One of the most commonly observed failures in stone masonry veneers is the formation of spalls at panel edges. These spalls typically originate at kerf cuts used for the lateral anchorage of the stone panel to the backup construction. The spalls often take the shape of a partial circle and are, therefore, commonly called “half-moon” spalls (Figure 1).
In historic masonry construction, lateral anchors in stone are often mild steel or galvanized steel. Since the anchors are located at the edges of stone panels, they are particularly vulnerable to corrosion if water penetrates through deteriorated joint materials. The corroding anchor imparts stresses into the stone, which is weak in tension and cracks and spalls at the anchor (Figure 2).
In recent decades, it has become common to use stainless steel anchors in stone masonry, avoiding this risk of corrosion. However, similar issues can also occur if expansive grout or other inappropriate materials are used to set the anchor into the stone, as discussed in our article “Failures: Expansive Grout as Anchor Fill” in the December 2015 issue of The Construction Specifier.
Repairs to address this type of stone distress need to consider both the cracked or spalled stone and the underlying failure of the lateral anchorage system. In some cases, repairs such as sealing the crack in the stone or patching the spalled fragment with mortar are implemented (Figure 3). This type of repair is insufficient to address the loss of lateral support for the panel and is often visually intrusive. A more appropriate repair includes both the repair of the spalled fragment of the stone, either with a dutchman repair or carefully installed patching mortar, as well as the removal of the corroded steel and the installation of new non-corroding supplemental lateral anchors. Depending on the configuration of the stone panel and the backup construction, new anchors such as helical pins, epoxied rods, or expansion anchors can be considered. For flat ashlar stone panels with larger spalls or where matching stone is readily available, consideration can also be given to replacing the entire panel. With this approach, it may be possible to preemptively remove and replace mild steel anchors not only at the replaced panel but also at the exposed edges of adjacent stone panels, reducing the risk of future distress.
Since inspection or preventive maintenance of the lateral anchors is usually impractical due to their concealed location and the large quantity of anchors, the best strategy to minimize the risk of corrosion and resulting stone distress is to maintain the joint materials (mortar or sealant) in good condition, thus minimizing the infiltration of water into the facade. In buildings with a history of corroded anchorage and spalling stone, facade inspection and maintenance every few years to address new spalls as they occur is an unfortunate necessity.
Kenneth Itle, AIA, is an architect and associate principal with Wiss, Janney, Elstner Associates (WJE) in Northbrook, Ill., specializing in historic preservation. He can be reached at kitle@wje.com.
Mike Ford is an architect and senior associate with Wiss, Janney, Elstner Associates (WJE) in Northbrook, Ill. He specializes in facade assessment and cleaning. He can be reached at mford@wje.com.
The opinions expressed in Failures are based on the authors’ experiences and do not necessarily reflect that of The Construction Specifier or CSI.
