Asphalt Frequently Asked Questions

What are the ASTM standards for Asphalt roads and paving?

STP 1433 Constructing Smooth Hot Mix Asphalt (HMA) Pavements
C 88 Standard Test Method for Soundness of Aggregates by Use of Sodium Sulfate or Magnesium Sulfate
D 1856 Standard Test Method for Recovery of Asphalt From Solution by Abson Method
D 2872 Standard Test Method for Effect of Heat and Air on a Moving Film of Asphalt (Rolling Thin-Film Oven Test)
D 6521 Standard Practice for Accelerated Aging of Asphalt Binder Using a Pressurized Aging Vessel (PAV)
D 6648 Test Method for Determining the Flexural Creep Stiffness of Asphalt Binder Using the Bending Beam Rheometer
D 6723 Test Method for Determining the Fracture Properties of Asphalt Binder in Direct Tension
D 6816 Practice for Determining Low-Temperature Performance Grade of Asphalt Binders.

What is hot-mix asphalt?

Asphalt pavement refers to any paved road surfaced with asphalt. Hot mix asphalt is a combination of approximately 95% stone, sand, or gravel bound together by asphalt cement, a product of crude oil.

What is a prime coat?

An application of a low viscosity to a granular base in preparation for an asphalt surface course.

What is a minimum temperature for asphalt mixes?

Mixes must be placed and compacted before they cool to 185 degrees F., so the minimum temperature will depend on the temperature of the layer upon which it is being placed as well as ambient conditions.

Why is a tack coat needed?

To assure a bond between the succeeding layers of pavement.

When is a tack coat necessary?

Almost always! On rare occasions when a pavement is being constructed which is not being used by traveling public and each succeeding lift is placed only a short time later, a tack coat may not be necessary.

What causes potholes?

This can be caused by a number of factors including, thin or weak pavement, poor drainage, and improper or lack of maintenance.

How does base failure occur?

This is usually caused by a consolidation of sub grade soils with the stone base. Other contributing factors are an overload of traffic in the area or poor drainage.

Why does asphalt bleed?

Normally caused by excess asphalt oil in the mix, low air voids, or excess prime or tack.

Why do depressions form?

A consolidation of sub grade soils combined with poor construction design are reasons. Also, poor drainage and excessive dense force over a small area (psi).

What causes edge cracks?

Usually this is from a lack of lateral support, i.e. there is an insufficient stable base adjacent to the edge. Additionally, there may be settlement of underlying material and a shrinkage of drying out soil.

What are the benefits of asphalt pavement?

- Asphalt pavement is economical as well as extremely durable.
- Asphalt is 100% recyclable.
- It is the material of choice on
U.S. roads with over 94% surfaced with asphalt (2.1 Million miles)
- Asphalt provides the best rate of investment return of any paving material.
- Asphalt retains heat more efficiently so that ice doesn't form as quickly and melts faster.
- Asphalt is impervious to de-icing salts and chemicals.

Are asphalt pavements safe?

Yes, as well as being an environmentally sound product, asphalt is basically an inert material, even in a water environment.

How thick should my pavement be?

This would depend on the amount and weight of the traffic that the pavement would be expected to bear. Typically, residential driveways are 2.5" to 3" thick and commercial parking lots are 3" to 4" thick.

How thick should the stone base be under my asphalt?

This answer is similar to the above. You would normally have thicker crushed aggregate base courses (CABC) under thicker asphalts. A normal combination would be 6" to 8" thick CABC under 3" to 4" pavements, although it is not abnormal to find 10" to 12" stone base under trucking terminals and the like.

What determines if I can resurface my existing pavement?

The condition of the asphalt mat as well as the condition of the stone base course will be primary factors.

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)))

where:

d is the specific gravity at 60F/60F

t is the temperature in F

K is the thermal conductivity (BTU-in)/(hour-ft2-F)

 

What is the Vapor Pressure of asphalt at typical storage temperatures?

 

It is estimated that at a typical inventory temperature of 325F, the vapor pressure of petroleum asphalt is less than 0.01 psia (1.5e-3 kPa).

 

Is there a problem with milling up and recycling asphalt mixes that used polymer modified binders?

 

Generally speaking, there should be no unique problems with using polymer modified mixes as RAP. There have been some individuals express environmental concerns about running millings which contain 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 mix temperature?

 

Mix temperature will be dependent on the grade of asphalt used in the mix. The less viscous the asphalt, the lower the temperatures should be. The more viscous the asphalt, the higher the temperature can be.

During mix design temperatures are specified for proper mixing and for compaction.

What is the typical thermal B.T.U. value for a pound of asphalt?

 

BTU varies by temperature and per cent mineral matter in the asphalt. Therefore, a range is usually quoted, but we have used approximately 158,500 BTU/gal. This value is an average for an AC-10 grade.

 

What is a typical value for the Specific Heat of Asphalt Cement?

 

0.22 calorie / gram - deg. C

 

What is an asphalt emulsion?

 

Asphalt emulsion is a combination of three basic ingredients, asphalt, water, and small amount of an emulsifying agent. The emulsifier, which is a surface-active agent, keeps the asphalt droplets in a stable suspension and controls the breaking time.

 

Why use asphalt emulsions?

 

Asphalt emulsion does not require a petroleum solvent to make it liquid and in most cases asphalt emulsions can be used without additional heat. Asphalt emulsions are environmentally friendly.

 

What are the primary reasons for testing and examining an asphalt emulsion?

 

There are three specific purposes for testing asphalt emulsions. They are as follows:

  • To provide data for specification requirements
  • To control the quality and uniformity of the product during manufacturing and use.
  • To predict and control the handling, storage, and field performance properties of the material.

 

Where can the specifications and test procedures for testing asphalt emulsions be found.

 

Two primary sources exist for finding specifications and test procedures for asphalt emulsions. The American Society For Testing and Materials (ASTM) and The American Association of State Highway and Transportation Officials (AASHTO) are generally the most widely accepted sources for determining specifications and test procedures. However, within many state highway organizations additional specification and test procedures may exist.

 

In what way can one asphalt emulsion be distinguished from another?

 

The particle charge test is used to identify the charge associated with a particular asphalt emulsion. Immersing a positive electrode and a negative electrode into an emulsion sample performs the test. The electrodes are then connected to a controlled direct-current electrical source. At the end of the test period, the electrodes are observed. If there is an appreciable layer of asphalt determined on the negative electrode then the emulsion is cationic.

 

Is there a test to determine the viscosity of a particular emulsion?

 

Viscosity is defined as a fluid's resistance to flow. For asphalt emulsions the Saybol Furol viscosity test is used as a measure of viscosity. Depending on the type of emulsion, one of two testing temperatures are used, 25oC and 50oC (77oF and 122oF).

 

Are there any tests used to determine the ability of an asphalt emulsion to coat an aggregate?

 

Yes, the field coating test and water resistance test are used to determine the ability of the asphalt emulsion to coat the job aggregate, the ability of the emulsion to withstand mixing, and the ability to resist the washing action of water after the completion of mixing. This test is primarily used to identify medium-setting asphalt emulsions suitable for mixing with coarse-graded aggregates.

 

How can it be determined exactly how much asphalt is present in a sample of asphalt emulsion?

 

A distillation is performed under controlled conditions to extract the light oil, and water from a sample of asphalt emulsion. What is leftover is the asphalt residue from the asphalt emulsion. This residue is expressed as a percentage by weight of the total sample. Additional tests may be run on the residue to determine the physical properties of the end-use asphalt.

 

What types of tests exist to determine the properties of the asphalt residue after it has been distilled?

 

There are many different types of tests that can be performed on asphalt emulsion residue. They measure the ductility, solubility, hardness, elasticity, and resistance to flow at elevated temperatures. AASHTO and ASTM have developed several tests to determine these properties.