Petrographical properties: structure, chemistry, mineral

STONE CLADDINGFor the majority of large commercial buildings, stone is used as a cladding material mechanically fixed to the structural system. The standards give a broad outline of the structural requirements, including fixings, back-up material and joints. The strength of the stone largely determines the appropriate cladding panel thickness. For granites, marbles and slate 40 mm slabs are usual for external elevations above ground-floor level, but for the softer limestones and sandstones a minimum thickness of 80 mm is frequently recommended. The standards relate flexural strength, span between fixings and external stone cladding thickness to high, medium or low wind exposure.Fixings must be manufactured from stainless steel or non-ferrous metal and sized to susussttaaiinn tthhee ddeeaadd llooaadd ooff tthhee ccllaaddddiinngg ttooggeetthheerr wwiitthh aapppplliieedd llooaadds s ffrroomm wwiinndd aanndd maintenance equipment. Movement joints are required to accept the differential structural movements of the frame and the thermal and moisture movements of the cladding. Horizontal compression joints of 15 mm minimum should be located at each floor level; vertical movement joints of 10 mm should be at approximately 6 m centers. Polysulphides, polyurethanes and silicones are used as joint sealants, although non-staining silicones should be used on stones which darken by absorption of silicone fluid. Stone-cladding systems should ideally be protected from impact damage at ground level by the design detailing.
Lightweight sttone cladding Thin-section stone (approximately 6 mm) may be bonded to lightweight backing materials to reduce the dead weight of stone cladding. The reduction in dead load is significant compared to thickstone sections which would require traditional stone cladding techniques. One such material, originally used in the aerospace industry, is a sandwich panel consisting of a core of honeycomb aluminum faced with glassfibre-reinforced epoxy resin skins. The polished stone facing is bonded to one face with epoxy resin, to create a lightweight stone-finished panel, which, if detailed appropriately, has all the visual qualities associated with solid stone masonry.
Rainscreen stone claddingRainscreen cladding systems incorporate stone panels fixed to the backing wall by a durable metal framing system, leaving a drained void space which may be partially filled with thermal insulation. The stone panels may be solid or a laminate of stone bonded to lightweight concrete. External and internal corner special units are prefabricated to match the facade. The standards describe the basis of both drained and ventilated rainscreens and pressure-equalised rainscreen cladding systems.