Engineering Optimal Asphalt Pavements
Asphalt pavements have always possessed Engineering, Economical, and Environmental benefits which are second to none in the roadway paving industry. These three "E's” help make asphalt the preferred pavement type in America. Asphalt has accomplished this in part through the superior engineering properties present in the product. Proof in point: the United States has 2.2 million miles of paved roads, and about 94 percent of them are surfaced with asphalt, America's choice.
The high population growth rate being experienced in Texas combined with national, state and local economic conditions – dwindling funding levels combined with high infrastructure needs – have elevated the importance of owners needing to insure that they are using a product with superior engineering properties. The asphalt paving industry through TXAPA's partnership with the Texas Department of Transportation (TxDOT) has stepped up to meet these demands by adding tools to a product that already has the best engineering properties available to owners. Specifications have been developed or are under development that will help asphalt pavements become stronger; more durable; achieve better in-place densities; improve ride quality; extend the paving season; reduce compactive effort; help with aggregate binder absorption, etc. All of this, along with other changes in TxDOT specifications, has helped to further improve the engineering properties of asphalt on Texas roadways.
The tools that have been added to these specifications are allowing the addition of recycled asphalt pavement (RAP), recycled asphalt shingles (RAS), substitute binders, and warm mix asphalt (WMA) to asphalt mixes. When used individually in asphalt mixes these additions can help asphalt pavements become stronger; more durable; achieve better in-place densities; improve ride quality; extend the paving season; reduce compactive effort; help with aggregate binder absorption, etc. When used together RAP, RAS, substitute binders, and WMA can help to produce a mixture that gains the above advantages while at the same time producing a mixture that is more affordable and more importantly more flexible.
RAP contains aggregate and binder while RAS contains binder. The asphalt binder present in RAP and RAS is highly oxidized. As a general rule of thumb the addition of 20% RAP or 5% RAS will have the same effect on asphalt pavement mixes as raising the binder grade by one binder grade. In other words it would raise the binder grade from a PG 64-22 to a PG 70-22. While this is in effect helps to strengthen the asphalt mixture, which is a good thing, it could create an asphalt mixture that is too stiff for the given usage. If you include 20% RAP and 5%RAS in the same mix you could effectively raise the binder grade by two binder grades (PG 64-22 to PG 76-22). The Hamburg Wheel Test (HWT) has confirmed this stiffening effect. Due to the above result it is often recommended that the binder grade be dropped by one or two grades (PG 76-22 to PG 70-22 or even PG 76-22) depending on the addition of RAP and/or RAS. The grade drop is often referred to as "grade dumping” and should be strongly considered when adding 20% RAP and/or 5% RAS. It is important to note that due to the above mentioned stiffening effect that TxDOT specifications limit the amount of RAP and RAS that can be used in various asphalt pavement mix types.
WMA is defined as additives or processes that allow a reduction in the temperature at which asphalt mixtures are produced and placed. The use of WMA is currently allowed on all TxDOT projects using dense graded mixes. By definition WMA is asphalt produced between 215 to 275 degrees Fahrenheit; however, the WMA additives can be used between 215 to 350 degrees Fahrenheit for other purposes such as a compaction aid; a placement aid during cooler weather; or to help facilitate long material hauls. WMA can be used in combination with RAP, RAS and binder substitution. The benefits to using WMA have been a more durable pavement, better in-place densities, improved ride quality and a wider paving window. WMA is a more durable pavement since the asphalt binder in the mix is less oxidized and the aggregate absorbs less of this same binder. Less aggregate absorption equates to more effective asphalt binder in the mix which helps produce better fatigue life in the mix. Better in-place densities help produce mixes that show lower permeability and an improved fatigue life. The ride quality is improved due to fewer bumps from thermal segregation and due to less swelling of rubberized crack seal on the existing roadway. WMA also helps extend the paving season into the winter and helps to facilitate night paving.
The addition of RAP, RAS, substitute binders, and WMA into TxDOT asphalt specifications should be considered by all owners (public and private). As seen above these tools offer excellent engineering benefits for asphalt paving mixes, as well as, offering excellent economic and environmental benefits.