The strengthened Ashton bridge. — The finished, strengthened bridge at Ashton, Greater Manchester © Plain Communications

The return of arched bridges

December 2014

Arch bridges are strong, durable and require little maintenance. However, very few had been built since the early 1900s until the FlexiArch was developed and launched in 2007. Now, there has been a minor renaissance for this ancient form of construction.

A crane and construction works around the Ashton bridge. — Each 1 m wide FlexiArch unit is lifted off by a single crane. As the FlexiArch unit is lifted, it centre sunder its own weight © Plain Communications

In 2004, Professor Adrian Long FREng of Queen’s University Belfast patented a system for building structures with all the positive attributes of conventional masonry arches, but which were able to be assembled quickly and economically.

The trouble with traditional masonry arches is that they are slow and expensive to construct. The voussoirs – the wedge-shaped stones forming the arch – are difficult to cut accurately and temporary timber centering is needed to support the arch while it is built. Using brick instead of masonry still requires centering, and rigid precast reinforced concrete arches can lead to long-term maintenance problems.

Long’s solution was a system of pre-casting concrete voussoirs for rapid assembly and ease of transport, making use of one of the main advantages of the arch: the principal forces are compressive, so no reinforcing steel is required. With help from various grants, Long worked with Macrete, precast concrete engineers from Northern Ireland, to produce a system that is user-friendly for contractors.

The voussoirs are first cast in un-reinforced concrete, with angles calculated to produce the correct curvature of the arch when completed. These are then laid side-by-side on the flat with their top, wider edges touching, and a layer of polymeric reinforcement is placed on top. Then, a 40 millimetre layer of screed (fine aggregate concrete) is placed on top and allowed to harden, holding the voussoirs together.

Tests show that the Macrete FlexiArch bridges have enormous reserves of strength, and early applications have confirmed that each unit can be placed on site in as little as 10 to 15 minutes

Several such slabs can then be transported easily to site in flat-pack form. When lifted from the truck at designated lifting points, gravity causes the arch to flex into its final form, governed by the angle of the voussoirs.

Tests show that the Macrete FlexiArch bridges have enormous reserves of strength, and early applications have confirmed that each unit can be placed on site in as little as 10 to 15 minutes. Over 50 FlexiArch 4 to 16-metre-span bridges have been installed in the UK and Ireland, with plans to extend this to Australasia, the US and Sweden with a view to sub-licensing the technology.

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This article has been adapted from 'FlexiArch', which originally appeared in the print edition of Ingenia 61 (December 2014).

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