Freeze/thaw likely culprit

Masonry Construction, March, 2006 by Norbert V. Krogstad

Q My company is having a problem with the brick parapet walls of our office building in Detroit, which was built in 1975. Over the years, the brick developed a network of cracks on the surface, primarily in the first few feet below the coping. In some areas, the exterior faces of these units are delaminating in layers.

The parapets stand about 2-feet above the roof, with two wythes of brick masonry and a partially filled collar joint. They are covered with a rowlock brick coping, which as best we can tell from areas where portions of the coping were removed, does not contain a flashing. Although there is no flashing beneath the coping, the top surface and front and back faces of the rowlock masonry are painted. I could not see any obvious cracks in the paint coating, so it doesn't seem that water is penetrating the coping.

We still have the original test reports for the brick from 1973. According to these reports, the brick units met the requirement for severe weathering units per ASTM C216.

Do you have any ideas about what is happening?

A As is the case with most masonry problems, there are many possible contributors. Because the failing brick units are primarily in the parapet, it is unlikely that the deterioration is the result of compressive stresses.

Based on the description of the problems and the location where they are occurring, it seems fairly clear that the units are failing from freeze-thaw degradation. Figures 1 and 2 show some examples of freeze-thaw damage on a building that I investigated in the past.

[FIGURES 1-2 OMITTED]

Surface crazing, or a network of cracks, appeared on the face of the brick units in the early stages of deterioration, as shown in Fig. 1, followed by delamination of the brick face, as shown in Fig. 2. Although grade SW brick units have better resistance to freeze thaw damage than moderate weathering (grade MW) brick, they are still susceptible to deterioration in severe exposures.

Brick units in parapets typically undergo very severe exposures. In northern climates like Detroit, snow collects on parapets and other ledges. During sunny days, even when the air temperature remains below freezing, some snow melts. Water from melting snow penetrates any voids or separations in the brick coping, or simply flows down the face of the wall. Once the sun goes down, the water quickly refreezes.

Also, why were the copings painted in the first place? It is unlikely that they were painted when the building was originally completed in 1975. It seems more likely that the paint coating were a past attempt to deal with a water leakage or deterioration problem. If this is the case, there may have been considerable damage even before the coating was applied.

Without a flashing beneath the rowlock copings, water from rain or melting snow can easily penetrate the masonry. However, even small quantities of water flowing down the face of the wall from melting snow can be absorbed by the exterior face of the masonry and refreeze at night. The masonry parapet can undergo numerous freeze-thaw cycles, as many as one per day. This continuous cycle of melting, wetting, and refreezing can quickly subject the brick units to more than 50 freeze-thaw cycles, which is the basis of the grade SW classification.

Water from rain or melting snow is a particular problem with copings, such as rowlock copings that are often built nearly flat and often do not project beyond the face of the wall. When copings do extend more that l-inch beyond the face of the wall and contain well formed drips on the underside, water flowing off the top surface from melting snow does not reach the face of the wall below.

If it is determined that freeze-thaw is the source of the deterioration, I recommend replacing the upper portion of the exterior wythe in areas where the spalling or surface crazing is severe and performing isolated repairs in areas where only a few units are damaged. The wall should be capped with a stone coping that sheds water onto the roof side and contains 1 1/2- to 2-inch overhangs with drips on the front and back faces. A continuous, watertight flashing that is fully sealed at splices and at the dowel penetrations should be installed beneath the coping.