Types Of Mastic Asphalt

Mastic asphalt is a blended bitumen-based product. It is manufactured either from the bitumen produced by the distillation of crude oil, or from lake asphalt, a naturally occurring blend of asphalt containing 36% by weight of finely divided clay, mainly imported from Trinidad. The bitumen is blended with limestone powder and fine limestone aggregate to produce the standard roofing types specified in BS 6925: 1988 (Table 6.2).

The effect of the finely divided clay particles within lake asphalt type BS 988T confers better laying characteristics and enhanced thermal properties; these are advantageous when the material is to be exposed to wide temperature changes, particularly in warm roof construction systems.

Mastic asphalt is usually delivered as blocks for melting on site prior to laying, although hot molten asphalt is occasionally supplied for larger contracts. Laid mastic asphalt is brittle when cold but softens in hot sunny weather. The hardness is increased by the re-melting process, and also by the addition of further limestone aggregate. Polymer-modified mastic asphalts, usually containing styrene butadiene styrene block copolymers, are more durable and have enhanced flexibility and extensibility at low temperatures, allowing for greater building movements and better resistance to thermal shock. Where mastic asphalt roofs are subjected to foot or vehicle access, then paving-grade mastic asphalt (BS 1447: 1988) should be applied as a wearing layer over the standard roofing grade material. Two key grades S and H are

Table 6.2 Mastic asphalt grades to BS 6925: 1988

Type

Composition

BS 988B

100% bitumen

BS 988T25

75% bitumen, 25% lake asphalt

BS 988T50

50% bitumen, 50% lake asphalt

Specified by manufacturers

polymer-modified grades

Fig. 6.4 Copper-faced bitumen roofing sheet available; the softer grade is suitable for footways and rooftop car parks, the harder grade for heavily stressed areas. For standard flooring, type F1076 mastic asphalt is required; for coloured flooring type F1451, and for tanking and damp-proof courses type T1097 is necessary. The flooring types of mastic asphalt are available in four grades (hard, light, medium and heavy duty) according to the required wearing properties.

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