on such surfaces to fill any slight gaps.
Usually, the weakest point of a gasket joint is the mitre formed at corners. Special care is needed to ensure that a sound weld is made.
TYPES OF JOINT
The two basic types of joint are the butt or push-pull joint (Fig. 1) and the lap or shear joint (Fig. 2).
The preferred type of joint for use with mastics or gaskets is the lap joint, in which the two surfaces being joined move parallel to each other, but remain the same distance apart. For equal movement of the surfaces, the extension of a mastic in a lap joint is only about half that in a butt joint, and the mastic is usual- ly better protected from the weather. Further, if for any reason the mastic fails in adhesion to one side of the joint or in cohesion. only slight leakage is likely to occur, since the mastic remains in place and con- tinues to fill the joint.
The convenience of butt joints is that they are easy to make and often easy to renew; however, if the mas- tic fails, the joint at its widest is in general only partly filled by the mastic and leakage may be severe.
Various joints are illustrated in Figs. 3 to 15 and are discussed later.
Figures 3 and 4 show joints suit- able for concrete slabs. the drip forming the first line of defence against rain penetration. In Fig. 5 the p.v.c. gasket performs this func-
inside the building, it may be diffi- If re- cult or impossible to renew. pair is necessary, both joints would probably best be treated by gunning in mastic from outside, in the case of Fig. 5 after removing the cover strip in the joint.
If a soft mastic were placed on the column before positioning the panel shown in Fig. 5, loadbearing spacers would be needed and the panel would need to be exactly placed at the first attempt to avoid damaging the seal. For these reasons, stiffer tape mastics, roofing felt or gaskets are often preferred; these give good service if the back of the panel is smooth and true.
The joint as shown in Fig. 5 is suitable where the edges of the panels bed on to the frame of a building but cover strips can be used at the back of the joint where this is not so. Where only vertical mem- hers are available to position clad- ding panels, the vertical joints may be as used in Fig. 5 and horizontal joints as in Fig. 3 or 4 but special care will then be needed where horizontal and vertical joints inter-
sect.
The lap joint of Fig. 5 is widely used in various forms both for thick and thin panels. In Fig 6. for example, its use in patent glazing is shown. The provision of drainage channels to deal with any rain pene- tration or condensation is to be noted. The system has a good re- cord in Britain, though in very cold countries, ice can build up in the channels.
Enlarged hole to allow for movement
-Load bearing rope
Soft bituminous mastic
tion and also prevents wind and snow entering the joint. Mortar is used in Figs. 3 and 4 to transfer load and to provide a stop against which the mastic can be firmly pressed home to ensure adhesion. The joint in these two examples is, however, a butt joint and the lap joint in Fib. 5 may make less de- mands on the mastic.
In Figs. 3 and 5 the mastic is shielded from light. which might cause it to harden, but because it is
116
Fig. 14 Joint for gutters or sheets
Figures 7 to 12 illustrate various forms of joint used in curtain walling, that in Figure 8 shows a form of neoprene gasket, in which
an
oversized tensioning strip is forced home when the window is installed to give the pressure requir ed to ensure watertightness.
Figure 9 shows a socket joint which combines the features of a butt and a lap joint. If the infill panel expands relative to the frame. either the mastic, being substantially
comes
incompressible, is extruded and be
or unsightly, the forces exerted during expansion may break the seal of a double-glazed unit or cause a plastic panel to buckle. To obviate such effects an expansion space may be left, preferably filled with a strip of closed-cell foamed plastic as shown in Fig. 10. The joint is then essentially of the lap type.
Figure 11 illustrates the use of "Thiokol' at the bottom of the joint. where the glass/"Thiokol" interface is shielded from sunlight, and a butyl mastic above. In practice, the 'Thiokol' is more usually gunned in at the top of the joint on the outside to give a surface resistant to damage by window-cleaners and children. Although this may prove less dur- able in the long run, there is as yet little evidence of unsatisfactory per- formance.
"Casket held in recess
Fig. 15 Flexible pipe joint. O-ring type
In Fig. 12 soft mastic such as a gunned oil or butyl mastic is used in the joint which also incorporates oval spacers to carry wind loads. The need for two mastics is thus avoided. The external bead is shown faired off at 40° to the horizontal, This has the advantage that the bot tom bead sheds rain easily and the upper bead is prevented from casting a shadow on the glass, thus reducing the risk of cracking when the curtain wall is most heated by the sun, i.e... when the sun is
is 40° above the horizon.
Mastics and gaskets play an im portant and useful part in buildings generally both in new construction and in repair. It is not possible to illustrate all their other applications to joints, but by way of example Fig. 13 shows their use in setting a window. Fig. 14 shows a joint for gutters or sheets, and Fig. 15 shows a flexible joint for a pipe using a rubber O-ring gasket to B.S.1737.
Prepared at the Building Research Station, Garston, Herts, Crown copyright.
THE HONG KONG & FAR EAST BUILDER-VOLUME 18. NUMBER 3
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