In my June column, we took a brief look at some of the factors which affect asphalt pavement performance. This time I am going to look at how mix type and materials selection matter to asphalt pavement performance, in a little more detail.
Mix type selection
Not all asphalt mixes perform the same. And they should not be expected to do so; they have different performance standards and design life periods. Mix types commonly used include dense-graded (both fine and coarse), open-graded or porous and stone matrix asphalt (SMA) mixes. Each of these mix types were developed to perform under differing environmental or climatic conditions and traffic levels. Think of it like this—subdivision streets need different mix properties than coal-haul routes.
Dense-graded mixes These mixes are well- or continuously-graded, meaning they have a relatively even distribution of aggregate particle sizes. These mixes are the most commonly used type and they provide structure plus serve as wearing surfaces.
Open-graded and porous mixes Open-graded mixes are just that—open. They have an open aggregate structure produced from a relatively uniform (one-sized) aggregate, with very little intermediate-sized particles. With their excellent friction properties and high skid resistance, open-graded friction courses (OGFC) serve as excellent wearing surfaces for high-speed applications. They are designed to reduce hydroplaning and pavement noise levels. In order to perform as intended, they must be designed and built with high air voids levels and must be highly permeable.
SMAs SMAs are gap-graded with a dominant coarse aggregate skeleton and an asphalt binder-rich mortar. These mixes depend upon stone-to-stone contact for strength and rutting resistance. They contain less of the sand fraction and more filler (aggregate passing the #200 sieve) than do dense-graded mixes.
Pavement layer thickness is another consideration in mix type selection. Traffic levels/volumes and make-up (percent trucks) should be considered in mix type selection. I would not choose an SMA or OGFC for a low volume route; the cost of the premium binders and aggregates used in these type mixes is not warranted. I also would not choose an SMA or OGFC from an inexperienced producer. These mixes handle differently than dense-graded mixes and a learning curve is involved for both the producer and the specifier/owner.
SMA is a good choice for intersections and high-stress areas, where maximum rutting resistance is needed. OGFCs are a good choice for high speed locations where hydroplaning is a concern. Care should be taken to construct them to be free draining. Because of potential icing concerns, they may not be suitable for cold weather locations.
Mixes for base layers are typically dense graded. Some specialty applications utilize a porous base to provide a highly permeable, free-draining pavement layer. Leveling courses are usually dense graded and tend to be relatively thin lift so mixes with smaller top sizes are generally better suited for these applications.
Choosing the appropriate ingredient materials for an asphalt mixture has direct bearing on pavement performance. The proper choices depend on the particular application—wearing surface, intermediate or binder courses, bases, and perhaps specialty applications (such as patching or leveling).
Due to environmental and vehicular exposure, wearing courses require the highest quality materials. Surface courses must be smooth, durable, skid-resistant and rut-resistant.
Stiffer, more rutting resistant mixes are appropriate for high stress applications and locations. For example, at signalized intersections (stop-lights), it is good practice to use a heavy duty binder, likely two PG high temperature grades more than that commonly used, for 1,000 feet before and 500 feet past the signal. If signals are closely spaced, use the heavy duty binder throughout the project?s length and limit the amount of rounded aggregate.
Here are some additional thoughts on choosing the ingredient materials:
Binder In the U.S., most asphalt binders are selected in accordance with Superpave guidelines, which are tied to traffic and environmental conditions. The choice of neat (unmodified) verses modified binders must be made. And some agencies choose to require a specific type of modification to meet their requirements.
Aggregate Since aggregate makes up about 95 percent of an asphalt mix, it has a huge influence on asphalt mix performance. The important aggregate properties include size and grading, shape and texture, affinity for asphalt, absorption, toughness and weathering resistance, cleanliness and consistency. Consistency is particularly important. Given a certain level of quality, if an aggregate is consistent, we can likely make appropriate adjustments to make it work. If it is inconsistent, we cannot produce a mix with acceptable performance.
Additives You have to decide or test whether any additives are needed. These could include anti-stripping additives, fibers for reinforcement or binder drain-down prevention, etc. The uniform addition and distribution of additives is critical to how the materials perform.
Reclaimed materials The use of reclaimed materials has grown greatly. Reclaimed Asphalt Pavement (RAP) and Recycled Asphalt Shingles (RAS) are probably the most widely used reclaimed materials. But there are many other reclaimed materials that are used on a local basis.
The way that RAP is processed influences both the materials costs and the mixture performance. RAP contributes both asphalt and aggregate, so it is important to know the binder properties and aggregate gradation. RAP has to be processed for size and consistency. Similar considerations apply to including RAS. Knowing the properties of the reclaimed asphalt binder is particularly important and additional testing is called for.
There are also materials handling considerations that influence performance, including asphalt storage and circulation, aggregate stockpiling and moisture content management, minimizing segregation, component metering/proportioning and more.
And if you are producing warm mix asphalt, that’s a whole other decision process to choose the type of production methodology—foaming, chemical, wax, etc.
By now, you should see that making good decisions on the mix type and the appropriate materials to be included in that mix goes a long way towards influencing how the mix will perform. Take your time and make these decisions carefully; the decisions made now will directly influence your pavement?s performance.