Tragverhalten von Stahlfaserbeton

Steel fibre reinforced concrete has been the subject of considerable research since the early 1960's. However, the use of steel fibre reinforced concrete in civil engineering has in general been limited to non-structural applications. This is due to the inherent softening behaviour of steel fibre reinforced concrete after cracking. To alleviate the problem steel fibre reinforced concrete can be used in combination with conventional reinforcement. This study aims at a better understanding of the load carrying behaviour of steel fibre reinforced concrete and reinforced steel fibre concrete. Accordingly, the study is organized in two main parts. The first part deals with (i) the pull-out behaviour of thin, flexible fibres from a cementitious matrix and (ii) the influence of steel fibres on the behaviour of concrete subjected to tension, compression and bending. In order to assess the fibre effectiveness analytical stress-crack width relationships are derived. These distinguish between an initial strain hardening behaviour (fibre activation) and a subsequent softening behaviour (fibre pullout). Provided that the fibres are pulled out without fracturing this model and its extension to bending allow to describe the fibre effectiveness with only two parameters which can be determined from simple tests. It is shown that the addition of steel fibres may result in an increase of the tensile strength of up to 25 %. However, due to the reduced workability of fresh concrete with high fibre contents such a strength increase is hardly achievable in practice. The second part presents a theoretical model capable of predicting the load-deformation response of reinforced steel fibre concrete. The model combines the tension chord model with the fibre effectiveness model developed in this study. It allows crack spacings and crack widths to be predicted with good approximation. It is verified by comparisons with results from tension and bending tests and a parameter study is performed to discuss the most important influences. The study shows that the addition of fibres has a favourable effect on crack spacings, crack widths and deformations provided that the conventional reinforcement remains elastic. However, the hardening response of ordinary reinforced concrete is changed to a softening response – even with low fibre contents – after onset of yielding of the conventional reinforcement. The softening response is associated with a localization of deformations that may reduce the deformation capacity of a structural member. Hence, in order to achieve a satisfactory behaviour of a structural member the contents and properties of steel fibres and conventional reinforcement have to be carefully selected based on a thorough investigation of the load-deformation behaviour. In order to facilitate this task approximate procedures for the design of structural members subjected to tension and bending are developed and complemented with simple relationships to account for the fibre effectiveness.

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