HMA consists of two basic ingredients: aggregate and asphalt binder. HMA mix design is the process of determining what aggregate to use, what asphalt binder to use and what the optimum combination of these two ingredients ought to be.
When aggregate and asphalt binder are combined to produce a homogenous substance, that substance, HMA, takes on new physical properties that are related to but not identical to the physical properties of its components. Mechanical laboratory tests can be used to characterize the basic mixture or predict mixture properties. HMA mix design has evolved as a laboratory procedure that uses several critical tests to make key characterizations of each trial HMA blend. Although these characterizations are not comprehensive, they can give the mix designer a good understanding of how a particular mix will perform in the field during construction and under subsequent traffic loading.
This section covers mix design fundamentals common to all mix design methods. First, two basic concepts (mix design as a simulation and weight-volume terms and relationships) are discussed to set a framework for subsequent discussion. Second, the variables that mix design may manipulate are presented. Third, the fundamental objectives of mix design are presented. Finally, a generic mix design procedure (which Hveem, Marshall and Superpave methods all use) is presented.
Before discussing any mix design specifics, it is important to understand a couple of basic mix design concepts:
Mix Design is a Simulation
First, and foremost, mix design is a laboratory simulation. Mix design is meant to simulate actual HMA manufacturing, construction and performance to the extent possible. Then, from this simulation we can predict (with reasonable certainty) what type of mix design is best for the particular application in question and how it will perform.
Being a simulation, mix design has its limitations. Specifically, there are substantial differences between laboratory and field conditions. Certainly, a small laboratory setup consisting of several 100 – 150 mm (4 – 6 inch) samples, a compaction machine and a couple of testing devices cannot fully recreate actual manufacturing, construction and performance conditions. For instance, mix design compaction should create the same general density (void content) to which the traffic will finally compact a mix in the field under service conditions (Roberts et al., 1996). However, it is difficult to calibrate a number of tamper blows (laboratory compaction) to a specific construction compaction and subsequent traffic loading (field compaction). Currently used correlations between these densities are empirical in nature and extremely rough (e.g., high, medium and low traffic categories). However, despite limitations such as the preceding, mix design procedures can provide a cost effective and reasonably accurate simulation that is useful in making mix design decisions.
Mix design, and specifically Superpave mix design, is volumetric in nature. That is, it seeks to combine aggregate and asphalt on a volume basis (as opposed to a weight basis). Volume measurements are usually made indirectly by determining a material’s weight and specific gravity and then calculating its volume. Therefore, mix design involves several different void and specific gravity measurements. It is important to have a clear understanding of these terms before proceeding.
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. In order to meet these demands, the mix designer can manipulate all of three variables:
- Aggregate. Items such as type (source), gradation and size, toughness and abrasion resistance, durability and soundness, shape and texture as well as cleanliness can be measured, judged and altered to some degree.
- Asphalt binder. Items such as type, durability, rheology, purity as well as additional modifying agents can be measured, judged and altered to some degree.
- 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.
Before embarking on a mix design procedure it is important to understand what its objectives are. 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 as well as durability 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. However, this section only addresses mix design issues.
- 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 (as can be done in the Superpave asphalt binder selection process) 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:
- The 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) increase HMA permeability and allow oxygen easier access to more asphalt binder thus accelerating 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. Excessive air voids can be either a mix design or a construction problem and this section only addresses the mix design problem.
- 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 asphalt binder modifier can be used.
- Air voids. When HMA air voids exceed about 8 percent by volume, 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. Excessive air voids can be either a mix design or a construction problem and this section only addresses the mix design problem.
- 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 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 can identify possible workability problems.
- Aggregate gradation. Gradations with excess fines (especially in the 0.60 to 0.30 mm (No. 30 to 50) 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 120 °C (250 °F)) 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 is generally good for workability but generally bad for 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 Superpave rotational viscometer 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.
HMA mix design is the process of determining what aggregate to use, what asphalt binder to use and what the optimum combination of these two ingredients ought to be. In order to meet the demands placed by the preceding desirable HMA properties, all mix design processes involve three basic steps:
- Aggregate selection. No matter the specific method, the overall mix design procedure begins with evaluation and selection of aggregate and asphalt binder sources. Different authorities specify different methods of aggregate acceptance. Typically, a battery of aggregate physical tests is run periodically on each particular aggregate source. Then, for each mix design, gradation and size requirements are checked. Normally, aggregate from more than one source is required to meet gradation requirements.
- Asphalt binder selection. Although different authorities can and do specify different methods of asphalt binder evaluation, the Superpave asphalt binder specification has been or will be adopted by most State DOTs as the standard (NHI, 2000).
- Optimum asphalt binder content determination. Mix design methods are generally distinguished by the method with which they determine the optimum asphalt binder content. This process can be subdivided as follows:
- Make several trial mixes with different asphalt binder contents.
- Compact these trial mixes in the laboratory. It is important to understand that this step is at best a rough simulation of field conditions.
- Run several laboratory tests to determine key sample characteristics. These tests represent a starting point for defining the mixture properties but they are not comprehensive nor are they exact reproductions of actual field conditions.
- Pick the asphalt binder content that best satisfies the mix design objectives.
The end result of a successful mix design is a recommended mixture of aggregate and asphalt binder. This recommended mixture, which also includes aggregate gradation and asphalt binder type is often referred to as the job mix formula (JMF) or recipe.
HMA mix design is a laboratory process used to determine the appropriate aggregate, asphalt binder and their proportions for use in HMA. Mix design is a process to manipulate three variables: (1) aggregate, (2) asphalt binder content and (3) the ratio of aggregate to asphalt binder with the objective of obtaining an HMA that is deformation resistant, fatigue resistant, low temperature crack resistant, durable, moisture damage resistant, skid resistant and workable. Although mix design has many limitations it has proven to be a cost-effective method to provide crucial information that can be used to formulate a high-performance HMA.