Measuring crack widths in reinforced concrete walls
Measuring crack widths in reinforced concrete walls
(OP)
I am interested in knowing how structural engineers measure crack widths in existing reinforced concrete walls. As long back as I can rememeber, we have used the CTL crack width gauge which is a transparent piece of plastic on which are marked lines of different widths from 0.20 mm to 2.0 mm. The user tries lining up the different line widths with the crack. I find when used with a magnifying glass, it is quite easy to make an exact detemination of the crack width. (I also have a calibrated maginfying "crack comparator" instrument but I find the palstic with the lines a litle easier to use).
Recently I met with an engineer on site after his reported crack width measurewments were generally twice what ours were when we measured the same crack at the same location along the crack; I found that he was using an electronic digital calipers to measure the crack widths, by holding it flat against the wall and adjusting it until it appeared that the gap between the prongs of the calipers was to equal the crack width.
- Has anyone ever heard of that method?
- What method is generally used?
Recently I met with an engineer on site after his reported crack width measurewments were generally twice what ours were when we measured the same crack at the same location along the crack; I found that he was using an electronic digital calipers to measure the crack widths, by holding it flat against the wall and adjusting it until it appeared that the gap between the prongs of the calipers was to equal the crack width.
- Has anyone ever heard of that method?
- What method is generally used?






RE: Measuring crack widths in reinforced concrete walls
Although I usually measure perpendicular to the crack, one must recognize that the actual displacement or rotation of the concrete may be at an angle to the crack, and so the actual movement can be greater than the perpendicular distance across the crack.
RE: Measuring crack widths in reinforced concrete walls
Dik
RE: Measuring crack widths in reinforced concrete walls
RE: Measuring crack widths in reinforced concrete walls
RE: Measuring crack widths in reinforced concrete walls
RE: Measuring crack widths in reinforced concrete walls
And I use the double business card trick that was described in Structure Magazine.
http://www.structuremag.org/OldArchives/2005/Septe...
I also like dik's idea with the nails. If you added a third nail to the group then you could calculate the x and y components of the displacement rather than just the magnitude of the separation.
RE: Measuring crack widths in reinforced concrete walls
RE: Measuring crack widths in reinforced concrete walls
Charliealphabravo - thanks for the information. Interetsting and much appreciated.
MAY I TAKE IT THAT NONE OF YOU USES OR PARTICULARLY RECOMMENDS DEMEC GAUGES FOR THIS PURPOSE? We are currently monitoring about 20 cracks, but likley we can eventually concentrate on about 6 of them.
RE: Measuring crack widths in reinforced concrete walls
I too have a pocket microscope with a calibrated gauge inscribed within, that I bought about 30 years ago, but find I need to use it with a flashlight to have sufficient light to read the calibrations, so is a bit awkward and not as easy to use as the CTL card.
For the CTL card, at my age, I find that a good magnifying glass nelps enormously in deciding which line matches the crack width.
RE: Measuring crack widths in reinforced concrete walls
I do not use DEMEC gauges. I have done dynamic strain monitoring across cracks, but that is not common.
RE: Measuring crack widths in reinforced concrete walls
www.PeirceEngineering.com
RE: Measuring crack widths in reinforced concrete walls
Our office just bought one of the microscopes dik mentioned. Pretty slick.
RE: Measuring crack widths in reinforced concrete walls
These cost about $10 and have a small battery light for illumination.
A human hair is approx 0.1mm in width - this scope easily magnifies a hair! Actually, when one of our children had head lice a few years ago my wife she used the scope to do daily checks on her scalp. Could see the little critters in FULL detail!
For details of these scopes see http://www.chemcosystems.com/Equipment.htm#Parts and the attached photo too.
HTH
RE: Measuring crack widths in reinforced concrete walls
RE: Measuring crack widths in reinforced concrete walls
This is not to answer you but to get your help. I am thinking to use Demec gauge for my research on expansion measurement. I could not find the information of the tolerance in gauge length. Will you help me about that? By tolerance, I mean- If I want to measure the expansion of a concrete cube of 100 mm size, the center to center distance between two steel discs at two ends will be less than 100 mm. In that case, can a 100 mm Demec gauge be used? Or simply, can a 100 mm Demec gauge be used to measure a gauge length of 95 mm or 105 mm? [I chose 100 as an example; the question will be the same for other gauge lengths as well]. Thanks.
RE: Measuring crack widths in reinforced concrete walls