March 1, 2013
Deborah Slaton & David S. Patterson, AIA
In winter, we become especially aware of the effects of exposure on everything around us. Cyclic wetting and drying, freezing and thawing, dramatic changes in temperature, wind, and cold all contribute to weathering and deterioration of the built environment’s elements (and, sometimes, its occupants).
The presence of repeated elements provides a unique opportunity to consider how exposure affects performance and durability. For example, limestone cladding panels on a shaded north-facing building wall may be covered with organic growth while those on a south-facing wall remain clean. Conversely, paint coatings on a west elevation can exhibit cracking associated with exposure to strong sunlight, while those on a shaded east elevation remain intact. Horizontal wood trim elements on which water can accumulate are subject to paint failure and other deterioration, while vertical wood trim elements are much less vulnerable to these types of distress.
Where built objects are apparently similar, exposure may be a key factor in how they weather differently. Similarly, differential weathering may indicate these objects are not as similar as they initially appear.
Exposure is a key factor in the example illustrated below, though in a different sense. A series of concrete benches surrounding a seating area in a Midwestern park are equally subject to sun, heat, rain, wind, snow, cold, and de-icing salts on adjacent paving. Although individual benches have been damaged in isolated areas by vandalism or skateboards, most have survived intact through many seasons or have only been minimally affected by concrete deterioration. However, one bench is significantly deteriorated, with extensive spalling of concrete and exposed corroding steel reinforcing bars.
In this example, all the benches experience similar environmental conditions. This suggests the deterioration of one bench among the others is related less to its overall exposure, and more to the apparent lack of sufficient concrete cover over the reinforcing steel. In other words, while all the benches look alike, an apparent problem in fabrication of the upper portion of one bench has left it more vulnerable to deterioration as water penetrates cracks in the concrete. The result is spalling of the concrete and corrosion of the reinforcing steel. Water penetration and concrete spalling has left more extensive areas of steel vulnerable to corrosion, further contributing to the concrete’s ongoing deterioration.
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 email@example.com.
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 contacted at firstname.lastname@example.org.
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