Structurally Sound

Taking a page from structural engineers, the asphalt industry aims to reduce the stain

Dave Newcomb, P.E., Ph.D. / June 20, 2005

The old adage about being penny-wise and pound-foolish is certainly applicable when it comes to cost considerations for pavement structures. On the one hand, if the pavement designed is inadequate for the traffic or soil conditions, then any amount of money saved in building it is soon overshadowed by the additional cost of massive repair or even replacement. On the other hand, if the pavement is so over-designed that resources are wasted in its construction, then too much is paid for it up front, eclipsing the savings that may be realized by minimizing future maintenance and rehabilitation. A properly designed perpetual pavement is one that is thick enough to ensure a long life with only periodic resurfacing while not consuming unnecessary materials and effort in its construction.

Conservatism is inherent in civil engineering design processes, and rightfully so—no one wants their project to wind up on the next episode of Pavement Engineering Disasters on some cable network. Seriously, civil engineers have a duty to safeguard life and property through proper application of design standards. However, they also have a responsibility to wisely design structures to minimize waste and promote sustainable development. The approach to designing a perpetual pavement that satisfies the need for conservatism while ensuring infrastructure sustainability.

Unwanted conservatism in pavement design often comes in the form of empirical design methods of the past. For instance, the 1993 AASHTO Guide on the Design of Pavement Structures was largely developed on the basis of results from the AASHTO Road Test of the late 1950s and early 1960s. The asphalt pavement structures included in this test were relatively thin (6 in. or less) asphalt mats over granular bases of various qualities. These were under traffic until they failed at 2 million equivalent single-axle loads (ESAL) or less. Thus, designs for traffic levels any higher than this required extrapolation of the results. Ever-increasing levels of truck traffic required more and more extrapolation, so thicker and thicker pavements were designed.

What this model does not take into account is that the weight of the truck traffic did not increase substantially, only the number of trucks. Once the asphalt pavement is thick enough, it doesn’t matter how many heavy loads run over it, because it simply won’t fail from the bottom up. It is important to recognize the limitations of our past design methods and develop a design strategy for the future that gives an appropriate pavement thickness for long-lasting asphalt roads. At higher traffic levels, the current approach results in much greater thicknesses than by using a perpetual pavement approach to design.

Bottoms up

The approach proposed by various U.S. researchers is the use of mechanistic analysis for the pavement structure. Simply put, mechanistic analysis means that the stresses and strains in the pavement structure are calculated just like they are for bridges or buildings. And just like in other structures, the idea is to limit the stresses and strains to the point where the most devastating forms of pavement distress (bottom-up fatigue cracking and deep structural rutting) do not occur. This avoids costly massive rehabilitation or even reconstruction in the future.

Prof. Carl Monismith and his team of researchers at the University of California Berkeley proposed this type of approach in the design of the I-710 freeway near Long Beach. This road, constructed in 2003, has a very high volume of heavy truck traffic coming from the port of Long Beach to nearby rail yards. They used a mechanistic approach that limited the bending strain at the bottom of the asphalt layer and compressive strain at the top of the subgrade. Additionally, they conducted full-scale accelerated loading tests on the surfacing material to ensure it would not prematurely rut in service.

Marshall Thompson, professor emeritus at the University of Illinois, has used the same approach in designing long-life asphalt pavements in Wisconsin, Kentucky and Kansas. The Kentucky pavement was constructed on I-64 in Louisville during 2001. The Wisconsin pavement is to be built in the summer of 2005 in the Milwaukee area on I-43, at the Marquette Interchange. The Kansas section, which also will be built this summer, will have instrumentation embedded in it to measure the strains caused by traffic. A perpetual pavement open house will be held at the site of the Kansas section, July 7-8 in Topeka. Information on this showcase can be obtained from the Kansas Asphalt Pavement Association at jjkapa @aol.com or 785/271-0132.

The Ohio DOT, in concert with Flexible Pavements of Ohio, designed a perpetual pavement that also is under construction this year. Located on U.S. 30 in Wayne County, the Ohio section is instrumented and is being monitored for performance by researchers. Unlike the other designs listed above, the limiting strain was applied to loads that are 20% above the current legal limits, resulting in a fairly conservative pavement section.

Per road basis

Dr. David Timm of Auburn University has developed design software for perpetual pavement. The software, PerRoad, is capable of performing a mechanistic analysis of a pavement to establish whether the design can be considered perpetual. It considers the same limiting strain criteria, but allows for the full spectrum of traffic loads to be considered, along with seasonal changes in material properties and variability in materials and layer thicknesses. The output gives an estimate of the amount of time the strain criteria are exceeded. For high-volume roads, the designer can minimize this percentage of high strains. In the case of low-volume roads, since heavy loads are infrequent, the designer can select a design life that is based upon minimal damage. This approach allows the design of perpetual pavements regardless of traffic volume.

The PerRoad program is being used to accomplish two types of goals. The first is the mechanistic pavement design training for agency engineers. Since it is a user-friendly program with a readily accessible help file, designers can come to appreciate the effect that changes in design input will have on the final pavement thickness. It also is being used to check perpetual pavement designs that are proposed, ensuring a realistic and economical design. PerRoad is available for free downloading from the Asphalt Pavement Alliance website (www.asphaltalliance.com).

The economic advantages of perpetual pavements extend beyond the initial cost of high-volume roads. Although they are more expensive initially for low-volume roads—as Figure 1 shows, they require more thickness—they more than pay for themselves in the long run. This is the conclusion that Prof. Steve Muench and his colleagues at the University of Washington came to when comparing long-lasting, low-volume asphalt pavements to what they called “disposable” pavements. Their study showed a 20% reduction in life-cycle costs for these roads in Washington state.

For high-volume roads, the consideration of user-delay costs merits attention. The ability to perform minor resurfacing activities in off-peak hours such as nights or weekends is a tremendous advantage. Returning the facility to service at the times of highest demand minimizes the economic detriment to the traveling public. While this is hard to quantify for a given situation and location, the Federal Highway Administration has guidelines that help to evaluate the benefits, at least on a relative scale. This, in combination with the avoidance of major rehabilitation or reconstruction costs, makes the concept extremely attractive for high-volume roadways.

Properly designed perpetual asphalt pavements are economically attractive for roads ranging from interstate freeways to low-volume roads. However, traditional empirical pavement design procedures do not have the means to properly approach the thickness determination. The PerRoad software allows engineers to use mechanistic design principles to design pavements to arrive at an appropriate design. Guidelines from FHWA can be used to assess the life-cycle cost benefits of perpetual pavements, including the initial cost, future maintenance and rehabilitation costs and potential user-delay costs. Software for performing life-cycle cost analyses is available free from the Asphalt Pavement Alliance.

About the Author

Newcomb is vice president for research and technology at the National Asphalt Pavement Association, Lanham, Md.

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