23 August 2002
23 August 2002
The potential benefits to the construction industry of combining composites with concrete in new-build applications were identified some while ago. However, there were some issues to be resolved and further investigated before the technology could be implemented, including:
1. The design basis for the technique.
2. The structural performance of elements combining composites with concrete.
3. The response of the materials to fire and mechanical damage.
4. How sensitive (or otherwise) the materials are to construction site environments.
5. How sensitive the manufacturing techniques are to construction skill levels.
The aim of the COMPCON research project, led by Taylor Woodrow, has been to address these broad issues. The test element, a 15m long beam, was designed to have a composite shell with a concrete ‘core’. The shell is moulded by laying the composite material over a steel mould and applying a vacuum. Heat is then applied to fully cure the resin and make it rigid.
The composite material used for the long span beam is a prepreg format, which means it is a mat of fibres with partly-cured resin, which is tacky. The word prepreg refers to the fibre mat being pre-impregnated with resin. The prepreg layers in the beam contain glass or carbon fibres, as appropriate. The glass material is mainly on the sides of the beam, where the shear loading needs to be resisted, and the carbon layers are all on the lower flange of the beam, where the tension resistance is needed to resist flexural bending.
A full-scale test to destruction, conducted by Taylor Woodrow as the culmination of its co-ordinated research, confirmed that the failure load of the beam compared well with the predicted ultimate capacity. The actual ultimate capacity was 5% greater than predicted. This is within normal experimental bounds of error.
The full-scale test confirmed that the traditional design assumptions, of plane sections remaining plane, apply. This means that the normal rules of flexural design can be applied to this type of composite construction.
Performance during fire was a major consideration in adopting composite materials for use in construction. Full-scale testing of a beam in a furnace demonstrated a fire rating in excess of one hour. This was with a fire protection board material placed on the beam. Improvements to the fire rating can be achieved using other protection materials, which are included in COMPCON’s assessment e.g. intumescent paints, phenolic coatings, etc.
The COMPCON project resulted in the formulation of a denser prepreg material than is typically used in aerospace and Formula 1 applications. In practice, this means that fewer layers of the material need to be laminated to create the section sizes appropriate for construction use. Also, a denser and stiffer resin allows effective transfer of the fibres’ mechanical properties throughout the laminate.
The mass of the 15-metre length of finished composite shell (without concrete) is approximately 200 kg (13 kg / metre), which is several orders of magnitude lighter than the masses of elements made of alternative construction materials. The test beam composite shell was transported by hand from the building in which it was manufactured to the structures testing lab.
Lighter elements have positive implications for delivery to, and manoeuvrability on, a construction site. For example, many composite shells could be stacked on a lorry and delivered to site, rather than just three or four steel or concrete beams. Placing the composite shell sections in position in a building frame and then casting the concrete into the shell may make installation weight savings and avoids having to transport heavy items on site.
The prepreg chosen for COMPCON is a so-called low temperature moulding (LTM) composite, which means the resin is cured at far lower temperatures than would be the case in the aerospace and Formula 1 industries. The LTM cure temperature was 65°C, achieved using a heating blanket placed over the laminated composite. No special plant is needed to achieve the resin cure.
Taylor Woodrow developed a simple method of laminating the prepreg material using a trolley system. This eliminates any sensitivity to unskilled personnel of using the advanced materials. Therefore, the project achieved the objective of transferring the specialist composites practice into the construction sector.
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