A New Form of Formwork

A New Form of Formwork

One aspect of bridge design – of which most people are not aware – is that a bridge’s major need for strength does not stem from the traffic it has to carry. The biggest load a bridge holds up is the weight of the bridge itself, which is usually much greater than the weight of its peak traffic.

One of a bridge’s heaviest elements, the concrete deck, is also one of the least structurally necessary, so it can be built last, after the main structure of the bridge is in place. The problem is that concrete casting requires formwork, which takes time to build. It also requires falsework to hold the formwork in place, which also takes time. And then, after you place the concrete, you have to wait for it to cure before you can remove the formwork and falsework, which takes time. Not to mention that building and removing all this formwork etc. means performing construction while hanging underneath a bridge, which is dangerous and slow.

And all this time, you’ve been impeding traffic in the river or canal underneath the bridge.

Designers in the Netherlands had a better idea. Rather than build the bridge’s structure in situ over the water, and then build traditional formwork in situ under the bridge, they decided to construct the bridge and its deck formwork on land, where it’s easy to build, then move it into position over the water, and then pour the concrete deck.

The key to this process was extremely strong, lightweight, stay-in-place formwork made of an FRP sandwich panel. This novel use of FRP leveraged its high strength-to-weight ratio, not for the bridge’s structural design, but simply to bear the weight of the steel-reinforced concrete decking while it was in its fluid state. Once cured, the deck bears its own weight.

The steel arch bridge design has two steel trusses, 3.8 m apart, which would be filled by the concrete deck. The sandwich panels had to bridge that distance and support concrete and steel reinforcement weighing 800kg/m2 (about 13 inches thickness of concrete and steel).

The designers, Ingenieursbureau Amsterdam, had two significant stipulations: the panels could only deflect by 25mm when loaded with concrete, and they could only protrude above the steel truss on which they rested by 20mm.

The solution designed by Poly Products, Werkendam, the Netherlands, is a sandwich panel of vacuum infused glass fiber reinforced polyester with a polyurethane foam core. The lip at the edge that bears all the weight is a mere 15mm thick. Achieving sufficient strength at the edge took some finessing, involving a 5 mm rubber strip on the steel to help spread the load and reduce interlaminar stress in the FRP.

The panels are slightly arched upwards during casting – by 25mm – so that when they deflect under loading, the bottoms are flat – an aesthetically desirable outcome, since the panels are going to stay in place permanently and form the underside of the bridge deck.

A detailed discussion of the engineering and fabrication of the panels, written by Dr. Ir. Albert ten Busschen, Technical Director of Poly Products, can be found in this paper.

It is believed to be the first time an FRP sandwich panel has been used as concrete bridge formwork in this way. It may well be an evolutionary step towards far more complex architectural formwork involving unique computer-generated shapes and digital fabrication. As designers break further away from rectilinear structures, the alliance of digital fabrication techniques and composite materials may provide the formwork for imaginative concrete structures that would be economically unfeasible using traditional formwork methods.


Photos via ommetjemetdetrike.wordpress.com and reinforcedplastics.com