Plastic fixing cuts costs and time

Dean Palmer reports on a novel all-plastic fixing system for aligning concrete tunnel segments that has huge potential in other industrial applications



A novel, all-plastic self-aligning fixing system for the assembly of concrete tunnel segments has been developed by UK company Bosworth Plastics. The new fastener, the Dowelock, is a patented design (in the UK and the US) and negates the need for steel bolts by exploiting the performance properties of two polymers.

Bosworth Plastics’ MD Peter Robertshaw told Eureka: “The fastener requires a minimal insertion force and operates at maximum pull-out force, even with joint gaps of up to 15mm, whilst its extension under load is a fraction of that of other dowel systems.”

Based in Hinckley, Bosworth designs and manufactures plastic components for the tunnelling and construction industries. It works closely with pre-casters and tunnel designers and is currently supplying 600,000 plastic fasteners for the Channel Tunnel Rail Link extension. The company has also supplied the likes of BAE Systems and Marconi, the latter with plastic missile connectors.

The firm’s latest invention, the Dowelock, is a breakthrough in tunnel segment fixing and is a great example of how to replace traditional metals with plastics. And because it’s injection moulded plastic, there are no rust problems either. The Dowelock is manufactured from Zytel, DuPont’s black, glass reinforced nylon resin, and is pushed by hand (up to 20kg push-in force) into a collet, which is moulded in Delrin, a white acetal resin from DuPont. The collet mechanically locks onto the moulded annular rings on the dowel without deformation of the rings. The fixings are pre-cast in the concrete tunnel segments.

Robertshaw explained to Eureka the reasons for re-designing the fixings in plastic: “In tunnel construction, there is an inherent problem of withdrawal forces of dowels. So we developed a self-aligning fixing system with a very low insertion force and a high pull-out force. If you have to push hard and ram a dowel into a segment, then you tend to destroy the very thing you need to be able to withdraw it again later.”

The tensile strength of the Dowelock enables it to hold tunnel segments in place whilst the strength of its inner collet (made of Delrin) means the dowel only has to be pushed in five notches to withstand up to six tonnes 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. And we can manufacture it in all sizes, to match any concrete segment geometry without any reduction in performance.”

According to Robertshaw, internal tests have 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 traditional steel dowel systems.

“When we designed the Dowelock it was clear that in order to improve on what was already out there, we needed a material that provided structural strength and minimised extension of the dowel, maintaining the gasket-filled gaps between concrete tunnel segments,” he added. “At the same time, we needed a compatible material to grip onto the dowel and match its performance. The Zytel/Delrin combination has enabled us to achieve our objective and to offer a strong, durable and cost-effective (approximately £2.75 per unit) alternative to metal fixings.”

There were other factors that needed careful consideration before development began. Robertshaw explained: “The tunnel construction company had to be able to build the segments fast and accurately. Second, the fixing system had to be as idiot-proof as possible and we also had to ensure that what we designed could actually be manufactured by the casting supplier.”

Dowelock 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 need to be filled with a mortar. The Dowelock is around half the size of traditional spear bolts. The logistics of delivering Dowelock to a tunnel’s location is also greatly reduced, particularly when you consider that only 12 fixing systems are needed per ring of a four to five metre diameter tunnel.

Dowelock could be used in a number of other applications. Cladding systems, bridge parapets, house construction, rail track assembly, garage construction and warehouse assembly are just some ideas we came up with at Eureka. If readers can think of any other applications, please email the Editor at dpalmer@findlay.co.uk

Pointers

* The Dowelock is injection moulded from a glass reinforced nylon resin with a collet moulded out of acetal

* The company claims the fixing is up to three times stronger than other plastic-based systems on the market and is up to five times stronger than traditional steel dowel systems

* The fastener requires a minimal insertion force and operates at maximum pull-out force, even with joint gaps of up to 15mm, whilst its extension under load is a fraction of that of other dowel systems

Author
Tom Shelley

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