Influence of experimental setups on the apparent uniaxial tensile load-bearing capacity of Textile Reinforced Concrete specimens

An important parameter for dimensioning of Textile Reinforced Concrete (TRC) structures and structural elements is the tensile load-bearing capacity of the composite. Respective values are usually derived from uniaxial tensile tests with overcritically reinforced TRC specimens. In this paper, influences from specimen geometry, e.g. plane and waisted specimens, and load application design, e.g. stiff glued steel plates or soft clamping constructions are investigated. Therefore, experimental results regarding the load-bearing capacity of the composite are statistically evaluated. The experimental observations are supported by results of numerical simulations with a one-dimensional model based on the Finite Element Method. These simulations provide stress distributions in concrete and reinforcing fibres as well as the tensile load-bearing capacity. Based on these results existing test setups for the derivation of the load-bearing capacity of the composite for dimensioning are assessed. As a result, plane plate specimens with a load application by means of friction is recommended for experimental determination.     

Durability model for AR-glass fibres in textile reinforced concrete

Textile reinforced concrete (TRC) is an innovative material for thin-walled, structural elements with a high load-bearing capacity. For a safe design of TRC load bearing structures comprehensive investigations were carried out to predict the time-dependent loss in strength of the AR-glass reinforcement embedded in fine grained concrete as a consequence of weathering. The present work describes two different approaches to calculate the amount of strength loss of the reinforcement as a function of material, humidity and temperature. Both models, however, do not take into account outdoor weathering of TRC components which leads to a strength loss that is too high to be realistic. One of these approaches is combined with the component temperature and humidity constantly measured during outdoor weathering to derive a basis for a durability model which can take into account arbitrarily complex weathering. Consequently, this work presents the basis for a quantitative prognosis of the loss in strength of AR-glass reinforcement in TRC adapted to the respective material and weathering conditions.     

Durability of textile reinforced concrete made with AR glass fibre: effect of the matrix composition

This paper presents the results of recent experimentation performed to study time-dependent changes in the mechanical performance of textile reinforced concrete (TRC) made with AR glass fibre and to specify the decisive mechanisms influencing the durability of this composite material. The effect of the matrix composition was investigated by varying hydration kinetics and alkalinity of the binder mix. At first, tensile tests on (accelerated) aged specimens made of TRC were performed. The results showed a pronounced decrease in the tensile strength and strain capacity for TRC whose matrix was most alkaline (Portland cement was used exclusively as binder in this composition). The performance of TRC made with modified, alkali reduced matrix composition was to a great extent unaffected by exposure to accelerated ageing. In order to investigate the mechanisms leading to such different behaviours, changes in the mechanical performance of the fibre–matrix bond were studied using double-sided pullout test specimens with under-critical fibre reinforcement after they had undergone accelerated ageing. Furthermore, the appearance of the microstructure in the interface between fibre and matrix was described by images obtained from SEM-investigations. Measured reductions in the toughness of the composite materials could be attributed mainly to the visual observed disadvantageous new formation of solid phases in the fibre–matrix interface, while the deterioration of the AR glass fibre seemed to play only a secondary role. It could be shown that the morphology of the formed solid hydration phases depends to a large extent on the matrix composition.     

Erratum to: Textile Reinforced Concrete: experimental investigation on design parameters

Textile Reinforced Concrete (TRC) is an advanced cement-based material in which fabrics used as reinforcement can bring significant loads in tension, allowing architects and engineers to use thin cross-sections. Previous research projects, developed during the last 10 years mainly in Germany, Israel and the USA, have shown the capabilities of such a material. In this paper an extensive experimental investigation of TRC is presented: tensile tests were carried out to obtain a complete mechanical characterization of the composite material under standard conditions, considering the influence of different variables such as reinforcement ratio, fabric geometry, curing conditions, displacement rate and specimen size.     

Textile Reinforced Concrete: experimental investigation on design parameters

Textile Reinforced Concrete (TRC) is an advanced cement-based material in which fabrics used as reinforcement can bring significant loads in tension, allowing architects and engineers to use thin cross-sections. Previous research projects, developed during the last 10 years mainly in Germany, Israel and the USA, have shown the capabilities of such a material. In this paper an extensive experimental investigation of TRC is presented: tensile tests were carried out to obtain a complete mechanical characterization of the composite material under standard conditions, considering the influence of different variables such as reinforcement ratio, fabric geometry, curing conditions, displacement rate and specimen size.     

Load–bearing behaviour and simulation of textile reinforced concrete

The new composite material Textile Reinforced Concrete (TRC) is a promising development which may open up entirely new fields for the application of the construction material concrete. The possible more filigree structures with high quality surfaces make TRC an attractive choice for the architect and give the engineer more freedom in design. However, the use of TRC requires design rules which are currently being developed at RWTH Aachen University, Germany. In this article, recent experimental results as well as modeling techniques are described.     

On the separation and recycling behaviour of textile reinforced concrete: an experimental study

This paper presents the results of recent experiments on the recyclability of the textile components in textile reinforced concrete (TRC). TRC as a multi-component system often contains organic ingredients such as carbon fibres and polymer impregnations. Consequently, the recycling of TRC is not trivial and has not yet been sufficiently clarified until now. In this study, an impregnated, bi-axially reinforced, and warp-knitted textiles made of carbon fibres was used in combination with a fine grained concrete. Flexural tests on TRC specimens containing recycled epoxy-impregnated carbon reinforcement were performed, whereby the recycling was simulated by a pre-treatment of the carbon fibre material in a jaw crusher. The results showed a pronounced decrease in flexural strength compared to untreated carbon reinforcement. Moreover, three different crushing methods were investigated with respect to their influence on the recovery of styrene-butadiene-rubber impregnated carbon textiles. Besides jaw crushing and impact milling, crushing with a hammer mill showed the best degree of purity but also caused the highest mechanical damage to the textile. The impact of material, structure of the composite and crushing methods on the separation behaviour could be deduced from the experiments.     

A comparison of engineered cementitious composites versus normal concrete in beam-column joints under reversed cyclic loading

Engineered cementitious composites (ECC) are a class of high-performance fiber reinforced cementitious composite with strain hardening and multiple cracking properties. For a reinforced concrete member, substitution of conventional concrete with ECC can significantly improve the deformation characteristics in terms of reinforced composite tensile or shear strength and energy dissipation ability. In this paper, a number of RC/ECC composite beam-column joints have been tested under reversed cyclic loading to study the effect of substitution of concrete with ECC in the joint zone on the seismic behaviors of composite members. The experimental parameters include shear reinforcement ratio in the joint zone, axial load level on the column and substitution of concrete with ECC or not. According to the test results, for the specimens without shear reinforcement in the joint zone, substitution of concrete with ECC in the joint zone cannot change the brittle shear failure in the joint zone, but can significantly increase the load capacity and ductility of the beam-column joint specimens, as well as the energy dissipation ability due to high ductility and shear strength of ECC material. For the specimens with insufficient or proper shear reinforcement ratio, substitution of concrete with ECC in the joint zone can lead to failure mode change from brittle shear failure in the joint zone to a more ductile failure mode, i.e. flexural failure at the base of the beam, with increased load capacity, ductility and energy dissipation ability. Increase of axial load on column and shear reinforcement in the joint zone have little effect on seismic behaviors of the members when they failed by flexural failure at the base of beam. In a word, the substitution of concrete with ECC in the joint zone was experimentally proved to be an effective method to increase the seismic resistance of beam-column joint specimens.     

Ultrasonic testing of reactive powder concrete

Concrete is a critical material for the construction of infrastructure facilities throughout the world. Traditional concretes consist of cement paste and aggregates ranging in size from 6 to 25 mm that form a heterogeneous material with substantial compressive strength and a very low tensile strength. Steel reinforcement is used to provide tensile strength for reinforced concrete structures and as a composite the material is useful for structural applications. A new material known as reactive powder concrete (RPC) is becoming available. It differs significantly from traditional concrete; RPC has no large aggregates, and contains small steel fibers that provide additional strength and, in some cases, can replace traditional steel reinforcement. Due to its high density and lack of aggregates, ultrasonic inspections at frequencies 10 to 20 times that of traditional concrete inspections are possible. This paper reports on the initial findings of research conducted to determine the applicability of ultrasonic testing techniques for the condition assessment of RPC. Pulse velocities for shear and longitudinal waves and ultrasonic measurement of the modulus of elasticity for RPC are reported. Ultrasonic crack detection for RPC also is investigated.     

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.     

Bond performance of reinforcing bars in inorganic polymer concrete (IPC)

The basic mechanical and chemical properties of fly-ash-based inorganic polymer concretes (IPC) have been studied widely, but, key engineering and structural properties of the material for instance modulus of elasticity, compressive, tensile, flexural strengths and bonding strength of the material to reinforcement have received little attention. Structural applications of reinforced IPC depend on the bond performance of the material to the reinforcement. Due to their difference with ordinary Portland cement (OPC) based concrete in terms of chemical reaction and matrix formation it is not known whether IPC exhibit different bonding performance with the reinforcement. Simply relying on compressive strength of the material and extrapolating models and equations meant for OPC based concrete may lead to unsafe design of structural members. To that end, 27 beam-end specimens, 58 cubic direct pullout type specimens and number of laboratory test specimens were tested to evaluate bonding performance of IPC with reinforcement. The results of beam-end specimens and direct pullout type specimens correlate favourably, although the results of direct pullout tests are in general more conservative than those of beam-end specimens. Overall, it can be concluded that bond performance of IPC mixes are comparable to OPC based concrete and therefore IPC and steel can be used as a composite material to resist tension in addition to compression.     

Influence of short dispersed and short integral glass fibres on the mechanical behaviour of textile-reinforced concrete

This article addresses the influence of the addition of short dispersed and short integral fibres made of alkali-resistant (AR) glass on the fracture behaviour of textile-reinforced concrete (TRC) subject to tensile loading. A series of uniaxial, deformation-controlled tension tests was performed to study the strength, deformation, and fracture behaviour of thin, narrow plates made of TRC, both with and without the addition of short fibres. Additionally, uniaxial tension tests on specimens reinforced with only short fibres were performed to figure out the difference in behaviour in the absence of textile reinforcement. Furthermore, multifilament-yarn and single-fibre pullout tests were carried out to gain a better understanding of bonding properties and crack-bridging behaviour. While pronounced enhancement of first-crack stress was achieved due to the addition of short dispersed fibres (the value increased by a factor of 2), a significant improvement in tensile strength was recorded for TRC specimens with the addition of integral glass fibres; the value increased by approximately 30 %. Moreover, TRC specimens reinforced with short dispersed glass fibres showed formation of more and finer cracks in comparison to the specimens with integral fibres. It was also found that short integral fibres can improve the bond between multifilament-yarns and the surrounding matrix by means of “special” cross-links. In TRC with short dispersed fibres this phenomenon was less pronounced. The investigations were accompanied by microscopical investigations which provided additional basis for an in-depth discussion of the decisive working mechanisms of hybrid reinforcement.     

Creep and cracking of concrete hinges: insight from centric and eccentric compression experiments

Existing design guidelines for concrete hinges consider bending-induced tensile cracking, but the structural behavior is oversimplified to be time-independent. This is the motivation to study creep and bending-induced tensile cracking of initially monolithic concrete hinges systematically. Material tests on plain concrete specimens and structural tests on marginally reinforced concrete hinges are performed. The experiments characterize material and structural creep under centric compression as well as bending-induced tensile cracking and the interaction between creep and cracking of concrete hinges. As for the latter two aims, three nominally identical concrete hinges are subjected to short-term and to longer-term eccentric compression tests. Obtained material and structural creep functions referring to centric compression are found to be very similar. The structural creep activity under eccentric compression is significantly larger because of the interaction between creep and cracking, i.e. bending-induced cracks progressively open and propagate under sustained eccentric loading. As for concrete hinges in frame-like integral bridge construction, it is concluded (i) that realistic simulation of variable loads requires consideration of the here-studied time-dependent behavior and (ii) that permanent compressive normal forces shall be limited by 45% of the ultimate load carrying capacity, in order to avoid damage of concrete hinges under sustained loading.     

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.     

Integrated structures and materials design

This paper introduces the concept of␣Integrated Structures and Materials Design (ISMD). ISMD combines materials engineering and structural engineering for the purpose of more effectively achieving targeted structural performance, by adopting material composite properties as the shared link. An application example, design of a bridge deck link-slab, is used to illustrate the essential elements of ISMD. It is shown that the composite hardened properties—tensile strain capacity, microcrack width, and Young’s Modulus, as well as composite self-consolidating fresh properties, are amongst the most important composite parameters that govern the targeted structural performance of safety, durability and ease of design and implementation. These are also properties that can be controlled in an Engineered Cementitious Composite—an ultra ductile concrete, by tailoring the ingredients for desired fiber, matrix and interface micromechanical parameters. Broad implications of ISMD on educational approach, research collaboration, and next generation infrastructure development, are briefly discussed.     

Influence of prestressing the textile on the tensile behaviour of textile reinforced concrete

In textile reinforced concrete (TRC), the yarns of the textiles are inherently wavy due to the manufacturing process. In most structural applications, this leads to delayed activation of textiles during loading and affects the composite performance negatively. In this context, the paper reports on the significance of mechanically prestressing or stretching the textiles before casting the TRC towards enhancing its load-carrying ability, which has been assessed by comparing the responses of specimens with textiles placed by manual and mechanical stretching. Studies were carried out to determine the uniaxial tensile behaviour of TRC with two types of alkali-resistant glass textiles, woven and bonded and their combinations. TRC with mechanically-stretched woven textile exhibits better performance compared to that of the manually stretched textile, in terms of load-carrying ability though the elongation at failure could be compromised for specimens with bonded textiles. Mechanically stretched textile lead to pronounced strain hardening behaviour, enhancement in the stress at first cracking and stress at peak, which also increase as the number of layers increases. The failure of TRC with manually stretched textile occurs with the pullout of the textile from the matrix contrasting with the rupture of textile when mechanically stretched. X-ray tomography images of the internal structure of TRC further revealed that there is less frictional bond loss and debonding of textile from the matrix for mechanically stretched textile.     

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。      

On the influence of the reinforcement structure of fibrous shells of revolution on the heat conduction in them

The initial boundary-value problem on the heat conduction in shells reinforced with fibers of constant cross section has been considered. It has been established that the specific, anisotropic, inhomogeneous properties of such a shell are determined by its heat conductivity dependent on the thermophysical properties of the phases of the composite material of the shell, the parameters of its reinforcement, and the geometry of this shell. The ways of reducing the three-dimensional problem on heat conduction to the two-dimensional one and the possibilities of reducing the dimension of this problem for thin shells of revolution reinforced symmetrically relative to their axis by two units have been determined. The stationary temperature fields of concrete thin shells of revolution with different Gaussian curvatures and different reinforcement structures have been compared. It is shown that the reinforcement structure and the geometry of a shell of revolution substantially influence the temperature distribution in this shell, which opens up a wide range of ways selecting designs of such shells with improved thermophysical parameters. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 79, No. 2, pp. 145–155, March–April, 2006.     

Modeling the dynamic properties of fibre-reinforced concrete with different coating technologies of multifilament yarns

The dynamic behaviour of fibre-reinforced, cementitious composite materials is gaining increasing interest. With respect to service life dynamic loading just under the elastic limit of the material at hand is relevant to practical applications, for the resulting (stress-)waves may be focused within regions of the heterogeneous composite material. This local overstraining of the material may lead to deterioration of the structural element. In this contribution, the effect of the set-up of the reinforcing fibres on the wave scattering behaviour is investigated. Special attention is paid to layered centric configurations of these fibres, as it occurs e. g. within textile-reinforced concrete (TRC). A mechanical model is developed and solved analytically providing an efficient and robust method to describe the dynamic behaviour of given fibre configurations. This method is needed for materials which have to be described mechanically before the manufacturing process – as it is the case for TRC. The proposed model also allows for planning experiments and thus is of additional value. It is shown that the inner structure of the fibres does influence the amplitude response spectra and consequently the proposed method also may be used for non-destructively detecting the inner structure of the multifilament yarns and other related objects.