No_3_October_1963 — Page 119

Cover strip sprung on

Gasket

Setting block

Expansion

Clip

space

Neoprene tensioning

strip insert -ed here

Neoprene gasket

Fig. 7. Joint in curtain walling. Tape Fig. 8. Tensioning strip gasket

mastic or gaskets under a pressure

strip.

OUTSIDE

Butyt Mastic

Thiokol

Foamed plastic or air to prevent

gop forces on edge of double glazed unit or Flexible plastic panel

Figs. 9 and 10. Socket joint combining features of a butt and lap joint

Setting block

OUTSIDE

Spacers

Selting block

Fig. 1. Curtain wall joint using

'Thiokol' and a butyl mastic

FIGS. 7-12. JOINTS FOR CURTAIN WALLING

Fig. 12. Curtain wall joint using plastic

mastic with spacers

This difficulty can

be overcome where necessary by providing spacers of more rigid materials to carry the load, for example, wind load or the weight of glass in opening lights.

The advantage of a plastic type of mastic is its ability to flow slowly with movement of the joint, thus avoiding too great a demand on the adhesion and cohesion of the mastic. Its ability to retain this property in use depends upon the speed rather than the extent of movement of the joint.

114

Elastic mastics (including mastics, based on Thiokol', 'Neoprene' sili-

cones

or urethane and some butyl mastics). Most of these compositions.

Mastic bedding Mastic pointing

Fig. 13. Joint for window frame

although plastic when applied, cure within a few days, producing a rub- ber like mass. The demands on the adhesion are then proportional to the extent of movement at the joint and are but little affected by its rate. When set, the material is able to carry wind loads and the weight of glass without being extruded. It is advantageous to use a material that cures to a rather soft rubber with some plastic flow since this less readily loses its adhesion when the joint moves. Rubber-like mastics may. however, be difficult to remove if reglazing becomes necessary.

GASKETS

Gaskets are preformed strips of material which are inserted in a joint and rely for their efficiency on con- tinuing pressure on the sides of a joint rather than on adhesion; hence. in a shear joint, they can tolerate large movements.

The functions of gaskets, and hence the properties needed in the joint, are essentially similar to those of mastics. Stiff tape mastics, oiled cords in patent glazing, or bitumen felt on which concrete panels are bedded are in many ways fulfilling the functions of a gasket.

True gaskets, however, are usually made of fully cured elastic com- pounds such as natural rubber. 'Neoprene' or p.v.c. In use, they are patent glazing, or bitumen felt on joint, under sufficient compression to ensure that they are not released by subsequent movements. Some gaskets are made of foamed material or contain pockets of air and can be compressed more readily into the joint. Such gaskets tolerate greater movement than solid ones.

as

Gaskets have the advantage that they require no hardening or curing period, but are fully functional soon as they are installed. They may be attached to fragile material such as glass to help to protect it during transport and storage.

On smooth surfaces, there is little need to clean away the last traces of grease before fitting gaskets as there is no need to obtain good adhesion with mastics.

Gaskets are, however, less able than mastics to cope with rough or irregular surfaces, and adhesives or mastics may be used with the gasket

THE HONG KONG & FAR EAST BUILDER-VOLUME 18, NUMBER 3