Flexural modelling of steel fibre reinforced sprayed concrete
A current limitation on the structural use of steel fibre reinforced sprayed concrete (that equally applies to cast steel fibre reinforced concrete) is a distinct lack of accepted design rationales and codes of practice. The research presented here describes the development of a model, based on conventional principles of mechanics, for predicting the flexure behaviour of a wet process sprayed concrete reinforced with deformed steel fibres. The model uses a stress-block diagram to represent the stresses (and resultant forces) that develop at a cracked section by three discrete stress zones: (a) a compressive zone; (b) an uncracked tensile zone; and (3) a cracked tensile zone. By using this concept it is shown that the stress-block diagram, and hence flexural behaviour, is a function of six principal parameters: the compressive stress-strain relation; the tensile stress-strain relation; fibre pull-out behaviour; the number and distribution of fibres across the crack in terms of their positions, orientations and embedment lengths; and the strain/crack-width profile in relation to the deflection of the beam. An experimental investigation was undertaken to obtain relationships for these parameters. Five tests were identified and developed as part of this investigation: a single fibre pull-out test; a compression test; a strain analysis test; a fibre distribution analysis test; and a flexural toughness test. The majority of the investigation used cast (as opposed to sprayed) specimens so that the test variables under investigation could be better controlled. Spraying trials were also successfully undertaken to demonstrate the pumpability and sprayability of the adopted mixes and to verify the use of the model for both cast and sprayed specimens. The results of the modelling analysis showed a reasonable agreement between the model predictions and experimental results in terms of the load-deflection response. However, the accuracy of the model is probably unacceptable for it to be currently used in design. A subsequent analysis highlighted the single fibre pull-out test and the sensitivity of the strain analysis tests as being the mai n cause of the discrepancies. As a result, recommendations are made for how the model might be improved. Overall this research has provided a valuable insight into the reinforcing mechanisms, fracture processes and characteristics of failure associated with the flexural behaviour of steel fibre reinforced concrete. It is envisaged that the proposed model could form the basis of a design rationale which requires only the matrix strength, fibre type, fibre content, beam size and loading geometry as design input parameters. Consequently, it could offer a much needed link between flexural toughness performance and structural design, by allowing designers to make informed choices regarding the mix design in order to meet the ultimate and serviceability requirements of a particular application.