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Composite technology is currently being exploited to significant effect by the automobile, aerospace, and wind energy industries. However, composite products used in these industries usually need to be cured at high pressures and temperatures, typically 6 atm and up to 180°C, which could limit the size of the parts that could be manufactured and significantly impact cost.
Composites manufacturers are now developing products that can be cured at low pressures and temperatures (typically 1 atm and as low as 30°C). With continued lower cost and ease of processing brought about by technology, the market for composites is projecting an upward trend for the industry, as well as new composite technologies that are reliant on data management, machine learning, and domain knowledge.
Structure of Composite Materials
The structure of composite materials is well known. Fibers are bonded together in a particular orientation by a polymer matrix, which transmits loads to the fibers and protects them from damage. The three main types of fibers currently-used are glass, aramid, and carbon.
Research into the use of composites for construction-related applications is mainly looking at pre-impregnated materials or ‘prepregs’, which should not be confused with traditional GRP or glass reinforced polyester materials. Polymer composites are usually used in construction; however, researchers found that the material is disadvantageous in terms of cost, lack of industry standardization, the absence of design codes, and relative experience working with the material. As such, new materials, in this case, prepregs, are being tested for their applicability.
Preparation of Prepregs
The first step in preparing a prepreg material is fiber manufacturing. Having identified the requirements of the fiber in terms of density, strength, modulus, strain to failure, and corrosion performance, the designer must decide on how to incorporate the fiber most effectively into the structure. There are two basic types of prepreg unidirectional and woven: an impregnated monolayer of aligned fiber and a resin-impregnated fabric. The impregnation process can be adjusted easily to give an extremely accurate fiber-to-resin ratio in the end-product, thus eliminating the possibility of inconsistent resin mixing.
Storage and Curing
Once formed, the prepreg has to be stored in such a way that the curing process is prevented until needed. This is because the resin is impregnated into the fiber along with all necessary additives, including the curing agent. Storage involves freezing the prepreg in a sealed plastic bag to prevent moisture uptake. Stored as such, the prepreg has a freezer life of about one year. When needed, the material is thawed out and removed from the bag. This process typically takes four to six hours.
Processing the prepreg involves a mold to impart the final shape. When preparing high-temperature composites, metal or a composite tool is needed, as well as an oven, autoclave, or press to provide the necessary pressure and temperature. By contrast, low-temperature molding materials (LTM) from the Advanced Composites Group can be used with simpler tools. For instance, a mold 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.
Temperature and vacuum requirements could be achieved by using 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 vast, complex structures to be designed and built for construction purposes.
Low-Temperature Moulding (LTM) Materials
How do LTM Materials and Processes Compare to Traditional Materials and Processes?
Composite materials made using the LTM method offer comparable properties to standard materials cured at higher temperatures and pressures. They have an optimized fiber-to-resin ratio, low void content, toughness, and accuracy. In addition, the LTM method has an economic attraction. High-temperature molding can be prohibitively expensive for prototypes, small production runs, and large structures, whereas LTM processing is affordable owing to lower tool cost, little to no capital expense and reduced energy use.
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 especially 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 are Suitable For LTM?
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 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 or sound insulation properties.
Composites in Construction
The economic benefits of using composite technology are not restricted to savings in terms of basic materials or production costs. The high strength and low weight of the materials, in combination with foam cores in monocoque shells, means that they could be used to form long span roofs in various infrastructures. 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 may suffer from corrosion of the steel bolted connections. As such, 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 with more design freedom compared 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 the heavy plant to lift them into place. In some applications, the weight of components could be reduced enough to allow them to be placed into position. This benefit is already being exploited by the construction industry wherein composite plates, instead of steel plates, are at times used to reinforce existing structures.
Behavior in Fire
Numerous practical aspects need to be addressed before the construction industry fully adopts composite technology. The primary consideration is the performance of the materials in a fire. Composites usually contain a high proportion of fibers and fillers, neither of which support combustion. Epoxy resin is not usually 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. Concerning the requirements of BS 476, companies and researchers are continuously conducting studies to ensure that regulations are met. Current areas of interest include measurement of flame spread, the use of radiation gradients, and fire-resistant elements.
There is no doubt that there are many applications in which complex shape, lightweight, and high durability are beneficial. Ultimately, the adoption of composite technology may well be the result of designers and architects perception that such technology provides promise in the future of many industries.
This article was updated on 7th February, 2019.