Hardened concrete containing macro synthetic fibres can generally be described as having post-crack energy absorption capacity. At normal fibre addition levels, the fibres should not themselves have any adverse effect on the compressive strength of the concrete. However, as previously mentioned, strength and durability will be compromised if a reduction in workability due to the fibres is recovered through the addition of extra water.
At higher fibre dosages, some types of macro
synthetic fibre may cause the plastic concrete to appear stiff and harsh. In some cases, it may be necessary to increase the slump of the concrete through the use of chemical admixtures in order to avoid any placement and/or compaction issues. In situations where coarse sand must be used, increasing the sand content may not be sufficient to overcome the harsh nature of the fibre reinforced concrete. In these cases, a higher cement content and/or the addition of a small amount of entrained air (2-3 percent) will create a workable fibre reinforced concrete mix.
According to CSTR 65, there is limited information on how the physical properties of macro synthetic fibres change over time and therefore how the longterm structural performance of the concrete may be affected. The modulus of elasticity and creep of the fibres should be kept in mind.
Macro synthetic fibres will soften when subjected to fire, and melt above around 150oC. They lose their mechanical properties and will no longer provide any structural capacity when they melt. It may be necessary to use passive fire protection (e.g. thermal barriers) to limit the temperature rise in the concrete