I read with interest the comments of Andrew Rideout T&TI, Dec 2003, p54. I would like to respond with some further observations in reference to the items raised in his letter and my article in T&TI, Sept 2003.

I agree that advances have been made in the development of Structural Synthetic Fibres (SSF) and that these now provide similar or slightly better performance in some areas than some types of steel fibres, but not all. There are also other technical and safety implications that must be taken into account when considering the use of fibre reinforced concrete where the risk of fire and the long term creep behaviour of the concrete has to be considered.

I raised the point of the behaviour of SSF in a fire situation, this was not covered to any degree in the letter. However, I have visited a number of web sites and the odd one does show that SSF can be slightly better than steel fibre alone, but no information was found regarding residual flexural strength of SSF concrete after a fire.

However, the inclusion of micro polypropylene fibres, at say 1kg/m, can virtually eliminate spalling. This is proven with micro polypropylene alone or blended with steel fibres. It has been recorded that significant residual flexural strength values are obtainable and projects are either currently underway or have been completed[Ref 1].

With regard to the behaviour of SSF and steel fibre concrete in creep, in the case of the latter I would refer readers to the T&TI’s website (www.tunnelsonline.info) for information on the papers that have been discovered, these relate to creep and micro-cracking in long term behaviour[Refs 2–9]. However having made a further search for data relating to SSF in creep and got nothing back, I have been given permission to publish results from our own laboratory via the T&TI web site[Ref 10].

With regard to price performance being the yardstick, this is correct up to a point – as there are other considerations, such as performance, technical support and consistency.

Furthermore, I should point out that there are now many hooked ended fibres available in the market and these are of variable quality in terms of base materials and production, aspect ratio and tensile strength. These factors do influence the performance of the fibre concrete.

At this point may I remind readers that good quality steel fibres should have a high aspect ratio. This will provide good performance at lower dosage rates, linked with consistent tensile strength, not only in the fibre length but also at the point of anchorage. Steel fibre concrete has toughness that redistributes the load as cracks in a micro behaviour, thus minimising crack widths and allowing possible auto-genous healing of the concrete to occur.

Structural polypropylene fibres will provide toughness at high deformations, but with much wider cracks in comparison.

John Greenhalgh

Bekaert Building Products