Thursday, December 28, 2000 - 09:16

Compacting Hot-Mix Asphalt Pavements: Part I

The single most important factor that affects the long-term durability of
a hot-mix asphalt (HMA) pavement is the density of the mix that is achieved
by the contractor at the time of construction. The density of a material
is defined as the weight of the material, which occupies a certain volume
of space. The compaction process causes the asphalt-concrete mix to be compressed
and its volume reduced. As the density of the HMA material increases, the
air-void content of the mix decreases (they are inversely proportional to
each other). Properly designed, a HMA mix should have an air-void content
in the range of 3% to 5%.

If the compacted mix has a high air-void content (greater than 8%), the
mix will not perform as well under traffic. Similarly, if the compacted
mix has a low air-void content (less than 3%), the mix will be susceptible
to permanent deformation or rutting and also to distortion under the applied
traffic loads. Thus, for the mix to perform as expected, the contractor
must be able to compact the mix to the desired level of density or air-void
content.

The density of the asphalt-concrete mix controls its durability. All of
the following factors are related to the air-void content of the HMA material:
fatigue life; permanent deformation; oxidation; moisture damage; distortion;
and disintegration.

As the air void of the HMA decreases, the fatigue life or number of repetitions
of load to failure of that mix increases. Tests have shown that reducing
the air-void content of a given asphalt-concrete mix from 8% to 5% can double
the fatigue life of the pavement. Thus, for a given thickness of HMA as
part of the pavement structure, the ability of the mix to carry load can
be increased significantly when the mix is compacted to a lower air-void
content.

The amount of permanent deformation or rutting that develops under load
in a HMA material also is directly related to the air-void content of the
mix. As the air-void content decreases, the amount of rutting that will
occur in that mix also decreases. If the mix design is proper, a well compacted
mix will not rut under the action of the traffic loads. If the mix design
is deficient in some aspect, proper compaction of the mixture can still
significantly reduce the amount of rutting and lateral distortion that will
occur under repeated load applications. If, however, the air-void content
of the mix is reduced to less than 3%, an increase in the rate of rutting
of the mix can result.

With time, the asphalt-cement binder in an asphalt-concrete mix will oxidize
and become more brittle. This oxidation or aging process causes the asphalt
cement to decrease in penetration and increase in viscosity. The rate of
oxidation is directly related to the air-void content of the mix. The lower
the air-void content, the less quickly the HMA material will age and become
stiffer.

Moisture damage or stripping occurs when water is able to enter the mix
and, under the repetitive action of traffic, works its way in between the
asphalt coating on the aggregate and the surface of the aggregate. The degree
of moisture damage is primarily related to the characteristics of the aggregate
used in the mix but also is directly related to the air-void content of
the mix. As the air-void content in the mix decreases, the amount of moisture
damage also decreases. Indeed, a mix that may strip badly at an air-void
content of 8% may not suffer any moisture damage if it can be compacted
to an air-void content below 4%.

Distortion or shoving is the displacement of the mix, typically in the longitudinal
direction, under the action of traffic. Distortion is primarily related
to the design and properties of the mix but also is related to the air-void
content. For a given mix, a decrease in the air-void content at the time
of construction will decrease the amount of distortion that the mix will
undergo when exposed to traffic loads, particularly stopping or turning
movements. An increase in the density of the mix (a decrease in the air-void
content) will increase the internal stability and strength of the mix and
may significantly reduce the amount of distortion that occurs under load.

Disintegration or raveling is directly related to the air-void content of
the mix. If a mix is properly compacted (to an air-void content of 8% or
less), the mix generally will not ravel if the asphalt content is correct.
If the same mix is compacted to a high air-void content, however, major
raveling may occur under the applied traffic loads. As the air-void content
of the mix decreases, the amount of raveling also will decrease.

An asphalt-concrete mix must be fully compacted before it cools to a temperature
of about 175 F. At temperatures above this value, the mix is normally still
warm enough for the compaction equipment to reorient the aggregate particles
into their densest configuration. Below that temperature, however, the mix
is generally too stiff to increase in density any significant amount with
continued rolling, although roller marks can often be removed below this
compaction cutoff temperature. The mix must, therefore, be compacted while
it is still hot. Five factors directly affect the rate of cooling of the
asphalt concrete mix when that material is placed on top of another existing
layer of the pavement structure. Those variables are: air tem- perature;
base temperature; mix laydown temperature; layer thickness; and wind velocity.

All other factors being equal, as the ambient air temperature increases,
the time available for compaction also increases. The mix will take longer
to cool to the cutoff temperature of 175 F on a warm day than on a cool
day. An increase in the air temperature allows more time for the compaction
equipment to achieve the desired density level in the mix.

More important than air temperature in the rate of cooling of the HMA mix
is the temperature of the surface of the layer on which the new mix is placed.
It is well known that heat in an asphalt concrete layer is lost in two directions.
The surface of the mixture cools as heat is transferred to the air. The
bottom portion of the mixture also cools as heat is transferred to the underlying
base material. There is more rapid cooling of the mix downward into the
base than upward into the ambient air.

Base temperature-the temperature of the layer on which the new asphalt concrete
mix is placed-is actually more important than air temperature in determining
the time available for compaction. An increase in the base temperature allows
more time for compaction.

As the temperature of the mix coming out from under the paver screed increases,
the time available for compaction also increases. A mix placed at a temperature
of 300 F, for a given lift thickness and other environmental factors, will
take longer to cool to the cutoff temperature of 175 F, than will the same
mix placed at a temperature of 250 F.

Probably the most important factor in the rate of cooling of an asphalt
concrete mix is the thickness of the layer being placed and compacted. As
the thickness of the layer increases, the time available for compaction
also increases. It takes considerably longer for a 3-in. thick layer of
HMA to cool to the cutoff temperature of 175 F than for a 1-in. layer to
cool to the same temperature. The cooling time is not directly proportional
to the lift thickness but is geometrically proportional. For example on
a 40 F day with the temperature of the base at the same value, a 3-in. thick
layer of HMA placed at a temperature of 250 F will take 19 minutes to cool
from the laydown temperature to the cutoff temperature of 175 F. On the
same 40 F day, with the same base temperature and for the same mix laydown
temperature of 250 F, a 1-in. thick HMA layer will cool to the cutoff temperature
in only 3 minutes.

A thin lift of asphalt-concrete mix will cool more quickly when exposed
to a high wind velocity than when there is little or no wind. Wind has a
much greater effect on the surface of the mix than at various depths within
the HMA layer. A strong wind can cause the surface to cool so rapidly that
a crust will form. This crust must be broken down by the rollers before
the compaction process can be accomplished. The higher the wind velocity,
the less time available for compaction, all other factors being constant.

Dickson and Corlew published a set of cooling curves for asphalt concrete
mixtures. Those curves show the amount of time available for compaction
under different combinations of conditions. The curves are reproduced in
Section Six of Part Three of the Hot Mix Asphalt Paving Handbook, available
from the National Asphalt Pavement Association. Three variables are used
to enter the graphs: Mix-laydown temperature, base temperature (which is
assumed to be equal to the air temperature), and compacted layer thickness.

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