Concrete: Freeflow

It was during a fact-finding mission to an indoor athletics track in Loughborough, Leicestershire, that Hugh Strange, an architect with Panter Hudspith, realised the full potential of Agilia concrete.

“It had a remarkable finish and we realised that we could do things with it that nobody else had ever done,” he says. “We tested how sensitive the concrete was by casting a leaf. It picked up absolutely everything, even the veins.”

Strange was looking at sample panels of Agilia, a self-compacting concrete that had been cast by Northfield Construction for the athletics venue. What he wanted was to find a material that could supply the finish he was seeking for a museum in Lincoln. Or, more accurately, the museum of Lincoln – a building to house 2 million artifacts that would tell the city’s history from 2005, when it was to be finished, back to “year zero”. As a result, the designs were influenced by the surrounding medieval streets and the mass of the stone walls at the Bishop’s Palace – the museum was to step up the slope towards the palace, moving from two levels to one as it crept up the 1 in 9 incline.

The exterior of the museum was to be faced in rough blocks of self-supporting limestone quarried from nearby Ancaster. By contrast, the door and window openings have been specified in pristine brass. “The museum is about precious human things contrasting with the rough mass of the landscape,” says Strange.

The concrete panels intended for the interior. Part of the attraction of using Agilia was its consistent surface finish, which meant that Strange could use exposed concrete inside. “The stone raw material of the exterior has been taken into the interior,” he says. To give this impression, the 75 mm depth of the concrete boardwork matches the courses in the limestone facade. It is here that the fineness of the detail achievable with Agilia really comes into its own. “Agilia is sharp enough to pick out the arrises between the boards,” says Strange. “With normal concrete, 2 mm arrises would crumble; at Lincoln they are very crisp.”

To reinforce the stratified appearance of the boardwork, Panter Hudspith planed planks of timber shuttering to depths varying between 18 and 21 mm. Strange says that by stepping the timber planks, the boardwork becomes more pronounced because the recesses offer a greater contrast between light and shade.

Originally Strange had been drawn to self-compacting concrete because it is paler than the ordinary variety. “We wanted light from the rooflights to reflect off the walls onto the artifacts,” he says. Panter Hudspith was also keen to use concrete because its high thermal mass would make it easier for the museum to maintain consistent temperatures.

Self-compacting concrete contains a super plastisizer to make it more fluid than normal concrete, and this allowed it to flow into Panter Hudspith’s complex forms. Although Agilia’s low viscosity means that air escapes without the help of noisy vibrating pokers, it is important to introduce as little air as possible during the pour to minimise the possibility of air bubbles being trapped. This can be avoided by supplying the concrete through a rubber tube placed at the base of the shuttering, known as a tremie pipe.

It is also critical that the correct release agent is used to ensure that air escapes from the concrete before it sets, and that the shuttering comes away cleanly from the finished concrete. Lafarge Readymix, which is part of Lafarge Aggregates, supplies the appropriate specification of the release agent for every application.

Lafarge says with the correct release agent the shuttering can be struck so cleanly from the concrete that it can be used up to 35 times. In reality, the manpower required to take out the fixings from the timber may not always make recycling feasible on this scale.

Something else that Northfield had to watch out for was the high pressures on the formwork caused by the greater pours achievable with Agilia. The greater weight of concrete and the hydrostatic pressure created means formwork has to be strengthened with thicker bolts at shorter centres.

Seamus Regan, Northfield’s construction director, says the formwork supplier must be informed of the concrete pressure to ensure that its design is robust and waterproof enough to withstand the pour.

Agilia was also specified for the sawtooth roof of the museum. Sloping concrete slabs of 22° and 28° made the roof challenging for Northfield. Even so, it managed to place the concrete in only nine 25 m3 pours.

The remarkable finishes that were obtained at Lincoln Museum should help Agilia and self-compacting concrete to attract a wider fanbase among architects and designers. At the moment Darren Williams, Agilia’s product manager, says the concrete is mainly specified by housebuilders who are impressed by the quality of the finish and the elimination of powerfloating (see box left). But there are already signs that Agilia is making an impression beyond the housebuilding sector – in Portsmouth it has been specified for the 165 m high Spinnaker Tower (see cover).