HMA Compaction.pptx

HIGHWAY CONSTRUCTION METHODS
HMA COMPACTION
ENG. S.A.S.T SALAWAVIDANA
CEng FIESL, MCIHT, MEng (Highway & Traffic), BSc.Eng (Hons)

INTRODUCTION
• It has been said that the top three factors in real
estate are “location, location, location”.
• It can also be said that the top three factors in HMA
pavement construction are “compaction,
compaction, compaction”.
• Compaction is the process by which the volume of
air in an HMA mixture is reduced by using external
forces to reorient the constituent aggregate
particles into a more closely spaced arrangement.
Eng. Suneth Thushara Salawavidana 2

INTRODUCTION
• This reduction of air volume in a mixture
produces a corresponding increase in HMA
unit weight, or density.
• Numerous researchers have stated that
compaction is the greatest determining factor
in dense graded pavement performance
Inadequate compaction results in a pavement
with decreased stiffness, reduced fatigue life,
accelerated aging/decreased durability,
rutting, raveling, and moisture damage.

Compaction Measurement and Reporting
• Compaction reduces the volume of air in HMA.
• Therefore, the characteristic of concern is the
volume of air within the compacted pavement.
• This volume is typically quantified as a percentage
of air voids by volume and expressed as “percent
air voids”.
• Percent air voids is calculated by comparing a test
specimen’s bulk density with its theoretical
maximum density (TMD) and assuming the
difference is due to air.

• Once TMD is known, portable devices can be used to
measure HMA density in-place.
• The terms “percent air voids” and “density” are often
used interchangeably.
• Although this is not wrong, since density is used to
calculate percent air voids, the fundamental parameter
of concern is always percent air voids.
• Percent air voids is typically calculated by using
AASHTO T 269, ASTM D 3203 or an equivalent
procedure (AASHTO, 2000).

• These procedures all use laboratory-
determined bulk specific gravity.
• These procedures require a small pavement
core (usually 100 - 150 mm (4 - 6 inches) in
diameter), which is extracted from the
compacted HMA.
• This type of air voids testing is generally
considered the most accurate but is also the
most time consuming and expensive.
Two Cores –
The Core on the Right has
Significantly Higher Air Voids

Compaction Importance
• The volume of air in an HMA pavement is
important because it has a profound effect on long-
term pavement performance.
• An approximate "rule-of-thumb" is for every 1
percent increase in air voids (above 6-7 percent),
about 10 percent of the pavement life may be lost.
• Keep in mind that this rule-of-thumb was
developed using limited project data, should be
used with extreme caution and applies to air voids
above 6 - 7 percent.
Core Extraction

• According to Roberts et al. (1996), there is
considerable evidence that dense graded mixes
should not exceed 8 percent nor fall below 3 percent
air voids during their service life.
• This is because high air void content (above 8 percent)
or low air void content (below 3 percent) can cause
the following pavement distresses (this list applies to
dense-graded HMA and not open-graded HMA or
SMA):

• Decreased stiffness and strength.
– Kennedy et al. (1984) concluded that tensile strength, static and resilient moduli, and stability
are reduced at high air void content.
• Reduced Fatigue Life.
– Several researchers have reported the relationship between increased air voids and reduced
fatigue life (Pell and Taylor, 1969; Epps and Monismith, 1969; Linden et. al., 1989). Finn et al.
(1973) concluded “...fatigue properties can be reduced by 30 to 40 percent for each one percent
increase in air void content.” Another study concluded that a reduction in air voids from eight
percent to three percent could more than double pavement fatigue life (Scherocman, 1984a).

• Accelerated Aging/Decreased Durability.
– In his Highway Research Board paper, McLeod (1967) concluded “compacting a well-designed
paving mixture to low air voids retards the rate of hardening of the asphalt binder, and results
in longer pavement life, lower pavement maintenance, and better all-around pavement
performance.”
• Raveling.
– Kandhal and Koehler (1984) found that raveling becomes a significant problem above about
eight percent air voids and becomes a severe problem above approximately 15 percent air
voids.

• Rutting.
– The amount of rutting which occurs in an asphalt pavement is inversely proportional to the air
void content (Scherocman, 1984a).
– Rutting can be caused by two different mechanisms: vertical consolidation and lateral
distortion. Vertical consolidation results from continued pavement compaction (reduction of air
voids) by traffic after construction.
– Lateral distortion – shoving of the pavement material sideways and a humping-up of the asphalt
concrete mixture outside the wheelpaths – is usually due to a mix design problem.
– Both types of rutting can occur more quickly if the HMA air void content is too low.

• Moisture Damage.
– Air voids in insufficiently compacted HMA are high and tend to be interconnected with each
other.
– Numerous and interconnected air voids allow for easy water entry (Kandhal and Koehler, 1984;
Cooley et al., 2002) which increases the likelihood of significant moisture damage.
– The relationship between permeability, nominal maximum aggregate size and lift thickness is
quite important and can change significantly as these parameters change.

• Air voids that are either too great or too low can cause a significant reduction in
pavement life.
• For dense graded HMA, air voids between 3 and 8 percent generally produce the
best compromise of pavement strength, fatigue life, durability, raveling, rutting and
moisture damage susceptibility.

Factors Affecting Compaction
• HMA compaction is influenced by a myriad of
factors; some related to the environment,
some determined by mix and structural design
and some under contractor and agency control
during construction (see Table 1)

• Environmental factors are determined by when and where paving occurs.
• Paving operations may have some float time, which allows a limited choice of
“when” but paving location is determined by road location so there is essentially
no choice of “where”.
• Mix and structural design factors are determined before construction and although
they should account for construction practices and the anticipated environment,
they often must compromise ease of construction and compaction to achieve
design objectives

• Obviously construction factors are the most controllable
and adaptable of all the factors affecting compaction.
• Although some factors like haul distance/time, HMA
production temperature, lift thickness and type/number of
rollers may be somewhat predetermined, other factors
associated with roller timing, speed, pattern and number of
passes can be manipulated as necessary to produce an
adequately compacted mat.
• This subsection discusses:
– Temperature (the environmental factor)
– Mix property factors

Temperature
• HMA temperature has a direct effect on the viscosity
of the asphalt cement binder and thus compaction.
• As HMA temperature decreases, its asphalt cement
binder becomes more viscous and resistant to
deformation, which results in a smaller reduction in
air voids for a given compactive effort.
• As the mix cools, the asphalt binder eventually
becomes stiff enough to effectively prevent any
further reduction in air voids regardless of the
applied compactive effort.

Temperature
• The temperature at which this occurs, commonly
referred to as cessation temperature, is a function of
the mix property factors. In some literature it is reported
to be about 79oC (175F) for dense-graded HMA
(Scherocman, 1984b; Hughes, 1989).
• Below cessation temperature rollers can still be
operated on the mat to improve smoothness and
surface texture but further compaction will generally not
occur.

Temperature
• Conversely, if the binder is too fluid and the aggregate structure is weak (e.g., at
high temperatures), roller loads will simply displace, or “shove” the mat rather
than compact it.
• In general, the combination of asphalt cement binder and aggregate needs to be
viscous enough to allow compaction but stiff enough to prevent excessive shoving.

Temperature
• Mat temperature then, is crucial to both the actual amount of air void reduction
for a given compactive effort, and the overall time available for compaction.
• If the initial temperature and cool-down rate are known, the temperature of the
mat at any time after laydown can be calculated.
• Based on this calculation rolling equipment and patterns can be employed to:

Temperature
• Take maximum advantage of available roller compactive
effort.
– Rollers can be used where the mat is most receptive to
compaction and avoided where the mat is susceptible
to excessive shoving.
• Ensure the mat is compacted to the desired air void content
before cessation temperature is reached.
– This can be done by calculating the time it takes the
mat to cool from initial temperature to cessation
temperature.
– All compaction must be accomplished within this
“time available for compaction”.

Time available for compaction
• Initial mat temperature.
– Higher initial mat temperatures require more time to cool down to cessation temperature,
thus increasing the time available for compaction.
– However, overheating the HMA will damage the asphalt binder and cause emissions.
• Mat or lift thickness.
– Thicker lifts have a smaller surface-to-volume ratio and thus lose heat more slowly, which
increases the time available for compaction.

Time available for compaction
• Temperature of the surface on which the mat is placed.
– Hotter surfaces will remove heat from the mat at a slower rate, increasing the time available
for compaction.
• Ambient temperature.
– Hotter air temperatures will remove heat from the mat at a slower rate, increasing the time
available for compaction.
• Wind speed.
– Lower wind speeds will decrease mat heat loss by convection, which will increase the time
available for compaction.

Mix Properties
• Mix aggregate and binder properties can also affect
compaction.
• They do so by affecting (1) the ease with which
aggregate will rearrange under roller loads and (2) the
viscosity of the binder at any given temperature.
• Gradation affects the way aggregate interlocks and thus
the ease with which aggregate can be rearranged under
roller loads.
• In general, aggregate effects on compaction can be
broken down by aggregate size.

Mix Properties
• Coarse aggregate.
– Surface texture, particle shape and the number of fractured faces can affect compaction.
– Rough surface textures, cubical or block shaped aggregate (as opposed to round aggregate)
and highly angular particles (high percentage of fractured faces) will all increase the required
compactive effort to achieve a specific density.
• Midsize fine aggregate (between the 0.60 and 0.30-mm (No. 30 and No. 50)
sieves).
– High amounts of midsize fine, rounded aggregate (natural sand) cause a mix to displace
laterally or shove under roller loads.

Mix Properties
– This occurs because the excess midsize fine, rounded
aggregate results in a mix with insufficient voids in the
mineral aggregate (VMA).
– This gives only a small void volume available for the asphalt
cement to fill.
– Therefore, if the binder content is just a bit high it completely
fills the voids and the excess serves to (1) resist compaction
by forcing the aggregate apart and (2) lubricate the aggregate
making it easy for the mix to laterally displace.
• Fines or dust (aggregate passing the 0.075-mm (No.
200) sieve).
– Generally, a mix with a high fines content will be more
difficult to compact than a mix with a low fines content.

Mix Properties
• The asphalt binder grade affects compaction through its
viscosity.
• A binder that is higher in viscosity will generally result in a
mix that is more resistant to compaction.
• Additionally, the more a binder hardens (or ages) during
production, the more resistant the mix is to compaction.
• Asphalt binder content also affects compaction. Asphalt
binder lubricates the aggregate during compaction and
therefore, mixes with low asphalt content are generally
difficult to compact because of inadequate lubrication,
whereas mixes with high asphalt content will compact easily
but may shove under roller loads (TRB, 2000).

Mix Properties
• Sometimes, a combination of mix design factors produces what is known as a
tender mix.
• Tender mixes are internally unstable mixes that tend to displace laterally and shove
rather than compact under roller loads.

STEEL WHEEL ROLLERS
• Steel wheel rollers are self-propelled compaction
devices that use steel drums to compress the
underlying HMA.
• They can have one, two or even three drums, although
tandem (2 drum) rollers are most often used.
• The drums can be either static or vibratory and usually
range from 86 to 215 cm (35 to 85 inches) in width and
50 to 150 cm (20 to 60 inches) in diameter.
• Roller weight is typically between 0.9 and 18 tonnes (1
and 20 tons).

Small Static Steel Wheel Roller
(1.32 tonnes (1.45 tons), 86 cm (34-
inch) wide drum)
Large Vibratory Steel Wheel Roller
(17 tonnes (18.7 tons), 213 cm (84-
inch) wide drum)

STEEL WHEEL ROLLERS
• In addition to their own weight, some steel wheel rollers can be
ballasted with either sand or water to increase their weight and
thus, compactive effort.
• Although this ballasting is a fairly simple process, it is usually done
before rolling operations start and rarely during rolling operations.
• Since asphalt cement binder sticks to steel wheels, most steel
wheel rollers spray water on the drums to prevent HMA from
sticking, and are equipped with a transverse bar on each drum to
wipe off HMA.
• Note, however, that this water will cool the HMA and can reduce
the time available for compaction.

Vibratory Steel Wheel Rollers
• Some steel wheel rollers are equipped with
vibratory drums.
• Drum vibration adds a dynamic load to the static
roller weight to create a greater total compactive
effort.
• Drum vibration also reduces friction and
aggregate interlock during compaction, which
allows aggregate particles to move into final
positions that produce greater friction and
interlock than could be achieved without
vibration.

• Roller drum vibration is produced using a rotating
eccentric weight located in the vibrating drum (or drums)
and the force it creates is proportional to the eccentric
moment of the rotating weight and the speed of rotation
(TRB, 2000).
• Operators can turn the vibrations on or off and can also
control amplitude (eccentric moment) and frequency
(speed of rotation).
• Vibration frequency and amplitude have a direct effect
on the dynamic force (and thus the compactive force) as
shown in Table 2.

• The ideal vibratory frequency and amplitude settings are a compromise based on desired mat
smoothness, HMA characteristics and lift thickness.
• Low vibration frequencies combined with high roller speeds will increase the distance between
surface impacts and create a rippled, unsmooth surface.
• In general, higher frequencies and lower roller speeds are preferred because they decrease the
distance between surface impacts, which (1) increases the compactive effort (more impacts per
unit of length) and (2) provides a smoother mat.
• The recommended impact spacing is 3 - 4 impacts per meter (10 - 12 impacts per foot).

• Vibratory steel wheel rollers offer potential compaction advantages over static
steel wheel rollers but they also require the operator to control more compaction
variables (amplitude, frequency and vibratory mode use) and there are certain
situations under which they must be used with caution (e.g., over shallow
underground utilities, in residential areas, thin overlays).

Pneumatic Tire Rollers
• The pneumatic tire roller is a self-propelled compaction device that uses pneumatic tires
to compact the underlying HMA.
• Pneumatic tire rollers employ a set of smooth (no tread) tires on each axle; typically four
on one axle and five on the other.
• The tires on the front axle are aligned with the gaps between tires on the rear axel to give
complete and uniform compaction coverage over the width of the roller.
• Compactive effort is controlled by varying tire pressure, which is typically set between 400
kPa (60 psi) and 800 kPa (120 psi) (TRB, 2000).

• Asphalt binder tends to stick to cold pneumatic tires but not to hot pneumatic tires.
• A release agent (like water) can be used to minimize this sticking, however if asphalt
binder pickup (the asphalt binder sticking to the tires) is not permanently damaging the
mat it is better to run the roller on the hot mat and let the tires heat up to near mat
temperature.
• Tires near mat temperature will not pick up an appreciable amount of asphalt binder.
• Insulating the tire area with rubber matting or plywood helps maintain the tires near mat
temperature while rolling.

• In addition to a static compressive
force, pneumatic tire rollers also
develop a kneading action between
the tires that tends to realign
aggregate within the HMA.
• This results in both advantages and
disadvantages when compared to
steel wheel rollers:
Pneumatic Tire Roller (notice rubber matting
insulation around tire area as well as tire marks
left in the new mat in front of the roller)

Advantages
• They provide a more uniform degree of compaction than steel wheel rollers.
• They provide a tighter, denser surface thus decreasing permeability of the layer.
• They provide increased density that many times cannot be obtained with steel
wheeled rollers.
• They compact the mixture without causing checking (hairline surface cracks) and
they help to remove any checking that is caused with steel wheeled rollers.

Disadvantages
• The individual tire arrangement may cause deformations in the mat that are
difficult or impossible to remove with further rolling.
• Thus, they should not be used for finish rolling.
• If the HMA binder contains a rubber modifier, HMA pickup (mix sticking to the
tires) may be so severe as to warrant discontinuing use of the roller.

HMA Compaction.pptx

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