Replacement of Historic Cast Stone Installations
Individual cast stone units, which are subject to repeated wetting (such as copings, railings and balusters) and exhibit severe failure due to spalling or reinforcement deterioration, may require replacement with new cast stone and can replicate deteriorated units in existing buildings.
 After removal from the mold, the new cast stone units are scrubbed to remove surface paste and expose underlying aggregate. End cast pieces, such as this sculpture, may have numberous air bubbles at the suface.Photo: Richard Pieper. |
Fortunately, a number of companies custom manufacture precast concrete units. The variables involved in manufacture are considerable, and it is wise to use a firm with experience in ornamental and custom work rather than a precast concrete firm which manufactures stock structural items, concrete pipe, or the like. Several trade organizations, including the Cast Stone Institute, the National Precast Concrete Association, and the Architectural Precast Association, have developed recommendations and/or guide specifications for the manufacture of cast stone and precast concrete. These specifications set standards for characteristics such as compressive strength and water absorptivity, and discuss additives such as air entraining agents and water reducing agents, which influence the longevity of new cast stone. Trade references and guide specifications should be consulted before contracting for replacement of historic cast stone.
 The holes on the surface of the casting (see above, left) are being filled with a mortar similar to the concrete mix used to cast the element. Photo: Richard Pieper. |
Fabrication defects in new cast stone. While the cement matrix coloration and aggregate considerations previously mentioned require the most careful attention, project staff should also look for defects which are common to cast stone fabrication:
Air bubbles. Small pits on the surface of the stone may form if the unit is not given adequate vibration to release trapped air during pouring. Bubbles can also be a problem when end casting long items such as columns or railings, where it is difficult to vibrate bubbles away from the finish surface of the unit.
Surface cracking or checking. Overly wet mixes and insufficient moisture during curing can result in surface cracking of large castings, such as columns. Such cracking dramatically reduces the durability of new cast stone. Small reinforced elements, such as balusters, also frequently crack at thin "necks" in the castings.
"The Maintenance, Repair and Replacement of Historic Cast Stone" an Historic Preservation Brief
