The idea of using fibres for construction materials reinforcement is hardly new since the Romans were using horsehair in mortar around 200 BC. But in 1960, Batelle in the US patented the production of straight fibre from wire for concrete reinforcement; and in 1970 Bekaert addedhooked ends to improve anchorage, followed five years later by the idea of gluing fibres together to improve handling and mixing.

Bekaert has been working with (mainly steel) wire since its founding in 1880 and is now the world’s largest producer of drawn steel wire products. At Bekaert’s plant, established in 1990 in Moen, Belgium, life can be quite lonely for operatives as the total workforce is only 75 in this highly automated and efficient production facility, producing millions of Dramix steel fibres every day. Is this article we follow the path of just one of them from rod to casting.

Various types of mild (bright) steel fibres, 30-60mm long and 0.55-1.05mm in diameter, are produced here for both precast and sprayed concrete tunnel lining, with another major application being large concrete floors without joints.

Bekaert first started producing steel fibres next to the current headquarters in Kortrijk, Belgium. Now there are nine Bekaert plants around the world producing wire construction products and related products such as brick ties. These include the recently acquired Korean plant with 3,000 employees, another in China with 7,000, and a probable new one in Colombia under discussion.

Moen is Bekaert’s highest producing plant in the world at 55,000t per year, but appears very unassuming from the outside. Inside, production equipment is laid out to promote efficient materials flow on parallel production lines through the works (see figure 1).

The operatives are organised into teams of eight plus one technician, and each has personal responsibility for a whole production line. Work periods are seven days on, seven days off. There are seven Dramix production lines with 35 dryprocess wire drawing machines using lubricated dies of steadily decreasing internal diameter. Each line runs 36 wires.

All machinery, except for standard packaging machinery, has been designed and manufactured by Bekaert itself.

The start
Material for fibres at Moen comes from several suppliers, such as Tata Steel (formerly Corus) in Germany, as coiled rod of 5.5 or 6.5mm diameter. The first job is to prepare the thick wire for the production lines by first taking scale off the rod.

Chemical descaling for 80 per cent of the supply is carried out at nearby Zwevegem by pickling in an acid bath, and lime applied to neutralise the acid, provide temporary rust protection, and act as a carrier for lubricant. The coils are then quickly delivered to Moen. Mechanical scaling at Moen is used for larger diameters and consists of rolling the rod to bend it in two directions, thus cracking the scale, and passing it through rotary wire brushes, leaving minimal contaminant to be washed off.

Drawing
The next preparation stage is to ensure sufficient lubrication to pass through the drawing dies. Dry soap of a particular type is used for lubrication but this has to be held onto the relatively smooth wire throughout the process. The rod is therefore passed through a warm bath that applies a crystalline lubricant carrier to the metal for a rough surface.

Rotating conical dies, with inserts made of Widia tungsten carbide or diamond, are used to reduce the diameter of the continuous wire by about 25 per cent each time. These inserts are changed once a week to maintain the correct diameter, but are also valuable and so are recycled at the Zwevegem plant for use in larger dies.

First three machines, designated CAZ, reduce the 6.60mm rod to 1.05 or 0.90mm diameter. The drawing process naturally elongates the rod, into wire by then, and also heats up the steel. The lubricant, forced into the wire by the die, must therefore be resistant to both pressure and heat. Next there are four ‘BAZ’ machines for further reduction from 5.50mm to 2.30mm diameter or 6.50mm down to 3.35mm.

The main and final reductions are achieved through 12 DAZ machines and 16 ‘CDZ’ machines to reduce 2.30mm-diameter wire to 0.62 or 0.55mm and 3.25mm wire to 1.05, 0.9, or 0.75mm-diameter fibre wire.

Typical elongation might be one metre of 5.5mm-diameter rod producing 30m of 1mm-diameter wire and eventually 100m of 0.55mm diameter fibre material. Lubricant soap is removed at the last die.

The drawing process not only reduces the wire diameter to fibre size but also increases the tensile strength of the steel. This must be greater or equal to 1200MPa.

The drawing line speeds can be up to 2,000m/min on eight lines, practically invisible to the naked eye and noted only by specialist instrumentation. This is a great increase on the 30m/min of the initial rod feed. The forces used in the drawing line have to be carefully controlled by a series of capstan and ‘dancer’ pulleys as well as the instrumentation. The fastest (CDZ) line is controlled electronically and has hydraulic brakes for safety. Excessive force could cause a line break and serious production problems. Continuous operation is essential to achieve high production rates, with even the coils of rod being welded together before entering the line to maximise continuity.

This reliably controlled high line speed is necessary to ensure maximum economical production. A typical line speed of 1,500m/min from a 1,500kg feed drum will produce 42,857 fibre lengths per minute. The only way production can be further increased at Moen is by more upgrades to line production since there is no room for more expansion of the facilities.

Once the wire has been drawn down through the series of dies to the required diameter of the fibre product, spools of the wire are transferred to the final production line for cutting, bending the ends and, if necessary, gluing together. A double payoff system at Moen operates two 36 spools, although this is not the biggest as two other Bekaert plants in Asia and Europe run with 48 wire lines.

Sticking together
First the wire is straightened in parallel runs as this makes it easier to glue the final fibre products together, using less material. The gluing process and nature of the adhesive is a closely guarded secret. The glue has to be water soluble after about one minute for quick and thorough dispersal of the fibres in the concrete mix, preventing fibre ‘balling’. The constituents of the glue must not affect the chemistry of the concrete. The glue used includes 50 per cent water when it is

applied but, once on the continuous fibre wires, the water is evaporated in an induction-heating oven and then cooled.

At the end of the line, immediately after the cooling process, the wire is cut and shaped in one operation (also secret) to form the familiar glued comb bundles of fibre, ready for packing and dispatch.

Packaging and dispatch
Bekaert fibres are designated by material, length and aspect ratio. In tunnelling the product designations commonly used are 65/35 and, for precast segments, 65/60 as with our ‘hero’ subject.

During packaging synthetic fibres can be added for Duomix mixed products for concrete protection in fires.

All packaging, into bags of various sizes, is carried out automatically, with final palletising and shrink-wrapping of 1,200kg for 20-kg bags, and 800- and 1,100-kg ‘big bags’ for bulk handling, and is all accomplished by robots.

Quality control
Bekaert rightly thinks of its fibres as structural elements produced to a clear engineering specification, rather than as a commodity. An equally clear, independent or independently audited testing and quality control procedure back up this claim.

Each consignment carries ink-jet coding for traceability according to Bekaert’s ISO 9001 (v2008) procedures, so that, in the unlikely event of a product fault, it can be checked back to the source of the problem. Bags carry the ink-jet print of date, time and line of manufacture, whereas pallets and one or half-tonne ‘big bags’ carry a forwarding label with the same information.

‘Big bags’ are used to charge dosing units at high capacity customer installations, mainly batching plants.

In addition to ISO 9001 compliance, the Moen plant has also achieved ISO 14001 registration (environmental standards) through Bureau Veritas; the first plant in Europe to do so.

Bekaert operates a standard documentation system covering product specifications, the work process and instructions to customers and users, amongst other things. The company’s quality control systems are integrated with this and cover all European operations. It operates at two levels, the first being at the (Moen) production plant and the second through separate testing at the concrete lab housed at Zwevegem, which also serves the rest of the world.

At the plant, quality control starts with checks on input supplies. The Bekaert purchasing department is mainly responsible for checking the incoming coiled rod, and there are similar checks on lubricant soap, glue, etc with a test report completed for each delivery. New suppliers have to go through a rigorous testing programme before approved.

On the production line operatives have whole-line responsibility and this includes quality checks such as possible wire ovality using a micrometer and length of cut fibres. Other production checks include fibre geometry using a magnifying profile projector, testing for tensile strength, and ductility during bending. More detailed testing is carried out in the concrete lab. Bekaert says that its quality control standards are higher than those required by the relevant EN European Standards.

CE product marking was achieved in 2008; the first manufacturer to do so. Under the CE certification all structural products, such as the steel fibres for precast concrete segments and sprayed concrete, are subject to Class One independent inspection. Non-structural products can be certified through in-house testing.

Quality control includes integration with customer claim handling so that any faults can be traced back.

Modern applications
Looking ahead, one of Bekaert’s emphases is continuous product development, even in ‘mature’ markets for wire. To carry out this aim there is a EUR 60M research and development budget employing 400 people worldwide.

In addition to the successful product development mentioned earlier there are Dramix Green, a galvanised fibre to eliminate corrosion, and stainless steel version for extreme demands. The latest innovation has been head of building products R&D Ann Lambrechts’ idea to flatten the ends of the fibres for better anchoring in the concrete. This increases the bending tensile strength of concrete by 32 per cent, with the better anchorage improving crack control and enhancing durability. The work won Lambrechts the European Inventor Award this year.

Recently there has been a priority to package products for easier handling and mixing, often demanded by government health and safety legislation. In addition to the glued fibres, various forms of dispenser or dosing systems are available including the latest Dramix RB belt conveyor dispenser for dosing smaller quantities (250g bags). Most Moen fibres go out in either ‘bulk bags’ for mechanical handling or palletted paper sacks.

Work continues in co-operation with leading concrete ready-mix producers such as Lafarge. The use of short, high-carbon steel fibres has resulted in their production of a ‘ductile concrete’ with strength in excess of 200MPa.

‘Sustainability’ is a common buzzword in discussions about modern construction practices, and tunnelling should be no exception. One aspect, as ever, is how structures can me made cheaper whilst maintaining or improving on the specified structural performance. Bekaert Dramix plays its part here too. Steel fibre reinforcement is claimed to be environmentally friendly (‘green’) as a basic concept, as it can be used to replace some or all of the thick rebar cages otherwise used in design to improve the tensile strength and toughness of concrete elements. This obviously saves on steel, mineral resources and the energy for manufacture. In addition, Bekaert is now using at least 20 per cent recycled steel in its production.

Leading tunnelling contractors and designers, notably Colin Eddie at Morgan Sindall, have been examining how concrete linings can be made thinner for the same performance such as at the London Heathrow Airport Baggage Tunnel – with sprayed concrete. It follows that concrete usage would then be lower; a major factor in reducing a project’s carbon footprint. Thinner linings using steel fibre reinforcement can require high-carbon steel material however, rather than mild steel, and this costs around twice as much, so the whole exercise is a matter of balance in financial as well as performance terms.

In the mix
Bekaert consults on fibre dosing procedures, including fibre distribution, for all sizes of concrete mixing, and supplies dosing equipment for many applications to ensure a steady feed according to the concrete specifications. The latest developments include the Dramix Booster automatic bulk dosing system and the RB belt dispenses that carries soluble paper bags of fibres, each weighing 250kg.

So, our ‘hero’ is one of many thousands of fibres in the hundreds of tonnes being delivered around the world, in this case to Costa Rica where segments are being produced for contractor Seli to line the Monte Lorio hydroelectric power project headrace tunnels just across the border in Panama. The segments are being made with only steel fibre reinforcement – no rebar cage – using Dramix 80/60 fibres (60mm long and 0.75mm in diameter).

Sample segments, 250mm thick, were independently tested to destruction for Seli by the Laboratorio di Strutture e Prove Materiali at the University of Rome (‘Tor Vergata’) to confirm maximum loading performance.


Line feed of chemically treated wire rod – the dust is lime Figure 1, production flow for the Moen plant from delivery and pretreatment to warehousing and dispatch Head of the main wire drawing lines at Moen Set up for testing the bending performance of a standard Dramix-reinforced concrete beam in the lab at Moen Dramix fibres on their way to the mixer at the Costa Rica segment casting plant for Seli’s hydropower tunnel contracts in Panama