Difference between revisions of "Asphalt"

Revision as of 23:24, May 20, 2008

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.

Standards

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

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 millenium. 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criteria, aggregate types and sizes, and asphalt binder (liquid asphalt), but all these changes continued to use the outdated Marshall method of mix design.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.^[1]

Grades

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.

For Example: 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.^[2]

References