Holdfast Training Services Trial Blog

First patented in 1874 and used extensively for bomb damage repair to airfields in WW2, SFRC is a well established construction material with a number of potential benefits, especially when used in situ to form composite slabs on profiled steel decking.

Steel fibres are graded I – V according to BS EN 14889-1

Steel fibres are often used to replace the steel mesh or fabric reinforcement in slabs. As well as composite slabs, they are used extensively in ground bearing slabs and pile-supported floors and can replace much, if not all, of the traditional welded reinforcement.

By mixing steel fibres into ready mix concrete at the batching plant you can avoid the need to pre-place traditional reinforcement. This gives considerable time and H&S benefits (manual handling, cutting, etc) as well as some secondary benefits such as reduced numbers of deliveries to site and less demand on crane time to lift bars and mesh into place. You are also able to better guarantee distribution of reinforcement through the depth of the slab, removing the risk of misplacing reinforcement within the depth of the slab.

On constrained sites (like mine) hook time is at a premium and there is limited lay down space for material. In addition, the build sequence only allows us to pour slabs once the two floors above have been steel decked to provide some measure of overhead protection (i.e. we cannot pour concrete on L01 until L03 has been completely decked by the steel erectors). On the other hand, once L02 and L03 are fully decked we have no way of craning our bars and mesh onto L01 as the lay downs will have been covered over. As a result the concrete contractor has to design, call-off , deliver and bulk lift all reinforcement for L01 eight weeks before he is able to access the level to place the reinforcement, run out pump lines and pour the concrete.

It seems to me that a large amount of this work could be rationalised through the use of SFRC in the concrete mix so I am wondering why it hasn’t been specified on this project. One issue could be that in order to provide a resisting stress under tension you need strain in the steel – in traditional (in plane) reinforcement the strain required is very low but in randomly oriented fibres there could be greater strain before the required stress is realised, resulting in higher deflections. Compression performance should be unaffected.

Steel fibres in a handful of the grey stuff

This project has 11 different composite slab combinations but the ‘generic’ case is a LC30/33 (Lytag) concrete on a profile sheet deck. The reinforcement is a H12 bar in every sheet trough (for fire resistance) and either A193 or A393 mesh in the top. Where this mesh is for crack prevention only I can’t (yet?) see why it couldn’t be replaced by fibres. The larger mesh is specified where the slab is designed to transfer diaphragm loads through the structure so there is more to be looked into there. I don’t propose replacing the fire bars.

Anyway, this is very much a new thought which may not go anywhere but I would welcome people’s thoughts or feedback from anyone who has seen this in action.

Oh, and do we still use it for airfield damage repair? I don’t remember using it.