In the high plateau lands of southwestern Colorado, the ancient stone cities of the Anasazi Indians are slowly wearing away. Brilliant sunlight, dusty winds, stinging rain and swirling snow take turns assaulting the sandstone blocks that make up the remains of these elaborate cliff dwellings.
Crumbling brick may signal acid rain damage as strongly as cracking and flaking concrete. Acid rain affects brick masonry by selectively dissolving the glassy fabric that holds the silica grains of a brick together. The resulting soluble sodium and calcium salts, carried by water, migrate through the porous brick. Salts are deposited wherever the water evaporates, leaving a white, powdery film that appears to ooze out of the masonry. Eventually, all that's left of a brick is a weakened "silica sponge."Limestone and marble are particularly vulnerable because acids attack their principal constituent, calcium carbonate. These reactions produce water-soluble substances that easily leach out and weaken and structure. The worn faces of marble tombstones (SN: 11/17/84, p. 313), limestone monuments and ancient buildings like the Acropolis in Athens all show the effects of air pollution and acid rain.The Mesa Verde project is only one of several ongoing studies designed to examine the effects of acid deposition on materials. Concern is mounting that acidic pollutants cause widespread damage to paint, stone, wood, fabrics, masonry, concrete and metals.Chemist A. Elena Charola of the Metropolitan Museum of Art in New York, however, warns and acid rain shouldn't be blamed for everything. In the case of bricks, neutral rainwater can be more damaging than acid rain. Rain by itself carves channels across the exposed faces of bricks, she says. "It's not the acid but the water that does the damage."Much of the building-wall damage now seen has been caused by efforts to clean or repair masonry, Charola says. Don't clean brick, she recommends. "It will last longer if you leave it dirty."Photo: Moving south, work crew squeezes into 61 strands the 27,572wires that make up each cable. View north shows 4,200 yawning feet between Golden Gate's towers"The materials problems are really formidable," says Robert Baboian, meeting chairman and head of the corrosion laboratory at Texas Instruments, Inc., in Attleboro, Mass. Not only may the damage be extensive, he notes, but very little is known about how the damage occurs and what can be done to prevent it.The picture is just as complicated for wood and paint. Factors other than immersion in occasional acid showers also contribute to deterioration. In some cases, the combination of acid rain and ultraviolet light from sunlight is much worse than either one by itself.For the next 10 years, researchers will analyze the chemistry of runoff water, monitor color changes, measure weight losses and perform other experiments in comparing protected and unprotected stone samples. Early results from the site at Research Triangle Park, N.C., show that even one heavy rainfall contains enough acid to wear away a few microns of the highly polished marble surface.Photo: Extending into space,sections of roadway seem to defy gravity as they approach each other. Safety netting is suspended belowThe GM Sunraycer was built to compete in a 1,950-mile race for solar-powered vehicles across central Australia from Darwin through Alice Springs to Adelaide. The race begins on Nov. 1 and will probably last at least six days. It features 25 solar vehicles from seven different nations. All vehicles must meet size constraints and have adequate braking, highway lighting and seat belts. They must also be stable in winds up to 40 miles per hour.Wood is a composite material made up of cellulose and hemicellulose fibrils held together by a natural glue called lignin. Chemically, the fibrils and the lignin behave quite differently. Cellulose, a polymeric sugar, is very sensitive to moisture and acids. Acid rain weakens these fibrils. On the other hand, lignin, a phenolic polymer, is susceptible to ultraviolet light. In bright sunlight, lignin decomposes, allowing the wood to start falling apart.Because temperatures can go as high as 120|F in the Australian sun, the GM Sunraycer's canopy is plated with a thin film of gold to protect the driver. The film blocks 90 percent of visible light and 98 percent of the infrared radiation. Because the car is driven only in daylight, enough light gets through to enable the driver to see the road.Much less is known about the effect of acid deposition on Portland cement and concrete. Nevertheless, the small amount of information available suggests that acids "significantly affect the durability of concrete," says Ronald P. Webster of Brookhaven National Laboratory in Upton, N.Y. Substances like sulfur dioxide, nitrogen oxides and hydrochloric acid are also known to accelerate the corrosion of steel rods used to reinforce concrete.California Historical Society, Shubert HallLibrary, Photographic Archives, 2099 Pacific Avenue. Open 1 to 5 Wednesdays through Saturdays; $3 for nonmembers. Lots of construction views, some postcard views, some photographs of opening-day festivities. Prices start at $13 for an 8-by-10 print.Peeling, cracked or discolored paint and disintegrating wood are not the only visible signs of possible acid rain damage. The rusting of reinforcing steel in bridge decks and other structures, ragged holes in metal automobile panels and the various ills of bronze and copper statues (SN: 6/29/85, p. 404) also signal problems that may be exacerbated if not directly caused by acid rain.To investigate the weathering of these materials, the National Park Service has established four sites where slabs of Salem limestone from Indiana and Shelburne marble from Vermont are exposed to the elements. After granite, limestone is the most commonly used building stone in the United States. Salem limestone, widely used for more than 100 years, makes up much of the Empire State Building in New York City. Shelburne marble has been used to face structures like the Jefferson Memorial in Washington, D.C.The car itself is 19.7 feet long, 6.6 feet wide and 3.3 feet high. Its aerodynamically shaped body is constructed from a lightweight, honeycombed composite material over a welded, aluminum-tube frame. The car, including its electric motor, battery, electronic components and solar panel, weighs merely 360 pounds.Fifty years later, the bridging of the GoldenGate still seems an extraordinary leap of the imagination. Step onto the walkway, or hike around Fort Point below the south tower, and your awe intensifies: the bridge moves as if alive. Its roadway vibrates with roaring traffic, the wind whistles through its cables--playing them, one observer said, like gigantic harp strings.The problem is particularly serious for modern bricks, says Charola, because these are typically fired only long enough to create a thin glassy surface layer. Once this layer is gone, a brick quickly disintegrates. Twelfth-century Venetian bricks, in contrast, sat in kilns much longer to create a glassy matrix throughout. These bricks, she says, have held up very well for centuries, despite high levels of pollution.San Francisco Archives, Third Floor, MainLibrary, Civic Center. Open 10 to 6 Tuesdays through Saturdays, 1 to 6 Wednesdays. Large selection includes construction views and opening-day scenes. Prices start at $5 for an 8-by-10. Through June 1, an exhibit on 1930s San Francisco displays vintage photographs and original bridge souvenirs.
In a recent study of automobile corrosion in Montreal, Detroit, Boston and Dallas, Montreal cars seemed to suffer the highest level of "body rot" although considerable corrosion damage also showed up in Detroit and Boston. Montreal happens to have highly acidic rain and allows heavy use of road salt during the winter. Laboratory tests, says Baboian, who conducted the survey, confirm that the combination of acidic moisture and salts like calcium chloride cause much more damage than either factor alone.
Author: Ivars Petersopn
