Macro synthetic fibres are typically used in structural concrete as replacement for nominal bar or fabric reinforcement; they do not replace structural steel and can not be part of the design in accordance with these Eurocodes. However, macro synthetic fibres can be used to provide the concrete with significant post-cracking capacity and hence can be used in some designs based on plastic analysis, such as for ground-supported slabs and for rock support using sprayed concrete. Design approaches are not as well developed as for steel fibres (see separate entry Design/Approach/Steel-fibre-reinforced concrete), though in many cases the same methods are used, making allowance for the differences in the stress:strain behaviour and in the long-term performance.
There is limited information on how the physical properties of macro synthetic fibres change with time and hence how the long-term structural performance of fibre-reinforced concrete may be affected. In situations in which the fibres are subjected to low levels of tensile stress in service, i.e. the concrete is uncracked, the strength of the fibre, and hence the moment capacity of the concrete element, will not be affected. However, where the in-service performance relies on the post-cracking capacity of the concrete and hence the fibres are subjected to sustained higher levels of stress, creep will be a significant design consideration. This is an aspect of behaviour that is not well understood at present and suggests that macro synthetic fibres may not be suitable for long-term loading in such applications.
When subjected to fire, macro fibres will soften as the temperature rises and will melt at a temperature of 150–160 degrees C. They will lose their mechanical properties and will no longer provide any structural capacity. It is therefore clearly important to avoid the use of macro synthetic fibres to provide structural capacity which may be lost in the event of fire with consequent collapse. The fire design of composite slabs with profiled metal sheeting, using specific combinations of fibres and profiled metal sheeting, has been determined by full-scale testing and a fire engineering model.
Further information may be found in Concrete Society Technical Report 65, Guidance on the use of macro-synthetic-fibre-reinforced concrete.