In the latest in our series on architects' details, Hugh Stewart, partner at Foster + Partners, explains the thinking and construction technique behind the rammed concrete walls of Narbo Via, a new museum of Roman antiquities in central Narbonne, France
What were you trying to achieve with the form of this building?
First and foremost, a degree of monumentality. This came in part from the subject matter of the museum, which is archaeology, but the stratified nature of archaeological investigation. In addition, from an environmental point of view, because Narbonne is in the hottest part of France, we based our approach on a substantially solid facade with relatively small amounts of glazing shaded by an over-sailing roof canopy. Once we'd taken these decisions, a conventional framed approach to the building proved inadequate.
Where does the technique of tamping concrete to form a wall originate?
We were researching how best to do a solid wall and that's what led us to use layers of dry-mixed concrete tamped into place on site.
We worked with a Canadian specialist, Meror Krayenhoff, who has been working in this field for a number of years. He calls the technique Sirewall [Structural Insulated Rammed Earth Wall]. He advised on the technique from the project's early days and supported the contractor during construction.
At Narbo Via, the 5.5m high walls are built up in 200mm tamped layers. They are the 800mm wide, with a 200mm wide insulated core sandwiched between 200mm of rammed concrete on the outside and 400mm of rammed concrete on the inside. Each layer of concrete is reinforced, and the two sides are tied together across the insulation.
The variations in layer colour are deliberate. We created four basic colours using iron oxides in differing quantities. We then mocked up the full-size walls in our office using paper prints until we contrived a seemingly organic 'layering' to the wall.
Are all the walls built using this technique?
Conceptually the building is split into two parts, display spaces and the research centre with a big internal street between the two. Both parts are defined by the coloured concrete walls with the primary structural columns also made of the same material. The walls support a grey concrete precast roof structure. Because the building is in a seismic zone, the walls are structurally connected using a wet concrete connection to the primary roof beams.
However, the basement walls are composed of conventional grey concrete – a decision taken by the contractor because the stratified walls takes longer to construct than conventional concrete walls.
How did you decide on an appropriate concrete mix?
The tamping technique relies on very careful grading of the aggregates. Normal concrete aggregate is about 15-20mm in diameter, along with sand, whereas this technique has a whole range of different sized aggregates, which the originator says makes it stronger. But in practice it is about as strong as a conventional grade concrete.
We built a prototype in a testing lab during the design process to convince ourselves of its viability.
Once a contractor was on board we produced a series of full-size prototypes. The concrete was mixed in a volumetric mixer along with the oxides for colouring and very little water. It was a very dry mix, you could lift it in your hand and nothing would stick. We used local aggregates, producing a neutral tone, and experimented to get the colour and texture of the layers to the level we wanted. With each variation we used a marginally different mix until we were satisfied with the outcome.
How are the walls constructed?
The walls were built between formwork, one layer at a time. One of the things we were particularly concerned about was to avoid having temporary tie rods linking the inner and outer formers, which would leave marks on the finished surface. To get round this the contractor, Vinci, used a full height former on one side and jump formwork on the other. The jump formwork was progressively lifted in sections and propped as the wall was constructed to avoid people having to work at height between the two formwork walls.
A hand-held pneumatic tamper was used to compact the placed concrete. It was a quasi-manual process in that respect. The tamping caused the larger aggregate to rise to the top, which gave that part of the layer a slightly grainier texture that added to the walls' character.
We were also insistent on the uniformity of layers, which was quite onerous for the builder. The tamping operatives in the formwork used a stick to measure and keep the layers horizontal and uniform in thickness.
Did the construction technique require any building regulation exemptions?
This is a project we did with our own in-house structural engineers. There were no major exemptions required to allow this technique to be used; it is regulation-compliant. It also complies with Eurocodes as a concrete with a prescribed composition. Cube tests were done every step of the way, including for each layer of concrete, to test its compressive strength.
How do the walls perform thermally?
The walls have a fantastic environmental performance, partly because of the internal layer of insulation and partly because of thermal mass, which is cathedral-like in its effect. For that mass, the thermal cycle is measured in months rather than in days, which would be the case for a lighter weight construction.
Interestingly, the walls' textured surface also allows a degree of acoustic absorption.
How does the embodied carbon compare to that of conventional concrete?
The cement content is similar to conventional concrete, however it uses local aggregates, which makes it of its place. Significantly, there is no cladding; in a conventional structure you have to invest in both internal and external finishes. With this technique the rammed structure is the finish.
What do you like most about the walls?
What I find very attractive about it is that compared to almost all contemporary construction you get exactly the same expression internally and externally. Most contemporary construction, because of the framed approach and thermal insulation, tends to be covered up. This technique is a pure expression of structure and there is a tremendous simplicity about it. The walls showcase how they provide the lateral and vertical structural support to the building.