The A, B, C's, of the Asphalt Pavement Criss- Crossing Our Country!

The National Interstate and Defense Highways Act of 1956, or ACT, established the interstate highway system throughout the United States and remains the largest public work project in the history of our country.  Asphalt, a/k/a blacktop was the predominant material used to surface the highways constructed under the ACT throughout the United States.  Of the 2.75 million miles of paved roads and highways in the United States, approximately 95% are surfaced with asphalt. 

Asphalt  pavement is manufactured or “batched” through different mixtures of aggregates and bitumen. Aggregates include crushed stone, gravel, sand, and slag created as a by-product of smelting iron.  Bitumen is a sticky, viscous, waterproof substance produced as a by-product during the distillation of crude oil when refining petroleum.  When different mixtures of aggregates and bitumen are batched together, they create asphalts having different strengths, durability, elasticity, resilience, and water-proofing.  Like concrete, any asphalt mixture should be designed and batched to best serve its use, strength, location, climate, and application. 

There are 3 principal methods used to batch asphalt.  Hot mix asphalt or HMA is the most commonly used batching method.  During the batching of HMA, both the aggregate and bitumen is heated up to 300-350 degrees Farenheit during the mixing process.  A high percentage of larger coarse-stone creates a stronger, more porous asphalt, with a void ratio in excess of 15%.  Whereas, relative percentages of course-graded stones and and fine-graded sands create an impermeable, smoother surface with a void ratio as low as 2%.  This later from of HMA asphalt is the most commonly used and used in road construction throughout the United States.  All HMA asphalts must be applied at a high temperature, spread, placed, compacted, and allowed to cool in a fixed and final location. Thus, HMA must be batched close to where it will be used or transported in heated trucks to its final destination.

Warm mix asphalt or WMA also heats the aggregate and bitumen during the batching process, but to far lesser temperatures in the range of 180-270 degrees Fahrenheit.  The WMA batching process includes water or chemical additives that reduce temperatures and consume less energy.  However, WMA's must also be placed at a warm temperature, spread, compacted, and allowed to cool in a fixed and final location.  But given its lower temperatures, WMA's are  more cost effective and environmentally friendly than HMA's  and allow longer travel times between the batching plant and final destination.  WMA's are growing in popularity due to their durability and versatility in a variety of uses and applications.  Still, WMA's use a relatively new batching process and there is a limited history and record of the durability of asphalts manufactured using this process. 

Finally, cold mix asphalt or CMA is batched without using any heat at all.  As a result CMA has very low strengths, and its application should be limited the the temporary repair of asphalt pavements or patching “pot-holes” in low-traffic areas.  CMA repairs should be replaced by HMA or WMA asphalts as soon as possible in order to avoid further degrading of the permanent asphalt pavement.  Unfortunately, we have all see situations where “pot-holes” continually return when they are repeatedly repaired using CMA asphalts. 

The thickness of asphalt pavements, is initially dependent upon the type of road and volume of traffic on the road. However, the asphalt mix design and thickness are also dependent upon soil conditions and availability of raw materials, such as sand and gravel.  On the average, asphalt pavement is placed in multiple layers or courses with a total thickness varying between 3"and 8".  The bottom courses of asphalt are generally thickest, ranging between 2" and 6" , have greater strength, and are batched using coarse aggregate resulting in a strong but porous mix.  The top courses of asphalt are thinner, ranging between 1" and 3", are non-porous producing a water tight surface, and smooth wearing surface.  Nowadays, it is common place to resurface existing asphalt pavements, by using a planning machine to remove the top course and then place and compact a new wearing surface without replacing the entire asphalt pavement.

Most folks are not curious about the paved roads over which they travel on a daily basis.  When you see a roller passing over freshly placed asphalt, you will understand that the mix must be places, spread and compacted in place in order to achieve strength and durability.  And next time to pull up to a stop light and your car traverses over rippled pavement, you will know the wrong asphalt mix design was used.  When your car falls into that same pothole that has been there for years, you will know why the repairs never last.  Understanding the limitations upon asphalt  pavements may just help you demand a proper repair from your local governments.  In the end, such knowledge could save you thousands in car repairs.