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Concrete In Australia : March 2008
NATIONAL PRECASTER NUMBER 47 • FEBRUARY 2008 internal conditions creates the level of heating or cooling required. This can be satisfied by passive (climatic thermal design) or active (HVAC) measures. Appropriate climatic thermal design of buildings can eliminate or significantly reduce building heating and cooling requirements. High thermal mass inside a building has its most significant impact on reducing cooling, a benefit for a world facing global warming. Design factors affecting the thermal design of buildings are: • Shape • Fabric • Fenestration • Ventilation. Concrete 100mm 200mm 300mm Time lag 3 hours 6 hours 9 hours Decrement 0.45 (55% reduction) 0.20 (80% reduction) 0.10 (90% reduction) For concrete to be an appropriate material for thermal design and energy efficiency in a building, it needs to be fully insulated from the outside climate. Section J of the BCA focuses on insulation and the conductivity of materials, as these reduce energy consumption and increase thermal comfort by increasing the mean radiant surface temperatures of the building envelope. As a result, less energy is consumed and therefore there are fewer greenhouse gas emissions contributing to climate change. Insulation on the outside of the building envelope also reduces the incidence of condensation. Any material with warmth on one side and ‘coolth’ on the other can have condensation occur on the warm side if the dew point is reached. Insulation and/or a vapour barrier will prevent condensation. For thermal mass to be effective, it should also be exposed to the interior and its occupants, not covered up with cosmetic finishes such as plasterboard. Design If heat inflows and outflows from a building are other than zero, the building will need heat input (heating) or removal (cooling). The difference between the climatic conditions and the nominated building extruded polystyrene insulation EXTERIOR INTERIOR Precast solutions for thermal performance It is widely accepted now that thermal bridging – conduction paths between the inside and the outside – has a significant detrimental effect on building performance. As well, the combination of thermal mass on the inside and insulation on the outside of a building envelope significantly increases comfort and also reduces energy consumption. Knowing this, the precast industry has developed a new way of detailing, to eliminate thermal bridging and to develop heavyweight, mass interiors that are insulated from the exterior: precast sandwich panels. Precast sandwich panels consist of two layers of concrete that are factory- made with a central layer of uninterrupted rigid insulation. They typically have a narrow (say 50 - 75mm) outer precast skin which is attached through the insulation to a wider (say 100 - 200mm) load bearing inner precast section, using non-conductive ties (connectors). Precast sandwich panels achieve the ideal thermal solution as they combine high internal mass insulated from the outside in a form of construction that has no thermal bridging. PRECAST SANDWICH WALL PANEL exterior wythe interior wythe To make precast concrete sandwich panels, a first concrete slab is formed, into which one end of the connectors are embedded. A layer of rigid insulation is positioned over the connectors and a second layer of concrete is cast to cover the protruding ends of the connectors. The connectors provide resistance to shear in at least two directions. 28 Concrete in Australia Vol 34 No 1