Jointing with Mastics and Gaskets — 2
DESIGN
THIS Digest considers the effects of thermal and moisture movements, restraints and tolerances, upon joints sealed with mastics and gaskets, and shows how such effects can be estimated quantitatively to assist in joint design. The previous Digest (October, Hong Kong & Far East Builder) discussed the composition and properties of mastics and gaskets and illustrated the main types of joint in which they are used.
Ideally, joints should be so de- signed that they meet the following requirements:
1. Wind rain and snow
cluded,
are ex-
2. Thermal and moisture move-
ments are allowed for.
3. The working loads do not
cause distortion.
4. The joints are durable.
5. The joints are easily repaired, should this prove necessary. 6. The thermal and sound insula. tion of the structure is pre- served.
7. The components are easily assembled with the necessary dimensional tolerances.
Provision for movement
Complete air and watertightness of the joints depends very largely on the continued adhesion of the mastic and this in turn will depend on the movement between the faces joined. It is essential, therefore, to form some estimate of the move- ments to be expected as a result of changes in temperature and, for non-metallic substances, moisture content also. Although thermal and moisture movements often oppose each other they can occasionally work together. The ranges of tem perature and moisture content ex- perienced by different parts of a building will vary according to the construction and exposure (Table 1).
Except in sheltered parts of a building, it should be assumed that porous building materials will un- dergo their full range of moisture movement from wet to dry; ma. terials cast in situ will, also, undergo initial drying shrinkage.
136
The steps taken in estimating movements are as follows:
1. Calculate the maximum thermal movements of panels etc. using the figures given in Tables 4-6 of Digest 99 (first series) with allowance for the size of panels and for the temperature ranges shown in Table 1 of the present Digest.
2. Add to this the calculated moisture movement from wet to dry, using the data of Digest 99 (first series) adjusted for the size of panels.
The result gives the maximum possible movement of a unit if it is erected at one extreme of tem- perature and moisture content, either cold and dry or warm and wet. The movement in the joint may be the total for the two units. separated by the joint.
3. From this figure for maxi.
mum
movement, the minimum mastic width (W) between the moving surfaces for lap or butt joints can be decided, using Table 2 as a guide. For satisfactory joints, this usually falls between in. and I in. The manufacturer
TOLERANCE ON PANEL POSITION
T in. T in.
MINIMUM MASTIC
WIDTH W in.
TOLERANCE ON PANEL POSITION
Tin.
Tin.
DESIGN JOINT WIDTH (W+27) In.
Fig. 1. Effect of tolerances on joints
of the mastic should be able to
advise on the performance his material will give. It is important to realise that this is not simply the extensibility of fresh mastic. It depends on adequate adhesion being maintained without failure while the aged mastic is stretched three or four hundred times a year through almost the full range of the combined thermal and moisture movements.
4. Variation in the size of units and the difficulty of fixing them in exact positions means that there is some uncertainty or tolerance (T) in the position of each unit. The design width of the joint between two units is then (W+2T), to ensure that no joint is less than the minimum mastic width as obtained above. In fact. the actual joint width may vary between (W) and (W+4T), see Fig. 1.
Examples 1 and 2 at the end of this Digest show how allowances can be made in practice for move- ments and tolerances.
Some relaxation of the above design recommendations may be permitted:
1. If it is known for certain that the units will not be erected at extremes of temperature or mois
ture content.
2. Where joint failure is unlike- ly to have serious consequences or where remedial measures are easy. 3. Where the units are firmly bonded to parts of the structure of the building that experience only relatively minor changes in temperature and moisture content. There are few available data on the nature of restraints exercised in this way, but as a rough guide,
THE HONG KONG & FAR EAST BUILDER-VOLUME 18, NUMBER 4
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