Hooked-end high-tensile steel fibres are added to concrete to make it tougher, creating a reinforced floor that can withstand heavy impact environments, says industrial concrete flooring expert Concrete Laser Flooring (CLF) director Peter Norton.

He explains that the different fibres used to reinforce concrete include synthetic fibres, steel fibres and structural synthetic fibres. The most common application for steel-fibre-reinforced concrete is in the production of slabs on grade, shotcrete walls, tunnel linings and precast elements.

Norfibre Steel Fibres, developed by CLF and used in CLF industrial flooring projects, is an economical way of replacing mesh by exploiting the fact that the fibres add ductility to concrete, but not flexural strength. They also increase concrete toughness and impact performance more than mesh does, as the reinforcing is three dimensional.

“To make the fibres, normal low-carbon steel is pulled through a series of dyes to give the wire strength of more than 1 100 MPa. Thereafter, the wire from 40 spools is fed to a glue line where water-dissolvable adhesive is applied to keep the fibres in strip form, much like rows of staplers. The hooked end of the fibre is designed to provide anchorage in a nonrigid way.

“The fibre cross section remains unchanged, so it can pull through the concrete at high loads to prevent brittle failure, owing to fibre breakage, and promote high energy absorption. Although there is no secondary anchorage, the hooks are specifically designed to maximise performance across the full range of concrete strengths,” explains Norton.

Aspect ratio is a key characteristic in determining performance. Norton says that high- aspect ratios lead to high performance; however, without collation, fibres tend to ball at aspect ratios of more than 50. He points out that 60 mm is the ideal length for readymix concrete, as it is long enough to ensure aggregate overlap and short enough not to block equipment.

“The fibres are packaged into bags of 20 kg. The bags are degradable and can be added directly to the mix without being opened. Each bag contains about 70 000 fibres. The paper bags are loaded into the mixer after the addition of all the other ingredients,” says Norton.

Although the function of bar and fibre reinforcement is the same, the method of achieving it does vary between the two different types of reinforcement.

Conventional (bar) reinforcement only controls the width of fully developed macrocracks that pass completely through the concrete section. “In other words, the reinforcement is holding together two or more individual pieces of concrete, with the crack width at the surface being dependent on the cover to the steel, the spacing between the individual pieces of reinforcement and the stress in the reinforcement. Conventional reinforcement forms a continuous tension-carrying element from one end to the other of the concrete element,” Norton explains.

In steel-fibre-reinforced concrete, the fibres are homogeneously distributed throughout the concrete matrix and are discreet from one another, which means they work completely differently in the way they control cracking, compared with conventional reinforcement.

“Firstly, this is because of the small size and distribution of steel-fibre-reinforced concrete. The fibres control cracking at the earlier micro- cracking, or crack development, stage, by preventing these macrocracks from developing into the visible macrocracks that will pass completely through the concrete section. Secondly, because of their discreet nature, the tensile stress that has to be carried from one end of the concrete ele- ment to the other has to pass from fibre to fibre. The concrete matrix itself is involved in transferring these stresses between fibres and it is obvious that the average spacing between fibres cannot be too great if this mechanism is going to work successfully,” concludes Norton.

However, not all fibres are the same and great care needs to be taken in assessing tensile strength, shape, length and diameter when designing with steel fibres. Polypropylene fibres do not perform the same functions, compared with those of steel fibres in concrete.