Medium
Nature of Mastic
Drying and non- drying oils
Plastic
Bitumen and rubber-bitumen
Table 1
COMPOSITION OF MASTICS
Mainly plastic, some partly elastic.
Usual Method of Application
Gun, knife or trowel, tape.
Gun, knife or trowel, tape, poured.
Butyl rubber
Mainly plastic, some partly elastic,
Gun, knife or trowel, tape.
Polyisobutylene
Plastic
1
Thiokol
Elastic
(polysulphide rubber)
Urethane
Elastic
1
Neoprene
Elastic
Silicone
Elastic
fluid to be poured into the joint. Mastics for pouring cold are based on 'Thiokol' or urethane.
Tape or strip. Mastics prepared in this form must be stiff enough to handle and are protected until im- mediately before use by a non- adherent wrapping.
Two-part. Most Thiokol' and some other mastics are normally provided in two separate parts, the hase and a curing or vulcanizing agent, which are mixed immediately before use. The resulting mixture is initially sticky and plastic and is best placed by gun. At temperatures above 45°F it stiffens slowly, remaining workable for a few hours and chang- ing to a soft rubber in the course of a few days. This change is delayed by low temperatures.
Exposed Face-
Mastic
Forced-Fit F.V.C.gasket Fig. 5. Vertical joint suitable for concrete
slabs
112
Tape, usually soft.
Mostly two-part, gun- applied or poured. Some single part materiais available.
Two-part, gun-applied or poured.
Two-part, gun-applied.
Single part, gun- applied.
Properties in the joint
Comments
The best are
Many harden aften a few years. probably the gun mastics which remain soft under a leathery skin for prolonged periods if used in adequate depth, preferably over in.
Cheap materials of good resistance to water and alkalies, many harden in a few years if exposed, Difficult to paint.
Recent materials retain softness well.
May lose adhesion to glass when sunlight gains access to glass/mastic interface,
Retains softness well. Adhesion to glass when sunlight gains access to the glass/mastic interface is relatively good.
Sets to a rubber in a day or so at temperatures above 45°F. Not tolerant of grease on surfaces. Good durability, but loses adhesion to glass when sunlight gains access to glass/mastics interface. Some materials can stain white marble.
Special mixers are required. Sets to a rubber in a day or so at temperatures above 45°F. Mainly recommended with primer for concrete.
Good reports from America experience.
Good reports from America experience.
The general properties required of a mastic are that it should be easy to apply and retain its form in the joint without slumping. Mastics for porous materials must not bleed into or stain them, and those for use with
Fig. 6. Vertical joint in patent glazing
alkalis in it. Alternatively, the joint may be primed. The mastic may be required to take paint after a few days. But the most important re- concrete must be resistant to the quirement is that the mastic shall seal the joint effectively, i.e., it must fill the
gap and in spite of movement continue to do so.
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British
little British
The following distinctions can be made between the characteristics of two broad groups of mastics.
Plastic mastics* (including most mastics based on oils, bitumen or polyisobutylene and some butyl mas- tics). Mastics of this type, although
OUTSIDE
Spring clip as first barrier
Drainage Channel
-Oiled cord or gasket
Condensation channel
they should be made stiff enough not to slump under their own weight, flow readily under stronger pressures and so may be extruded from joints if subjected to a continuing load.
*The term 'plastic' is used here to denote a mastic capable of plastic flow, and does not imply that it is based on plastics.
THE HONG KONG & FAR EAST BUILDER-VOLUME 18, NUMBER 3
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