We drive on asphalt roadways most every day. After all ninety-four percent of U.S. roadways are surfaced with asphalt. You may have even seen asphalt being laid on a roadway. Truckloads of hot, black asphalt mix are sent to the site to be laid down and compacted. But what is asphalt? What are the parts in that mix that produce a smooth, safe roadway surface to drive on? Let's take a look. A typical asphalt mix is made up of approximately ninety-five percent aggregates, meaning stone sand and reclaimed products. The type of stone can vary across the country depending on what type of stone is locally available to producers. The other five percent of the mix is the liquid asphalt glue. Asphalt is refined from the heavy components of crude oil and does not come from tar, which is a coa derivative. In the industry it is known as binder since it binds the mix together. Here are some examples of different aggregate sizes that get used in asphalt mixes. These vary in diameter from one-and-a-half inches all the way down to a fine-grain sand and dust sized particles. Asphalt roads are designed for many different purposes. A small two-lane rural road has a different "recipe" than a major interstate highway - so these aggregates are used in different quantities to meet the specifications for the desired roadway. A stiffer, less flexible payment can be achieved by using larger aggregates and fewer fines, which creates more stone to stone contact. Using more fine aggregates allows for a more flexible pavement because individual stones are surrounded with more binder. This is a core drilled from a roadway that shows a cross-section of the asphalt within. You can see a course mix of asphalt in the lower section or base course and a finer mix design for the surface layer. This fine mix is less permeable and keeps water from penetrating down and causing cracks to form. In many states, specifications allow for the use of reclaimed asphalt pavement, or RAP, and recycled asphalt roofing shingles (RAS) in the permitted asphalt mixes that get placed on roadways. One hundred percent of asphalt millings can be reused in new surfaces. This results in many thousands of tons of recycled asphalt being used in the United States annually. It's actually the most recycled product in the country (U.S.A.). That brings us to the liquid asphalt binder itself. It only makes up five percent of the mix but it is the glue that holds everything together. It is designed to be a strong yet somewhat elastic adhesive. Polymers and other additives may be used to change its properties so that it behaves differently depending on roadway conditions. These ingredients are heated and mixed together to be placed on the roadway and roller compacted. Once cooled the new asphalt road surface is ready to ride on in just minutes!
