A number of subject specific terms are useful in order to discuss concisely frc structure and behaviour. The term frc itself can refer to either fibre-reinforced cement or fibre-reinforced concrete. There are a number of distinctions between the two. The first normally refers to thin-sheet material produced with high cement content (often 1:1 by weight with fine aggregate but occasionally just cement) and no coarse aggregate. The fibre content is relatively high (up to 5% by volume) and the fibres are intended to provide primary reinforcement, i.e. to provide increased bending strength, tensile strength and/or toughness. It will be referred to as primary frc. The second refers to concrete with cement plus fine and coarse aggregate, to which relatively small amounts of fibre (<1% by volume) are added as secondary reinforcement to help control shrinkage cracking or provide post-failure integrity for safety reasons (i.e., to prevent spalling after accidental overload). It will be referred to as secondary frc. There is some overlap between the two classes. Many authors use the two `frc’ terms interchangeably and in many contexts, the difference is unimportant, but where it is relevant it has been explicitly made clear.
As mentioned above, the unit reinforcement element in most frc other than steel-frc is a bundled group of filaments with space between them, which may or may not be filled with hydration product depending on the age of the composite and the nature of the matrix. Such a group of filaments is referred to as a `strand’. A group of strands is referred to as a `tow’ or a `roving’. A strand may be natural, as in sisal or coir-frc where the strand is a naturally occurring bundle of cellulose fibres containing both inter-filamental space and lumen pores, or artificial as in glass fibres, where strands of up to 200 filaments are grouped together during fibre manufacturing and held together by a soluble or partially soluble coating known as a `size’ (often derived from organics such as polyhydroxy-phenol). This size may also have an effect on the hydration at the interface and is normally designed to improve durability. Occasionally, frc is made with more than one type of fibre, e.g. glass and polypropylene. Such frc is referred to as hybrid-frc.
The amount of fibre in a given frc is normally expressed as fibre volume fraction Vf , defined as the volume of fibres divided by the volume of composite, and varies between about 0.1% and 10%. The limiting factor on Vf is generally its adverse effect on fresh concrete workability and/or compaction. Some analyses also require reference to matrix volume fraction Vm, which is 1 ÿ Vf .
As with ordinary concrete, most frc is made with matrices based on Portland cement (PC), i.e. that based on 3CaO.SiO2 (alite) and 2CaO.SiO2 (belite) as the main hydrating compounds. However, the use of pozzolanic additives and non- Portland cements is widespread, especially as the use of such matrices may confer durability benefits. A matrix made from PC with additions of additives such as metakaolin, condensed silica fume, pfa or ggbs is referred to as a modified matrix. A matrix based on a different chemistry, such as high-alumina cement or calcium sulpho-aluminate cement is referred to as a non-Portland (nP) matrix. There is some overlap between the modified and nP matrices, particularly in the addition of calcium sulpho-aluminates such as synthetic ye’elemite (4CaO.3Al2O3.SO3) to PC. Although an additive such as this would normally suggest that the matrix be referred to as modified, the additive radically alters the hydration behaviour and hydrated phase assemblage such that it should really be considered as nP.