Asphalt (known in the industry as Hot Mix Asphalt, or HMA), is a combination of aggregates (usually crushed stone and some sand), filler (cement, hydrated lime or stone dust) and a bituminous binder called "asphalt cement". Additives are sometimes used (e.g. rubber from old car tires) . Reclaimed "asphalt" from old roads, called RAP (recycled asphalt pavement), is also, increasingly, being used.
When hot, HMA is pliable and can be rolled flat to create a smooth, durable, water proof substrate.
Hot Mix Asphalts are defined by the gradation of the aggregate. This includes the maximum size of crushed stone present in the mix. Most asphalt specifications state what mix type to use for various projects and give the range of particle size that the aggregate gradation must fall within.
Asphalt mix types are referred to by the gradation limits. Each state, city or municipality has their own designation for asphalt mixes suitable for use in their jurisdiction. For example Florida DOT refers to S1 mixes while Texas DOT uses Type A, B C or D mixes.
To adopt a national standard for HMA all States are moving to the SuperPave method of describing asphalt mixes.
SuperPave is not a type of hot mix asphalt. SuperPave is a method of designing Hot Mix Asphalt. SuperPave stands for Superior Performing HMA Pavements.
Prior to SuperPave, there were two recognized methods of asphalt mix design, the Hveem and Marshall methods.
Both of these design methods are named after their inventors. Both methods describe a series of standardized tests and testing equipment that is to be used to determine the optimum aggregate and asphalt binder (liquid asphalt) blend needed for an asphalt mix to perform. The Marshal method of mix design is the most widely used.
There have been many minor changes to the types of asphalt mixes since the turn of the millennium. These changes were needed to ensure that the performance of HMA meets today's traffic counts and loads. All of these changes, described different mix criteria, aggregate types and sizes, and asphalt binder (liquid asphalt), but all these changes continued to use the outdated Marshall method of mix design. SuperPave is a new design method was developed to adopt a national standard for HMA pavements. This standards allows our industry to measure HMA performance on a national level. Therefore a new design method was needed that better simulated the traffic loads expected; one that would perform better under those loads.
It was also decided that the asphalt binder used needed to do a better job of fitting to the climate in which the mix was to be laid. This resulted in the development of the "performance graded" (PG) asphalt binder.
PG Graded Asphalt Binder
Binder is the new name for asphalt cements. It is the glue that holds the aggregate together and makes asphalt black. PG graded asphalt binder is a new method of specifying binder appropriate for the climate where the asphalt mixture is to be laid. The binder is tested to make sure it gives the greatest amount of flexibility during cold temperatures (to resist cracking) and the greatest amount of rigidity during warmer temperatures (to resist rutting).
The type of PG graded asphalt binder used depends on the temperatures of the area. For example, in Florida, binders need to be able to deal with extended periods of heat. In Alaska binders need to deal with prolonged periods of cold. In Wisconsin the binders need to have flexibility to deal with both types of climates.
Not all agencies have adopted SuperPave mix design technology or the PG graded asphalt binder system, and continue to use the old terminology for describing asphalt binders. The following describes three methods still widely used to describe asphalt binders:
All of the above methods of describing the asphalt binder are based on some measure of the asphalt viscosity.
Penetration Grades (such as pen 85 -100) is based on how deep a needle will push into the asphalt under a certain weight and at a certain temperature. Viscosity Grades are based on the measured viscosity at a certain temperature, and AC Grades are also based on measured viscosity, but after aging or hardening the asphalt in an oven to simulate the effect of heat on the asphalt during making of the mixture.
Compaction: This is the process of densifying the HMA while it is in a hot, pliable state, to pack the aggregate particles together and to remove as much of the air voids as possible. Compaction gives the mix strength, resistance to rutting (i.e. resistance to additional densification under the action of vehicular traffic), it seals the surface from the ingress of water, and gives a smooth riding surface.
During compaction of asphalt hot mix three very important factors that will affect pavement performance take place. The asphalt coated aggregate particles are pressed together, air voids are reduced, and mix density (weight to volume ratio) increases.
- This squeezing together of the aggregates increases their surface to surface contact and interparticle friction, resulting in higher mix stability and pavement strength.
- The reduction of air voids to the optimum level in the mix produces a pavement that is nearly impermeable. In an under compacted mix the voids tend to be interconnected thereby permitting the intrusion of air and water into the pavement structure. The air oxidizes the asphalt binder, making it brittle and causing the mix to crack under the repeated deflections due to traffic loads. Water permeating into and through the pavement can lead to stripping of the asphalt from the aggregate, weakening of the base and subgrade soils under the pavement, or freeze-thaw damage in colder climates. Premature pavement distress can occur in any of these situations.
- Finally, if a high level of compaction (concurrently with items 1 and 2 above) is not achieved during construction, subsequent traffic will further consolidate the mix. This will occur principally in the wheel paths and the resulting ruts could be a safety hazard to traffic using the road.
The goal of compacting an asphalt concrete pavement is to achieve the optimum air void content and provide a smooth riding surface. Behind the paver, the mat has 15 to 20% air voids. It is the task of the rollers to reduce that void content to 8% or less for dense-graded mixes. At that level, the voids are usually not interconnected and the detrimental effects of air and water are avoided. Caution must be exercised, however, not to compact the pavement to a very low air void level. This minimum void level is necessary to allow for thermal expansion.
Even with a properly designed mix, compaction of the mat is possible only if correct mix temperature exists.
At temperatures between 85°C (185°F) and 150°C (300°F), the asphalt binder is fluid enough to act as a lubricant in the compaction process aiding movement of the mix aggregates into a dense configuration. As it cools below this temperature range, asphalt becomes very stiff and sticky making further mix densification extremely difficult. Therefore, compaction must be completed before the mix cools to 85°C (185°F).
Asphalt cement or "binder" is a complex hydrocarbon found as a natural deposit or as a residue from distilling crude oil to remove petroleum, motor oils, naphtha and kerosene. Asphalt cement is dark brown or black in color and at normal temperature is a solid but as the temperature is increased it becomes sticky and then oily. Asphalt cement makes up about 5% of the volume of the mix and is used to hold together (or cement or glue) the aggregate particles together in the HMA.
Aggregates are crushed stone, sand and fine ground rock, mixed in a predetermined proportion to obtain desired properties in the HMA. Aggregates and fillers make up about 90% of the volume of the mix.
Filler is a fine dust used to harden the asphalt cement, and improve adhesion of the asphalt cement to the aggregates.
Aggregate gradation HMA is graded by the percentages of different-size aggregate particles it contains. Aggregate gradation is a way of describing the proportions of the various sizes of crushed stone, sand and filler, by passing the aggregate through a set of sieves and measuring the weight retained on each sieve.
Compaction this is the process of densifying the HMA while it is in a hot, pliable state, to pack the aggregate particles together and to remove as much of the air voids as possible. Compaction gives the mix strength, resistance to rutting (the additional densification under the action of vehicular traffic), it seals the surface from the ingress of water and gives a smooth riding surface.
SuperPave is a method of designing Hot Mix Asphalt.