No_4_December_1963 — Page 148

Table 2

CHOICE OF MASTIC ACCORDING TO CALCULATED

Movement as a

mastic width*

Butt

percentage of minimum

Lap joint

5

joint

I

Under 10

Under 40

IO 25

40-75

Over 25 Over 75

MOVEMENT

Mastics

suggested

Putties are adequate, but must be painted.

Any soft mastic or gasket. Oil or bitumen-based mas- tics are adequate if shielded or easily renewed.

a

Plastic or elastic mastics or gaskets are suitable. Where movement is slow (less than I in./h) or if the joint is likely to spend long periods at full extension, plastic mastic (e.g. butyl, polyisobutylene or oil-based gun mastic) is better. Where there is stick-slip move- ment or only short periods are spent at full exten- sion, an elastic mastic should be used (e.g. 'Thiokol', urethane). The greater the movement, the softer the mastic should be,

It is probably better to rely on gaskets in shear on specially designed joints without mastics.

от

* As defined on page 1, col. 2.

NOTE

3. Edges of the glass must be clean cut; chipping in hand- ling or erection and nibbling to adjust size must avoided.

be

4. Not more than a in. wide margin at the edge of the glass should be covered by the bead.

5. Ideally, the top external bead should not cast a shadow on the glass (and thus in effect increase the edge cover) when the glass is at its hottest, that is when the sun is directly opposite the wall and 40° above the horizon. External top beads for coloured glasses are sometimes faired off for this reason

see Fig. 12 of Digest 36 (second series) and normally other beads would be similarly shaped. Fairing off the botton bead is also useful since it reduces the risk of standing water.

Installation of factory produced insulated glazing units, corsisting of two panels of glass fixed about in. apart, requires great care and may differ with different units. A com- bined guarantee should if possible be obtained from the manufacturers of the units, frames and mastics and from the glazing contractor.

The choice of whether beads should be used externally or intern-

140

Gaskets can be used for all joints

ally depends largely on the prac ticability and cost of scaffolding and on whether subsequent re-glazing is best carried out from outside or inside the building where there are back-up walls, access to internal beads would, of course, be impos- sible. Apart from this, there is much to be said for internal glazing since wooden beads often warp out- doors and hollow aluminium beads are difficult to seal without using large amounts of mastics. In- ternally, also, there is no reed to shape the top bead as described above

Provision against failure

Mastic joints may fail in the course of time either because of ageing or because of defects in

erection design,

or application. Where possible, complete reliance against rain and wind penetration should not be placed on the mastic seal, and the use of cover strips, overhangs or drips in front of the seal and of a drained and ventilated cavity behind it are wise precau- tions. Such a cavity also prevents access of moisture to the further side and thermal insulation, which loses efficiency when wet, or any material that can corrode or rot easily may safely be put there.

PRECAUTIONS ON SITE

Satisfactory jointing depends at least as much on the workmanship as on the design and the materials used. On large jobs in particular, therefore, it is advisable to employ specialist teams trained for efficient working on the particular form of construction perhaps by means

of a trial run on a mock-up.

Two-part jointing compounds must be very thoroughly mixed; it is often difficult to ensure this unless the two components are of roughly equal volume, or of con- trasting colours. The use of a cen tral mixing plant with reliable operatives may assist.

Surfaces to be joined with mastics must be clean and dry. When aluminium surfaces are de-greased with solvents and clean rags, the rags must be changed frequently to avoid redistributing the grease. Special care must be taken to ensure that angles are completely cleaned.

It may be necessary, especially on porous or dusty materials, to prime the surface first. to prevent:

1. Undue absorption of medium from the mastic and conse- quent loss of adhesion.

2. Staining of the surrounding

material

3. Attack on the mastic by al- kaline constituents of cement products

4. Lack of adhesion.

The primer must be supplied by the maker of the mastic.

In applying the mastic it must be forced firmly into contact with the surfaces to give good adhesion. With mastics applied by gun or trowel, this can usually only be done if the joint is partly filled, for exam. ple, by fibreboard or mortar filling.

For curtain walling the panels and glass are usually cut to size off site. It may be helpful to prepare templates sufficiently oversized to allow for the necessary clearances and tolerances and use them during erection to make sure that the open- ings are of the correct size and free from fixing devices which might later cause the glass to crack.

THE HONG KONG & FAR EAST BUILDER-VOLUME 18, NUMBER 4