If you’re reading this blog, you probably know that asphalt is the glue that holds roadways together. But how do we produce asphalt, and how do we make sure if holds up to our quality standards? In the most recent RoadReady newsletter, we explored how asphalt grades are used to describe different binders. In this edition of the blog, we’ll explore how asphalt is produced to meet specifications.
Asphalt is one of many constituents found in naturally occurring petroleum. Crude oils contain different concentrations of each of these constituents, and as a result oils exist that are composed almost entirely of asphalt. In the early days of asphalt road paving, Trinidad Lake, a naturally occurring asphalt seepage, provided a huge majority of the asphalt consumed worldwide. This lake contained primarily asphalt, and was the first source of asphalt for the U.S.
Components of Petroleum
Today, asphalt is usually obtained from the refinement of petroleum. Petroleum can be separated into paraffin, gasoline, naptha, kerosene, and diesel as well as asphalt. Separation of these constituents is performed by distilling the crude oil. This essentially means that the oil is heated and volatile compounds are allowed to evaporate at different rates. Asphalt is the heaviest constituent of crude oil and will be left as a residue following the distillation process. Modern sources of petroleum are characterized by their American Petroleum Institute (API) gravity value, a dimensionless indicator of density. Asphalt has a low API gravity relative to unrefined petroleum, and this fact can be used to estimate the asphalt content of petroleum. Oil with a high API gravity will contain a low percentage of asphalt, with oil with a low API will contain a high percentage of asphalt.
Distillation can separate the different components of crude oil
Apart from total asphalt content, the characteristics of the crude oil will impact the performance of the final asphalt binder. Asphalt is composed of three primary components, including:
- Asphaltenes – Heavy compounds that provide structure, strength, and stiffness to the asphalt
- Heavy Oils – Non-polar molecules that impart fluidity and viscosity to the asphalt
- Resins - semi-solid to solid compounds that cause ductility and malleability in the asphalt.
The relative proportion of each of these materials in the asphalt is dependent on the makeup of the crude oil, and will have important effects on the physical characteristics of the binder. For instance, the higher the asphaltene content, the lower the penetration value and higher the softening point, as asphaltenes form much of the structure of the asphalt. Asphalts with high resin contents will typically be very pliable. Each of these properties is important when determining the grade of an asphalt, and determining whether it is suitable for a given roadway project. Asphalts produced with a specific grade in mind are commonly created through blending of different asphalts, and understanding the effects of chemical composition on physical properties is important when selecting which asphalts to combine. Other characteristics of the crude oil can predict its suitability for paving asphalt, such as the initial concentration of wax and sulfur.
Polymer additives can reduce cracking in cold weather
When the desired properties are not provided by the asphalt residue, additives are used to bring the asphalt to grade. This is typically done through the addition of polymers, a class of compounds made up of small, repeating chemical units. When used in pavements, the addition of polymers is usually done to address thermal cracking at low temperatures or rutting at high temperatures. These polymers can be divided into plastomers and elastomers. The former creates a rigid lattice structure while the latter improves the elasticity of the binder.
The sophistication of asphalt grading has required the production of higher quality asphalt. Fortunately, the demand for ever tightening specifications has been accompanied by a greater understanding of refinement, crude oil, and additives. This combination of knowledge and expectations has led to a generation of better asphalt binders in our roadways.
For more information on asphalt binder production and grading, visit the following links:
Superpave Performance Grading
Asphalt Production and Oil Refining