While there are an infinite number of questions that can be asked, we compiled a list of those questions that have been directed to us the most. These FAQs are categorized into subject areas listed in the contents dropdown box below.
We tried to keep both the questions and answers concise. Additional information is referenced where applicable for those seeking more in-depth information on a given subject.
We also recommend that you attend our Asphalt Academy courses at sites throughout the country to obtain expert instruction on asphalt topics.
Mix and Thickness Design
- Is the pavement structure (subgrade, subbase, base, and all asphalt layers) adequate to support the loads? You need to purchase our MS-23 Manual, Thickness Design of Asphalt Pavements for Heavy Wheel Loads.
- Is the hot mix asphalt surface stiff enough to resist deformation (ruts or indentations)? This is dependent on many factors, such as stiffness of the original mixture, age of the mix (gets stiffer over time), temperature of the mix during loading, loading itself, duration of applied load, etc. While not usually a problem, when it occurs it can typically be resolved by placing some steel (or other rigid material) plates below the point load to distribute the load across a wider area.
The maximum lift thickness is dependent also upon the type of compaction equipment that is being used. When static steel-wheeled rollers are used, the maximum lift thickness that can be properly compacted is 3 inches. When pneumatic or vibratory rollers are used, the maximum thickness of lift that can be compacted is almost unlimited. Generally, lift thicknesses are limited to 6 or 8 inches. Proper placement becomes a problem in lifts thicker than 6 or 8 inches.
For open-graded mixes, compaction is not an issue since it is intended that these types of mixes remain very open. Therefore, the maximum size aggregate can be as much as 80 percent of the lift thickness.
- Specifying a percentage of the unit weight from the laboratory mix design. Example: 96% of the Marshall unit weight
- Establishing a value based on results achieved on a project-site test strip. Example: 98% of test strip density.
- Specifying a percentage of the maximum unit weight. Example: 93% of the maximum unit weight.
Specifying some minimum percent of the maximum unit weight has gained acceptance with many specifying agencies. The maximum unit weight is also sometimes called the “solid density”. This value is based on the asphalt mixture’s maximum specific gravity – also known as the Rice value or G mm in Superpave. The maximum unit weight is determined by multiplying the Rice value by 62.4 pounds per cubic foot (PCF). For example, 2.500 is a typical Rice value. 2.500 X 62.4 = 156.0 PCF. Then, if 95% compaction is specified, the minimum acceptable unit weight is: 0.95 X 156.0 = 148.2 PCF. If 93% of solid is specified, or a maximum of 7% air voids are allowed in the compacted mat, then the minimum target value would be 145.1 PCF (0.93 X 156.0).
The thickness of the course being compacted does influence its compactability. Too thin a mat does not have sufficient workability, and too thick a mat may be unstable. In order to be compacted, the mixture must have controlled workability. Typically, for dense-graded mixes, a lift thickness of 3 to 4 times the nominal maximum size (NMS) of the aggregate is needed. For example, a mix containing ½-inch NMS stone should be placed at a compacted depth of at least 1-½ to 2 inches. If a ½ -inch top-size mix is placed at 1 inch compacted depth, the mat may pull and tear and the stones may be broken by the rollers. Thus, the “depth of paving” does influence the ability to obtain proper compaction. The target value for compaction, based on a materials property – the maximum specific gravity, does not change but the likelihood of meeting the target density is changed.
In some cases, after exhausting all reasonable efforts to achieve the desired density, the project engineer may establish a new target value based on attainable values achieved on that project. This reduced density should only be allowed after all rolling pattern and other adjustments have been unsuccessful. Performance of asphalt mixtures is directly related to density.
- When 15% or less RAP is used: “The binder grade for the mixture is selected for the environment and traffic conditions the same as for a virgin mix. No grade adjustment is made to compensate for the stiffness of the asphalt in the RAP”.
- When 16 to 25% RAP is used: “The selected binder grade for the new asphalt is one grade lower for both the high and low temperature stiffness than the binder grade required for a virgin asphalt. For example, if the specified binder grade for the virgin mix is a PG 64-22, the required grade for the recycled mix would be a PG 58-28”.
- When more than 25% RAP is used: “The binder grade for the new asphalt binder is selected using an appropriate blending chart for high and low temperature. The low temperature grade is one grade lower than the binder grade required for a virgin asphalt”.
Normally, the above guidelines would be applied to both new and existing pavements. If a warranty was applied to a project, a more conservative approach – such as the use of blending charts – might be taken.
The current trend is toward allowing an increase in the amount of RAP in a mix, however, it is suggested that you contact the local state highway agency and/or asphalt binder supplier for the prevailing local practices.
Reheated samples can be utilized to give an overall check of the original sample results. Before any significant precision is attributed to reheated sample results, a correlation should be developed for reheated sample air voids and original sample air voids by performing a series of comparative tests.
- d is the specific gravity at 60F/60F
- t is the temperature in F
- K is the thermal conductivity (BTU-in)/(hour-ft2-F)This information comes from page 870 of Asphalts and Allied Substances, 4th Edition , by Herbert Abraham (published in 1938).
- c = specific heat in BTU per pound per °F or calories per gram per °C
- d = specific gravity of the asphalt at 60/60°F
- T = temperature, F A typical value for specific heat for a paving grade asphalt binder at 300°F is 0.515. This assumes a specific gravity of 1.030.This information comes from page 870 of Asphalts and Allied Substances, 4th Edition , by Herbert Abraham (published in 1938).
- Our publication IS-220, Polyphosphoric Acid Modification of Asphalt. You can order off our website.
- A workshop on Polyphosphoric Acid Modification of Asphalt Binders was conducted in Minneapolis, MN on April 7-8, 2009. This workshop was jointly sponsored by the Transportation Research Board (TRB), Federal Highway Administration, Minnesota Department of Transportation, TERRA, Association of Modified Asphalt Producers, Innophos, ICL Performance Products, and the Asphalt Institute. All the speaker presentations (both slides and videos) are now available for public viewing at the following link. https://engineering.purdue.edu/NCSC/PPA%20Workshop/2009/index.html
Use a naturally colored aggregate. As the asphalt binder wears way from the surface with traffic, the color of the aggregate is exposed.
Use an additive in the asphalt binder. Various iron compounds can impart a red, green, yellow or orange tint to a pavement, while other colors can be achieved using different metal additives. A special “synthetic” binder that contains no asphaltenes has been used because it takes color more readily. This method of tinting the mix allows color to permeate the entire depth of the material, so there are no surface wear-off concerns.
Coat the surface with a material that penetrates the voids and bonds well to asphalt pavement, such as an epoxy-fortified acrylic emulsion. Many colors are available. Care should be taken to ensure that surface friction is not compromised, especially if the pavement is used for vehicular traffic. One possible disadvantage of this method is that the surface may wear off with time and need to be renewed.
You can also visit a web page on the University of Kentucky website where you can download papers, PowerPoints and also the computer program called KENTRACK, which is computer program for hot mix asphalt and conventional ballast railway trackbeds.
Additional AI Resources
- Mix Design Technology Certification
- Optimizing Volumetrics and HMA Compactability
- An Introduction to the Bailey Method (Optimizing Volumetrics and HMA Compactability)
- Asphalt Mix Design Basics
- Asphalt Mix Design Using RAP & RAS
- Interpreting Asphalt Test Results
- Laboratory Performance Testing of Asphalt Mixtures
- MS-1 Thickness Design-Highways & Streets
- MS-2 Mix Design Methods for Asphalt Concrete and Other Hot-Mix Types
- MS-4 The Asphalt Handbook
- MS-17 Asphalt Overlays for Highway and Street Rehabilitation
- MS-22 Construction of Hot Mix Asphalt Pavements
- MS-22S Principios de Construccion de Pavimentos de Mezcla Asfaltica en Caliente
- MS-23 Thickness Design – Asphalt Pavements for Heavy Wheel Loads