APAI Article Archive

The National Asphalt Paving industry takes pride in the fact that over 90% of our nation’s roads have an asphalt surface. At first glace, that is a great testimonial for asphalt usage since the public can easily confirm it is asphalt if it is black on top. For the most part the nation’s asphalt surfaces are excellent pavements. The problem with a significant percentage of asphalt surfaces in Iowa is that just because it is black on top does not mean it is a quality, load-carrying structure.

A large portion of public perception of full depth asphalt pavements is the broken surface of the local shopping center’s parking lot or other thin asphalt surfaces over rock. Many surfaces with asphalt on top are corrugated, badly rutted and produce a black substance which tracks into home carpets. In part, the public’s perception is correct - just because it is black on top it isn’t necessarily a quality full depth structure and may include non-designed and cosmetic thin overlays on little or no base. Also included in the black on top surfaces are maintenance treatments such as asphalt stabilized roadways, armor or seal-coated and even built up dust treatments from crank case oil.
With the exception of the Department of Transportation’s (DOT) projects, the common thread of quality Hot Mix Asphalt (HMA) paving design and specification did not exist in Iowa at the turn of the millennium. This fact coupled with competitive pavement industry and public influence greatly affected the local agency engineer’s confidence in full-depth HMA for long term performance. Predominantly, at those levels, HMA pavements were limited to overlays and rehabilitation.

In the middle 1980s the National and State Paving Associations and member contractors, along with transportation officials, who over the years had tested design and material usage in HMA pavements, realized that new research was necessary to develop specifications that would address directly the correction of field-encountered problems. In 1985, the problem was comprehensively studied in the Strategic Highway Asphalt Research Project (SHRP). Congress established a five-year $50,000,000 research project with the objective of developing asphalt specifications that would be directly related to field performance and produce a superior performing asphalt pavement (Superpave).
The results of this research produced specifications for asphalt binders, mix design and testing that were revolutionary and initially very complicated and intimidating. So called asphalt experts were immediately rendered as novices with the new material testing methods and mixed design procedures. In Iowa, it was not until the mid-1990s that the first Superpave HMA projects were constructed. The first pure Superpave test project was built in Des Moines, Iowa; however, Superpave tests were predominantly limited to DOT highways and interstates.
In the late 1990s, Superpave was receiving high marks throughout all parts of the United States and the Iowa DOT was greatly encouraged by the performance of its test pavements. However, reluctance to change to Superpave specifications from known Marshall design procedures still existed in the consulting, urban and county engineers’ offices.

Since the beginning of the year 2000, there is no more definitive term for material selection, usage and the design of full-depth HMA for all agency levels than “The Asphalt Revolution.” In the last two years, what began in 1985 with the SHRP research, has finally been put to use in a simplified form that is acceptable to city, county, state, and commercial full-depth HMA projects.

Here are some elements of why the local agency designer is now gaining increasing confidence that full-depth HMA will perform and compete during acceptable life-cycles.

Asphalt Binder Selection
Performance graded binders (PG), formerly viscosity rated asphalt cements, are now the standard for Iowa as well as most of the United States. Binder selection charts are readily available from liquid asphalt suppliers and are selected for project location. The Iowa binders such as PG 64-22 indicate it meets physical property requirements of the Superpave specifications at the high and low temperature grade of 64 degrees centigrade and -22 degrees centigrade. Binders selected for temperature extremes at a particular geographic location can also be modified for unusually high traffic, high speed stopping and turning, and heavy standing loads. Rigid testing of binders is conducted by the supplier, cross-checked by the DOT, and finally tested again at the hot mix plant location. The PG binder gives the local designer a good feeling that it will perform well for all temperature extremes at the project location. Finally, binder selection confidence is enhanced by the fact that physical property requirement testing is related directly to in-place, long-term field performance and to diminish the effects of rutting, oxidation aging and low temperature cracking.
Aggregate Mix Composition
Quality aggregates in Iowa, both pit run and quarried, are abundant and are classified by the State in terms of A and B sources. It has long been known that increasing percentages of crushed aggregate is necessary for HMA to be successful in the increasing traffic counts encountered in new pavement design. The engineer is keenly aware of the need for fractured aggregate particles for HMA pavement strength. Aggregates, both bank run or pit run materials, usually produce rounded particles with low internal friction and resistance to movement. Aggregate mix composition requires processed aggregates to be quarried, crushed, washed and separated into distinct sizes. Full-depth HMA pavement shear strength is dependent on internal friction and aggregate interlock. A selected percentage of crushed particles for the coarse portion avoids pavement deformation (pushing, rutting) and low temperature cracking. The Superpave methods place new emphasis on the fine aggregate fraction of the mix. By specifying coarse crushed content and fine aggregate angularity, the sharp specifications achieve high internal friction and aggregate interlock throughout the total mix. The Superpave mix composition methods avoid fine sand and specify a minimum fine aggregate angularity. Fine aggregate angularity is measured by percent of air voids in the fine aggregate (smaller than 2.36 mm). Higher percentage of voids means more fractured faces in the fine aggregate. For higher traffic loading, washed manufactured fine aggregate is necessary to meet the requirement.
Gyratory Mix Design Specifications(1)
Gyratory compactor mix design process utilizes valuable information acquired from SHRP research and Superpave projects in a simplified version that eliminates the apprehension and reluctance of engineers for conversion from the Marshall process. The Gyratory Mix Design Specification provides lab results that are more closely aligned with field applications. More importantly it eliminates the conception that Superpave design is not adaptable to urban and rural Hot Mix projects.
The Gyratory Compactor is used to provide specimens for volumetric analysis and to measure densities. It compacts trial blends of aggregates and binders in the densest possible arrangement by constant pressure and gyrations that are similar to traffic wheel loading pressure.
Quality Management Asphalt
Augmentation of revolutionary HMA pavement design during the 1990s was enhanced with the development and enactment of the state of Iowa’s Quality Management - Asphalt (QM-A) program. The program provides uniform standards and delineates the contractor’s responsibility for all aspects of project mix design, quality control, and testing. Contractor laboratories and technicians are certified by the DOT. Although QM-A was initially limited to DOT HMA projects, engineers at all levels recognized the advantages of uniform and standard HMA quality control. The program has a unique aspect. It promoted the promulgation of quality control knowledge in the contracting industry.
Standard HMA Specifications
In the late 1990s practically every city, county, and consulting engineer had separate and subtly different construction specifications for HMA roadway projects. Specifications proliferated as agencies made minor changes to improve HMA performance. Specification requirements for local agencies complicated the HMA industry’s efforts to produce standard thicknesses for full-depth competitive pavement. Armed with the new Superpave technology, it was obvious that specifications required revision and standardization. The only document that was current with technology and updated constantly was the DOT specification. Opposition to a standard specification for HMA design and construction at all agency levels is now diminished. Statewide steering committee sanctioned by the Center for Transportation and Education (CTRE) was formed to provide policy guidance and to update the Urban Standard Design and Specifications manual.(2) As the result of that committee’s efforts the asphalt section of the manual has been revised, and except for minor changes, refers to the DOT Standard Specification for Highway and Bridge Construction.(3) In effect it is now recognized that one specification for HMA design and quality control now exists for the state of Iowa.
Full Depth HMA Design Technology
Armed with new SHRP technology for performance-graded binders and mix design, the QM-A program, uniform and standard specifications, the HMA industry in Iowa turned its attention to technical transfer of information. Education in the new tools for HMA design is the catalyst for confidence building of engineers.