Asphalt Frequently Asked Questions
What are the ASTM
standards for Asphalt roads and paving?
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
- 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 325°F, 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.
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:
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.