Masonry Magazine October 2006 Page. 18
MASONRY CHIMNEYS
The strength of the masonry and the thickness of the structural chimney shell had to be sufficient to withstand the weight of the structure and the wind forces associated with its height and projected wall area.
In cases where the chimney was integrally constructed with the masonry walls of the factory, school or other building that it serviced, it had to aesthetically match the rest of the structure.
What usually met these specified design goals would be a tall rectangular, square or sometimes circular masonry shell that started as a segment of the building wall and then projected above the wall at the roofline. In the region between the roof and the base of the chimney in the boiler room below, the masonry shell would also have an additional refractory brick lining to protect it from the direct heat of the flue gases at the level where they entered the interior of the chimney.
Generally the exterior of a masonry chimney consists of vitrified clay brick, which is very hard and nonabsorbent. Because it is much more costly than common brick, it is often part of a veneered wall, but it is not uncommon to see chimneys constructed of all face brick where economics allowed it. Similar to other building masonry, the brick is commonly laid up in mortar consisting of Portland cement, sand, lime and water.
On the interior, the usual lining wall was constructed of the cheapest type of brick available because it was not expected to withstand weathering conditions. It was also generally laid up in common mortar, unless it was subjected to very hot temperatures, such as with chimneys having incinerators attached to them.
In combination, the two walls sufficed to convey the hot, dry, coal-fired gases without too much trouble. Of course, since coal-fired gases contain acidic oxides of sulphur and nitrogen, they tended to condense acidic moisture at the tops of the chimneys causing the need for periodic maintenance in the form of repointing and occasional replacement of their corroded metal chimney caps over the years.
As cities became more populated and industrialized, the concept of having hundreds or even thousands of coal-fired boilers in operation in a city posed a great threat to the health of their inhabitants. Fortunately, oil- and natural-gas-fired boilers were developed, which alleviated the pollution problem.
What has then occurred is that nearly every chimney that was built for gas-fired conditions is now oversized and too cool to expel the boiler exhaust gas without allowing it to condense inside the chimney. This factor, although non-aggressive in nature, has led to the need for investigation and most likely some form of restoration. Most of the problems that occur on masonry chimneys are related to the presence of moisture and its effects on the masonry and any metallic appurtenances attached to them.
Masonry Deterioration
WITH RESPECT TO masonry deterioration, the conditions are often different for the exterior wall surfaces than those of the interior. On the exterior, where problems tend to be noticed more readily and are more often anticipated by building maintenance personnel, the chimney shells experience mortar deterioration due to weathering effects, such as freeze/thaw cycles, erosion of joints due to water run-off, and some wind erosion. At the top of the wall, where flue gas is in direct contact with the atmosphere and is likely to condense as a somewhat weak acid, the conditions are at their worst. Both the masonry itself and the chimney cap, which is generally either a cast-in-place concrete ring or a cast iron segmented ring assembly, will be deteriorated by chemical attack.
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