Keywords: textile-reinforced concrete, multi-filament yarn, multi-layer model, bond stress-slip relation, tension-stiffening, non-linear analysis.

The novel composite material named "textile-reinforced concrete" combines the ample compressive strength of concrete with the corrosion resistance and the tensile strength of the fibres. Some industrial units have already started with the production of formwork-systems, storing boxes and facade-elements with integrated heat insulation [4]. Textile-reinforced concrete can be used instead of conventional composite building materials for many new applications.

The composite material consists of multi-filament yarn embedded in a cementitious matrix. The yarn can be considered as being made up of two components, namely the inner and the outer filaments. While the latter are in direct contact with the cementitious matrix, the former do not, thus exhibiting a different load-carrying behaviour. The load-carrying capacity and damage behaviour of textile-reinforced structural members is heavily dependent upon the bond characteristics between the filaments themselves and also between the outer filaments and the cementitious matrix. Normally, the filaments as constituents of the yarn are arranged randomly within its cross-section and along its longitudinal direction. The penetration depth of the matrix into the yarn is another variable in the longitudinal direction. For the evaluation of the bonding behaviour of textile-reinforced concrete it is, therefore, necessary to numerically describe the local failure mechanisms of textile-reinforced concrete occurring within the yarn, in the bonding layer and in the cementitious matrix; especially a detailed analysis of the yarn and the bond layer between yarn and matrix is essential. The consideration of the propagation of discrete cracks developing within the cementitious matrix requires large amounts of computing time, even for the simulation of one-dimensional stress states.

For this reason, the numerical model presented allows a simplified simulation of the global bearing behaviour of textile-reinforced concrete structures without resorting to a separate modelling of the components mentioned above. For the numerical simulation an assumed strain finite element based on the Reissner-Mindlin shell theory for finite rotations is applied. The four node element describes the geometry and the deformation with isoparametric shape functions. The nonlinearity of the material is idealized by a multi-layer model. A model of Darwin and Pecknold [2] is used to specify the concrete's characteristics for each layer. The smeared crack formulation of the concrete takes the effects of tension stiffening and tension softening into account. To model the textile, layers of equal thickness with uniaxial properties are used. The basic model assumption for the crack formation is the supposition of an average crack-spacing and the equality of each crack-element. The tension stiffening effect is considered as a modified stress-strain-relationship. The tension stiffening factor is evaluated iteratively for a one dimensional reinforced bar under tensile load. Afterwards the tension stiffening factor is transformed into the two dimensional plane of the shell.

In the paper, two bond models with different numbers of bond interfaces within the composite specimen are described and compared. The underlying bond slip relationships both for the internal (filament-filament) and external (filamant-matrix) interfaces were taken from Littwin [3]. The first model with two bond-interfaces is based on the splitting of the filaments of a yarn cross-section into two components. The first component has direct contact to the surrounding matrix and therefore represents a heterogeneous bond-layer. The second component consists of the inner filaments with no contact to the matrix. The second model dispenses with the need of distinguishing between internal and external filaments and is based on a rigid internal bond. The load capacity of textile-reinforced concrete decreases, if the reinforcement direction differs from the principle stress direction. To take this into account the load-capacity is reduced by an angle depending factor based on Bartos [1].

Finally the paper presents simulation results of textile-reinforced tensile bars and four-point-bending tests and their comparison to experimental data. The simulation results demonstrate the robustness and efficiency of the presented concept and visualise a good correlation with the experimental data.

1

Bartos, P., "Bond in Glass Reinforced Cements", Bond in Concrete, Elsevier. Applied Science, London, 1982.

2

Darwin, D., Pecknold, D.A. "Nonlinear biaxial stress-strain law for concrete", Journal of the Engineering Mechanics Division 103(EM2), 229- 241, 1977.

3

Littwin, R., "Verifizierung und Optimierung eines Verbundgesetzes", Textilbeton- 1. Fachkolloquium der Sonderforschungsbereiche 528 und 532. RWTH-Aachen, 2001.

4

Forschungsantrag SFB 532: "Textilbewehrter Beton � Grundlagen f�r die Entwicklung einer neuartigen Technologie", RWTH-Aachen, 2002.

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