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Asphalt Pavement Thickness and Mix Design

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.

For further information see the Thickness & Mix Design Documents, other engineering areas, Asphalt magazine and APA websites, and the links page for other information related to these topic area.

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

For an asphalt pavement in a container terminal, is there any rules of thumb as to what the maximum load could be without causing damage?
No rule of thumb answers to your question, but two issues should be considered:

  • 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.
Is there a problem with milling up and recycling asphalt mixes that used polymer modified binders?
Generally speaking, there are no unique problems with using polymer modified mixes as RAP. Some individuals have expressed environmental concerns about running millings containing ground tire rubber (GTR) through a drum plant. Florida uses a small percentage of GTR on most of their highway surface mixes. California and Arizona also use GTR frequently.
What is the proper nominal aggregate size to use?
Nominal aggregate size dictates lift thickness. Minimum lift thickness should be at least 3 times the nominal maximum aggregate size to ensure aggregate can align themselves during compaction to achieve required density and also to ensure mix is impermeable. Therefore, the desired lift thickness can direct the decision on nominal aggregate size to use.

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.

What is the recommended air void content for compaction of asphalt pavements?
Efforts should be made to control compacted air voids between 7% and 3%. Once voids reach 8% or higher, you get interconnected voids which allow air and moisture to permeate the pavement which reduces the durability of the pavement. On the other hand, if air voids fall below 3%, there will be inadequate room for expansion of the asphalt binder in hot weather and when the void content drops to 2% or less, the mix becomes plastic and unstable.
How is air void content controlled?
Air voids is a reverse proportion of the density of the compacted mix. By specifying a density requirement, the voids are inversely controlled. Keep in mind that density is a relative term, compared to a target density of either lab compacted mix, a maximum theoretical density, or a control strip density. Procedures for using the three methods are spelled out on Page 7-17 to 7-21 of the new MS-22 and Page 241 of the old MS-22.
What is the process or how is Density Target Value established or determined?
There are several ways to establish density targets. Some of the more common approaches include:

  • 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.

Is there a limit on the percentage of RAP utilized in new installations.What about RAP use for resurfacing of old asphalt roads? Any limits?If there are limits on the use of RAP in new or resurfacing installations, who sets the limits?
The Asphalt Institute strongly endorses the use of RAP in asphalt mixtures. RAP has a history of positive performance.Regarding limiting the RAP content, that is the decision of the specifying agency or owner. Almost all of the state highway departments now allow the use of RAP. A few restrict its use in wearing courses; even fewer (one or two) do not allow its use at all. Most agencies have developed a means of accomodating the stiffness of the reclaimed asphalt from the RAP by the selection of the particular grade of the virgin binder.The FHWA Asphalt Mixture Expert Task Group developed recommendations that are being considered by the Association of State Highway and Transportation Officials (AASHTO) to provide guidance in asphalt binder grade selection when using RAP. These recommendations are summarized below.

  • 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.

What is the proper mix temperature?
Mix temperature is dependent on the grade of asphalt used in the mix. Less viscous the asphalt requires lower the temperatures, while more viscous asphalt requires higher temperatures.At the start of a mix design project, target temperatures are specified for proper mixing and compaction. These temperatures should be adjusted for project conditions (weather, haul distances, etc.). Whenever possible, variances of more than 25 degrees from the mix temperature should be avoided.Note: When working with modified binder, the binder supplier should provide mix temperature recommendations.
How do the lab-compacted air voids of reheated asphalt mixture samples compare to the air voids of original mixture samples, as-produced, not reheated?
There is not a predictable value or rule-of-thumb number for the difference in air void content of original and reheated samples. The general trend would be for the reheated samples to have higher air voids than the original, compacted specimens. Absorption and hardening or stiffening of the asphalt binder in the reheated samples likely causes this difference.

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.

Liquid Asphalt

How do you determine the thermal conductivity of asphalt cement?
The equation we have used for thermal conductivity is: K = (0.813/d)*(1-(0.0003*(t-32)))

  • 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).
What is the vapor pressure of asphalt at typical storage temperatures?
It is estimated that at a typical inventory temperature of 325°F, the vapor pressure of petroleum asphalt is less than 0.01 psia (1.5e-3 kPa).
What is the typical thermal BTU value for a pound of asphalt?
BTU varies by temperature and per cent mineral matter in the asphalt. A range is usually quoted, but we have used approximately 158,500 BTU/gal. This value is an average for an AC-10 grade. Most refineries will have a calorimeter somewhere in the lab that is used to run this test. Each company should run its own test on the specific product that is being used.
What is a typical value for the specific heat of asphalt cement?
The conventional method for determining specific heat for asphalt is listed as follows: c = (0.388 + 0.00045*T)/(d 0.5 )

  • 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).
How can I learn more about polyphosphoric acid modification of asphalt?
Two excellent resources:

  1. Our publication IS-220, Polyphosphoric Acid Modification of Asphalt. You can order off our website.
  2. 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.

Special Applications

Is there a way to color an asphalt pavement other than shades of black and grey?
While not widely used, there are ways to color an asphalt pavement other than the common blacks and greys. The second and third options below are considered specialty products and more information can be obtained by contacting individual manufacturers.

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.


Does AI have any information about asphalt and its use in railroad beds?
Railroad information can be found in the Construction, Thickness and Design, Maintenance and Rehabilitation, and Pavement Performance Documents pages.

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.

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