Although radial bricks are no longer used to construct industrial chimneys, regular maintenance and repair are necessary for continued operations. Throughout the United States, industrial chimneys - usually built of brick with heights ranging from 50 to 250 feet - serve many of the nation's facilities by venting the gaseous by-products of on-site power generation or waste incineration. As such, these structures, particularly those constructed of radial brick, require regular maintenance and repair to remain operational. With a thorough understanding of the environmental effects and changes in operating conditions, most facility owners can maximize the useful life of their radial chimneys.

History of radial brick While rectangular brick can be used for chimneys, radial brick - invented in Europe in the 1800s - made brick chimney construction more efficient and economical. As the name implies, radial bricks are curved on their outer face to a specific radius, making round chimneys easier to construct. The bricks have hollow, cored interiors that provide a keying effect between each course, allowing mortar to partially fill the cores. The tighter circles permitted by the radius of the brick faces, coupled with the added strength of the keyed joints, allow radial brick chimneys to achieve greater heights with smaller diameters at the base. This means a radial brick chimney requires less area at ground level than a common brick chimney of the same height. Further, radial bricks measure 5 inches high by 7 inches wide (and from 4 inches to 8 5/8 inches deep), requiring fewer bricks to be laid compared with a common brick chimney of the same dimensions.

From the time radial brick chimney construction first appeared in the United States in the late 1800s, it became the design of choice for many universities, hospitals, and small boiler installations. At that time, reinforced concrete chimney construction was not widespread, nor was it economical for smaller chimneys. Gradually, concrete was used in place of brick for chimneys taller than 200 feet, and radial brick chimney construction declined sharply in the 1970s. Few, if any, radial brick chimneys are built in the United States today, primarily because the scaffolding systems used in their construction are not considered safe by current standards. In the 1950s, 12 brick manufacturing companies in the United States produced radial brick for chimney construction; today there is only one, and its production is used only for repairs and partial rebuilding of existing chimneys. Indeed, there is a large inventory of radial brick chimneys throughout the United States that require regular maintenance.

Maintenance concerns

To understand the maintenance problems facing owners of radial brick chimneys, it is necessary to better understand how the structures were constructed. Designed to handle dry gases with temperatures ranging from 300 to 1,500°F, most radial chimneys were built without a liner or with only a partial liner extending from the bottom of the breeching opening to about 20 feet above it. Where gas temperatures exceeded 600°F, full-height liners were installed. The liners generally were 4 inches thick and built with radial brick laid in the same mortar as the main chimney column.

The chimneys were equipped with standard lightning protection systems (LPS) consisting of two or more air terminals at the top, connected by a cable extending down the outside of the chimney to a solid, 10-foot rod driven into the soil near the base. A cast-iron cap was installed over the brickwork at the top of the chimney.

Effects from exposure - Like all exposed structures, radial brick chimneys deteriorate over time from the effects of the elements. Wind and rain erode the mortar joints. Wind loads or thermal stresses can cause cracks to develop in the mortar, and freeze-thaw cycles cause brick faces to spall. Spalled brick faces are especially damaging to radial brick because they expose the interior cores of the brick, allowing atmospheric moisture to penetrate directly into them and seep down into the lower brick courses.

Required maintenance to combat environmental exposure includes periodic pointing of the mortar joints, caulking of cracks, and grouting new brick faces over spalled areas prevents deterioration from degrading the chimney's structural integrity. Exterior steel bands often are installed on radial brick chimneys to limit and contain vertical cracks. It is rare to find any radial brick chimney now in service without several such tension bands in the upper portion of its column. However, unless the segments of a chimney band are sized and joined properly by connectors designed to develop the full strength of the band steel, they may be little more than decorations.

Lightning damage - Lightning damage is far more prevalent to radial brick chimneys than to concrete chimneys for three reasons. First, the LPS downlead cables are exposed more often on radial brick chimneys, making them more susceptible to atmospheric corrosion. Second, there is no internal reinforcing steel in brick chimneys that can be made part of the ground path, so any lightning strike not conducted to ground by the exterior cables will go directly into the brick itself, causing significant damage. Third, mechanical damage to the grounding system at grade can also cause LPS impairment on radial brick chimneys. For example, it is not unusual for vehicles to damage the ground cables at the chimney's base, resulting in loss of continuity in the LPS.

Unlike the deterioration of brick and mortar from prolonged exposure to the atmosphere, lightning damage occurs immediately and often is severe, if not catastrophic. A lightning strike that is not safely conducted to ground inevitably creates major cracks in the chimney column. Superheated by the energy of the electrical discharge, the expansion of moisture within the brick may produce a sizeable hole when a large portion of brickwork literally is blown out. The frequent inspection and maintenance of LPS components should be a priority for radial brick chimneys.

Changes in operating conditions - The most insidious threat to the integrity of the hundreds of radial brick chimneys still functioning in the United States comes from changes in operating conditions. Currently, nearly all radial brick chimneys are subject to very different conditions than those in effect when they first were placed into service. For example, fuels have changed, from coal to oil to gas, or a combination of fuels.

Additionally, while radial brick can withstand high temperatures, it is not acid resistant. When flue gas temperatures fall below 350°F, acids more readily form because of condensation of the gas. Radial brick are laid up with 1/2-inch mortar joints, using mortar composed of one part Portland cement, two parts hydrated lime, and five parts sand. Portland cement mortars, particularly those containing lime, have very poor resistance to acid attack. The high sulfur content of some fuels used during the last 60 years has led to severe acid attack of the brickwork in these chimneys. Fuel oil used prior to the onset of air pollution controls produced especially damaging concentrations of these acids.

Also, if the boilers that vent out through these chimneys were converted at a later date to natural gas, high-water vapor content was introduced to the combustion products. This vapor condenses as the flue gas cools when it rises through the chimney. The resulting moisture catalyzes the formation of acidic ions in the residue that remains on the brickwork's inner surface from previously burned fuels.

Repair of a radial brick chimney requires highly skilled equipment and expertise.

Because acid attack occurs from the inside, its presence often is not seen until the acid permeates the entire thickness of the chimney wall. It may then be observed as a greenish or yellowish coloration of the mortar, and the mortar joints may swell because of sulfuric acid present in the flue gas. Not only is the strength of the mortar diminished by this sulfation (and with it the integrity of the bond between the brick), but the swelling of the joints also imposes an upward lifting force that may cause large areas of the brickwork to bulge outward.

The entire, upper portion of the chimney may lean as a result of expanding mortar throughout the brick column. This lean cannot be corrected by removing the upper portion of the chimney, since removing brick at the top reduces the weight against which the expanding joints at lower levels act; bulging and leaning usually recur in time. Instead, steps must be taken to reduce the acid attack throughout the chimney by protecting the mortar joints with acid-resistant gunite or other suitable materials.

Thermal stress - Yet another source of damage to radial brick chimneys is thermal stress from fires set deliberately inside the base. Chimneys are provided with a clean-out door at grade for the periodic removal of fly ash or other combustion residue that may build up in the base. Some facilities have used this in lieu of an incinerator to burn trash directly inside the chimney's base. This practice severely damages the brick, which cannot withstand such high temperatures and direct flames. The walls at the base of chimneys abused in this manner may disintegrate until less than 50 percent of the original wall thickness remains. The structural integrity of the chimney is, of course, severely compromised in such a situation. There are no maintenance procedures to correct such misuse, aside from ceasing the practice.

Conclusion

The unique dynamics of radial brick chimneys require specific attention to maintenance. Not only are many causes of deterioration unique to these structures, but maintenance and repair requires highly specialized equipment and expertise. Annual (or at least biennial) inspections will help ensure a chimney's continued service. Even a cursory, binocular inspection of a brick chimney by an experienced inspector can reveal signs of interior problems not yet apparent to untrained eyes. Repairs made early will be far less extensive - and far less expensive - than those required when deterioration is visible to all.

Bill Harkins is a project coordinator with Pullman Power LLC in Kansas City, Mo. He has more than 43 years of experience in the chimney industry. He can be reached at bharkins@structural.net.