Rebar corrosion in concrete: Acoustic emission as a detection method
Corrosion of rebar (steel reinforcing) in both cast in place and precast concrete is responsible for a major part of the actual deterioration of structural concrete. Unfortunately, field assessment methods lack accuracy and cannot provide information before significant corrosion has taken place. One exception might be the acoustic emission (AE) technique which detects weak stress. The advantage AE has over other nondestructive evaluation methods (NDE) is that it directly detect the process of a flaw growth. Corrosion itself cannot be detected effectively at this point in time. But its by-products cover the steel and swell. This applies pressure to the surrounding concrete, which in turn creates micro-cracks. During the expansion stress waves are generated when the pressure is high enough to break the interface layer. The growth of these micro-cracks is roughly proportional to the amount of corrosion by-product of a corroding rebar (except perhaps in the incipient stages). Therefore, the degree of the corrosion can assessed by detecting the AE wave rate and their amplitude.
Both the theoretical prediction and experimental results have shown that AE can detect rebar corrosion at relatively early corrosion stages. Although the concept is still new, this field method can localize and characterize the change of a corroded rebar and to help make timely maintenance and repair decisions.
This is a real problem both for inside spaces and the exterior of building envelopes, and is equally annoying and difficult to deal with in construction, mobile homes, auto-vehicles, as well as recreational vehicles (RVs) such as campers.
In an existing enclosed space located in any of the above, mildew may look (should one be lucky enough to see it) as black or gray spots. It may be killed (or at least its growth be stunned) by using either regular or scented household bleach in a suitable dilution for the particular environment. For example one can usually use straight bleach on glazed tiles, only 1:9 on visible wood. Safety, test areas and procedures should be followed as per manufacturer directions or any other reliable source. Note: just because some procedure is described in a published book does not guaranty its correctness! While the success is far from certain, it is quite likely that most surfaces treated with bleach will change color and/or texture. This is function of the concentration used, material applied to, and its finish.
On the outside of buildings and RVs pressure washing or scraping are commonly used. While these methods can make a wall or roof look like new, they have some major drawbaks:
To further discuss the biodegradation, corrosion or concrete/masonry damage issues related to your project, please e-mail us at RoofDoc@usa.net
