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Concrete In Australia : June 2008
NATIONAL PRECASTER NUMBER 48 • MAY 2008 Open-drained joints Joints in Precast Concrete Buildings Face-sealed joints The open-drained joint is recommended for high-rise construction. It consists of a rain barrier in the form of an expansion chamber with a loose-fitting baffle and an air-seal at the interior face of the panel (see diagram below). The baffle prevents direct entry of the wind-driven rainwater. The pressure in the chamber between the baffle and the internal air seal is at external air pressure. There is, therefore, no pressure differential to drive rain past the baffle. The air-seal is the demarcation barrier between outside and internal air pressures. Water that enters the joint in front of the baffle is drained downwards. At every intersection between the vertical and horizontal joints, a short length of flashing (300 mm) is used to ensure water is discharged to the outside. Advantages – Can tolerate relatively large movements. – The rear sealant is protected from UV light and weather. – Can be installed from inside the building (no scaffold required). – Long maintenance-free life. – Best for medium- and high-rise construction. Rear Disadvantages – Careful supervision is required during installation as it is difficult to remedy defects due to poor workmanship. – Not suitable for tall vertical panels (> 9.0 m in height). – Cannot accommodate joint gap tolerances > 5 mm. Drainage zone, 50 mm minimum Install neoprene baffle strip after erection of next level of panels. See lapping detail on right for treatment at horizontal joints Vertical air-seal‡ ‡ Air-seal may be a sealant with backing rod,closed-cell sponge or square neoprene strip Minimum 20 Fix flashing in place with flexible seal under back and sides Horizontal air-seal‡ Upstand* 300 Minimum 20 * 50 mm typical, 75 mm in exposed locations Flashing Bottom baffle lapped under flashing Minimum 120 Preferred 150 Top baffle Air-seal sealant optional Sealant with backing-rod These joints are simple, economical and are most suited to low-rise construction (see diagram above). They are sealed by a single run of gun-applied sealant close to the exterior surface of the joint. A backing- rod forms the rear of the sealant. The external face seal should, where practical, be supplemented by a seal near the inside face of the panel. Advantages – Panel edges can have simple profiles, no grooves required. – Can be used for complex panel shapes (angled or curved). – Can have a rear seal as a second line of defence. – Lowest first cost. – Can be readily inspected, repaired or replaced. – Best for low-rise construction. Disadvantages – Must be applied from external scaffolding or other form of access. – Sealant is exposed to UV light and weather – needs more maintenance. – In a single-seal system even a small failure may allow water penetration due to capillary effects and pressure differentials. Minimum 15 Preferred 20 Maximum 30 Sealant with backing-rod Rear sealant optional Minimum 15 Preferred 20 Maximum 30 For more information about joints in precast concrete buildings refer to the Precast Concrete Handbook, available for online purchase at http://www.nationalprecast.com.au/services/publications. 28 Concrete in Australia Vol 34 No 2 50