Superpave Evaluation of Gradation

by

Gerald A. Huber
Heritage Research
Indianapolis, Indiana

The purpose of this paper is to explain the method of evaluating gradation within Superpave and the reason for selecting the approach used. Specifically, the following topics will be discussed.

Sieve Sizes

In the United States today there is not one universally used set of sieve sizes. A number of states use a standard set of sieves is specified by AASHTO. Several other states use different sets.


The standard set of sieves, in SI units, are from smallest to largest:
0.075 mm 2.36 mm 25.0 mm
0.150 mm 4.75 mm 37.5 mm
0.300 mm 9.5 mm 50.0 mm
0.600 mm 12.5 mm
1.18 mm 19.0 mm


The sieve sizes are not a soft metric conversion of English sieve sizes, but are defined in ASTM E-11 based on the metric size. Terms that are currently used to describe sieve size such as #8, 1/2 inch, etc. are shown in ASTM as approximate English equivalents. Hence #8 sieves in use today actually are metric sieves with 2.36 mm sieve opening.

Now consider the AASHTO standard sieve sizes and another sieve set commonly used which are listed in Table 1. Note the sieve opening size. The standard AASHTO set of sieves was specifically chosen to match sieve sizes that were used by researchers such as Nijboer, Goode and Lufsey (AAPT 1961), Richard Davis (AAPT 1965), and Huber and Shuler (ASTM 1991). The particle size increases in a geometric ratio of 2. Hence, starting from the 0.075 mm sieve, each next size is twice, or nearly so, the size of the previous sieve.


Table 1. Comparison of Standard and Non-Standard Sieve Sizes
Standard Non-Standard
English Equivalent SI English Equivalent SI
2 inch 50.0 mm 2 inch 50.0 mm
1-1/2 inch 37.5. mm 1-1/2 inch 50.0 mm
1 inch 37.5 mm 1-1/2 inch 37.5 mm
3/4 inch 19.0 mm 7/8 inch 22.4 mm
1/2 inch 12.5 mm 5/8 inch 16.0 mm
#4 4.75 mm 3/8 inch 9.5 mm
#8 2.36 mm 1/4 inch 6.3 mm
#16 1.18 mm #10 2.00 mm
#30 0.600 mm #20 0.850 mm
#50 0.300 mm #40 0.425 mm
#100 0.150 mm #80 0.180 mm
#200 0.075 mm #200 0.075 mm



The system works well from a practical use point of view until the sizes start to become large and the jump from one sieve to the next becomes too large. Jumping from 9.5 mm to 19.0 mm to 37.5 mm provides too large a gap between sieves. Consequently an intermediate set of sieve sizes is nested inside. This set of sizes 12.5 mm, 25.0 mm and 50.0 mm, itself based on a factor of 2, provides an intermediate step in the primary series.

Now consider the non-standard sizes. Each successive size is not a constant ratio larger than the preceding size. In the smaller size sieves, less than 9.5 mm, the ratio of one sieve size to the next ranges from 1.5 to 3.2 instead of a consistent factor of 2.0. Inconsistencies between research done on maximum density lines and the non-standard set of sieves required adoption of the standard set of sieves. Superpave adopted the Asphalt Institute method of drawing the maximum density line and defining the nominal maximum size and maximum size. The Asphalt Institute approach is based on the standard set of sieve sizes.

0.45 Power Charts

Superpave adopted the 0.45 power chart for graphical display of gradation as currently recommended by the FHWA. No evidence of either published or unpublished data was discovered which would support the adoption of any power value other than 0.45. Some reports have circulated in the industry that plotting the sieve opening raised to the 0.45 power may not be universally applicable for all aggregates. Specifically, it is claimed that the power should be larger, 0.50 or 0.60 for some aggregates, particularly crushed aggregates.

SHRP investigated the history of the 0.45 power chart before adoption. The 0.45 power chart as used today is based upon work of Nijboer from the Netherlands and from Goode and Lufsey of the Bureau of Public Roads. Nijboer evaluated the packing of both quarried aggregates and uncrushed gravel. He found that the densest configuration occurred for a straight line gradation plotted on a 0.45 power chart. Goode and Lufsey validated the work of Nijboer for aggregates in the United States and further investigated the packing of various "typical" gradations used in the United States.

Maximum Density Line

Currently in the asphalt industry the practice of drawing maximum density lines is confusing. Different methods are used with varying degrees of success. SHRP investigated the history of defining maximum density lines and evaluated the current status of maximum density lines in the industry today.

The work of Goode and Lufsey validated the 0.45 power chart and investigated one specific maximum density line for contrived typical gradations. The method proposed by Goode and Lufsey in their 1962 AAPT paper for determining where to draw the maximum density line is cumbersome and is not used by any agencies today.

Concurrent with SHRP, the FHWA formed an Expert Task Group on volumetric properties of asphalt mixes. The group investigated two methods of drawing maximum density lines. One method draws a line from the percent passing the 0.075 mm sieve to the first sieve passing 100%. The other method contained in the Asphalt Institute publications requires the line to be drawn from the origin to the maximum sieve size. Background and research supporting the Asphalt Institute method is published in ASTM Special Technical Publication No. 1147.

Within the SHRP research effort an expert task group was assigned to evaluate empirical mixture and aggregate properties. Using a modified Delphi process, consensus was reached to draw a maximum density line according to the method proposed by the Asphalt Institute.

Nominal Maximum and Maximum Size

Integrated with the Asphalt Institute definition of maximum density line is the definition of nominal maximum size and maximum size. Specifically the definitions are:

Nominal maximum size - one size larger than the first sieve to retain more than 10% Maximum size - one size larger than nominal maximum size.
These definitions are consistent with ASTM definitions except they are more specific. Published research (ASTM, STP 1147) demonstrates the importance of correctly defining the maximum size to ensure a valid maximum density line is drawn.

Gradation Control Mechanism

Historically gradation specifications have consisted of gradation master bands in which minimum and maximum percent passing has been specified for all or most sieve sizes. Such gradation specifications have an inherent undesirable feature. Mixes can be designed within the specification which demonstrate poor performance in service. Specifically, these mixes have a poor aggregate structure and are susceptible to permanent deformation. Within the same specification mixes can be designed which are sufficiently dense to reduce asphalt content below desirable levels and, hence, lead to durability problems.

Based upon the history of aggregate gradation specification SHRP focused on the desired attributes of asphalt mixes and developed a method of gradation control using control points and a restricted zone instead of historical aggregate gradation specifications. An example gradation control is shown in Figure 1 for a 12.5 mm nominal maximum size mixture. Significant features of the gradation control include a restricted zone sitting atop the maximum density line between the 2.36 mm and 0.300 mm sieves and various control points.

The control points were selected to accomplish specific objectives. The four upper control points, minimum 100% passing maximum sieve size, 90 to 100% passing nominal maximum sieve size and maximum 90% passing sieve smaller than nominal maximum size, are a result of the definition of nominal maximum and maximum sieve size. For example, in Figure 1 by definition, if a gradation is not 100% passing 19.0 mm sieve, between 90 to 100% passing 12.5 mm sieve, and less than 90% passing 9.5 mm sieve, the gradation will not be a 12.5 mm nominal maximum size.





Figure 1. Typical Gradation Control for 12.5 mm Mixture


The lower 4 control points serve a different purpose. The minimum and maximum requirement on the 0.075 mm sieve obtained from ASTM D3515, is typical for dense graded asphalt mixes. Other mixes such as a Stone Matrix Asphalt (SMA) which typically has 10 to 14% passing the 0.075 mm sieve would not meet this specification.

Gradation control points on the 2.36 mm sieve control the amount of sand sized particles in the mixture. The upper control point limits the amount of sand in the mixture to exclude sand-asphalt mixes which cannot be made which meet the specification. The lower control point ensures adequate sand is contained in the mix to ensure a dense graded mixture. An open graded, porous asphalt mixture cannot meet this gradation control point.

The restricted zone has been specified to ensure adequate aggregate structure is developed in the mixture. Gradation requirements specify that mixes must plot either above the restricted zone or below the restricted zone as specified by the agency. Mixes which pass above the restricted zone will tend to be sandier and have a weaker aggregate structure than mixes which pass below the restricted zone.

Mixes which pass below the restricted zone are constrained by the minimum percent passing the 2.36 mm sieve producing a narrow region within which the gradation must lie. Hence all mixes which pass below the restricted zone will tend to have similar sand contents.

The restricted zone encourages development of gap graded mixes. Mixes which pass below the restricted zone are gap graded. They develop an aggregate structure with the load carrying capacity provided by the large aggregate particles. Sand size particles play a more passive role.

Gap graded mix technology is highlighted in European mixture technology. SMA is a gap graded mixture in which load is carried on the stone to stone contact of coarse aggregate and the outer void space is filled with a filler-asphalt mastic. The role of the stone skeleton is to carry load and create high resistance to permanent deformation. The role of the mastic is to provide durability and long life for the pavement. Ideally the mastic should carry no load but should act as a passive filling in the spaces inside the coarse aggregate skeleton. Another example of gap graded mixture technology is draining asphalt which contains 20% air voids in place. Load carrying capability is provided by the stone on stone contact of coarse aggregate particles. The inter void space in this case contains air.

Mixtures meeting the Superpave gradation bands below the restricted zone are gap graded mixes in which load carrying capability is predominantly located in the aggregate interlock of the coarse aggregate particles. Space between the interlocked coarse aggregate particles is filled with a sand asphalt mixture. Hence, load carrying capabilities of Superpave mixes below the restricted zone are enhanced. If additional sand was added moving the gradation up into the restricted zone, sand particles would begin to separate the coarse aggregate particles reducing the load carrying capability of the mixture.

For more information on Superpave gradations, contact Gerry Huber or Bob McGennis.



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