤维束的应力-应变关系。试验结果发现, 即使经环氧树脂浸渍, 也不能完全发挥碳纤维束的抗拉强度; 单轴拉伸碳纤维束获得的应力-应变曲线几乎是完全线性的, 在达到其极限强度80%左右出现一定的非线性特征。假定细粒混凝土开裂后只有纤维编织网承担荷载, 从而由TRC薄板试件单轴拉伸的荷载-变形关系获得的碳纤维束的应力-应变关系可合理简化为双线性的形式, 并针对本文中研究的纤维编织网给出了其相应控制参数点的取值。基于受弯构件计算理论, 采用本文纤维束拉伸模型得到的两种系列受弯构件的计算值与试验值吻合较好。     

Anchoring of shear strengthening for T-beams made of textile reinforced concrete (TRC)

Test results indicate that thin layers of concrete utilizing textile reinforcement can be used as shear strengthening for Reinforced Concrete (RC)-T-beams thereby increasing the shear capacity of these particular types of beams. Serviceability properties are also improved; in particular, the amount of deflection is reduced, as well as crack widths. This article provides a comprehensive discussion of the detail of force transfer from the textile reinforced strengthening layer to the existing concrete of RC-members, as well as the characteristics of the individual failure levels. In addition, the effects from adhesive tensile bond and shear bond are described in detail, and the interrelationship between the transferable bond force and the associated bond length is shown. The load carrying capacity of the bond represents a critical load for highly reinforced strengthening layers. Furthermore, this article shows how, in these particular cases, the effects of strengthening can be further increased by the application of additional mechanical anchoring in the pressure zone.

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Résumé Il est montré avec les résultats d’essai, que les minces couches avec une armature textile peuvent être utilisée pour un renforcement de force transversale des poutres en T en béton armé. La capacité portante de relevé peut être agrandie par un renforcement des poutres en T. Il se montre, que la qualité d’état de service peut être bonifiée. Ce sont surtout les flèches réduites et les larges de fissure plus petits. Le détail de la transmission de la force du renforcement armé textile au vieux béton est examiné pleinement. On évoque les particularités des différents modes de défaillance. Les effets d’adhérence de tension adhésif et de ciseaux sont décrits en détail, et le rapport entre la force d’adhérence transférable et la longueur d’ancrage adéquat est montré. Avec des couches haut-renforcées de renforcement la capacité portante d’adhérence représente une charge critique. On montre comment l’effet de renforcement en ces cas peut être encore augmenté par une ancrage mécanique additionnel dans la zone comprimée.

     

A computational model for failure analysis of fibre reinforced concrete with discrete treatment of fibres

Failure patterns and mechanical behaviour of high-performance fibre reinforced cementitious composites depend on the distribution of fibres within a specimen. In this contribution, we propose a novel computational approach to describe failure processes in fibre reinforced concrete. A discrete treatment of fibres enables us to study the influence of various fibre distributions on the mechanical properties of the material. To ensure numerical efficiency, fibres are not explicitly discretized but they are modelled by applying discrete forces to a background mesh. The background mesh represents the matrix while the discrete forces represent the interaction between fibres and matrix. These forces are assumed to be equal to fibre pull-out forces. With this approach experimental data or micro mechanical models, including detailed information about the fibre-matrix interface, can be directly incorporated into the model.     

低配网率纤维编织网增强混凝土轴拉力学性能

通过单束纤维与纤维编织网增强混凝土(Textile reinforced concrete, TRC)薄板的单轴拉伸试验, 研究了纤维编织网增强混凝土这种新型材料的受力特征和影响其极限承载力的主要因素, 提出临界配网率的概念。以配网率为变化参数, 讨论了在低配网率(配网率小于1%)时材料的力学性能。当配网率大于临界配网率时, 纤维编织网增强混凝土薄板的极限承载力大于其开裂荷载, 加载过程中没有承载力突降; 随着配网率增加, 纤维的利用率随着配网率增加呈线性降低, 即纤维丝的强度并不能完全发挥。低配网率时, 随着配网率增加, 薄板的裂缝间距减小, 裂缝宽度下降。根据混合定律和ACK模型把TRC薄板的拉伸应力-应变曲线简化成三线型模型, 从宏观上提出了考虑配网率影响的极限承载力计算公式。      

Correlation of constitutive response of hybrid textile reinforced concrete from tensile and flexural tests

This papers addresses the disparities that exist in measuring the constitutive properties of thin section cement composites using a combination of tensile and flexural tests. It is shown that when the test results are analyzed using a simplified linear analysis, the variability between the results of tensile and flexural strength can be as high as 200–300%. Experimental results of tension and flexural tests of laminated Textile Reinforced Concrete (TRC) composites with alkali resistant (AR) glass, carbon, aramid, polypropylene textile fabrics, and a hybrid reinforcing system with aramid and polypropylene are presented. Correlation of material properties is studied analytically using a parametric model for simulation of flexural behavior using a closed form solution based on tensile stress–strain constitutive relation. The flexural load carrying capacity of TRC composites is computed using a back-calculation approach, and parameters for a strain hardening material model are obtained using the closed form equations. While the parametric model over predicts the simulated tensile response for carbon and polypropylene TRCs, predictions are however consistent with experimental trends for aramid and glass TRCs. Detailed discussion of the differences between backcalculated and experimental tensile properties is presented. Results can be implemented as average moment–curvature relationship in the structural design and analysis of cement composites.     

Durability issues of FRP rebars in reinforced concrete members

The use of fibre reinforced polymers (FRPs) as rebars in reinforced concrete (RC) elements is a viable means to prevent corrosion effects that reduce the service life of members employing steel reinforcement. However, durability of FRP rebars is not straightforward as it is related to material properties as well as bar–concrete interaction. A state of the art of durability of FRP rebars is presented herein in order to highlight issues related to the material properties and interaction mechanisms which influence the service life of RC elements. The design approach implemented in international codes is discussed and the reduction factors taking into account the durability performances are summarized.     

Durability and simulated ageing of new matrix glass fibre reinforced concrete

This paper applies a model of glass fibre reinforced concrete (grc) degradation, based on the glass technology principle of static fatigue, to a large body of diverse data, some collected from both industry and open literature and some generated by the authors. Activation energies for the strength loss process in OPC and modified matrix grcs are derived and it is shown that the modified matrix grcs degrade by a different mechanism to OPC grc. In consequence, using traditional acceleration factors to predict the long term behaviour of modified matrix grc can lead to severe over-estimation of durability. It is also concluded that hot water ageing is not suitable for polymer modified grc and that ageing at temperatures over 65 °C may lead to the activation of strength loss processes not encountered during natural weathering in some matrices.     

Concepts for textile reinforcements for timber structures

This paper presents general remarks on the development of timber structures as a result of technological changes. It discusses drawbacks of wood with respect to structural application. Shear and transversal strengths are very low due to the anisotropy of wood. Various techniques are applicable to cope with this drawback. This paper presents the reinforcement by means of tailor made textiles as comprehensive technology to deal with this kind of difficulties in many design situations. Particular importance is attached to the material productivity during the transformation of raw wood into structural cross sections. It is shown that cut offs in the sawmill and the predominating full section lead to a poor use of the material compared to technical profiles. A new understanding of the wood as a cellular solid is presented that allows a thermo-hygro-mechanical forming of cross sections from solid wood panels.

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Résumé Cet article présente des considérations générales sur l’évolution de constructions en bois en␣relation avec les changements technologiques. Il␣discute les inconvénients de l’utilisation structurale du bois. Vu l’anisotropie du matériau, la résistance au cisaillement et celle perpendiculaire aux fibres sont très faibles. Diverses techniques ont été proposées dans le passé pour pallier ces inconvénients. Les renforcements textiles sur mesure sont présentés comme technologie universelle permettant de surmonter ces difficultés dans la construction. Un accent particulier est mis sur la productivité du matériau à l’égard de la transformation de la grume dans une section portante. Il est démontré que les déchets dans la scierie ainsi que les sections solides mènent à une mauvaise exploitation du matériau bois comparée à celle des profilés métalliques et plastiques. Une nouvelle conception du bois comme solide cellulaire est présentée∼; elle permet un formage thermo-hygro-mécanique des sections tubulaires à partir des panneaux de bois massifs.

     

Durability characteristics of nano-GFRP composite reinforcing bars for concrete structures in moist and alkaline environments

In this study, nano glass fibre-reinforced polymer (nano-GFRP) composite reinforcing bars (rebar) for concrete structures were fabricated, and their performance during prolonged exposure to water and alkaline solutions was evaluated. Nano-GFRP rebar was prepared using the pultrusion method by adding nanomaterials to vinyl ester resin. The nanomaterials, including silica (SiO₂), alumina (Al₂O₃), and silicon nitride (Si₃N₄), were added to vinyl ester resin at 1, 3, and 5 wt.%. After exposure to water and alkaline solutions for 50 and 115 days, tensile strength tests were carried out, and the moisture diffusion coefficient was determined and compared with that of general GFRP rebar. The results indicated that the tensile residual strengths of nano-GFRP rebar and general GFRP rebar were almost the same after exposure to water, whereas the tensile residual strength of nano-GFRP rebar was higher than that of general GFRP rebar after exposure to alkaline solution. The moisture diffusion coefficient of the alumina and silicon nitride samples increased slightly after exposure to alkaline solution, but the silica sample had a higher moisture diffusion coefficient; however, the moisture diffusion coefficients of all nano-GFRP rebar samples were comparable to that of general GFRP rebar.     

Statistical modelling of carbonation in reinforced concrete

As time passes the properties of concrete change as a result of its interaction with the environment and durability is affected. Reinforcement corrosion is singled out in various studies as being mainly responsible for reinforced concrete degradation. Concrete alkalinity protects the reinforcement bars from corrosion but the carbonation phenomenon significantly contributes to the destruction of their passive coating, thus favouring the corrosion onset. Therefore, concrete carbonation is considered an important problem both in Civil Engineering and in Materials Science. This study’s main objective is to try to quantify the contribution of potential conditioning factors to concrete carbonation’s rate. This study addresses the statistical modelling of the concrete carbonation phenomenon, using a large number of results (964 case studies), collected in the literature. A computational method (multiple linear regression analysis) is used to define a mathematical model that can estimate the carbonation coefficient as a function of a set of conditioning factors. These models allow the estimation of the carbonation coefficient, and consequently the carbonation as a function of the variables considered statistically significant in explaining this phenomenon. Two distinct models are proposed to suit predictions to two environmental exposure relative humidity ranges.     

Constitutive model for fibre reinforced concrete based on the Barcelona test

Several constitutive models for fibre reinforced concrete (FRC) have been reported in the past years based on the flexural performance obtained in a bending test. The Barcelona test was presented as an alternative to characterise the tensile properties of FRC; however, no constitutive model was derived from it. In this article, a formulation to predict the tensile behaviour of FRC is developed based on the results of the Barcelona test. The constitutive model proposed is validated by simulating the results of an experimental program involving different types of fibres and fibre contents by means of finite element software. Moreover, the simplified formulation proposed is compared with constitutive models from European codes and guidelines.     

Fatigue life prediction of fiber reinforced concrete under flexural load

This paper presents a semi-analytical method to predict fatigue behavior in flexure of fiber reinforced concrete (FRC) based on the equilibrium of force in the critical cracked section. The model relies on the cyclic bridging law, the so-called stress–crack width relationship under cyclic tensile load as the fundamental constitutive relationship in tension. The numerical results in terms of fatigue crack length and crack mouth opening displacement as a function of load cycles are obtained for given maximum and minimum flexure load levels. Good correlation between experiments and the model predictions is found. Furthermore, the minimum load effect on the fatigue life of beams under bending load, which has been studied experimentally in the past, is simulated and a mechanism-based explanation is provided in theory. This basic analysis leads to the conclusion that the fatigue performance in flexure of FRC materials is strongly influenced by the cyclic stress–crack width relationship within the fracture zone. The optimum fatigue behavior of FRC structures in bending can be achieved by optimising the bond properties of aggregate–matrix and fiber–matrix interfaces.     

Thermo-mechanical properties of fiber reinforced raw perlite concrete

The effect of hooked steel, wavy steel and polypropylene fibers on the thermo-mechanical properties of raw perlite aggregate concrete was investigated. In order to determine the effect of fiber ratio on the thermo-mechanical properties of 100% raw perlite concrete, 0.25%, 0.75%, 1.25%, and 1.75% fiber ratios were used by volume of the sample and also, 350 kg/m³ cement dosage and 3 ± 1 cm slump were used. When compared to the control sample that contains no fiber, (1) with the increase of steel fiber ratio in the mixtures thermal conductivity (TC), unit weight, splitting-tensile strength, and flexural strength of concretes increased, (2) with the increase of steel fiber ratio in the mixtures compressive strength of concretes decreased, and (3) with the increase of polypropylene fiber ratio in the mixtures TC, unit weight, compressive strength, splitting-tensile strength, and flexural strength of concretes decreased.     

水泥基复合材料中纤维拉拔的变位约束细观力学模型

从细观尺度研究受拉水泥基复合材料中倾斜纤维的桥联行为,以分析承受轴向拉伸的随机各向分布纤维增强混凝土的力学性能。将单根随机纤维的理论解在三维空间进行积分运算有效描述受拉纤维混凝土的受力过程。给出了拉拔力、拉拔总功与裂纹张开位移和纤维倾斜角的函数关系。结果表明,增加纤维的倾斜角度会降低纤维的裂纹桥联力,拉拔力和拉拔总功的理论计算结果与试验结果有较好的一致性。      

Modelling the loss in strength of AR-glass fibres in textile-reinforced concrete

AR-glass-reinforcement in concrete, which for example is used in the new composite material textile-reinforced concrete, loses strength in the long term as a consequence of weathering. The knowledge of this time-dependant development of strength loss is an essential aspect for the application of textile-reinforced components in structurally relevant areas. The present work describes investigations about the development of strength loss of AR-glass-reinforcement in concrete caused by weathering. First of all, the main reason for the loss in strength of AR-glass in concrete was determined [1]. Based on this, a mathematical approach to describe the loss in strength was derived for constant climatic conditions. This model was verified with numerous test results acquired by various investigation methods. The influence of humidity, temperature, pH-value and glass composition on the loss in strength of the AR-glass-reinforcement was investigated. The loss in strength of the reinforcement starts above a critical humidity and increases exponentially with higher temperatures. Additionally combined loads were analysed to finally derive a basis for a durability-model which can take arbitrarily complex weathering into account. Pull-out investigations underline that the bond between AR-glass-reinforcement and matrix is independent of the long-term weathering.     

Corrosion process and abatement in reinforced concrete wrapped by fiber reinforced polymer

The corrosion performance of steel reinforcement embedded in concrete samples encased by carbon fiber reinforced polymer (CFRP) wraps was investigated experimentally. Concrete samples were wrapped with 0−3 fabric layers impregnated with one of two different epoxies. To accelerate corrosion, samples were subjected to an impressed current and a high salinity solution. Current flow measurements dynamically monitored corrosion activity during exposure, while reinforcement mass losses were measured following exposure. Theoretical predictions of total mass loss were compared with actual corrosion mass loss values. Test results indicated that CFRP wrapped specimens had prolonged test life, decreased reinforcement mass loss, and lower corrosion rates. The performance of wrapped specimens was superior to that of either control samples or those coated only with epoxy. Results indicated that the level of corrosion abatement provided by the CFRP wraps was influenced both by the type of epoxy used and the number of wrap layers.     

A plasticity based model and an adaptive algorithm for finite element analysis of reinforced concrete panels

This paper deals with an adaptive finite element procedure for the analysis of plain and reinforced concrete panels in a state of plane stress. Therefore, we will present a plasticity based model for plain concrete which captures the two failure modes of concrete within one formulation. In spite of a simple formulation the model is capable to describe the different mechanisms for tensile failure as well as for compression fracture. To restrict the time discretization error and the spatial discretization error to certain tolerances, the constitutive model is embedded in an adaptive algorithm which controls the size of the incremental load steps and leads to a hierarchical mesh refinement if necessary. The application of the model will be shown by various numerical examples. Copyright © 2002 John Wiley & Sons, Ltd.     

Strain distribution along lapped joints in reinforced concrete

Strain measurements along lapped joints in reinforced concrete tension members were obtained using a method of internally strain gauging the reinforcing rods. Strain concentration gauges, installed at the lap ends of some specimens, demonstrated the localised nature of force transfer prior to crack formation.
Results from ten test specimens of varying lap length and rod diameter have been produced. The changes observed in the longitudinal strain distributions, and the associated bond and concrete stresses as cracks developed are presented.