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User Beware By Mark Buncher, PhD, PE; Mark Blow, PE; and Dwight Walker, PE Why would you use a concrete thickness program to design an asphalt pavement? The Asphalt Institute and many other well-established asphalt pavement indus- try associations have invested significant amounts of capital, time and effort in bringing the transportation industry credible, high value engineering tools, technology and information regarding asphalt pavement. Design professionals should use these sources exclusively for their asphalt pavement design needs. Likewise, the concrete pavement industry promotes their product by developing tools for using concrete in pavement applications. One such tool, a thickness design program called StreetPave that is sold by the American Concrete Pavement Association (ACPA), goes beyond the scope of designing concrete pavement and attempts to replicate the Asphalt Institute’s thickness design methods. A critical flaw in this replication of the Institute’s method needs to be exposed. The ACPA website describes StreetPave as follows: “StreetPave is the latest in thickness design technology for streets and local road pavements. This software utilizes new engineering analyses to produce optimized concrete pavement thick- nesses for city, municipal, county, and state roadways. It includes an asphalt cross-section design process (based on the Asphalt Institute method) to cre- ate an equivalent asphalt design for the load carrying capacity require- ment. A “Life Cycle Cost Analysis” module allows you to perform a detailed cost/benefit analysis and make informed decisions on your pavement design project. With one pavement design tool, you can design equivalent concrete and asphalt sec- tions and evaluate the best possible solution(s) for your pavement needs.” The problem with this description is that the claim of equivalent asphalt and con- crete sections is false. StreetPave takes the single subgrade strength value input by the user (only one value is allowed) and inappropriately reduces it prior to running the asphalt thickness design calculation. No similar reduction is performed with the concrete design. Thus, the asphalt sec- tion is based on a subgrade strength that is significantly less than the user input value and is different from the subgrade strength used in the concrete design. The result is an asphalt section that is thicker than necessary, and more costly than the equivalent concrete section. Resilient Modulus Input The Asphalt Institute methods allow the pavement designer to use one of two practices to determine a single Design Subgrade Modulus (MR) value that is used in conjunction with the thickness design curves. One practice evaluates a group of individual subgrade modulus tests, and based on the test method’s vari- ability and a desired level of reliability, determines an appropriate Design Subgrade MR value. This procedure is based on normal statistical variation and is clearly described in our MS-1 manual, SW-1 software and Research Report 82- 2. The second practice simply allows the pavement designer to assess all known subgrade condition information and then apply conservative engineering judgment to assign a single Design Subgrade MR value to be used with the design curves. ACPA’s StreetPave, however, queries the user for a single Subgrade MR value, pre- sumed by the user to be the design value, and then in a “black box” manner further reduces it with a statistical calculation using default variability and reliability values. This forced reduction is unseen and does not occur as a separate, notice- able step, but only as a hidden part of calculating the asphalt pavement thick- ness. If the user does not access a second- ary help screen, he will not be aware that the single MR design value was reduced. StreetPave covertly applies an additional and inappropriate factor of safety unbe- knownst to the user, which results in excessive asphalt thickness. SW-1 versus StreetPave Comparison Perhaps the best way to illustrate the problem with StreetPave is to apply it to one of ACPA’s own examples. In a recent ACPA marketing brochure, StreetPave is used to design equivalent concrete and asphalt pavement sections for a residential street. Figure 1 shows how the Institute’s actual design proce- dure (using our SW-1 software) compares Ohio Asphalt Winter 2008 9PDF Image | Porous Asphalt Pavement
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