Construction Pathology: Biodegradation
Building Pathology: Biodegradation
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@ A. Sebastian Engineering and Investigation Services
Construction materials are under a continuous attack from Mother Nature. Biodegradation will occur! It is only a matter of time. Algae, fungi (molds), and mosses are opportunistic. We are learning only now how to live with them, and perhaps benefit in some ways such as in environmental clean-ups. But we do not even begin to understand the other side of the coin.
Fly ash, fungi and metal leaching
Let us use as an example a fungus that extracts heavy metals.
A technique developed at the University of Zurich's Institute for Plant Biology (Zurich, Switzerland) by Helmut Brandl uses aspergillus niger to leach metals from industrial fly ash. This fungus leached up to 50% of copper, cadmium, lead and zinc after just one day of incubation and 80% to 90% after 20 days.
When using sucrose as a carbon source at a 30°C ambient temperature the fungus produces gluconic and citric acids, plus surfactants and emulsifiers that enhance the leaching efficiency.
The metals stay in solution and can be recovered either electrolytically or by ion exchange.
This is wonderful if we view the fly ash as a difficult waste from coal burning thermo-generating stations. But the same fly ash is (or at least could be) a major component of many concretes (up to 30% of the total cementetious material in some mixes). At this point we do not even have an educated guess about what the same aspergillus niger might be able to do to fresh or hardened concrete. Would concrete behavior, resistance to environmental and chemical attack (which is one of the main reasons for using fly ash in the mix designs in the first place), or physical characteristics change? Given the fact that living organisms use up oxygen in order to survive, would we loose entrained air from the fresh concrete, which in turn may render a hardened outside pavement useless in front of chemical deicers? Would we have additional or fewer micro-voids and cracking? Would we loose the improved tensile strength brought by the fly ash? Would we see segregation of aggregate due to increased slump (promoted perhaps by the release of surfactants and emulsifiers).
"Chemical Engineering", Vol. 103, No. 10, October 1996.
Floods and mold
Another example: flooded buildings are prone to severe mold contamination, which in turn can trigger serious problems for people with allergies or asthma.
Various studies have some scary findings.
Washington University School of Medicine in St. Louis researchers surveyed buildings flooded during the summer of 1993.
Buildings in which no clean-up was done had mold counts of 13,600 to 453,740 spores pcm (allergic reactions can occur in humans at only 3,000 spores per cubic meter). In buildings with some clean-up (such as carpet and dry-wall removal) mold counts ranged from 23,400 to 77,574 spores pcm. Even buildings that had been gutted, decontaminated with bleach and rebuilt had over 7,700 spores pcm.
In contrast, mold counts in control buildings were less than 750 spores pcm.
The mold problem begins with the boarding up flood-damaged buildings after
an evacuation, which creates perfect conditions for mold growth.
And once mold is established, its near-microscopic, reproductive spores
permeate the building, making complete decontamination practically difficult.
Partial clean-up for mold seems to be a bad idea as well. The cleaning has to be almost pathological : tearing out everything but the exterior walls and the framing, then treat all the surfaces with a fungicidal agent such as bleach seems to be the passable solution.
But even these extreme measures are of little use if flood-damaged items that harbor spores are brought back or remain inside the building.
So, short of demolition (or fire perhaps), there is no practical solution at this point.
The problem becomes even more difficult if we consider "human made" floods, anything from leaking windows to roofs, exterior walls, crawl spaces, basements and much more.
Wayne Hearn: "When flood waters recede, allergenic mold moves in", "American Medical News", Nov 14, 1994 v37 n42 p26(1).
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