There probably aren’t many domed concrete structures where you live and there’s a reason for that – they’re difficult to build. Doing so usually requires the construction of a supporting wooden structure that holds the concrete in place while it hardens. Now, however, a team at the Vienna University of Technology has devised a system that allows concrete shell structures to simply be ‘inflated’ and cinched together with a steel cable.
Called the pneumatic wedge method, the technique has been likened to peeling an orange and lying its skin flat on a table – except in reverse. Here’s how it works:
First, a number of flat rebar forms are laid out side-by-side on the ground at the building site, on top of a deflated plastic air cushion.
Concrete is then poured into those forms and allowed to harden, resulting in a series of flat slabs. Metal beams and a steel cable are subsequently attached to those slabs, linking them all together.
Next, the dome-shaped air cushion is inflated. As it fills with air, it lifts the slabs up from underneath, causing them to bend with it in the process. At the same time, the cable is tightened in order to snug them in against one another, with the connecting beams ensuring that they all move in unison. All of the slabs have wedge-shaped edges that allow them to securely interlock with their neighbours.
Once the shell is up, the air cushion is deflated and removed, and the beams and cable are taken off. The bending of the hardened concrete does cause some small cracks to appear, but these reportedly don’t affect the stability of the structure. “If the shape is right, each stone holds the others in place and the construction is stable,” said project co-leader Prof. Johann Kollegger.
Finally, the whole thing can be covered with a layer of plaster, adding a bit more strength (and perhaps also keeping people from wondering about those cracks).
In a test of the system, a concrete dome measuring 2.9 m in height was built in approximately two hours. According to Kollegger, shells of up to 50 m in diameter should be doable. The other project leader, Benjamin Kromoser, believes that the technique should reduce the construction costs of such buildings by about 50% – along with savings in time and materials.
Austrian Federal Railways has already commissioned a design for a wildlife overpass that incorporates the technology.The technique could also be used for building band shelters.
