Masonry Magazine December 2005 Page. 27
Concrete should never be poured against unprotected cast stone. Where poured-in-place concrete is placed against cast stone sills, separate with appropriate material prior to pouring concrete.
Prior to 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 stone and face of the back-up structure.
Head joints at coping and sills, 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, who should prime the joints, insert a properly sized backup rod, and apply the sealant with a gun.
Cast stone should be handled in a way to minimize chipping. Care must be taken to not bump the stone into anything. Handle stones with the wide portion of the cross section in the vertical position to minimize breakage. After setting, columns, pilasters, entry jambs, windowsills and all stone with projecting profiles should be protected during the remaining construction.
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 begins.
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. Do a test to make sure that the water repellent does not affect the color or texture when dry.
Regardless of the degree of care exercised during construction, a final wash down will be needed. 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 durability exceeding 100 years.
Bill Russell is the chairman of ASTM Committee C 27.20 on architectural and structural concrete products and president of Continental Cast Stone East, by Russell Inc. in Berlin, N.J. He is a founder and past president of the Cast Stone Institute, has served as its technical director for 14 years, and presently chairs its Committee on Educational Standards.
Reprinted with permission. Copyright © 2005, STRUCTURE magazine a joint publication of the National Council of Structural Engineer's Associations (NCSEA), ASCE Structural Engineering Institute (SEI), and the Council of American Structural Engineers (CASE). This article appeared in the May 2005 issue which used on various masonry pics pertinent to the industry. For more information, please visit www.structuremag.org
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