Asphalt concrete

Asphalt concrete is a composite material
commonly used in construction projects such as
road surfaces , airports and parking lots . It
consists of asphalt (used as a binder ) and
mineral aggregate mixed together, then laid
down in layers and compacted. It is also
increasingly used as the core for embankment
dams. [1]
The terms "asphalt (or asphaltic) concrete",
"bituminous asphalt concrete" and the
abbreviation "AC" are typically used only in
engineering and construction documents and
literature. Asphalt concrete pavements are often
called just "asphalt " by laypersons who tend to
associate the term concrete with Portland
cement concrete only. The engineering
definition of concrete is any composite material
composed of mineral aggregate glued together
with a binder, whether that binder is Portland
cement , asphalt or even epoxy . Informally,
asphalt concrete is also referred to as
"blacktop", particularly in North America.
Mixture formulations
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Mixing of asphalt and aggregate is accomplished
in one of several ways:
Hot mix asphalt concrete (commonly
abbreviated as HMAC or HMA) is produced by
heating the asphalt binder to decrease its
viscosity, and drying the aggregate to remove
moisture from it prior to mixing. Mixing is
generally performed with the aggregate at
about 300 °F (roughly 150 °C) for virgin
asphalt and 330 °F (166 °C) for polymer
modified asphalt, and the asphalt cement at
200 °F (95 °C). Paving and compaction must
be performed while the asphalt is sufficiently
hot. In many countries paving is restricted to
summer months because in winter the
compacted base will cool the asphalt too
much before it is packed to the optimal air
content. HMAC is the form of asphalt concrete
most commonly used on highly trafficked
pavements such as those on major highways ,
racetracks and airfields.
Superpave , short for "superior performing
asphalt pavement," is a pavement system
designed to provide longer lasting roadways.
Key components of the system are careful
selection of binders and aggregates,
volumetric proportioning of ingredients, and
evaluation of the finished product.
Warm mix asphalt concrete (commonly
abbreviated as WMA) is produced by adding
either zeolites , waxes , or asphalt emulsions to
the mix. This allows significantly lower mixing
and laying temperatures and results in lower
consumption of fossil fuels, thus releasing less
carbon dioxide, aerosols and vapors. Not only
are working conditions improved, but the
lower laying-temperature also leads to more
rapid availability of the surface for use, which
is important for construction sites with critical
time schedules. The usage of these additives in
hot mixed asphalt (above) may afford easier
compaction and allow cold weather paving or
longer hauls.
Cold mix asphalt concrete is produced by
emulsifying the asphalt in water with
(essentially) soap prior to mixing with the
aggregate. While in its emulsified state the
asphalt is less viscous and the mixture is easy
to work and compact. The emulsion will break
after enough water evaporates and the cold
mix will, ideally, take on the properties of cold
HMAC. Cold mix is commonly used as a
patching material and on lesser trafficked
service roads.
Cut-back asphalt concrete is produced by
dissolving the binder in kerosene or another
lighter fraction of petroleum prior to mixing
with the aggregate. While in its dissolved state
the asphalt is less viscous and the mix is easy
to work and compact. After the mix is laid
down the lighter fraction evaporates. Because
of concerns with pollution from the volatile
organic compounds in the ligher fraction, cut-
back asphalt has been largely replaced by
asphalt emulsion. [2]
Mastic asphalt concrete or sheet asphalt is
produced by heating hard grade blown
bitumen ( oxidation ) in a green cooker (mixer)
until it has become a viscous liquid after which
the aggregate mix is then added.
The bitumen aggregate mixture is cooked
(matured) for around 6-8 hours and once it is
ready the mastic asphalt mixer is transported
to the work site where experienced layers
empty the mixer and either machine or hand
lay the mastic asphalt contents on to the road.
Mastic asphalt concrete is generally laid to a
thickness of around 3 ⁄ 4 –1  3 ⁄ 16 inches (20-30
mm) for footpath and road applications and
around 3 ⁄8 of an inch (10 mm) for flooring or
roof applications.
In addition to the asphalt and aggregate,
additives, such as polymers , and antistripping
agents may be added to improve the
properties of the final product.
Natural asphalt concrete can be produced
from bituminous rock, found in some parts of
the world, where porous sedimentary rock
near the surface has been impregnated with
upwelling bitumen.
An airport taxiway , one of the uses of
asphalt concrete
Asphalt concrete pavements—especially those
at airfields—are sometimes called tarmac for
historical reasons, although they do not contain
tar and are not constructed using the macadam
process.
Performance characteristics
Asphalt concrete has different performance
characteristics in terms of surface durability, tire
wear, braking efficiency and roadway noise. The
appropriate asphalt performance characteristic
is obtained by the traffic level amount in
categories A,B,C,D,E, and friction coarse (FC-5).
Asphalt concrete generates less roadway noise
than Portland cement concrete surfacing, and is
typically less noisy than chip seal surfaces. Tire
noise effects are amplified at higher operating
speeds. Noise is generated through the
conversion of kinetic energy to sound waves .
The idea that highway design could be
influenced by acoustical engineering
considerations including selection of surface
paving types arose in the very early 1970s. [3]
[4]
Asphalt damaged by frost heave , or
freezing of groundwater.
Asphalt concrete degradation and
restoration
Asphalt deterioration can include crocodile
cracking , potholes , upheaval, raveling, rutting ,
shoving, stripping, and grade depressions. In
cold climates, freezing of the groundwater
underneath can crack asphalt even in one winter
(by frost heaving). Filling the cracks with
bitumen can temporarily fix the cracks, but only
proper construction, i.e. allowing water to drain
away from under the road, can slow this
process.
Factors that cause asphalt concrete to
deteriorate over time mostly fall into one of two
categories: Environmental factors and traffic
loads. Often, damage results from combinations
of factors in both categories.
Environmental factors include heat and cold,
the presence of water in the subbase or
subgrade soil underlying the pavement, and
frost heaves .
High temperatures soften the asphalt binder,
allowing heavy tire loads to deform the
pavement into ruts. Paradoxically, high heat
and strong sunlight also causes the asphalt to
oxidize, becoming stiffer, less resilient and
cracking. Cold temperatures can cause cracks as
the asphalt contracts. Cold asphalt is also less
resilient and more vulnerable to cracking.
Water trapped under the pavement softens the
subbase and subgrade, making the road more
vulnerable to traffic loads. Water under the road
freezes and expand in cold weather, causing and
enlarging cracks to form. In spring thaw, the
ground thaws from the top down, so water is
trapped between the pavement above and the
still-frozen soil underneath. This layer of
saturated soil provides little support for the road
above, leading to the formation of potholes .
This is more of a problem for silty or clay soils
than sandy or gravelly soils. Some jurisdictions
pass frost laws to reduce the allowable weight
of trucks during the spring thaw season and
protect their roads.
Traffic damage mostly results from trucks and
buses. The damage a vehicle causes is
proportional to the axle load raised to the fourth
power [5] , so doubling the weight an axle
carries actually causes 16 times as much
damage. Wheels cause the road to flex slightly,
resulting in fatigue cracking, which often leads
to crocodile cracking . Vehicle speed also plays a
role. Slowly moving vehicles stress the road
over a longer period of time, increasing ruts,
cracking, and corrugations in the asphalt
pavement.
Other causes of damage include heat damage
from vehicle fires, or solvent action from
chemical spills.
Prevention and repair of degradation
The life of a road can be prolonged through
good design, construction and maintenance
practices. During design, engineers measure the
traffic on a road, paying special attention to the
number and types of trucks. They also evaluate
the subsoil to see how much load it can
withstand. The pavement and subbase
thicknesses are designed to withstand the wheel
loads. Sometimes, geogrids are used to
reinforce the subbase and further strengthen the
roads. Drainage, including ditches, storm drains
and underdrains are used to remove water from
the roadbed, preventing it from weakening the
subbase and subsoil.
Good maintenance practices center on keeping
water out of the pavement, subbase and
subsoil. Maintaining and cleaning ditches and
storm drains will extend the life of the road at
low cost. Sealing small cracks with bituminous
crack sealer prevents water from enlarging
cracks through frost weathering , or percolating
down to the subbase and softening it. For
somewhat more distressed roads, a chip seal or
similar surface treatment may be applied. As the
number, width and length of cracks increases,
more intensive repairs are needed. In order of
generally increasing expense, these include thin
asphalt overlays, multicourse overlays, grinding
off the top course and overlaying, in-place
recycling, or full-depth reconstruction of the
roadway.
It is far less expensive to keep a road in good
condition than it is to repair it once it has
deteriorated. This is why some agencies place
the priority on preventive maintenance of roads
in good condition, rather than reconstructing
roads in poor condition. Poor roads are
upgraded as resources and budget allow. In
terms of lifetime cost and long term pavement
conditions, this will result in better system
performance. Agencies that concentrate on
restoring their bad roads often find that by the
time they've repaired them all, the roads that
were in good condition have deteriorated. [6]
Some agencies use a pavement management
system to help prioritize maintenance and
repairs.
Recycling
Asphalt concrete is often touted as being 100%
recyclable . Very little asphalt concrete is actually
disposed of in landfills.
Several in-place recycling techniques have been
developed to rejuvenate oxidized binders and
remove cracking, although the recycled material
is generally not very water-tight or smooth and
should be overlaid with a new layer of asphalt
concrete. Cold in-place recycling mills off the
top layers of asphalt concrete and mixes the
resulting loose millings with asphalt emulsion.
The mixture is then placed back down on the
roadway and compacted. The water in the
emulsion is allowed to evaporate for a week or
so, and new hot-mix asphalt is laid on top.
Asphalt concrete that is removed from a
pavement is usually stockpiled for later use as
aggregate for new hot mix asphalt. This
reclaimed material, commonly known by the
acronym 'RAP' for recycled or reclaimed asphalt
pavement, is crushed to a consistent gradation
and added to the HMA mixing process.
Sometimes waste materials, such as asphalt
roofing shingles, crushed glass, or rubber from
old tires, are added to asphalt concrete as is the
case with rubberized asphalt , but there is a
concern that the hybrid material may not be
recyclable .