Unlike in conventional construction where the stresses can be predicted and the steel placed accordingly, the fibres are spread throughout the shotcrete underground roof support mix. This provides a support system for a situation where one cannot predict at which exact point it will be most required.
The fibres reinforcing the shotcrete provide an energy absorbing capacity as they give ductility to the shotcrete, allowing cracks to form and the shotcrete to deflect, providing a kind of ‘crumple-zone’. The energy absorption buys time and retains support essential for a stressed area to be evacuated if necessary, and mines base their specification calculations on how much work the fibre in the shotcrete does after a crack forms. The fibre-reinforced shotcrete can be applied in a single step, which is another advantage of this system.
One of the major providers of this metal fibre to shotcrete suppliers in Southern Africa’s mining industry is Metalloy Fibres, part of the Nimag Group of Companies. Nimag was originally a part of the metallurgical processing division of Impala Platinum, but when the platinum mining group disposed of what it saw as non-core assets it was purchased by its current management. However, Metalloy Fibres’ origins in the mining sector have placed it well to produce and develop fibres used in roof support concrete.
Metalloy Fibres employs 11 out of the 86 staff at Nimag, located near Magliesburg, and manufactures two distinct metallic fibre products. Only one of these is incorporated into shotcrete, this being ‘slit sheet’ fibre, which is cut from coils of high tensile steel obtained from Mital (Ispat Iscor). The other is a cast stainless steel fibre, used in refractory applications such as furnace and ladle linings. Metalloy Fibres is one of only about five companies worldwide that possess the ‘melt extract’ technology to cast fibres; this method sees the fibres spun from a molten bath of stainless steel by a rotating wheel. Rapid solidification and cooling results in a fine metallic grain size, which is important for tensile strength and oxidation resistance. Some 60% to 70% of the cast fibre is exported to regions such as the European Union (EU), Australia and the Middle East.
“The group has positioned itself as an expert in melting and casting products of stainless steel, and after a search for a diversified product, cast fibres were deemed to be a fit for the company,” says Brian Dillon of Metalloy Fibres.
In May 2001 the factory’s cast fibre equipment was hot commissioned and cast fibres were placed initially into the local market, with export opportunities exploited at a later stage.
However, more directly relevant to the mining sector, is Metalloy Fibres’ slit sheet fibre production.
“We can manufacture fibres in any grade of low carbon or stainless steel that the customer requires,” says Metalloy Fibres marketing director, Ian Ball, “but fibres for shotcrete reinforcement are produced from steel with the highest possible tensile strength, a grade known by its automotive specification, SAE 10/12.”
The company purchases 22 tonne coils from Mital and slits these into smaller coils in a conventional decoiling and slitting line. The coils are fed into cutting machines which have rotating tungsten carbide cutting wheels and which slice the coils, in a perpendicular direction, into fibres. However, while the slit sheet process is conceptually quite simple, in practice it requires equipment built and operated with near micron precision to manufacture fibres to the required ASTM A820 Type II specification. The cross sectional area of the fibre, and hence its aspect ratio, is determined by the rate at which the coils are fed into the cutting machines and the thickness of the coils. Length is determined by the width of the final slit. At the same time as cutting, an end deformation or a full-length crimp is added to each fibre. The shape and configuration of the fibre is designed to ensure there is sufficient bond within the shotcrete to take peak load, while allowing a controlled pull out of the fibre from the shotcrete to maximise energy absorption.
For mining applications as a rule of thumb the fibre length must be no greater than 60% of the diameter of the shotcrete hose used, usually 50 mm, to minimise potential blockage of the equipment.
Metalloy Fibres is ISO 9001 accredited and each batch of steel fibre produced is traceable back to the coil from which it was manufactured.
“Because of the safety aspect of the application in shotcrete there must be traceability,” Dillon says. This is important as there are products on the market that use cheaper steel that is not ideal for shotcrete applications.
“Metalloy Fibres supplies some 30 customers with slit sheet fibre. Not all of these are mines or shotcrete producers and applicators – slit sheet fibres have many above ground applications in concrete. Steel fibres greatly improve impact resistance in applications such as safes and vaults (including ATMs), industrial floors and impact absorbing highway barriers. Steel fibres assist in the placement of concrete for very flat warehouse floors and allow a reduction in the thickness of such floors by up to 30% with subsequent savings in concrete. Our fibres are even used in the reinforcement of cast concrete garden furniture and ornaments, although I couldn’t say this is our biggest market!” says Ball.
Unlike with cast fibre, the slit sheet fibre is supplied mostly to the local market, as it is in essence a bulk commodity with a large number of manufacturers around the world. It is a commodity where local manufacturers have an advantage because of transport economics and the barriers to entry at a local level are due to the precision machines required and the knowledge required regarding the shape and appropriate steel selection.
Mines – indirectly through the shotcrete suppliers – are by far the largest customers of Metalloy Fibres’ slit sheet fibre output and account for about 85% of volume.
In general though, the market for steel fibre in concrete in South Africa remains small. “In the EU fibre is used in reinforced concrete applications as an accepted time and labour saving solution, as these are expensive in comparison to raw materials,” Ball says. “In comparison South African labour costs remain relatively low although this may not remain the case in the long term.”
Dillon says there have been surges in demand for steel fibre over and above the steady mining industry demand when major projects are on the go, such as when South Deep shaft was being constructed (lined with fibre reinforced shotcrete), and during the construction of the Lesotho Highlands projects where many kilometres of tunnels were fibre reinforced shotcrete lined.
THE DIFFERENT FIBRE OPTIONS FOR ROOF SUPPORT SHOTCRETE
Slit sheet fibre competes with the more expensive drawn wire steel fibre as a source of steel fibre for shotcrete applications. The choice here is often dependant on the type of shotcreting system used, with drawn fibres typically used in wetcreting, and slit sheet fibre typically used in the dry application of shotcrete. Drycrete (where the water is added to the pneumatically conveyed shotcrete at the nozzle of the machine) is often used where space and access are problems as the machines are smaller than wetcrete machines. In wetcrete applications, the shotcrete is batched remote from the machine’s nozzle and pumped wet to the nozzle. This method of application requires larger pumps and other equipment than drycrete but has the advantage of better control over the water/cement ratio, which is important for the best strength of concrete possible. An operational downside of wetcrete is that great care must be taken to clean pumps and flush pipes after cessation of shotcreting so that no shotcrete is allowed to set in the equipment.
In addition to being a supplier of slit sheet fibre for concrete reinforcement, Metalloy Fibres is investigating the manufacture of a polypropylene fibre to displace imported fibres of the same type also mainly used in shotcrete. Dillon says that while polypropylene fibres have been manufactured, this project is in the development stage, and only when the product has been fully tested and ready for commercial production will it go onto the market. This development will allow Metalloy Fibres the ability to offer a range of fibres suitable for all customers.
Some of the mines specify polypropylene fibres because of a concern about corrosion where the ground water is bad. Ball says that the perceived corrosion risk would be more valid for where traditional reinforcing mesh was being used, where galvanic corrosion can be set up due to the continuous nature of the mesh, but with the fibres being small and discontinuously spread throughout the shotcrete the corrosion risks are significantly lower.
“However, there is a health and safety factor with steel fibre. When sprayed on, the fibres can protrude from the concrete and these are sharp enough to cut.”
The addition of polypropylene fibres give shotcrete somewhat different performance characteristics than steel. When destructively testing a panel of shotcrete to determine the all important energy absorbing quality of the shotcrete, steel fibre reinforcement allows the shotcrete to support a higher initial load than polypropylene, but the load can reduce substantially as the deformation increases. Panels made with polypropylene fibre reinforcement generally show a lower initial load but with less fall-off as the deformation increases. This can be explained by the softer, more extensible nature of polypropylene.
“A combination of steel and polypropylene fibre appears to have merit. In fact we are looking into this and have done tests with the aim of getting the best of both worlds,” Dillon says.
In terms of price the two options balance out, as polypropylene is two or three times more expensive than steel fibre, but requires less than a half to two thirds in an application.MRA