Technical Questions

ACI Committees, Membership, and Staff have answered common questions on a variety of concrete related topics.

Alternative methods for measuring crack width

Q. Can feeler-type gauges, such as gap tools or wire gauges, be used to measure crack widths in concrete members in lieu of traditional crack comparator cards or optical devices listed in ACI 224.1R-07? In my experience, feeler gauges can be better tools in low light conditions and where there is a lot of dust/debris on the concrete surface.


A. While evaluating a concrete structure, accurate determination of crack widths will help in identifying the possible causes and, if needed, repair methods for cracks. As stated in your question, ACI 224.1R-07 recommends two devices for measuring crack widths: a crack comparator— “a small, hand-held microscope with a scale on the lens closest to the surface being viewed”; and a clear card with lines of various widths.

The document does not mention the option of using feeler-type gauges, however, we’ve asked members of ACI Committee 224, Cracking, for their thoughts on the subject. Here are their comments:

- “We never use them. You cannot readily measure crack width away from the surface, primarily because crack widths and surfaces tend to be so irregular. Crack widths also vary with length and depth. A flat object, such as a feeler gauge, would get hung up on the irregularities. Maybe a wire would work to probe for depth, but I could see it getting deformed quickly and no longer being accurate. When it gets hung up on surface irregularities, it would underestimate ‘measured’ crack widths. Given the surface of concrete, optical is easily best.”

- “My experience with cracks is that they are not straight enough to fit a feeler gauge into.”

- “I’ve never used a feeler gauge, as it seemed impractical to obtain decent results.”

- “Wire thickness gauges are better than flat pieces of material due to the typical irregularity of the fractured concrete within the crack, which can make it hard to read the crack width.”

- “The only accurate measurement is [through] the use of a lighted magnifying microscope. But this is easy to use in the laboratory by a person with experience in reading several widths at reasonable distances along a crack path. For the novice and for field observation, the plastic graduated transparent pocket card is typically appropriate to compare the actual crack width with the different graduated lines widths on the card. It is easy to use in the field by moving the graduated transparent plastic card along the length of the crack, estimating the possible varying widths as you compare against the different widths of the graduated lines on the card and averaging the readings as a good practical value of the crack width for decision making. Remember, we are talking about measuring crack widths in the range of 0.005 to 0.05 in. (0.127 to 1.27 mm).”

- “Feeler gauges have a plate-like flat surface to measure minimal distances between two smooth surfaces. As you know, they are used effectively in the steel industry, in conditions where you have two parallel planes (usually smooth surface) facing each other. That is not the case with concrete cracks. Concrete cracks typically form an irregular random surface not readily accessible with a feeler gauge. Therefore, it is nearly impossible to get accurate readings with feeler gauges when measuring concrete cracking.”

- “A feeler gauge is only good for measuring cracks that do not have any side spalls at the top, so you are measuring the crack width at the surface of the crack and not having to insert the feeler gauge into the crack at a great depth. Once you have to start inserting the gauge into the crack, there is a possibility of obtaining a narrower gap width, if you have a V-shaped crack.”

- “Since surface cracks typically ravel at their edges and exhibit a notched profile, my experience has been that a visual assessment of the crack is necessary. A feeler gauge will not give the user information about the width of the actual crack, but only the width of the notch at the surface. A crack comparator card generally has sufficient accuracy to determine crack widths for repairs since the recommended limit for epoxy injecting a crack is 0.01 in. (0.25 mm). In my experience, optical microscopes can be challenging to use even when evaluations are performed in direct sunlight because the models I’ve used only allow light from one side of the scope. If you require this level of accuracy, I recommend using the optical microscope coupled with your own light source to ensure direct lighting.”

- “If you insist on using flat pieces of metal to feel crack widths, the old-style feeler gauges used for manually adjusting carburetors on internal combustion engines work well. Take multiple readings at each general location—taking 5 to 10 readings and averaging them is easy to do.”

As you can see from the provided comments, the majority of the respondents from ACI Committee 224 are skeptical of obtaining accurate crack widths with feeler gauges; however, the ultimate decision on using them is yours.


References: ACI 224.1R-07; ACI RAP-1

Topics in Concrete: Cracking in Concrete; Testing of Concrete; Troubleshooting Concrete

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