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Builder and Contractors (Apr 10) CCANZ Updates IB39 Fibre Reinforced Concrete

02 Apr 2010

CCANZ Updates IB39 Fibre Reinforced Concrete

Alan Kirby – Cement & Concrete Association of New Zealand (CCANZ)

By outlining the different fibre types and different fibre reinforced concretes available in New Zealand, the paragraphs below (based on the revised IB39) seek to address various points of confusion about when and where each fibre should be used.

What is the purpose of using fibres in concrete?
Unreinforced concrete has excellent compressive strength but is a brittle material that deforms elastically under tension until it cracks. At this point it rapidly loses its load carrying capacity, i.e. it does not have any post-crack ductility. Steel reinforcement bars or mesh usually provides this ductility, but fibres can also provide it.

The basic concept of FRC is that fibres arrest the growth of cracking and/or provide post crack ductility. Steel and macro synthetic fibres, like conventional reinforcement, only become effective when concrete has cracked in its hardened state. It is how and at what stage this cracking is restrained that differentiates fibre types, and should be considered in selecting the most appropriate fibre for a particular application. Micro synthetic and cellulous fibres help to prevent plastic shrinkage cracking. They do not provide any significant amount of post-crack ductility.

Fibres can also be used to, amongst other things, improve impact and abrasion resistance, and reduce spalling of concrete.

Types of Fibres
Fibres range in type based on their base material (e.g. steel, synthetic, or cellulose), their size, and their shape.

Steel and macro synthetic fibres enhance the toughness, ductility, and energy absorption capacity of hardened concrete. However, in the case of macro synthetic fibres, these properties may only be realised at crack widths approaching or exceeding those recommended at the serviceability limit state, as a result of their lower Modulus of Elasticity (MoE): steel has an MoE of approximately 200 GPa; polypropylene has an MoE between 4 to 10 GPa.

Long-term performance, due to the difference in creep performance of steel and polypropylene fibres should also be kept in mind. Both fibre types can be used to contribute to the load bearing capacity of a concrete member. At normal dosages they do not affect the flexural strength, nor do they contribute significantly to reducing or eliminating plastic shrinkage cracking. However, they do control drying shrinkage cracking.

Micro synthetic fibres and cellulose fibres at appropriate dosages are particularly useful at limiting the widths of micro cracking in concrete in its plastic state. These fibres also lower the permeability of concrete and improve its impact and abrasion resistance. Micro synthetic fibres in concrete in fire situations are used to improve fire

performance, as they melt at about 150-160°C leaving voids for the passage of moisture vapour that would otherwise lead to explosive spalling.

Generally, these products provide no level of capacity / crack control if the concrete cracks in its hardened state.

Fibres blends, i.e. blends of steel or macro synthetic fibres and micro or cellulose fibres, can be used to provide a combination of benefits to both plastic and hardened concrete.

Not all fibres are the same - Horses for courses!
It is very important to note that:

  • No two fibres are the same: A fibre should be objectively selected for a particular application and the design based on the properties of the FRC pertaining to the selected fibre.
  • The effects of fibres on the concretes that they reinforce depend on a number of factors – the fibre type, its dimensions, its properties, the dosage of fibres used in the concrete, and the properties of the matrix the fibres are contained in.
  • Empirical design guides e.g. CSTR 63, ACI 544.4R and NZS 3101 have been developed and published for steel reinforced concrete. However, these guides may not necessarily be suitable for macro synthetic fibre concrete in consideration of its different long-term performance.

It is imperative to recognise that a FRC designed and tested with a particular type and brand of fibre cannot be assumed to have the same properties as a FRC made from different fibres. This includes fibres that may look the same but come from different manufacturers or suppliers. Test reports and/or certification of the FRC are essential.

IB39 Fibre Reinforced Concrete is available from the CCANZ website.