Proper Design ESAL Selection Is Critical for Superpave Mix Design Success

By Phillip Blankenship
BlankenP@kochind.com
National AC Technical Support, Koch Materials Company

Article submitted to FOCUS

As Superpave Volumetric Mix Design is implemented, the asphalt paving industry is learning to use the system. As one might expect, there is still some confusion as to the origin and definition of certain key terms that have a critical impact on materials selection. For example, a recent DOT experience with a Superpave mix design for a high volume pavement emphasizes the need to clearly differentiate Superpave's use of the term 20 year Design ESAL's from the agencies historical expected design life for a given mixture. Although an agency may assume a 10-year life expectancy for a typical surface mix, using 10 year design ESAL's may have an unexpected negative impact on material quality and resulting pavement performance. While Superpave mix designs cover a wide range of traffic loads (e.g. remote rural roads to urban interstates), all Superpave mixture designs require the use of 20 year traffic projections to calculate design ESAL's.

Superpave assumes that asphalt mixtures will achieve design density within three to five summers under traffic. All mixes used to determine the relationship between density and gyrations (NDesign) were from pavements greater than 12 years old, and all had reached their final density. A misconception is that asphalt mixtures gradually densify over the entire life of the pavement. Pavements usually resist further densification after three years due to age hardening of the asphalt cement. So, if a Superpave mix design is designated for 10 years of traffic rather than 20, one will underestimate the effects of traffic during those first three critical years of pavement life.

TABLE 1

Design YearsDesign ESALs, Million NDesignMinimum Course Aggregate Angularity*, % Minimum Fine Aggregate Angularity, %Minimum Sand Equivalent, Approximate %AC for 9.5mm Surface
20596 85/804545 5.5
10286 75/-4040 5.5

* 85/80 denotes that 85% of the coarse aggregate has one or more fractured faces and 80% has two or more fractured faces.

As an example, notice from Table 1 how design ESAL's affect Fine Aggregate Angularity (FAA) and NDesign for a typical surface mix (Table 1). A 20-year design of 5 million ESAL's will yield a mix that requires a minimum FAA of 45% and an NDesign of 96 gyrations. Typically, mixes requiring 45% FAA will only allow 5 to 15% natural sand. The same mixture designed for 10 years may fall in the 2 million ESAL category. This will allow a minimum FAA of only 40% and an NDesign of 86 gyrations. This lower FAA may allow the use of 25% natural sand. The percent asphalt cement (%AC) may or may not change depending upon the allowable Voids in the Mineral Aggregate (VMA). In addition, as the compactive energy (NDesign) is increased during mixture design, the mixture skeletal strength is increased.

While the intention of using 10-year design ESAL's is to produce a mixture that is designed for 10 years, the result is a lower quality, weaker mixture that may be susceptible to rutting under the expected traffic loads. Whether we use 10 or 20 year design ESAL's, the pavement will still experience the same traffic in the first 3 years of service.

Choosing the appropriate design traffic level is an engineering decision. To better achieve the desired performance of a Superpave mixture, use 20 year traffic projections to calculate design ESAL's.

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