Concrete cloth - flexible fibrous cement

Concrete, Feb 2008 by Crawford, William

Four years ago, my business partner and I entered a design competition run by the British Cement Association. At the time we were students studying for a postgraduate degree in Industrial Design Engineering in London and we had no idea that our entry for a rapidly deployable emergency shelter would result in the launch of our own technology development company and involve research trips to disaster zones around the world including Uganda and New Orleans.

Four years on and the concept has matured into a material technology that has applications far beyond emergency shelter. Following 12 months of development, funded through a combination of private equity investment and grants, the company has now relocated to larger premises in south Wales and is setting up the volume production facility for concrete canvas shelters (CCS) and concrete cloth.

Concrete cloth

The shelters have been enabled by the development of a core material technology called concrete cloth, a unique proprietary material that we now believe has a very wide range of applications throughout the building and civil engineering industry.

The cloth consists of a three-dimensional fibre matrix, containing a specially formulated dry concrete mix. A PVC backing on one surface of the cloth ensures the material is completely waterproof, while hydrophilic fibres on the opposite surface aid hydration by drawing water into the cement.

The cloth has the following key advantages:

* Rapid - the material can be hydrated either by being sprayed or by being fully immersed in water. Once hydrated, it remains workable for four hours and hardens to 80% strength within 24 hours. This time can be reduced by adding accelerants into the dry mix at the point of manufacture.

* Easy to use - dry concrete cloth can be cut or tailored using simple hand tools such as Stanley knives. The PVC side can be supplied with an adhesive backing and the fibrous side bonds well to concrete or brick surfaces when set. It can be easily repaired or upgraded using existing cement products.

* Flexible - concrete ciotti can be easily nailed or stapled through before setting. It has good drape characteristics, allowing it to take up the shape of complex surfaces including those with a double curvature.

* Strong - the fibre reinforcement acts to prevent cracking, absorbs energy from impacts and provides a stable failure mode.

* Fireproof - concrete cloth is a ceramic-based material and will not bum (Figure 5).

* Waterproof - the PVC backing on one surface ensures that concrete cloth is completely waterproof.

* Adaptable - concrete cloth is currently supplied on 1.2m-wide rolls but can be manufactured with a roll width of up to 5m. The cloth can be produced in a range of ttiicknesses from 5-20mm.

* Durable - concrete cloth is chemically resistant, has good weathering performance and will not degrade in UV.

Deployable shelters

The original concept was to create rapidly deployable hardened shelters that require only water and air for construction. The key to the idea was the use of inflation to create a surface that is optimised for compressive loading. This allowed thin-walled concrete structures to be formed, which are both robust and lightweight.

The shelter is deployed in four stages:

* Delivery (Figure 1) - CCS is supplied folded and sealed in a sack. The 16m^sup 2^ variant is light enough to transport in a pick-up truck or light aircraft.

* Inflation - once delivered, an electric fan is activated which inflates the plastic inner to lift the structure until it is self-supporting. The shelter is then pegged down with ground anchors around the base (Figure 2).

* Hydration - the 54m^sup 2^ shelter is then sprayed with water (with smaller variants, hydration takes place by filling the sack with water. The volume of the sack controls the waterxement ratio). Hydration is aided by the fibre matrix, which wicks water into the cement (Figure 3).

* Setting - the concrete cloth cures in the shape of the inflated inner and 24 hours later the structure is ready to use. Access holes allow the installation of services; water, power, air conditioning and heating units (Figure 4).

The structures are designed as part of a modular system; units can be easily linked together enabling the space to be tailored to the application. If required they can be demolished using basic tools. The thin-walled structure has a very low mass, leaving little material for disposal.

The University of Bath has conducted finite element analysis on the shelters, showing that the structures can withstand a very high distributed compressive load, enabling sandbags, earth or snow to be piled on top. This gives the shelters excellent ttiermal properties and can provide protection against shrapnel, blasts and small arms fire. Concrete canvas shelters are specified to withstand 0.75m of wet sand on the sides (sufficient to stop 7.62mm rounds) and 0.5m on the roof (to protect against shell fragments).

Concluding remarks

In summary, concrete cloth can be used to rapidly createwaterproof, fireproof, fibre-reinforced thin concrete forms across a wide range of applications. Some of these applications include: rapid trackway or landing surfaces, structural reinforcement, back blinding, ground stabilisation, tunnel lining and even improvement of ballistic protection. The British Army has recently placed an order to test the cloth as sandbag wall reinforcement on operational trials.