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Aggregate Structure Design
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Superpave mix design does not specify a strict aggregate structure. Rather, it specifies aggregate gradation control points based on nominal maximum aggregate size (NMAS). Although these control points are straightforward, they are minimal and quite broad, leaving substantial room for any number of markedly different gradations. Therefore, the mix designer is left to consult other sources outside the Superpave mix design specification in order to develop an aggregate gradation that will contribute to satisfactory HMA performance.
There are few formally defined standard processes for aggregate structure design. In practice, the process involves (1) determining an aggregate gradation that will produce an acceptable aggregate structure, and (2) determining the blending ratio of different aggregate stockpiles in order to achieve the desired gradation.
This section discusses some of the guidance and rules-of-thumb typically used in aggregate structure design.
Generally the NMAS is selected based on the pavement's intended use. Smaller NMAS (9.5mm (0.375 inch) and smaller) are generally used for HMA pavement surface courses in moderate to low traffic conditions. They are typically easier to finish and have a smooth microtexture. These are often used for HMA pavements intended for heavy foot traffic (e.g., walking paths, parking lots) because their smooth microtexture gives a smoother surface and is more pleasing to view from close range.
Larger NMAS (12.5 mm (0.5 inch) and larger) are generally used for HMA pavement surfaces and base courses in moderate to high traffic conditions. Surface courses with NMAS > 12.5 mm (0.5 inches) are generally limited to extreme heavy-duty conditions because of problems with permeability, segregation and placement.
NMAS also affects HMA pavement permeability. Mixes with larger NMASs tend to have larger sized voids, which are more likely to be interconnected. Therefore, at the same air void level, mixes having larger nominal maximum aggregate sizes have more potential for high permeability than mixes of smaller nominal maximum aggregate sizes. Critical density values for different nominal maximum aggregate sizes are (Cooley et al., 2001):
Figure 2 shows that, in general, the larger the NMAS, the greater the permeability for a given density or in place air void content. Figure 3 shows that, in general, as lift thickness increases, it becomes less likely that a series of interconnected voids can span the entire lift depth. Therefore, as lift thickness increases, permeability decreases.
Gradation selection is really a selection of the best aggregate structure for the intended purpose. Superpave does not have a formal method of making this selection; the gradation control points only provide broad limits making it necessary to use further guidance in selecting a particular gradation. Furthermore, in Superpave mix design, each aggregate characteristic is generally considered in isolation, there is little formal consideration given to the interaction of these characteristics.
Therefore, aggregate gradation selection tends to rely on a combination of general concepts and local experience. To an extent, these concepts have evolved as means to avoid detrimental gradation properties or interactions that are not controlled elsewhere. The following is some generally accepted guidance to follow when selecting aggregate gradation (see the Gradation Guidance Graph).
Natural sand (Figure 5) is defined as fine aggregate that comes from naturally occurring sources as opposed to aggregate crushing (manufactured sand). Natural sand tends to be smooth rather than angular and can be poorly graded. Natural sand is generally less expensive than manufactured sand and therefore is preferable if it does not detrimentally affect HMA performance.
The general consensus is that because of its rounded rather than angular nature excess natural sand can cause HMA stability problems and lead to rutting and possibly compaction difficulties. Therefore, many specifications place a limit on natural sand content within the aggregate. This has been done in one or both of the following ways:
All of these methods are overly restrictive since they do not directly address the properties of concern: fine aggregate angularity and gradation. Rather, they indirectly address natural sands, which tend to contain smooth and/or poorly graded particles. The FAA test is an attempt to remedy this problem.
In general, gradations that closely follow the FHWA 0.45 power curve maximum density line should be avoided because they tend to have low VMA. This VMA could be low enough that it does not allow adequate room for the asphalt binder to coat the aggregate particles. If this is the case, the mixture may be susceptible to (1) deformation as the asphalt binder forces the aggregate particles apart or (2) bleeding as the asphalt binder is squeezed out of the mixture.
Although the restricted zone is now only a guideline (rather than part of the specification), it provides a convenient means to differentiate between coarse and fine mixes. This is important because coarse and fine mixes are fundamentally different in the way in which they transmit loads through the aggregate structure:
Low mineral filler content should be avoided. Mineral filler tends to combine with the asphalt binder making it more viscous. A low filler content may result in a less viscous asphalt binder and cause the mixture to behave like one with too much asphalt binder (an "over-asphalted" mixture). Conversely, too much mineral filler can make the mixture too viscous making it difficult to place and compact. Also, too much filler could drastically increase the aggregate surface area that the asphalt binder needs to cover and lead to an excessively thin asphalt binder film around each particle.
The Bailey method is a formal procedure used to design aggregate gradation to ensure adequate coarse aggregate interlock. Developed by Robert D. Bailey of the Illinois Department of Transportation (IDOT) and refined by Vavrik and Pine, this method provides a systematic approach to aggregate gradation selection for dense graded mixtures. It is one of the few well-defined methods for gradation selection in the U.S.
(To install the stockpile blender, insert the VSL CD-ROM into your computer's CD-ROM drive and select the "Install the Stockpile Blender Tool" button from the automatic startup menu. You can also access this menu by double-clicking the "Start the VSL.exe" file on the VSL CD-ROM)
Generally, Superpave mix design aggregate gradations cannot be achieved using one aggregate stockpile. Rather, several different aggregate stockpiles are blended together in a ratio that will produce an acceptable final blended gradation. It is quite common to find a Superpave mix design that uses 3 or 4 different aggregate stockpiles.
Typically, several aggregate blends are evaluated prior to performing a complete mix design. Evaluations are done by preparing an HMA sample of each blend at the estimated optimum asphalt binder content then compacting it. Results from this evaluation can show whether or not a particular blend will meet minimum VMA requirements and Ninitial or Nmax requirements.
Aggregate stockpile blending is typically done via trial-and-error. It generally only takes between 2 and 4 trials to get an adequate blend. Because blends usually take place between coarse and fine aggregate stockpiles, it often works out that the No. 8 (2.36 mm) sieve is the critical sieve. The second most critical sieve to check when blending is usually the No. 200 (0.075 mm) sieve. If a blend ratio results in these two sieves being at the desired percent passing, it is often the case that the other sieves are also at the desired percent passing.
