Masonry Magazine December 2002 Page. 24
COVER STORY
Prior to setting, insure climatic conditions are within thermal limitations of the mortar.
Ensure that all specified flashing and damp proofing is installed. Flashing pierced by stone anchors must be sealed either by metal thimble, grommet, or approved sealant. Flashing locations are the responsibility of the architect and should not be assumed by the mason under any circumstances.
Concrete should never be poured against unprotected cast stone. Where poured in place concrete is placed against cast stone sills, be sure to use the appropriate material to separate them prior to pouring concrete.
Before setting, guarantee that the surfaces set in mortar are drenched with water. This will secure a good bond and help to prevent mortar shrinkage. Weep holes must be installed over windows, at relieving angles and at the bottom of walls. No mortar drippings should be allowed in the wythe between the back of the stone and the face of back-up structure.
Quite often, it is not the mason's responsibility to fill all of the joints between stones. All head joints at coping and sills, and joints at column covers, soffits, and, in general, all stone sections with projecting profiles, exposed top joints or rigid suspension connections to the supporting structure should have sealant joints applied by the caulking contractor. This contractor should prime the joints, insert a properly sized backup rod and gun in the sealant.
Chipped cast stone must be patched as the setting proceeds because waiting too long can cause color problems with the patch. A test patch should be approved before general patching is to begin.
Planter coping, fountain coping, swimming pool coping, treads, risers, stone pieces at grade, and pavers should be treated with a silane or siloxane water repellent after setting. This will minimize the likelihood of dirt and groundwater staining the surface of the stone, a frequent cause of staining, efflorescence and enhancement of crazing. Test the water repellent on a non-visible area to make sure it does not affect color or texture when dry.
Load bearing units should be reinforced as necessary. Remember, they may be designed to be handled only in the same orientation as they will be installed in the structure. Lintels and large panels must be kept vertical or they will be prone to cracking.
Regardless of the degree of care exercised during construction, a final wash down will be needed and, normally, whatever is specified to clean the brickwork will adequately clean the cast stone. A variety of commercial cleaners are available and most contain detergents combined with mild solutions of phosphoric and/or muriatic acids.
Extreme care should be taken when applying acidic cleaners to areas where joints are left open or where sealant is used as jointing material. The sealant manufacturer should be contacted to be sure of compatibility with cleaning materials. Acids left behind the stone on masonry wythe may cause corrosion problems later on.
Cast stone is a time-tested material of enduring architectural beauty that, when properly selected and installed, should result in a project with infinite durability which in concrete terms means a time period exceeding 100 years. With proper attention to the details of specifications, workmanship and design, the construction team has the ability and opportunity to build monumental structures with cast stone at reasonable cost for use and enjoyment by generations to follow.
Bill Russell is the Chairman of ASTM Committee C 27.2 on architectural and structural concrete products and President of Continental Cast Stone East, by Russell, Inc., Berlin, N.J. He is a past president of the Cast Stone Institute and served as Its Technical Director for 14 years.
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Georgetown Visitation Preparatory School, Washington, D.C.