The decision on whether to use mortar with pointed joints or sealant
joints between stones is a common one. All head joints at coping
stones and joints at column covers, cornices, platforms, soffits,
under all relief angles, and in general, all stone sections
with projecting profiles, exposed top joints or rigid suspension
connections to the
supporting structure, should be "soft" sealant joints.
Hard mortar joints are best suited for masonry-bound trim items
such as belt courses, lintels, window surrounds, date stones,
inscription block, quoins, keystones and similar applications.
Always rake and point mortar joints rather than full-bed setting
and finishing in one operation.
Sealant joints allow for movement at the vertical joints. Leave
head joints dry when setting. It is a mistake to use mortar
in combination with a sealant in the same joint. An allowance
for compression is required for the system to be effective. After setting, prime the ends of the stones, insert properly sized foam backup rod and gun in sealant.
Since sealant systems are not intended to bear weight, use plastic setting pads or lead shims when setting the stones on a soft bed joint. The sealant is not intended to adhere to the foam backer rod. The sealant should adhere to the parallel surfaces only. The foam rod should be placed to a depth approximately equal to the width of the joint.
Sealants are specified under section 07920. The most common type are
one-part "moisture cure" or "air cure." Two-part systems are also available
which require the mixing of materials together to allow chemically induced
The inherent properties of silicone products make them excellent sealant
materials. Silicones provide superior weathering resistance and perform
over a wide range of service temperatures. They are easy to apply, have
low shrinkage rates, and can accommodate high movement. While organic materials tend to crack, dry up, and become brittle or even revert with age, silicones remain flexible and durable.
Two component, polyurethane sealants are tough and elastic, allowing for movement of up to 50% of the joint width. They are also durable, flexible and form a watertight bond with most building materials. According to the manufacturers, these formulations offer weather tight seals in caulking joiongs today for as long as 20 years under normal application conditions and ten years under severe conditions.
Allowance for thermal and other movement should be at least 25% of the joint
size. For instance, a normal 3/8" joint should be expected to compress to appproximately
1/4" and expand to approximately 1/2" during elongation.
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