Deborah Slaton and David S. Patterson, AIA,
When designing repairs to existing buildings and structures, emphasis is typically on durability, with the goal of achieving the longest-possible service life. However, there are some cases in which it is desirable to design repairs that are reversible or removable.
This issue is probably encountered most often in work on historic buildings. Precepts such as the Secretary of the Interior’s Standards for the Treatment of Historic Properties—the primary standards used in preservation—note the desirability of reversible treatments. Dictionary.com defines “reversible” as “capable of reestablishing the original condition after a change by the reverse of the change.” Why would this be desirable in work on historic structures?
The Secretary of the Interior’s Standards emphasize preservation of historic fabric, repair versus replacement, and avoiding damage to, or removal of, original materials. For example, the standards note additions to historic structures should be designed and implemented such that, if removed in the future, the historic form and character of the property would remain unimpaired. The standards also note chemical or physical treatments causing damage to historic materials shall not be used. (In many cases, such damage is irreversible.)
Perhaps the best known example of a treatment damaging to historic materials is sandblasting. This process, the application of sand at high pressures as an abrasive cleaning medium, was popular during the 1960s. It was highly damaging to stone façades and typically removed the hard-fired surface of brick masonry, exposing the softer interior to erosion. It also left the masonry surface more vulnerable to deterioration and accumulation of soiling. Today, micro-abrasive cleaning techniques are available that use fine particulates at very low pressures. While considered a better and much less aggressive approach to abrasive cleaning, these treatments may also still be considered ‘irreversible’ as they can remove a small portion of the exterior masonry surface, together with soiling. Although the amount of original material removed is at a micro level, its removal is not reversible.
Other types of potentially irreversible repairs include application of sealers and consolidants that penetrate and/or react chemically with stone, brick masonry, or concrete, and cannot be removed. Consequently, there is additional importance to careful study and trial repairs prior to application of the treatment, whether to a historic façade or to more contemporary construction. If the application does not achieve the desired result, or if it causes damage, it cannot be removed.
Using sealant as a joint fill may be considered reversible—after all, the sealant can be removed from the joint. However, where silicone sealant has been employed and the decision is made later to replace the sealant with mortar, joint preparation can be challenging and it can be very difficult to obtain sufficient mortar bond. Thus, the initial repair may not be so reversible after all.
Deborah Slaton is an architectural conservator and principal with Wiss, Janney, Elstner Associates (WJE) in Northbrook, Illinois, specializing in historic preservation and materials conservation. She can be reached at firstname.lastname@example.org.
David S. Patterson, AIA, is an architect and senior principal with the Princeton, New Jersey, office of WJE, specializing in investigation and repair of the building envelope. He can be e-mailed at email@example.com.
The opinions expressed in Failures are based on the authors’ experiences and do not necessarily reflect those of The Construction Specifier or CSI.