HMA is a combination of approximately 95% aggregate bound together with approximately 5% asphalt binder, a product of crude oil. The asphalt cement is heated to approximately 340° F and blended with the stone in an asphalt plant. The hot-mix asphalt is loaded into trucks, hauled to the job, dumped into the asphalt paver, and laid onto the rock base or existing pavement in thin layers (1”-3”) or “lifts”. The asphalt cement gives the HMA the flexible nature to the pavement while the use of multiple lifts of asphalt increases the strength of the pavement. Hot Mix Asphalt (HMA) is the material of choice for a variety of projects that improve our daily lives and our environment. A few of the many benefits it can provide are: • Reduced traffic noise • Increased wet-pavement friction for improved safety • And it’s 100% recyclable, which saves our natural resources. The versatility of HMA goes far beyond pavements, driveways and parking lots. It is also being used for liners of fish rearing ponds and domestic water reservoirs, for recreational paths and for industrial sites. It is the pavement of choice for an increasing variety of recreational and environmental purposes such as golf cart paths, sludge-drying beds, silage pits, work platforms at agricultural sites, and as revetments protecting roadway slopes and populated areas against flood damage. HMA has proved to be a versatile, environmentally friendly construction material. For additional information regarding the benefits of HMA, visit the Why Asphalt section.
Warm-mix asphalt technologies allow the producers of asphalt pavement material to lower the temperatures at which the material is mixed and placed on the road. Reductions of 50° to 100° Fahrenheit have been documented. Such drastic reductions have the obvious benefits of cutting fuel consumption and decreasing the production of greenhouse gases. In addition, engineering benefits include better compaction on the road, the ability to haul paving mix for longer distances, and extending the paving season by being able to pave at lower temperatures. For more information about WMA.
The asphalt binder makes up about 5 percent of the typical asphalt pavement. Also referred to as “liquid asphalt,” “asphalt cement,” or “bitumen,” asphalt binder is the glue that holds aggregates together in a pavement. Liquid asphalt forms naturally, such as at Pitch Lake in Trinidad, but most of the asphalt binder used is derived during the refining process that converts crude oil to fuel. In some cases, asphalt binders are modified through the addition of polymers, ground tire rubber, or other materials to adjust physical and chemical properties of the asphalt binder and altering how it acts within a pavement. In the U.S., performance grading is used to characterize asphalt binders under the Superpave design method. They are usually identified by the high and low surface temperatures, in Celsius, the pavement will experience. For example a PG 64−22 binder would be expected to yield good performance at surface temperatures from −22°C to 64°C (−8°F to 147°F).
Aggregates are hard, inert materials that typically make up about 95 percent of an asphalt pavement. Aggregates can be rocks or gravel of various, controlled sizes, as well as sand and dust. A mix of different size and types of aggregates is used to achieve desired pavement characteristics. Some pavements use reclaimed asphalt pavement (RAP) or other recycled materials or byproducts, such as slag, fly ash, and even glass, for a portion of the aggregate. Most often aggregates are sourced locally, sometimes at a quarry that is co-located with the asphalt mix production facility, but in some cases desired chemical or physical properties are not inherent in local aggregates and they have to be sourced from further away.
Recycled Asphalt Pavement (RAP) and Fractionated Recycled Asphalt Pavement (FRAP) are millings from asphalt pavements that have been processed for use in new pavements. Both products are controlled according to Illinois Department of Transportation’s specifications. FRAP is separated into 2 or more fractions based on particle size. The fractionated material provides the contractor with more control over their production.
Recycled Asphalt Shingles are either manufacturer waste (Type I) or post-consumer waste (Type II) shingles that have a Beneficial Use Determination (BUD) from the Illinois Environment Protection Agency. Both products are controlled according to Illinois Department of Transportation’s specifications.
Rubblization is a cost-effective means of rehabilitating deteriorated portland cement concrete (PCC) pavements. The concrete is broken into pieces, and then it is overlaid with asphalt pavement. It minimizes delays and allows for construction during off-peak hours. The rubblized roadbed is left in place, so that it does not have to be trucked off to a landfill. This not only saves landfill space, it eliminates many trips by trucks, saving diesel fuel and reducing traffic congestion. The new asphalt pavement will remain smooth, safe, and quiet for years.
Most asphalt pavements are dense-graded and are used effectively in all pavement layers and for all traffic conditions. Dense graded asphalt pavements are commonly designed using the Superpave design method, including performance-grading of the asphalt binder. Like all asphalt pavements, dense-graded mixes can provide a smooth, high performance surface that is easy to maintain.
While most pavements are impervious, open-graded asphalt pavements are designed specifically to allow water to drain through the pavement. These can be constructed as full-depth porous pavements, where water drains through the pavement to the soil; or they can be constructed as an open-graded friction course (OGFC), which helps move water to the side of a pavement, improving friction while reducing both road spray and noise. Full-depth porous pavements are an EPA best practice for stormwater management and they can help reduce pollutant concentrations. In fact, even OGFCs have been demonstrated to help filter possible pollutants from highway runoff.
Stone-matrix asphalt (SMA) is a gap-graded asphalt pavement designed to improve rut resistance and durability through the use of a stable stone-on-stone skeleton held together by a rich mixture of asphalt cement, along with stabilizing agents such as fibers and/or asphalt modifiers. SMA is primarily used to pave high-volume U.S. interstates and highways, achieving high levels of rutting resistance and durability. In addition to improved durability and rutting resistance, SMAs have very good friction characteristics. They have been shown to be effective in reducing road spray and traffic noise. SMAs have also been successfully used on high-volume urban roadways with heavy bus and truck traffic.
Superpave (SUperior PERforming asphalt PAVEment) is a comprehensive system for the design of paving mixes that are tailored to the unique performance requirements dictated by the traffic, environment (climate), and structural section at a pavement site. It enhances pavement performance through the selection and combination of the most suitable asphalt binder and aggregate. From October 1987 through March 1993, the Strategic Highway Research Program (SHRP) conducted a $50 million research effort to develop new ways to specify, test, and design asphalt materials. Superpave represents the integration of several products of the SHRP asphalt research program into a single system for the design and analysis of paving mixes. It encompasses new material specifications, test methods, equipment, software, and mixture design method. Superpave was devised to replace the diverse material specifications and mixture design methods used by all states with a single system that can provide results tailored to the distinct environmental and traffic conditions found at any given location in the United States. Superpave was developed to address and minimize permanent deformation, fatigue cracking, low temperature cracking, and it considers how the effects of aging and moisture damage contribute to the development of these three distresses.