Design of fibre-reinforced external in-situ concrete paving
Many thousands of square metres of fibre-reinforced concrete warehouse floors and areas of external concrete paving have been successfully constructed in the United Kingdom over the past five to ten years. The applications to date principally use steel fibres but there are also examples of macro synthetic fibre-reinforced slabs (see Concrete Society Technical Reports 63 and 65). Fibres at normal dosages do not improve the tensile strength of concrete. However improvements in ductility and hence in post-cracking behaviour can be achieved if an appropriate type of fibre is used at a suitable dosage. The use of fibres also means that it is not necessary to place and fix fabric reinforcement and possibly thereby simplifies the construction process. Fibres may protrude from or become detached from the surface. Corroded steel fibres may be visible in the surface.
At the present time, there is no single universally-adopted design procedure for fibre-reinforced external paving, although a method has been published by the Centre for Civil Engineering Research and Codes (CUR) in the Netherlands. Design of fibre-reinforced paving is often carried out by the suppliers of the fibres; in this situation it is essential that it is clear which party takes design responsibility. Different fibre suppliers have adopted different procedures; whichever method is used, it is necessary to include partial factors of safety which recognise the possible effects of fatigue under repetitive loading and take into account the effects of temperature and shrinkage.
A simple approach adopted by some fibre suppliers, is to first carry out a conventional design and replace the area of fabric reinforcement with an equivalent amount of fibre while keeping the slab thickness unchanged. The required dosage of fibre is determined using standard bending theory such that the fibre-reinforced cross-section has the same post-cracking bending moment capacity as the fabric-reinforced cross-section. An alternative is to use a plastic design approach, as given in the Third Edition of Concrete Society Technical Report 34, Concrete industrial ground floors. Although this is specifically for internal industrial floors, some fibre suppliers adopt the same design approach for external paving, applying an increased load factor to the wheel load to account for repeated applications. This approach has been adopted for a number of contracts in the UK.