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News

All plastic fixing system uses DuPont

Du Pont Engineering Polymers : 30 October, 2003  (Company News)
A new all-plastic, self-aligning system for the assembly of concrete tunnel segments has been launched by UK company Bosworth Plastics Limited.
The Dowelock’s patented design sets new standards in the fixing of tunnel segments without the need for bolts by exploiting the performance properties of two engineering polymers from DuPont. It requires a minimal insertion force and operates at maximum pull-out force, even with joint gaps of up to 15 mm, whilst its extension under load is a fraction of that of competing dowel systems.

The Dowelock’s design addresses the problems associated with many boltless dowel type systems. Using the Dowelock system a dowel, moulded in DuPont’s black, glass-reinforced Zytel® nylon resin, is pushed by hand (or 20 kg push-in force) into a collet, which mechanically locks onto the moulded annular rings on the dowel without deformation of the rings. The collet is moulded in DuPont’s white Delrin® acetal resin.

The tensile strength of the dowel made of Zytel® enables it to hold the tunnel segments in place, whilst the strength of the collet made of Delrin® needs the dowel to be pushed in only five notches to withstand up to a six-tonne pull force. The performance of these materials is also critical in terms of creep and shear, as they need to withstand running loads of up to 2-3 tonnes per metre with minimal extension.

Overall, internal testing at Bosworth Plastics has shown the Dowelock to be up to three times stronger than similar plastic-based systems on the market and up to five times stronger than the traditional steel dowel systems. Due to the efficiency of its design and the choice of material, the Dowelock can be made in all sizes as well as for all concrete segment geometries, without any reduction in performance.

“We are convinced that we could only have succeeded with engineering polymers from DuPont,” says Peter Robertshaw, managing director of Bosworth Plastics and inventor of the Dowelock. “Together with the support of DuPont material specialists, we have pushed the boundaries of the polymers to maximise their performance properties. When we designed the Dowelock it was clear that in order to improve on what is already out there, we needed a material which provided structural strength and minimised extension of the dowel, maintaining the gasket-filled gaps between concrete tunnel segments. At the same time, we needed a compatible material to grip onto the dowel and match its performance. The Zytel®/Delrin® combination has allowed us to achieve our objective and to offer a strong, durable and cost-effective alternative to metal fixings.”

Further advantages are in the Dowelock’s size and ease of installation. It provides quick and clean installation without internal pockets – in contrast to a bolt-based system which requires the incorporation of pockets in the concrete segments to allow the bolts to be tightened, and which also needs to be filled with a mortar. Approximately half the size of spear bolts, the logistics of delivering the Dowelock to the tunnel's location is also considerably reduced, particularly when one considers that 12 fixing systems are required per ring of a four to five metre diameter tunnel.
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