Variables & Objectives

HMA is a rather complex material upon which many different, and sometimes conflicting, performance demands are placed. It must resist deformation and cracking, be durable over time, resist water damage, provide a good tractive surface, and yet be inexpensive, readily made and easily placed (Figure 2).

Variables

In order to meet these demands, the mix designer can manipulate all of three variables:

• Aggregate. Items such as type (source), gradation and size, abrasion resistance, durability and soundness, shape and texture (FAA, CAA, flat and elongated particles) as well as cleanliness can be measured, judged and altered.
• Asphalt binder. Items such as type, durability, rheology, purity as well as additional modifying agents can be measured, judged and altered.
• The ratio of asphalt binder to aggregate. Usually expressed in terms of percent asphalt binder by total weight of HMA, this ratio has a profound effect on HMA pavement performance. Because of the wide differences in aggregate specific gravity, the proportion of asphalt binder expressed as a percentage of total weight can vary widely even though the volume of asphalt binder as a percentage of total volume remains quite constant.

Objectives

This section presents the typical qualities of a well-made HMA mix. By manipulating the variables of aggregate, asphalt binder and the ratio between the two, mix design seeks to achieve the following qualities in the final HMA product (Roberts et al., 1996):

• Deformation resistance (stability). HMA should not distort (rut) or deform (shove) under traffic loading. HMA deformation is related to one or more of the following:
  • Aggregate surface and abrasion characteristics. Rounded particles tend to slip by one another causing HMA distortion under load while angular particles interlock with one another providing a good deformation resistant structure. Brittle particles cause mix distortion because they tend to break apart under agitation or load. Tests for particle shape and texture (CAA, FAA, flat and elongated particles) as well as durability (L.A. abrasion) and soundness can identify problem aggregate sources. These sources can be avoided, or at a minimum, aggregate with good surface and abrasion characteristics can be blended in to provide better overall characteristics.
  • Aggregate gradation. Gradations with excessive fines (either naturally occurring or caused by excessive abrasion) cause distortion because the large amount of fine particles tend to push the larger particles apart and act as lubricating ball-bearings between these larger particles. A gradation resulting in low VMA or excessive asphalt binder content can have the same effect. Gradation specifications are used to ensure acceptable aggregate gradation.
  • Asphalt binder content. Excess asphalt binder content tends to lubricate and push aggregate particles apart making their rearrangement under load easier. The optimum asphalt binder content as determined by mix design should prevent this.
  • Asphalt binder viscosity at high temperatures. In the hot summer months, asphalt binder viscosity is at its lowest and the pavement will deform more easily under load. Specifying an asphalt binder with a minimum high temperature viscosity (as can be done in the Superpave asphalt binder selection process) ensures adequate high temperature viscosity.
• Fatigue resistance. HMA should not crack when subjected to repeated loads over time. HMA fatigue cracking is related to asphalt binder content and stiffness. Higher asphalt binder contents will result in a mix that has a greater tendency to deform elastically (or at least deform) rather than fracture under repeated load. The optimum asphalt binder content as determined by mix design should be high enough to prevent excessive fatigue cracking. The use of an asphalt binder with a lower stiffness will increase a mixture's fatigue life by providing greater flexibility. However, the potential for rutting must also be considered in the selection of an asphalt binder. Note that fatigue resistance is also highly dependent upon the relationship between structural layer thickness and loading.
• Low temperature cracking resistance. HMA should not crack when subjected to low ambient temperatures. Low temperature cracking is primarily a function of the asphalt binder low temperature stiffness. Specifying asphalt binder with adequate low temperature properties (using the BBR and DTT results) should prevent, or at least limit, low temperature cracking.
• Durability. HMA should not suffer excessive aging during production and service life. HMA durability is related to one or more of the following:
  • Asphalt binder film thickness around each aggregate particle. If the film thickness surrounding the aggregate particles is insufficient, it is possible that the aggregate may become accessible to water through holes in the film. If the aggregate is hydrophilic, water will displace the asphalt film and asphalt-aggregate cohesion will be lost. This process is typically referred to as stripping. The optimum asphalt binder content as determined by mix design should provide adequate film thickness.
  • Air voids. Excessive air voids (on the order of 8 percent or more in a dense-graded HMA) increase HMA permeability and allow oxygen easier access to more asphalt binder thus accelerating its oxidation and volatilization. To address this, HMA mix design seeks to adjust items such as asphalt content and aggregate gradation to produce design air voids of about 4 percent.
• Moisture damage resistance. HMA should not degrade substantially from moisture penetration into the mix. Moisture damage resistance is related to one or more of the following:
• Aggregate mineral and chemical properties. Some aggregates attract moisture to their surfaces, which can cause stripping. To address this, either stripping-susceptible aggregates can be avoided or an anti-stripping additive can be used.
• Air voids. When HMA air voids exceed about 8 percent by volume (for a typical dense-graded HMA), they may become interconnected and allow water to easily penetrate the HMA and cause moisture damage through pore pressure or ice expansion. To address this, HMA mix design adjusts asphalt binder content and aggregate gradation to produce design air voids of about 4 percent.
• Skid resistance. HMA placed as a surface course should provide sufficient friction when in contact with a vehicle's tire. Low skid resistance is generally related to one or more of the following:
• Aggregate characteristics such as texture, shape, size and resistance to polish. Smooth, rounded or polish-susceptible aggregates are less skid resistant. Tests for particle shape and texture (CAA, FAA, flat and elongated particles) can identify problem aggregate sources. These sources can be avoided, or at a minimum, aggregate with good surface and abrasion characteristics can be blended in to provide better overall characteristics.
• Asphalt binder content. Excessive asphalt binder can cause HMA bleeding. Using the optimum asphalt binder content as determined by mix design should prevent bleeding.
• Workability. HMA must be capable of being placed and compacted with reasonable effort. Workability is generally related to one or both of the following:
  • Aggregate texture, shape and size. Flat, elongated or angular particles tend to interlock rather than slip by one another making placement and compaction more difficult (notice that this is almost in direct contrast with the desirable aggregate properties for deformation resistance). Although no specific mix design tests are available to quantify workability, tests for particle shape and texture (CAA, FAA, flat and elongated particles) can identify possible workability problems.
  • Aggregate gradation. Gradations with excess fines (especially in the No. 30 to 50 (0.60 to 0.30 mm) size range when using natural, rounded sand) can cause a tender mix. A gradation resulting in low VMA or excess asphalt binder content can have the same effect. Gradation specifications are used to ensure acceptable aggregate gradation.
  • Asphalt binder content. At laydown temperatures (above about 250 °F (120 °C)) asphalt binder works as a lubricant between aggregate particles as they are compacted. Therefore, low asphalt binder content reduces this lubrication resulting in a less workable mix. Note that a higher asphalt binder content enhances workability but generally harms deformation resistance.
  • Asphalt binder viscosity at mixing/laydown temperatures. If the asphalt binder viscosity is too high at mixing and laydown temperatures, the HMA becomes difficult to dump, spread and compact. The RV specifically tests for mixing/laydown temperature asphalt binder viscosity.

Knowing these objectives, the challenge in mix design is then to develop a relatively simple procedure with a minimal amount of tests and samples that will produce a mix with all the above HMA qualities.