HMA – Mix Design Procedures
HMA is composed of two basic ingredients – aggregate & asphalt binder. The objective of the asphalt mix design is to achieve the following qualities in the HMA: 1. Deformation resistance (stability) 2. Fatigue resistance. 3. Low temperature cracking resistance. 4. Durability 5. Moisture damage resistance 6. Skid resistance 7. Workability
These requirements may be conflicting and require compromises to be made in the mix design process.
HMA – Mix Design Procedures
HMA is composed of two basic ingredients – aggregate & asphalt binder. 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. • If binder content too low – not all the aggregate particles are coated resulting in reduced durability • If the binder content is too high – the aggregate particles have too much lubrication and may move relative to one another
1. 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.
2. 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. 3. 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.
4. 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.
5. 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 strippingsusceptible 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.
6. 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.
7. 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.
3. Optimum asphalt binder content determination. Mix Basic Mix-Design Procedure design methods are generally distinguished by the HMA mix design is the process of determining what asphalt method with which they determine the optimum aggregate use, what asphalt binder and whatasthe bindertocontent. This process can to beuse subdivided optimum combination of these two ingredients ought to be. follows: Three common procedures used in North America: • Make several trial mixes with different asphalt • Hveem binder contents. • Marshall • Compact these trial mixes in the laboratory. It is • Superpave important to understand that this step is at best a rough simulation of field conditions. All mix design processes involve three basic steps: • Run several laboratory tests to determine key sample characteristics. These tests represent a starting point forNo defining 1. Aggregate selection. matterthe themixture specificproperties method, but they not comprehensive nor with are they the overall mixaredesign procedure begins evaluation exact reproductions actual fieldbinder conditions. and selection of aggregateofand asphalt sources. Different specify methods • Pick the asphaltauthorities binder content thatdifferent best satisfies the mix acceptance. design objectives. of aggregate 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. 2. 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 and Provincial DOTs as the standard (NHI, 2000).
