262.
pdf
A SunCam online continuing education course
An Introduction to Asphalt Pavement
Construction
by
John E. Kristensen PE., PLS., PMP.
262.pdf
An Introduction to Asphalt Pavement Construction
A SunCam online continuing education course
Getting Started
The material and work associated with placing what will be termed as
"Asphalt Paving" goes by many other names. "Hot mix", "Bituminous
Asphalt", "Bituminous Concrete Pavement", or just plain "mix" are some of
the more common. Asphalt paving is classified as a flexible pavement for
design purposes.
There are also relatively new buzzwords in Asphalt pavement, mostly lead
by the word "Super pave". Super pave is the federal government's
attempt to improve and "nationalize" the performance of Asphalt
pavement. Costs have increased drastically in order to manufacture and
place this mix and the long term results are still being evaluated, but
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An Introduction to Asphalt Pavement Construction
A SunCam online continuing education course
generally, it is looked upon as beneficial and will be discussed further in
other parts of this course.
When we talk about Asphalt pavements, there are many different test
procedures used to determine acceptability, involving among other things,
measurement for air voids and compaction. The typical unit of
measurement for payment is the Ton or SY. These subjects are covered
further as the course proceeds.
Asphalt pavement sections are placed in multiple lifts. They are usually
classified by the purpose they serve, being surface, binder or base.
Super pave terminology has changed that some. It lists size of aggregate
in mm in the pavement description, and depending on that number, with
some owners, you distinguish the location of placement. As an example,
19 mm paving is commonly used as a surface mix.
Asphalt plants are a LOT harder to get permits for than concrete plants.
Things like Stack Tests and other technical testing will have to be
performed by independent testing labs. When planning to set up a
portable Hot Mix plant for a specific project, it is imperative to confirm with
local and state authorities that permits are available, and the approximate
time frame it will take to issue them. Some areas have even set a
moratorium on issuing any permits at all for new or temporary Asphalt
plants.
(asphalt drum plant) (asphalt batch plant)
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An Introduction to Asphalt Pavement Construction
A SunCam online continuing education course
Why do Owners choose Asphalt Pavements?
There are several reasons Owners choose Asphalt, the most obvious
answer is it is less expensive in the short term than the concrete
pavement alternative. But there is more to it than just cost.
A big consideration is traffic. An asphalt paved roadway can have traffic on it
within hours, as opposed to 14 to 28 days with concrete. This isn't as
much of a factor with new construction but it can be critical with
rehabilitation and reconstruction work.
Asphalt also provides for a smoother, quieter riding surface. Even with the
new technologies for profiling and micro-grinding new concrete pavements
(which has greatly improved concrete pavement rideability), the multiple,
thinner lifts of the asphalt placement process allow for corrections to be
made several times if needed, rather than at the top of a deep section as
is the case with concrete.
Another significant reason is the riding surface can be milled and replaced
at reasonable costs. This allows the road surface to be renewed more
frequently.
Types of Asphalt Paving
Asphalt pavements are distinguished and categorized one of three ways:
• By where they go and what they do (Surface, Binder, Base, Open
Graded, Patch, etc.)
• By the type of liquid asphalt that is used to make the mix (AC-20, PG-
64-22, etc.)
• By maximum nominal size aggregate (1", 19 mm, etc.)
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262.pdf
An Introduction to Asphalt Pavement Construction
A SunCam online continuing education course
Generally, you can tell if the mix is classified as a super pave or not by the
description. Any performance grade asphalt (PG) or any metric size
aggregate (19 mm) designations are usually super pave mixes.
Surface Mix — The riding layer of pavement, also referred to as the
“wearing course", usually contains aggregates no larger than nominal
3/4"stone. Surface mix in most states has a "friction factor rating" or similar
worded requirement that restricts course aggregate to a maximum LA
abrasion value of no more than 30 +/-. Many states now require the use of
a material transfer device to place the surface mix.
Binder Mix - Is an intermediate layer mix between surface and base.
Base Mix - Usually the first course of mix to be placed on a
graded/prepared subgrade or subbase material. Although total depth can
be 12" or more, usually lifts are restricted to no more than 6" for compaction
requirements for density.
Super Pave- (Short for Superior Performing Asphalt Pavement) A
performance based method of preparing Mix designs for HMA. The method
takes into account, among other things, the project's climate and design
traffic count. (ESAL-Equivalent Single Axle Loads)
Open-graded Mix— Has a dual role. First, as an intermediate layer mix
that consists primarily of gap-graded course aggregate and liquid asphalt.
Used as a drainage layer, typically under concrete pavement, this type mix
openly flows water away from the roadway to the shoulder or under drain
system provided. Second, as a surface mix, which provides excellent
drainage qualities, and is very skid resistance. Known as "Popcorn Mix" it
is very unstable and is effective for only 2-5 years. It almost always must
be replaced within 10 years.
FAA - A specialty mix used for airport runway paving. Consistency of
production and compaction densities are much harder to obtain than with
roadway mixes.
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An Introduction to Asphalt Pavement Construction
A SunCam online continuing education course
Cold Patch — a special mixture of emulsified asphalt and aggregates,
which is typically mixed in a central mix plant, but can be mixed in place. It
is typically used for wintertime repair of potholes.
Chip Seal Coat (single pass or multiple pass) - is simply liquid asphalt
applied to the roadway with a distributor truck, followed closely behind by
an application of 3/8" stone.
HMA — (Hot Mix Asphalt) a general term for all hot mix asphalt pavements
SMA (Stone Matrix Asphalt) - Is a premium-wearing course adapted from
European technology that uses a predominance of one-size course
aggregate, with little fine aggregate. Coupled with a high asphalt binder,
mineral filler content, and fibers to induce stabilization, the design creates
a mix that is rut resistant and durable. Some states have used this mix
on their major interstate projects since the early 90's with consistent
success. Other states have developed test sections, with some states
adapting the mix and others still evaluating.
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262.pdf
An Introduction to Asphalt Pavement Construction
A SunCam online continuing education course
(asphalt plant pictured above shows aggregate storage piles, silos,
recycled asphalt piles, plant, baghouse, drum, control house, QC lab trailer,
and truck haul route layout.)
Ingredients
Liquid Asphalt- as it applies to the paving industry, is a black
thermoplastic material consisting mostly of bitumen hydrocarbons. As it is
heated it becomes a liquid, enabling it to coat solid aggregate. As the
asphalt cools, it hardens and provides cohesion for the coated
aggregates. Since virtually all asphalt is produced at a refinery, different
grades of asphalt can have different properties. All asphalt is waterproof,
and resistant to many chemicals, including acids.
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An Introduction to Asphalt Pavement Construction
A SunCam online continuing education course
Cutbacks- a term used to describe a number of diluted forms of liquid
asphalt. The most common forms and the diluents used are listed:
Rapid Cure (RC) naphtha or gasoline high volatility
Medium Cure (MC) kerosene medium volatility
Slow Cure (SC) oils low volatility
Anti-Strip- A liquid additive that improves the adhesion of the binder
material to the aggregates. It also helps makes the mix resistance to
moisture damage. Anti-strip can be blended into the liquid asphalt at one
of several locations. It can be added as the liquid asphalt is being loaded
into the delivery truck, or as the material is being pumped into the storage
tanks. Most commonly it is added as an in-line blend as the liquid asphalt
is being introduced to the plant as the mix is produced.
Asphalt Emulsions- A combination of liquid asphalt, water and an
emulsifying agent. Since the two main ingredients don't mix well, the
globules of asphalt stay suspended in the water. Emulsions are either
anionic or cationic in nature.
RAP- (Recycled Asphalt Pavement) the asphalt and stone material
which is removed from an existing roadway by mill or other methods, to be
recycled into new mix to save money. The RAP needs to be crushed to
aggregate size before being introduced into the plant dryer. It is then
mixed with the other aggregates and coasted with liquid asphalt to produce
the new mix. The amount of RAP that can be used depends on type of
plant and type of mix being produced. Usually 15% -20% RAP is all that
can be incorporated into the new mix using a batch plant for surface
course. However, for base course in a drum dryer, up to 40% can be used
in most circumstances.
Aggregates — A hard, inert material such as sand, gravel, crushed stone
or slag, used as a fill material for pavement applications. For asphalt,
the # 8 sieve is the divide between coarse and fine aggregate, whereas in
Concrete the # 4 sieve is that dividing size.
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An Introduction to Asphalt Pavement Construction
A SunCam online continuing education course
Mix Design
With HMA there are four criteria that influence design. They are:
• Mix Density
• Air Voids
• Voids in the mineral Aggregate
• Asphalt Content
Mix Density. High-density mix in the finished pavement is considered
essential for a long lasting, well performing mix. Density is expressed in
pounds or kilograms per cubic foot or cubic meter, and is calculated by
multiplying the bulk specific gravity for the in-place mix by the density of
water. Densities are obtained by establishing certain rolling patterns that
maximize compactive effort.
(a nuclear density gauge (shown above) emits radioactive particles and measures the
number of particles that pass through a substance in a given period of time to determine
the density of the in place material)
Air voids in the in-place mix allow for thermal expansion during seasonal
temperature variations and to allow for slight compaction and movement of
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An Introduction to Asphalt Pavement Construction
A SunCam online continuing education course
the mix under traffic loading. An allowable range for these voids, between
coated particles, usually ranges between 3% to 5%. A mix that has an air
void content too low will "bleed", one with a high air void content provides
passage for potentially damaging air and water into the layer. Density and
air voids are directly related; the higher the density the lower the air void
content and vice versa. Lower air void contents provide for a more durable
mix.
Voids in the Mineral Aggregate also known as VMA is the void space
available between and in aggregate particles in a compacted mix. This
space is available to be filled with liquid asphalt, but also contains air voids.
VMA is expressed as a percentage of the total bulk volume of the
compacted mix, and is the sum of the voids filled with asphalt (VFA) and
the air void measurements. The more VMA in the dry aggregate, the more
opportunity for all aggregate to be coated by asphalt, hence a more
durable mix. If the density of the gradation of the compacted hot
mix provides for a below minimum VMA value, the mix will tend to be less
durable.
Optimum Asphalt Content is directly related to aggregate gradation. The
finer the mix, the larger the total aggregate surface is available to be
coated with asphalt, therefore increasing the need to increase the amount
of asphalt to be added to the mix design. Aggregate absorption must be
determined in the mix design procedure in order to optimize asphalt
content. Asphalt content is expressed two ways in HMA terminology: total
asphalt content and effective asphalt content. The first is the total amount
added to the mix, the second is all that is not absorbed by the aggregates.
That gives a good indication of the bonding value of the asphalt to the
aggregate.
Applications and Uses
HMA can be used in a variety of ways ranging from a 3/8" overlay of existing
pavements (either concrete or hot mix), to a level-up course, to a full depth
pavement section.
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An Introduction to Asphalt Pavement Construction
A SunCam online continuing education course
The single most important factor in the placement of asphalt pavements,
other than handwork, is to provide "uniformity in a continual operation". The
smoothness of the pavement suffers whenever the paver stops.
Therefore, the speed of the paver and plant production/delivery must be
coordinated to maximize production and minimize stops. This is not to say
that maintaining good rolling patterns and ensuring required and
consistent densities aren't also very important.
HMA can be placed using several grade control methods, depending on
the owner's specification. It can be placed in a constant lift, say 4", which
just follows the line and grade of the existing roadway. It can be placed
using a "ski" which is a 30' long aluminum bar that skids along the existing
pavement next to the paver. The ski takes the bumps out of the ride and
directs the screed to a smooth transition attainable over the 30' length. It
can be placed by setting the screed to a particular depth and cross slope,
or it can be laid by means automated grade control. GPS survey
receivers, robotic survey lasers, or string line control sensors from the
paver are examples of grade control automation.
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An Introduction to Asphalt Pavement Construction
A SunCam online continuing education course
Types of Joints
Most joints in asphalt pavement, whether longitudinal or transverse, are
butt to butt and are vertical. Some states allow a new "wedge joint", for a
longitudinal joint. It actually is a sloped 45° degree tapering of the joint to
be matched against the next lane. The overlap from the paving of the next
lane then completes the required pavement thickness. This approach,
allows for better compaction of the HMA at the joint.
Longitudinal joints of successive layers need to be staggered so that they
do not rest on top of each other (similar to the bonded layers of bricks in a
wall). Usually alternating each lift 6" each side of the previous layer will
effectively avoid problems. Additionally bonding agents are applied to the
joints to help seal and bond the fresh asphalt to the old or previously placed
joint material.
Joints need special attention to insure they are constructed to the same
density as the rest of the mat. They are a source of constant
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An Introduction to Asphalt Pavement Construction
A SunCam online continuing education course
maintenance as the roadway ages, and typically lead to the source of
pavement deterioration that necessitates the pavement to be replaced.
Method of Placement
Hot Mix Pavements
Temperature is very critical in the placement of asphalt pavements, much
more so than it is to the placement of concrete pavements.lthough
different mixes are manufactured and placed at different temperatures,
most are manufactured above 275 ° degrees F and must be placed
above 225° degrees F. This is due to the fact Hot Mix must be rolled
(compacted) in stages throughout its cooling period. If hot mix is not rolled
when it attains the proper temperature, the desired densities at completion
will not be achieved. As a guide, all HMA should be completely rolled
before it cools below 180 °degrees.
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An Introduction to Asphalt Pavement Construction
A SunCam online continuing education course
Rolling hot mix asphalt is an art and should never be overlooked in its overall
importance in providing a quality asphalt placement. Different owners have
different sequences of rollers and different roller patterns that they believe give
them the best results. Some owners use two rollers, some use three to
complete their rolling sequence.
Roller Operators must be skilled in reading the pavement, knowing when the
pavement is ready for the next rolling makes a difference in the ultimate density
obtained.
Most Owners require a "knockdown" rolling. This is usually one or two
passes by a double drum vibratory roller. The knock down pass is right
behind the paver, so the mix is still quite hot.
The second rolling is usually a pneumatic tire roller. Known for their many -
wheeled front and back axles, these rollers simulate tire pressure from
vehicles, and knit the pavement together.
Following the pneumatic tire roller is a static rolling. Again, a double drum
steel wheel roller, without any vibration, would perform this work.
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An Introduction to Asphalt Pavement Construction
A SunCam online continuing education course
Ultimately, pavement densities are taken by a nuclear density meter or
similar machine, and correlated to lab results by taking in-place cores.
Density requirements vary from agency to agency, but typically are
between 95% -100% of the lab established density.
Equipment Needs
Asphalt Pavers
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An Introduction to Asphalt Pavement Construction
A SunCam online continuing education course
Asphalt pavers, like it's concrete counterparts, come in many different
shapes and sizes. They can track driven or ruber tired.
Cat AP-1050 Track Mounted Asphalt Paver
Although there are several manufacturers of paving equipment, there are
typically two styles; Wheel and Track. Both have their advantages and
disadvantages, and although they have specific advantages to placing
certain types of mixes, either can be used in most situations, depending
on availability. However, track pavers are typically used for thicker layers
that take more tons/If of roadway, hence a slower travel speed, and for
base course HMA where placement is on stone or gravel. Rubber tire
pavers are more frequently used for thinner lifts of binder or surface
courses than are placed on top of other lifts of HMA. Rubber tire pavers
can travel much faster than a track machine, and are more maneuverable
when pullbacks in the paving operation are required.
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An Introduction to Asphalt Pavement Construction
A SunCam online continuing education course
Material Transfer Devices (MTD) are becoming more popular and are
indeed required by many state DOT specifications. Used to store up to
several loads of hot mix, these vehicles proceed in front of the paver, and
feed the paver in a more continuous manner than the starting and
stopping required sometimes with direct truck delivery into the paver. It
also helps cycle time of the trucks since they do not have to wait to dump
into the paver. Additionally, the storage capacity helps maintain consistency
and temperature for the delivered mix.
Asphalt Rollers
Rollers also come in many makes and models. Rollers for hot mix differ
from rollers for soil or stone compaction. Hot Mix Rollers should not be
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An Introduction to Asphalt Pavement Construction
A SunCam online continuing education course
used for any other type of rolling, since other materials can damage the
rollers and may affect its performance on the mix.
Typically four styles of rollers are used:
• Static Steel Wheel
• Vibrator Steel Wheel
• Pneumatic Rubber Tire
• Walk behind for handwork
Refer to the placement section for the use of each style.
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An Introduction to Asphalt Pavement Construction
A SunCam online continuing education course
Distributor Truck
The liquid asphalt placed directly on the roadway to be used for prime,
tack or chip seal is placed by use of a distributor truck. Depending on the
size of tank desired, (2,500 gal size is standard) the distributor truck
consists of the desired tank mounted on a ten-wheel truck frame
(although some are mounted on a six wheel frame) with rear mounted
spray bar and pumping and metering equipment. The truck is capable of
spraying a 15' +1- roadway with the desired application rate of product. A
hand wand is attached to do small or irregular sections. Usually MC
asphalt is used for this purpose. CAUTION shall be taken when changing
liquids in the truck. If MC asphalt is desired to be used after an emulsion,
all the water MUST be out of the tank before putting the MC material in.
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An Introduction to Asphalt Pavement Construction
A SunCam online continuing education course
The Construction Engineer's Role
The engineer's role in hot mix paving is crucial.
The plant engineer is responsible for:
➢ Preparing Asphalt Pavement mix designs
➢ Assuring a testing lab/Quality Control is in place
➢ Preparing grade sheets or 3D Models for paving profile grading.
➢ Monitoring raw material stockpiles for need, costs, and yield rates
➢ Assuring all raw material contracts with the major suppliers are fully
executed, and that each supplier has a letter advising them of schedule,
total quantity needs, and for delivery rates and times.
➢ Ordering #2 fuel for the plant operation. Increasingly, projects are using
waste oil to help mitigate plant-operating costs.
➢ Assuring contracts for truckers are in place, including delivery of the
liquid asphalt.
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An Introduction to Asphalt Pavement Construction
A SunCam online continuing education course
The field paving engineer is responsible for:
➢ Recording quantities and calculate yields. With liquid asphalt, it is
imperative that the engineer account for any escalators provided for in the
contract by the owner. The liquid asphalt can be paid as a separate item,
or its cost can be incidental to the mix. Particularly with the super pave
items; there will be multiple types of liquid asphalts needed and records of
each quantity must be kept separately.
➢ Assuring that all quantity control is being performed as required by the
specification, including but not limited to Marshall tests, cores, extraction
testing, density testing etc.
➢ Smoothness testing of final riding surface.
➢ Ensuring that the distribution truck is scheduled as may be needed.
➢ Scheduling hauling units
Both Field and Plant Engineers will have a role in estimating and pricing,
quotes and proposals for bid work.
Lastly, asphalt pavement can be used for LEED (Leadership in Energy and
Environmental Design) construction. Permeable asphalt helps control pavement run-
off, provides root space and irrigation for trees and plantings, and controls surface
transported pollutants. Asphalt pavement is a highly re-useable product by itself.
Milling and crushing existing asphalt pavement allows it to be introduced into new
asphalt product up to 40% recycled. Additionally many of the ingredients can be made
from recycled products, The asphalt material can be extracted from used roofing shingle
materials, and the aggregate and filler materials can be made from ground recycled
glass and slag products.
ASPHALT FUN FACTS FROM VARIOUS INTERNET SOURCES
Asphalt pavements are the most recycled materials (80 million tons annually) in the
United States—more than glass, metal, tires, paper or any other material. We crush
and recycle the reclaimed asphalt pavement (RAP) right back into our mixes every day,
making asphalt pavements a truly sustainable product.
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An Introduction to Asphalt Pavement Construction
A SunCam online continuing education course
Liquid asphalt was used by ancient Egyptians to embalm mummies, as they recognized
the lasting qualities of asphalt.
The Phoenicians caulked the seams of their merchant ships with liquid asphalt, the best
waterproofing material available.
The United States has 2.6 million miles of paved roads, and about 94% of them are
surfaced with asphalt pavement. Asphalt pavements are, and continue to be, America’s
pavement of choice
The first road use of asphalt occurred in 1824, when asphalt blocks were placed on the
Champs-Élysées in Paris, France.
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