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FindArticles / Business / Masonry Construction / May, 2004

Radial brick chim neys: constant attacksfrom Mother Natur e, lightning, andcom bustion byprodu cts m ake a regularm aintenance progra m for these oldstructures essential

by Bill Harkins

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Chimneys built of brick--with heights ranging from 50-250 feet--serve numerous types of facilities byventing the gaseous byproducts of onsite power generation or incineration of hazardous waste.However, for all of their seeming permanence, these chimneys, particularly those constructed of radialbrick, require regular maintenance and repair to remain operational and to maximize their useful life.

Efficient and economical

While rectangular brick can be used for chimneys, radial brick--invented in Europe during the 1800s-made chimney construction more efficient and economical. As the name implies, radial brick arecurved on their outer face to a specific radius, making round chimneys easier to construct.

Radial brick have hollow, cored interiors that provide a keying effect between each course throughmortar partially filling the openings. 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 greaterheights with smaller diameters at the base. In other words, a radial brick chimney requires less area atground level than a brick chimney of the same height. Since radial brick measure 5-inches high x 7-inches wide (and from 4-inches--8 5/8-inches deep), fewer units are needed, compared with acommon brick chimney of the same overall dimensions.

From the time radial brick chimney construction first appeared in the United States during the late1800s, it became the design of choice for many universities, hospitals, and small boiler installations.At that time, reinforced concrete chimney construction was not widespread and that approach was noeconomical for smaller stacks. Concrete gradually replaced brick for chimneys taller than 200 feet,and radial brick construction declined sharply in the 1970s. Few, if any, radial brick chimneys are builin the United Status today, primarily because the scaffolding systems used in their construction arenot safe by current standards.

In the 1950s, 12 manufacturing companies in the United States produced radial brick for chimneyconstruction. Today there is only one, and its production is used only for repair and partial rebuildingof existing chimneys. However, despite the absence of new radial brick chimney construction, thelarge inventory already built throughout the United States requires a focus on maintenance.

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Maintain the inventory

To comprehend the maintenance problems facing owners of radial brick chimneys, it is necessary tobetter understand how they were designed and built. Constructed to handle dry gases withtemperatures from 300[degrees] F--1500[degrees] F, most radial brick chimneys were built without aliner, or with only a partial liner extending from the bottom of the breeching opening to about 20-feetabove it. If gas temperatures exceeded 600[degrees] F, full height liners were installed. The linerswere generally 4-inches thick and built with radial brick laid in the same mortar as the main chimneycolumn.

The chimneys were equipped with standard lightning protection systems (LPS) consisting of two or

more air terminals at the top of the chimney, connected by a cable extending down the outside to asolid, 10-foot rod driven into the soil near the base. A cast iron cap was installed over the brickwork atthe top of the chimney.

Effects from Mother Nature

Like all exposed structures, radial brick chimneys deteriorate over time from the effects of theelements. Wind and rain erode the mortar joints. Movement of the brick column from varying windloads, coupled with thermal stresses, can form cracks in the mortar, while freeze-thaw cycles in wintercause brick faces to spall. Spalled faces are especially damaging to radial brick because they expose theinterior cores and allow atmospheric moisture to penetrate directly into it and seep down into the

lower courses. Periodic pointing of the mortar joints, caulking of cracks, and grouting new brick facesover spalled areas prevent deterioration from degrading the chimney's structural integrity.

Exterior steel bands often are installed on radial brick chimneys to limit and contain vertical cracks. Itis rare to find any radial brick chimney now in service without several such tension bands in the upperportion of its column. However, unless the segments of a chimney band are properly sized and joinedby connectors designed to develop the lull strength of the band steel, they may be little more thandecorations.

Lightning damage

Lightning damage is far more prevalent to radial brick chimneys than to concrete versions for three

reasons.

First, the LPS downlead cables are exposed more often on radial brick chimneys, making them moresusceptible to atmospheric corrosion. Second, there is no internal reinforcing steel in brick chimneysthat can be made part of the ground path, so any lightning strike not conducted to ground by theexterior down cables goes directly into the brick itself, causing significant damage. Third, the LPS maybe impaired because of mechanical damage to the grounding system at grade. Radial brick chimneysoften are located in areas of the facility with frequent vehicle traffic (loading docks, delivery trucks,etc.). It is not unusual for vehicles to damage the ground cables at the chimney's base, resulting in theloss of continuity in the LPS.

Unlike the deterioration of brick and mortar from prolonged exposure to the atmosphere, lightningdamage occurs immediately and is often severe, if not catastrophic. A lightning strike that is not safelyconducted to ground but is, instead, diverted into the chimney brickwork by a faulty protectionsystem, inevitably creates major cracks in the column. It may produce a sizeable hole as a largeportion of brickwork literally is blown out by the expansion of moisture within the brick, which aresuperheated by the energy of the electrical discharge.

Frequent inspection and maintenance of LPS components should be a priority for radial brickchimneys.

Changes in operating conditions

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The most insidious threat to the integrity of the radial brick chimneys serving boiler plants comesfrom changes in operating conditions over the years since their original construction. Currently,almost all chimneys are subject to very different conditions than those in effect when they were firstplaced in service. Fuels have changed, from coal to oil to gas, or a combination.

While radial brick can withstand high temperatures, it is not acid resistant. When flue gastemperatures fall below 350[degrees] F, acids more readily form because of gas condensation. Radialbrick are laid up with mortar composed of one part portland cement, two parts hydrated lime, and fiveparts sand. Portland cement mortars, particularly those containing lime, have very poor resistance toacid attack.

The high sulfur content of some fuels used over the last 60 years has led to severe acid attack of thebrickwork in these chimneys. Fuel oil used prior to the onset of air pollution controls producedespecially damaging concentrations of these acids.

If the boilers were converted at a later date to natural gas, a high water vapor content was introducedto the combustion byproducts. This vapor condenses as the flue gas cools when it rises through thechimney. The resulting moisture catalyzes the formation of acidic ions in the residue from previouslyburned fuels that remain on the brickwork's inner surface.

Because acid attack occurs from the inside, its presence often is not seen until the problem permeatesthe entire thickness of the chimney wall. It then may be observed as a greenish or yellowish coloration

of the mortar. The mortar joints may swell because of expansion caused by sulfation of the cement bysulfuric 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 imposes anupward lifting force that may cause large areas of the brickwork to bulge outward.

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