Composite technology is being exploited to great effect by the automobile and aerospace industries. However, the composite products used in these industries normally need to be cured at high pressures and temperatures (typically 6 atm and up to 180°C). This can limit the size of the parts that can be manufactured, as well as having a significant impact on cost. Now composites are being developed that can be cured at low pressures and temperatures (typically 1 atm and as low as 30°C). Thanks to lower cost and easier processing, the technology is becoming a practical proposition for consideration by the building industry.
Structure of Composite Materials
The structure of composite materials is well known - essentially, fibres are bonded together in a particular orientation by a polymer matrix, which transmits loads to the fibres and protects them from damage. The three main types of fibres currently used are glass, aramid and carbon. Research into the use of composites for construction related applications is mainly looking at preimpregnated materials, or ‘prepregs’, which should not be confused with traditional GRP or glass reinforced polyester materials.
Preparation of Prepregs
The first step in preparing a prepreg material is the manufacture of the fibre. Having identified the requirements of the fibre in terms of density, strength, modulus, strain to failure and corrosion performance etc, the designer must decide how to incorporate the fibre most effectively into the structure. There are two basic types of prepreg unidirectional and woven. The former is an impregnated monolayer of aligned fibre and the latter is a resin-impregnated fabric. The impregnation process can be adjusted easily to give an extremely accurate fibre-to-resin ratio in the end product, so eliminating the possibility of inconsistent resin mixing.
Storage and Curing
Once formed, the prepreg has to be stored in such a way as to prevent the curing process from happening until the material is needed for use. This is because the resin is impregnated into the fibre along with all the necessary additives, including the curing agent. Storage involves freezing the prepreg in a sealed plastic bag to prevent moisture uptake. Stored in this way; the prepreg has a freezer life of about one year. When it is needed, the material is simply thawed out and removed from the bag, a process that normally takes four to six hours.
Processing the prepreg involves a mould to impart the final shape. When processing high temperature composites, a metal or composite tool is needed, along with an oven, autoclave or press to provide the necessary pressure and temperature. By contrast, low temperature moulding materials (LTM) from the Advanced Composites Group can be used with much simpler tools - a mould can be made of anything from resin to concrete or timber, as long as the material can withstand temperatures of 30-60°C and maintain a vacuum, figure 1.
Figure 1. Vacuum only processing of a large curvature structure.
The temperature and vacuum requirements can be achieved using nothing more than a tent, portable pumps and electric space heaters. This simple technology is key to allowing designers and manufacturers to use LTM composite materials on the construction site, away from autoclaves and composites manufacturing shops. Such on-site technology could enable very large, complex structures to be designed and built for construction purposes, figure 2.
Figure 2. Large marine structure under vacuum bag and temporary oven..
Low Temperature Moulding (LTM) Materials
How do LTM Materials and Processes Compare to Traditonal Materials and Processes?
Composite materials made by the LTM method offer comparable properties to standard materials cured at higher temperature and pressure. They have an optimised fibre-to-resin ratio, low void content, toughness and accuracy. In addition, the LTM method has an economic attraction. High temperature moulding can be prohibitively expensive for prototypes, small production runs and large structures, whereas LTM processing is affordable owing to lower tool cost, low/no capital expense and reduced energy use, figure 3.
Figure 3. Marine component being removed from tooling and vacuum only cure at low temperature.
What Benefits can LTM Offer the Construction Industry?
LTM has many potential benefits in construction applications including flexibility of form, structural efficiency and construction, productivity and life cycle costing. Composite materials also have advantages over steel and reinforced concrete in terms of maintenance and durability - this is true for a range of applications, such as bridges, storage reservoirs, swimming pools and coastal structures, in which steel corrosion is an issue.
What Types of Articles is LTM Suited To?
The LTM process is particularly useful for manufacturing large structures with specific design requirements. Reinforcement can be built into the structure at the exact point where it is required by increasing the local laminate thickness. The laminate ‘architecture’ can also be tailored to produce stiff, lightweight coverings for large areas. Such applications call for a thin-skinned, cored laminate, which is easy to make by sandwiching a core of honeycomb or foam material, say, between two layers of laminate. The same material system could be used to make panels of cladding material with built-in weather protection and thermal/sound insulation properties.
Composites in Construction
The economic benefits of using composite technology more widely in the construction industry are not restricted to savings in the basic materials or production costs. The high strength and low weight of the materials, in combination with foam cores in monocoque shells, mean that they could be used to form long span roofs in sports stadia, for example. Reducing the weight of this type of roof in turn offers savings in the supporting structure and foundations. It also offers an alternative to the use of laminated timber beams, which can suffer from corrosion of the steel bolted connections. So life cycle and maintenance costs are reduced too.
Composites are flexible and can form irregular shapes, varying sections or detail features. This allows an architect more design freedom than with other materials. Applications benefiting from such flexibility include styled entrances to commercial, retail and hotel buildings and shell roofs in atria, shopping malls and leisure facilities.
Manufacturing large, lightweight monocoque structures on site would reduce the need for heavy plant to lift them into place. In some applications, the weight of components could be reduced enough to allow them to be man handled into position. This benefit is already being exploited by the construction industry - composite plates are sometimes used to reinforce existing structures instead of steel plates.
Behaviour in Fire
Of course, many practical aspects need to be addressed before composite technology is fully adopted by the construction industry. The main consideration is the performance of the materials in a fire. Composites usually contain a high proportion of fibres and fillers, neither of which support combustion. Epoxy resin is not normally fire resistant, but it can be mixed with fire-retardant additives to make the matrix self-extinguishing, with low smoke emission. Comprehensive fire tests have been carried out relating to the use of composites in railway carriages (both internally and externally) and for aerospace applications. However, these test results have yet to be compared to the requirements of BS 476. This is one aspect that researchers will be exploring.
There is no doubt that there are many applications in which complex shape, light weight and high durability are beneficial - the opportunity is there for the construction industry to exploit. Ultimately, the adoption of composite technology may well be the result of designers and architects doing what they do best - stretching the ingenuity of engineers to make their imagination a constructed reality.