Masonry Magazine October 2006 Page. 24
MASONRY CHIMNEYS
One of the primary causes of vertical crack formation in masonry chimneys is that of corrosion-related expansion of embedded metal features in their walls. The concept is that as iron or steel oxidizes, it expands similar to the expansion of trapped moisture as it freezes. Here again, as it expands, the internal forces of expansion are much greater than the strength of the masonry, and cracking/crushing occurs at the interface between the embedded metal surface and its surrounding material. There is a significant difference, however, between freezing-related expansion and oxidation expansion of iron, in that it can result in the corroded material being enlarged by many times its original thickness. Although the material in its corroded state has little or no strength when ultimately corroded, it developed tremendous expansion force as it expanded.
This phenomenon often occurs along the vertical plane of embedded climbing rungs, which for many years were installed on chimneys so that the use of scaling ladders was unnecessary. Masons are well advised to avoid climbing any such embedded rungs if they show any sign of corrosion whatsoever. In almost every chimney observed that had iron or mild steel rungs embedded in their walls, the rungs had corroded and expanded, causing vertical cracks which traversed from rung to rung for the entire height of the chimney. Not only are they unsafe to climb, they are also causing the chimney to develop wider and wider cracks as they corrode further.
growth, and when they are installed loosely, the masonry wall will generally expand until they are tight, thus allowing further widening of the previously repaired cracks.
Recommendations for Restoration
SINCE THE MAIN FACTOR affecting the degradation of masonry chimneys is that of continued moisture absorption and surface corrosion, the best solution for oversized, overly cool chimneys is to contain the flue gas inside a flue that will not be subject to the damages described. In the case of large chimneys, where the cross section is several feet across, the usual material of choice is that of a stainless steel flue. These flues are typically much smaller in diameter and can be inserted from the top of the chimney with enough space around them to attach the new flue to the inlet duct of the boiler. Once the new flue is installed, the chimney shell will no longer be subjected to moisture from the boiler gases. It will, however, continue to experience freeze/thaw damage from rainwater or other weather conditions unless a roof-type cover is placed over its top. In those situations where a stainless steel flue is installed, it is also recommended that a concrete roof be placed over the formerly open space to not only protect the interior of the chimney, but to also structurally stabilize the top portion of the wall. It is suggested that the concrete roof slab be reinforced with steel rods and dowelled into the masonry wall at the top.
If the cost of an interior steel flue is prohibitive, the chimney walls should be thoroughly inspected to determine the depth of mortar degradation, and repair in the form of repointing or shotcreting should be performed.
In extreme cases, where the damage is so bad as to affect the stability of the upper region of the wall, it is quite possible that partial demolition of the top of the chimney will not deter from its ability to adequately vent the boiler gas from a lower elevation. Here the act of removing badly damaged material from the top of the chimney not only solves the problem of deterioration, but also results in the overall load and resultant stress on the chimney walls being lowered.
Regarding embedded steel climbing rungs, either on the outer face or the inside, it is best to have them removed. Not just cut off, but actually cut or drilled out of the surrounding masonry wall. As mentioned earlier, they are not only unsafe to climb, but they are also causing more serious cracking problems.
WE BRIDGE THE GAP
BETWEEN PROFIT AND PRODUCTIVITY.
Are you.....
- Still looking for the safe, economical way to keep material moving?
- Still using manual labor to bring material to your work level?
- Ready for a change?
Models available with capacities from 20 to 2000 lbs.
Lift heights to 400 feet at speeds up to 80 fpm.
Scaffold, Monorail & I-Beam Mounting - Available on All Models
Get more productive... Call us today and see how a Beta Max Hoist System can lift your profits!