Bag to the future

Precast concrete is most often used to produce elements that are structurally efficient, have quality of finish and are manufactured under controlled conditions. Using fabric formwork could enhance the benefits of precast technique considerably. It could help construction teams produce elements that are structurally efficient and architecturally exciting - and in a relatively inexpensive and practical manner.

Fabric-cast concrete involves casting concrete in a flexible formwork, usually a woven fabric of some kind. By carefully shaping the fabric it is possible to produce complex shapes that would otherwise be difficult to manufacture using conventional formwork systems.

Possibly the earliest recorded application of fabric-cast concrete was by Spanish architect Felix Candela in Mexico in the early 1950s. Candela used sackcloth draped over timber profiles to

build shell structures for school buildings.

In the 1960s Miguel Fisac built a number of buildings and structures using flexible plastic formwork to create sculpted, repeatable precast cladding panels.

Fabric fantastic

Where Fisac tended to work with two-dimensional panels with one exposed surface, Professor Mark West at the University of Manitoba's Centre for Architectural Structures has pioneered complex, three-dimensional beams and columns with continuously varying cross-sections. His most ambitious project to date has been a 12m beam with a 7m main span and cantilever spans of 2.5m at either end. The beam was shown to have saved 50% of the concrete and 40% of the reinforcement that an equivalent rectangular beam would have needed.

Casting concrete in fabric brings real advantages.

• Inexpensive formwork: The most efficient way of carrying any force is by axial tension. Fabric will adopt the most efficient geometry to carry the weight of the wet concrete cast into it. The formwork needs little or no additional bracing other than the direct support of the fabric itself. The key is in controlling the geometry and deformation of the fabric to achieve the required shape. The fabric itself is relatively inexpensive and many different types are suitable for use.

• Structurally efficient forms: The fabric can be shaped to produce form-active structural components such as beams and columns. The fabric can be used to create elements with variable cross-sections that change in both depth and breadth (top picture, this page) to follow the principal structural actions. The quantity of concrete used and hence the self-weight of the structure can therefore be considerably reduced compared with conventional rectangular cross-sections.

Fabric will automatically adopt the most efficient geometry to carry the weight of wet concrete cast into it

• Improved quality of concrete: The porous nature of the fabric aids compaction curing and the surface texture of the concrete. The benefits of permeable formwork liners in conventional concrete are now becoming established. During casting, excess water and air is allowed to bleed through the fabric. This reduces the number and size of blemishes and air holes, with consequent improvements in surface texture and appearance. The density, strength and durability of the surface concrete are therefore all improved by fabric casting.

Here in the architecture workshop at the University of Edinburgh we started by casting simple rectangular panels. We stretched fabric across a rectangular steel frame and raised it off the ground, placing various forms under the fabric. When the concrete was cast, the fabric deformed over the forms underneath to produce contoured surfaces. The fabrics used were simple textile fabrics: cotton and polyester woven and non-woven materials. All the fabrics we tested gave good-quality finishes although the non-woven proved difficult to strip.

Taking it further, we organised a five-day workshop with 10 senior students to produce precast elements, and invited Professor West over from Canada to inaugurate the project. I will describe two of the elements here, a column and a beam.

Pillar talk

For the column, we manufactured an adaptable casting rig, consisting of two horizontal steel frames covered with plywood (see picture above, top right). A tube of fabric was stretched between the upper and lower platforms; jacking the top frame upwards removed all slack from the fabric. We then poured concrete through the top hole, which was cut in the shape of a figure-of-eight, and found that with sufficient tension the fabric would not bulge during casting. We achieved a twist by rotating the top panel by 90 degrees.

We then cast the beam shown on page 16. It is essentially a T-section in which the web tapers from a maximum depth and breadth at the mid-span to zero at the supports. The form was produced from a rectangular piece of fabric, folded over. The curve of the web was defined by a parabolic curve stitched into the folded fabric. The rectangular flange was formed using plywood sheet.

An elongated elliptical hole was cut along the central axis of the flange defining the horizontal section of the web where it meets the flange. We pushed the fabric through the hole and attached the surplus to the upper side of the plywood, creating a seal. We weighted the fabric below the plywood to achieve sufficient pre-tension.

We used a geotextile fabric that had excellent strength and tear resistance, and was easily stripped off the cured beam. The geometry of the beam follows the shape of the bending moment diagram for a uniformly distributed load. The beam is now undergoing structural tests.

These preliminary studies show that fabric-cast concrete can produce good-quality precast elements in forms that would otherwise require complex and expensive formwork. Further research is under way to consider improved methods of construction, patterning of fabric, estimating the extension of the fabric and selecting the fabric and mix design for the optimum construction process.