Measuring crack widths in reinforced concrete walls

[ajk1]

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?

 

[charliealphabravo]

I use both tools...but I wouldn't recommend using a caliper like you described seeing. The ID caliper on the backside of the tool can usually be inserted into the crack to get a precise measurement. If the crack is too small for the ID then I take a macro photograph of a scale across the crack and determine the width in the office.

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.

 

[dik]

I use a pocket microscope for visual; it is graduated for 0.1mm and you can visually break the 0.1mm stadia into 1/4's reasonably well for 0.025 level of 'accuracy'. Usually 0.1mm is adequate for everything. For measuring growth in crackwidth, I use 1/8" concrete nails 1" or so on either side of the crack (drilled and tapped with hammer) and then use vernier calipers.

Dik

 

[ajk1]

The project geotechncial engineer has suggested demec gauges. I used them in the lab in undergraduate days, but never in a building during my engineering practice. It seems to me they are more bother than the CTL calibrated card I normally use, which is calibrated down to 0.15 mm (0.005") which seems to be quite sufficient considering that we are monitoring the building for possible movement due to the adjacent deep excavation work and caisson wall/tie-back instalaltion. The geotechniocal engineer is saying the way to do it should be by demec gauges. There are quite a few existing cracks to monitor. Any comment on the pros and cons of demec gauges vs. the CTL cards?

 

[cvg]

the CTL cards are adequate and easier to use. I would recommend digital photograph of both the crack and the card.

 

[ajk1]

cvg - my thoughts exactly. Thanks.

 

[charliealphabravo]

Monitoring is a different story. I have used the CrackMON gauges that you epoxy to the surface. I keep a few with me.

And I use the double business card trick that was described in Structure Magazine.

http://www.structuremag.org/OldArchives/2005/September 2005/From-Experience.pdf

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.

 

[Ron]

Agree with cvg and dik...I've used the CTL card for years and found it to be adequate. For monitoring movement, I use Avongard crack monitors, but for initial assessment, I use the CTL card. I also use scales laid across the crack and macrophotography for documentation.

 

[ajk1]

Ron - I do have the Avongard gauges in place. Looks like great minds think alike. But I am also monitoring with the CTL card because it is more finely divided at 0.15 mm vs 1 mm for the Avongard. Also concerned a little that vandals can remove the Avongard so don't want to rely totally on it.

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.

 

[ajk1]

dik - thanks for the idea of the nails. Cool.

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.

 

[Ron]

ajk1...you, dik and I will have to share the magnifying glass!!

I do not use DEMEC gauges. I have done dynamic strain monitoring across cracks, but that is not common.

 

[PEinc]

Most often, I just make two thin pencil marks on the wall, one on either side of the crack, 1 or 2 inches apart. Then, just measure the distance between the marks. It's a lot cheaper than buying crack gauges. This works well for short term monitoring (weeks, not months or years).

http://www.PeirceEngineering.com

 

[MrHershey]

Normally CTL card and an adequate resolution photograph to really take a look.

Our office just bought one of the microscopes dik mentioned. Pretty slick.

 

[Ingenuity]

We use a 50X magnified and graduated crack scope with 0.05mm graduations from CHEMCO SYSTEMS in Calif. A manufacturer of epoxy injection resins and equipment. Very technical-based team.

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

 

[ajk1]

Thank you Ingenuity. That sounds like a wonderful instrument. I am going to follow up on it. Probably the shipping cost here to Toronto will exceed the cost of the instument!

 

[bgautam]

Dear ajk1,
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.

 

[ajk1]

bgautum: sorry that i do not know the answer to your question. maybe best if you start a new string, with this question.