SHRP Performance Graded Asphalt Binders

Introduction

In 1987, the Strategic Highway Research Program (SHRP) began developing a new system for specifying asphalt materials. The final product of the SHRP asphalt research program is a new system referred to as "Superpave" which stands for Superior Performing Asphalt Pavements. Superpave software is a computer program that assists engineers in materials selection and mix design. However, the term "Superpave" refers to more than just the computer program. It represents an improved system for specifying component materials, asphalt mixture design and analysis, and pavement performance prediction. The system includes test equipment, test methods, and criteria.

Asphalt Binders

One portion of Superpave is a new asphalt binder specification witha new set of tests to match. The document is called a binder specification because it is intended to function equally well for modified as well as unmodified asphalts.

The new system for specifying asphalt binders is unique in that it is a performance based specification. It specifies binders on the basis of the climate and attendant pavement tempertures in which the binder is expected to serve. Physical property requirements remain the same, but the temperature at which the binder must attain the properties changes. For example, the high temperature, unaged binder stiffness (G*/sin d) is required to be at least 1.00 kPa. But this requirement must be achieved at higher temperatures of the binder is expected to serve in a hot climate.

Performance graded (PG) binders are graded such as PG 64-22. The first number, 64, is often called the "high temperature grade." This means that the binder would possess adequate physical properties at least up to 64°C. This would be the high pavement temperature corresponding to the climate in which the binder is actually expected to serve. Likewise, the second number (-22) is often called the "low temperature grade" and means that the binder would possesss adequate physical properties in pavements at least down to -22° C. Additional considerations given to the time of loading (open highway, city streets, intersections, etc.) and number of loads (heavy trucks). The table below shows the current binder grades in the SHRP binder specification.

High Temperature GradeLow Temperature Grade
PG 46-34,40,46
PG 52-10,16,22,28,34,40,46
PG 58-16,22,28,34,40
PG 64-10,16,22,28,34,40
PG 70-10,16,22,28,34,40
PG 76-10,16,22,28,34
PG 82-10,16,22,28,34

In this table, the PG 76 and 82 grades are used only to accommodate slow transient or standing loads, or excessive truck traffic.

A module in the Superpave software assists users in selecting binder grades. Superpave contains three methods by which the user can select an asphalt binder grade:

Superpave Weather Database

Superpave software contains a database of weather information for 6500 reporting stations in the US and Canada, which allows users to select binder grades for the climate specific to project location. For each year a weather station has been in operation the hottest seven day period is determined and the average maximum air temperature for those seven consecutive days is calculated. For all the years of record (stations with less than 20 years of records were not used) a mean and standard deviation are calculated. Likewise the coldest day of each year is identified and the mean and standard deviation are calculated.

Reliability

As used in Superpave, reliability is the percent probability in a single year that the actual temperture will not exceed the design temperature. Superpave binder selection is very flexible in that a different level of reliability can be assigned to high and low temperature grades. Consider summer air temperatures in Dallas, Texas, which has a mean seven-day maximum of 38° C and a standard deviation of 2° C. In an average year there is a 50 percent chance the seven-day maximum air temperature will exceed 38°C. However, only a two percent chance exists that the temperature will exceed 42° C; hence, a design air temperature of 42°C will provide 98 percent reliability.

Start with Air Temperature

To see how the binder selection works, assume that an asphalt mixture is being designed for Dallas. In a normal summer, the average seven-day maximum air temperature is 38°C with a standard deviation of 2°C. In a normal winter, the average coldest temperature is -11°C with a standard deviation of 3°C. For a very cold winter the temperature is -17°C.

Convert to Pavement Temperature

Superpave software calculates high pavement temperature 20 mm below the pavement surface and low temperature at the pavement surface. For a wearing course at the top of a pavement section, the pavement temperatures in Dallas are 60° and -11° for 50 percent reliability and 64° (60° + 2 standard deviations) and -17°C for 98 percent reliability.

Select Binder Grade

For a reliability of at least 50 percent, the high temperature grade must be PG 64. Selection a PG 64 would actually result in a higher level of reliability of 98 percent because of the "rounding up" t the next standard grade. The low temperature grade must be a PG-11. Likewise, rounding to this standard low temperature grade results in 95 percent reliability. For 98 percent reliability, the required high temperature grade remains PG 64; the low temperature grade is PG-22.

Manipulating temperature frequency distributions is not a task that the designer need worry about. Superpave software handles the calculations. For any site, the user can enter a minimum reliability and Superpave will calculate the required asphalt binder grade. Alternately the user can specify a desired asphalt binder grade and Superpave will calculate the reliability obtained.

Effect on Loading Rate in Binder Selection

Superpave binder selection by climate only assumes that a binder will be used in a mixture subjected to fast moving loads. The loading rate used by the dynamic shear rheometer is 10 radians per second, which corresponds to a traffic speed of approximately 90 kilometers per hour. Much slower loading rates are experienced by pavements near intersections, toll booths, etc. In some cases, loads are not moving but rather are stationary. In these cases, a binder would have to exhibit a higher stiffness to overcome the slower loading rate.

To accomodate these situations, the high temperature grade should be increased by at least one or two grades. For example, assume that a temperature based selection such as the previous example for Dallas, Texas resulted in a desired binder grade of PG 64-22. To account for slow transient loads, the designer would select one grade higher binder, a PG 70-22. If standing loads were anticipated, the designer would select a PG 76-22. Loading rate has no effect on the selected low temperature grade. Pavement design temperatures of 76° or 82°C do not correspond to any climate zone in North America. Specifying this grade is simply a means of ensuring that the binder will have higher stiffness at 64°C, the actual high pavement design temperature. Because the highest possible pavement temperature in North America is about 70°C, two additional high temperature grades, PG 76 and PG 82, were necessary to accomodate binder selection based on slow loading rates.

Effect on Traffic Level on Binder Selection

Superpave recommends that traffic level be considered when selecting binders. When the design traffic level exceeds 10 million ESALs, the designer is encouraged to consider increasing the high temperature grade by one grade. When the design traffic level exceeds 30 million ESALs, the designer is required to increase the high temperature grade by one grade. As with loading rate, there is no effect of traffic level on low temperature grade. For the Dallas example where the temperature based selection required a PG 64-22, a project with a high number of ESALs would therefore require a PG 70-16 or PG 70-22. However, a knowledgeable designer would probably select a PG 70-16 for the Dallas area. That is because it is very possible that a PG 70-16 might be available at a lower cost when compared to a PG 70-22. Furthermore, there is no significant compromise using PG 70-16 when compared to a PG 70-22 for Dallas because the PG 70-16 still results in 95 percent reliability for low temperature grade. Low temperature cracking is likely not a distress exhibited by pavements in Dallas.

Mineral Aggregates

SHRP researchers also believed that mineral aggregates played a key role in HMA performence. While they did not develop any new aggregate test procedures, they refined existing procedures to fit within the Superpave system. Two types of aggregate properties are specified in the Superpave system: consensus properties and source properties.

Consensus properties are those which the SHRP researchers belived were critical in achieving high performance HMA. These properties must be met at various levels depending on traffic level and position within the pavement (appendix C). High traffic levels and surfaces mixtres (i.e., shallow pavement position) require more strict values for consnsus properties. Many agencies already use these properties as quality requirements for aggregates used in HMA. These properties are:

By specifiying coarse and fine angularity, SHRP researchers were seeking to achieve HMA with a high degree of internal friction and thus, high shear strength for rutting resistance. Limiting elongated pieces ensures that the HMA will not be as susceptible to aggregate breakage during handling and construction and under traffic. By limiting the amount of clay in aggregate, the adhesive bond between asphalt binder and aggregate is strengthened and otherwise enhanced.

Source properties are those which agencies often use to qualify local sources of aggregate. The SHRP reseachers beleved that achieving these properties was important, but did not specify critical values since they are so source specific. The source properties are:

Toughness is measured by the LA abrasion test. Soundness is measured by the sodium or magnesium sulfate soundness test. Deleterious materials are measured by the clay lumps and friable particles test. These tests are already in common use by most agencies.

To specify aggregate gradation, SHRP reseachers refined an approach already in wide use by many agencies. It uses the 0.45 power gradation chart with control limits and a restricted zone to develop a design aggregate structure.

A Superpave design aggregate structure must pass between the control points while avoiding the restricted zone. The maximum density gradation is drawn from the 100 percent passing the maximum aggregate size through the origin. Maximum aggregate size is definded as one siza larger than the nominal maximum aggregate size. Nominal maximum size is defined as one size larger than the first seve size to retain more that 10 percent. The restricted zone is used by SHRP Superpave to avoid mixtures that have a high proportion of fine sand relative to total sand and gradations that follow the 0.45 power line, which do not normally have adequate voids in the mineral aggregate (VMA). In many instances, the restricted zone will discharge the use of fine natural sand in an aggregate blend. It will encourage the use of clean manufactured sand. The design aggregate structure approach ensures that the aggregate will deveop a strong, stone skeleton to enhance resistance to permanent deformation while achieving sufficient void space for mixture durability.

Putting it all together

Because Superpave mixture design and analysis is more complex than those in current use, the extent of its use depends on the traffic level or functional classification of the pavement for which it is being used. Consequently, three levels of Superpave mixture design were developed.

While much of the resources in SHRP were devoted to developing the SST, IDT, their protocols, and performance prediction models, volumetric mix design occupies a key role in Superpave mix design. Volumetric design, which is all that is required by a Level I mixture design, entails fabrication of test specimens using the SGC and selecting asphalt content on the basis of air voids, voids in the mineral aggregate (VMA), voids filled with asphalt (VFA), and the ratio of dust to effective asphalt content. Consensus and source aggregate properties must be acheived.

A level II mixture design uses a volumetric mix design as a starting point. A battery of SST and IDT tests are performed to arrive at a series of go/no go performance predictions.

A level III mixture design encompasses most of the facets of Levels I and II. Additional SST and IDT tests are performed at a wider variety of temperatures. Level III design is the only protocol that utilizes SST confined specimen testing. Because of the more comprehensive range of tests and results, Level III design offers an enhanced and more reliable level of performance prediction.