For more than a year now, we've been reporting on the development of the “Standard Measuring Protocol for Evaluation of Concrete Elements for Conformance to Specified Tolerances.” Colin Milberg of San Diego State University has been developing the plan and the processes for gathering as-built data on concrete construction projects for use as a basis for construction tolerances. He also has lined up the technological tools to make this large undertaking possible, including two laser scanners and a powerful set of computational tools to do the number crunching. (For additional background information on this project, see How Close Are We Getting; from the July 2007 issue of Concrete Construction). The laser scanners already have enabled the rapid collection of precise and accurate benchmark data. Sophisticated software and some clever programming provide a base for calibrating the overall system.

Using that knowledge base, Milberg has been able to set data collection parameters for statistically sampling projects going forward. Rather than looking for a complete and comprehensive survey of every concrete element—which is what the laser scanners would be capable of doing, given an ideal schedule —about 5% of the building elements will be checked.

Help needed

Now that the methods and processes have been set up, it's time to ask for help. The prototyping has been accomplished, but to reach full production Milberg needs more data from more contractors in the field. Although this requires a commitment of time and effort, it can be done with a standard total station. No need to buy that laser scanner quite yet.

The pitch

When one is asking for volunteers, it's only fair to explain why they should participate and what's involved. The “why” is pretty straightforward. By looking at how closely as-built walls, floors, columns, and other building elements are matching the design, the ACI 117 committee will be able to verify the applicability of existing tolerances or come up with new ones that are more realistic. But making these types of decisions—which will affect concrete contractors for years to come—requires a large amount of reliable field data. Milberg reports that three contractors currently are sending in field data collected using total stations, and a fourth is providing him with laser scan data. That, combined with the data he and his team of students have gathered, has served well to get the kinks out of the system. Now it's time to go into high production.



Figure 1. The data-entry program calculates coordinates for the design location of 30 randomly selected points on the surface. As-built measurements are entered in the blue area of the grid.
Figure 1. The data-entry program calculates coordinates for the design location of 30 randomly selected points on the surface. As-built measurements are entered in the blue area of the grid.

The process

An interactive spreadsheet makes the complex data collection process as simple as possible. That document, and an accompanying commentary on its use, have been available on the ACI Web site for some time, but only to 117 committee members. Nonmembers can request a copy by contacting Milberg directly at cmilberg@sdsu.edu.

The data entry form also is now available in an online version. Although it's quicker for real data entry, the fact that you only see one screen at a time, and in order, makes it difficult to get a process overview. Fortunately, Milberg has put together a brief Web-based presentation that guides future contributors through the entire process and effectively provides that overview. Like the “why” explanation, the actual data collection process is straightforward, too. The first step is selecting the building elements to be used. For example, consider the vertical elements in a simple five-story building. If there are 16 columns, you would multiply 16 times 0.05 and get 0.8. Rounding up, that would mean looking at just one randomly selected column on each floor. The number of walls per floor would be determined in a similar fashion, again rounding up to one per floor if the calculated number is less than one.

Next comes the initial setup, which consists of entering coordinate data from the project plans for each element. Because these values will be integrated into a larger body of data, this and subsequent steps require following a few standard procedures, including naming conventions and the use of project and local coordinate systems. For example, a plain wall might have only one face that is of interest. The corners are identified beginning with 1 as the lower right-hand corner, then in sequence moving clockwise around the surface. Openings such as doors and windows introduce additional corners, all numbered in order. You then supply coordinate data for each of the corners as distances from a datum point using the project coordinate system. For consistency, the project coordinate system follows the right-hand rule and uses the convention of the positive X axis indicating east, positive Y pointing north, and Z showing elevation where up is positive.

Figure 2. The online version of the data entry form gives the random point locations in both the project and surface coordinate systems. As-built measurements are entered in the three right columns.
Figure 2. The online version of the data entry form gives the random point locations in both the project and surface coordinate systems. As-built measurements are entered in the three right columns.

After the input data have been entered, the spreadsheet generates a set of coordinates for 30 random points on the surface under consideration. These are the points at which as-built measurements are to be taken. Their locations are given in the surface coordinate system as horizontal and vertical measurements from the point identified as corner 1. The initial spreadsheet (see Figure 1) was assembled to show the design coordinates in the surface coordinate system. However, it asks for as-built measurements in the project coordinate system, which makes it difficult to do a quick check to confirm that the numbers you're entering look reasonable.

In the online version of the form now in use (see Figure 2), the design location data appear in the surface coordinate system but also are presented in the project coordinate system—all the calculations are done behind the scenes—which allows you to quickly compare your as-built measurements to the design values and get an immediate sense how close you've come.

Get involved!

If you think this sounds like a lot of work, you may be right. But remember that it's only through this type of investment by a lot of good-hearted, unnamed, underappreciated folks that the industry ever makes any progress. If you're willing to consider contributing to this effort, drop an e-mail to Colin Milberg. Then let the data transmission begin.