Our customer has asked us to help him evaluate some cracking that occurred on a recent pour of a large parking lot. We believe that the cracks are caused by dry shrinkage, but the owner is concerned that the cracks are structural in nature. We have agreed to monitor the cracks for a year or so to help calm the owner's concerns. What is the best way measure any change in the cracks' width in order to monitor the slab's performance?

The best place to start is by reading the current version of ACI 224, "Causes, Evaluations and Repair of Cracks." Committee members provide a thorough review of current practices in evaluating cracks in Chapter 2.

Given the situation you've described, you'll probably begin by locating each crack on a drawing of the parking lot's plan view. At selected points along each crack's path, you will next need to measure its width. The key is to begin with an accurate measurement. For a fresh crack, the measurement should be at the parking lot's surface. For a weathered or spalled crack, you will need to measure both the depth of the spalling, as well as the actual crack's width, as long as it's not more than 1/8 inch below the original surface. If the crack is in an area where spalling or weathering has occurred, you will need to note the depth of the surface deterioration. If this depth is less than 1/8 inch from the original surface, you need only measure the width of the crack below the deterioration.

After measuring the cracks, you can determine whether they are tolerable by referring to Table 1 in ACI 224 ("Control of Cracking in Concrete Structures"), which lists tolerances for cracks in reinforced concrete. If the crack widths are smaller than the listed widths, there probably is not a structural problem. If larger, you may need to call in an expert.

The committee lists several methods of measuring crack width, but from the sound of your problem, the best tool to use is a comparator. It's an optical measurement device consisting of a microscope and measuring scale.

We spoke with a staff engineer at Edmund Scientific Co., a Barrington, N.J.-based manufacturer of precision instruments, and asked what would be the correct model for a typical concrete crack field examination. He recommended a comparator with a stock number of K02-047. It has an accuracy of 0.005 inch with its 9X magnification. The unit includes the reticle, the glass slide that serves as a scale. The engineer said the cost is about $70.

The engineer recommended the purchase of a 50X direct-measuring pocket microscope if you are looking for a device that offers a little more accuracy. Its reticle is equipped with 0.001-inch divisions and is priced about the same as the comparator.

You can receive a catalog by calling Edmund at 856-573-6280. Several lab-equipment suppliers offer similar devices. To learn more on this subject, click on "Search Article Archive" under "Other Resources" on the left-hand navigation column. On the Search form, type "comparators" in the keyword field, and the search result will yield an excellent reference, "Evaluating Concrete Cracking by Measuring Crack Width," written by consulting engineer Douglas Haavik.