Question: What causes the dark discoloration seen on many old concrete surfaces, particularly bridges? Why would one span of a bridge railing be free of this discoloration while adjacent spans are blackened by it?

Answer: James C. Porter, an engineer with the Louisiana Dept. of Transportation, recently noticed the unusual condition of a 60-year-old bridge in the urban environment of Baton Rouge, La. The bridge is a railroad overpass with several reinforced concrete girder spans. Exposed to rain and other environmental effects, it shows the gray to black staining common to bridge railings and architectural concrete after a period of service.

However, there is one span (shown in Mr. Porter's photos) whose railings show no such staining.

Mr. Porter wonders what has caused this difference. He says, "If the phenomenon is explainable it may be a simple matter of mix or finish control that caused the happy result. If so, its practice may solve a costly, aesthetic, concrete surface maintenance problem that plagues owners who place high priority on the appearance of facilities with exposed concrete surfaces."

Algae and lichen on concrete

Algae are primitive plant-like organisms ranging in size from single cells to large seaweeds. They are widespread in soil and water, and are found on wood and stone throughout the world. Since there are about 25,000 species ranging in size from 3 microns to 62 meters (203 feet), they are hard to define precisely. When certain algae grow in combination with specialized fungi, the result is lichen. Both algae and lichen can grow on concrete, particularly in moist environments.

Although they may adversely affect the appearance of the concrete, they do not usually present a serious threat to durability (see article in January 1991 issue, page 44). Thus, according to Nick Rabalais of the Louisiana Transportation Research Center, the Louisiana Dept. of Transportation and Development does not plan to investigate what appears to be a unique bridge condition.Nonetheless, both Mr. Porter and Mr. Rabalais are looking for an explanation. They have verified that the bridge was built by the State of Louisiana in 1938 and maintained by the state until 1982, when ownership was transferred to the East Baton Rouge Parish. They have found no record of any differences in the construction or maintenance history of the unique span that would set it apart from its neighbors. What is the most plausible explanation?

Detailed observation of the bridge

Mr. Porter has carefully examined the bridge railings and finds uniform erosion of surface mortar on all the spans, whether with algae growth or without. This has exposed the aggregate in all of the railings. Since the aggregates seem alike from span to span and mortar erosion is similar for all spans, he believes that the span without black algae growth was built at the same time as the rest of the bridge.A close-up examination shows that the span described as algae-free does have some trace of black algae growth, so slight that it is imperceptible at a distance. There is a yellow growth that appears intermittently and consistently on all railings, with little impact on appearance.

Porter speculates that this may be lichen. He describes the black growth more fully: "The black algae consistently covers all the other rails on the bridge including those immediately adjacent to the rails that have no black algae growth. There is a much less, almost imperceptible, difference in the black algae growth on the surfaces of supporting beams on all the spans. This can be from the fact that there is much less total black algae growth on these protected areas. They are apparently not exposed to the conditions that better support black algae growth on the rails.

Follow up:

"In Problem Clinic (May 1992, p. 383) we showed a 60-year-old bridge span in Baton Rouge, La., that, unlike adjacent spans, is not discolored by algae growth. This apparent algae-resistant span baffled engineers from the Louisiana Dept. of Transportation (LDOT). They remained confused even after checking the construction, ownership, and maintenance history of the bridge and carefully examining the bridge railings. Our editors were equally puzzled.We invited readers to share their opinions as to what caused this algae-free section. One reader suggested that the cement in the algae-coated sections may contain nutritive phosphorous, a plant fertilizer that could promote algae growth.

While this may be possible, there's a simpler explanation. Kenny Sexton opened his Colonels Club restaurant next to the bridge in the summer of 1990. There are tables in front of the restaurant beneath the bridge and a parking lot on the other side of the bridge. To prepare the restaurant for its opening 2 years ago, Mr. Sexton decided to power-wash the outside of his building. Then, apparently turned off by the dirty appearance of the bridge, he put a half gallon of bleach in a 55-gallon drum of water and turned his spray on the bridge. He says he filled the drum several times but only the first drumload contained bleach. He says he wanted to clean the other two adjacent spans but he "never got around to it."Mr. Sexton says he was approached by one of his customers (an engineer) who was holding the May issue. He then called LDOT to explain the situation.