Masonry Magazine October 2006 Page. 20
MASONRY CHIMNEYS
On the interior of the chimney, particularly at the top region just below the chimney cap, the effect of chemical attack combined with freeze/thaw deterioration is also the most significant factor. And, because this region of the chimney is only visible when inspected from the interior at the top, it often goes unnoticed for many years until conditions worsen to the point that major damage is present.
Obviously, if portions of the exterior chimney wall were to collapse due to mortar deterioration, the effect would not only be noticeable, but also dire. As mentioned earlier, this rarely happens because deeply recessed mortar joints are most likely noticed by plant or building maintenance personnel and masonry maintenance is performed long before such extreme circumstances occur. However, it is not unusual for interior portions of chimneys to remain unattended, even when masons have repointed the exterior surface of chimneys. This is due to the fact that most masonry repair firms have equipment, such as scaffolding, that only allows access to the outside of the chimney, or the work is often performed while the chimney is in operation, thereby preventing access to the interior. In general, it can be assumed that, even if a chimney has been regularly maintained through external repointing, it is likely that some degree of masonry deterioration is present on the interior.
Some of the more extreme results of the failure to recognize these conditions are as follows:
It is not unusual for mortar deterioration to be so severe that an interior inspector can literally see daylight through vacant mortar joints.
Cast iron caps have been observed to be "teetering" atop chimney walls where the bolts, which had previously joined them into rings, had corroded completely away. In the case of concrete chimney caps that did not contain steel reinforcement, radial cracks had occurred that had separated the concrete into pieces that were loose and ready to topple off the wall during strong winds.
On the interior linings, entire sections had collapsed into the chimney where they filled the chimney base with several feet of brick rubble.
Even where collapse had not occurred, it is most often noted that the interior base of masonry chimneys contain large amounts of brick and mortar fragments that have sloughed off of the walls. It is expected that, when a small amount of such fragments are observed on the chimney floor, there is a much greater amount still present on the walls.
Metallic Corrosion Effect
ASIDE FROM THE USUAL concern of having metal attachments to a chimney rust and stain the exterior wall, there are some other major factors related to the presence of corroded metal on chimneys.
At the very least, each tall chimney is (or should be) equipped with a lightning protection system. The usual configuration for such protection consists of a group of rods or arrester "points" that protrude above the top of the structure while being attached in series to an encircling cable and one or two conducting cables running down the side of the structure to grounding connections. These systems are generally attached to the masonry walls through the use of drilled or embedded brass fittings. These fittings generally hold up well to corrosion conditions, but the cables often break free from their attachment anchors due to possible metal fatigue or other causes of stress.
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