Mix placement and compaction are the two most important elements of highway construction using hot mix asphalt (HMA). Placement involves using specialized equipment like pavers to lay down the HMA at the desired thickness and involves considerations like lift thickness, longitudinal joints, and hand placement. The basic components of an asphalt paver are the tractor system which accepts HMA and spreads it using a hopper, conveyors, and augers, and the screed which levels and compacts the material. Maintaining a constant head of material in front of the screed and avoiding emptying the hopper are important for ensuring proper placement.

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

2. INTRODUCTION
• Mix placement and compaction are the two most
important elements in HMA pavement construction.
• Mix placement involves any equipment or procedures
used to place the delivered HMA on the desired
surface at the desired thickness.
• Mix placement can involve complicated asphalt paver
operations or simple manual shoveling.
• This section provides a basic description of HMA
placement operations.
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3. Placement Considerations
• There are, of course, many considerations to take into
account when placing HMA.
• Many are dependent upon local materials, weather, crew
knowledge and training, and individual experience.
• Lift thickness. A "lift" refers to a layer of pavement as
placed by the asphalt paver. In order to avoid mat tearing
(which generally shows up as a series of longitudinal
streaks) a good rule-of-thumb is that the depth of the
compacted lift should be at least twice the maximum
aggregate size or three times the nominal maximum
aggregate size.
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4. Placement Considerations
• Longitudinal joints. The interface between two
adjacent and parallel HMA mats.
• Improperly constructed longitudinal joints can
cause premature deterioration of multilane HMA
pavements in the form of cracking and raveling.
Handwork.
• HMA can be placed by hand in situations where the
paver cannot place it adequately.
• This can often occur around utilities, around
intersection corners and in other tight spaces.
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5. Placement Considerations
– Hand-placing should be minimized because it is prone to
aggregate segregation and results in a slightly rough
surface texture.
– If hand placement is necessary the following precautions
should be taken:
– Place the HMA in a pile far enough away from the
placement area that the whole pile must be moved.
– If the pile is located in the placement area its
appearance, density or aggregate distribution may be
slightly different than the surrounding handworked mat.
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6. Placement Considerations
• Carefully deposit the material with shovels and then
spread with lutes. Do not broadcast (scoop and pitch)
the HMA with shovels - this is likely to cause aggregate
segregation.
• All material should be thoroughly loosened and evenly
distributed. Chunks of HMA that do not easily break
apart should be removed and discarded.
• Check the handworked surface with a straightedge or
template before rolling to ensure uniformity.
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7. Placement Considerations
• Mat problems. The asphalt paver, MTV, rollers, mix
design and manufacturing introduce many variables
into flexible pavement construction.
• A familiarity with common causes of the more typical
mat problems can help improve construction quality.
• Some common mat problems are microcracking, fat
spots, joint problems, non-uniform texture, roller
marks, shoving, surface waves, tearing (streaks) and
transverse screed marks.
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8. Asphalt Paver
• In 1934 Barber-Greene introduced the Model 79 asphalt laydown
machine, a self-propelled formless laydown machine with a floating
screed.
• Since then, the basic concept of the asphalt paver has remained
relatively unchanged: HMA is loaded in the front, carried to the rear
by a set of flight feeders (conveyor belts), spread out by a set of
augers, then leveled and compacted by a screed.
• This set of functions can be divided into two main systems:
– The tractor (or material feed system)
– The screed
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9. Tractor (Material Feed System)
• The tractor contains the material feed system,
which accepts the HMA at the front of the
paver, moves it to the rear and spreads it out to
the desired width in preparation for screed
leveling and compaction.
• The basic tractor components are:
– Push Roller and Truck Hitch
– Hopper
– Conveyor
– Auger
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10. Push Roller and Truck Hitch
• The push roller is the portion of the paver that
contacts the transport vehicle and the truck hitch
holds the transport vehicle in contact with the
paver.
• They are located on the front of the hopper.
Eng. Suneth Thushara Salawavidana 10
Push Roller and Truck Hitch
Truck Hitch Engaged

11. Hopper
• The hopper is used as a temporary storage area for
HMA delivered by the transport vehicle.
• Therefore, the paver can accept more material than is
immediately needed and can use the volume in the
hopper to compensate for fluctuating material demands
created by such things as paving over irregular grades,
utility access openings or irregular intersection shapes.
• Hopper sides (or “wings”) can be tilted up (or “folded”)
to force material to the middle where it is carried to the
rear by the conveyor system.
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12. Hopper
• Hoppers can also be fit with inserts to
allow them to carry more HMA.
• These inserts are typically used in
conjunction with a material transfer
vehicle (MTV).
Eng. Suneth Thushara Salawavidana 12
Hopper with Wings Folded Up
Caterpillar AP-1055B with Hopper Insert

13. Conveyor
• The conveyor mechanism carries the
HMA from the hopper, under the chassis
and engine, then to the augers.
• The amount of HMA carried back by the
conveyors is regulated by either variable
speed conveyors and augers or flow
gates, which can be raised or lowered by
the operator or, more often, by an
automatic feed control system.
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Conveyors
(seen from the rear of a partially built paver)

14. Auger
• The auger receives HMA from the conveyor and
spreads it out evenly over the width to be paved.
• There is one auger for each side of the paver and
they can be operated independently.
• Some pavers allow the augers to be operated in
reverse direction so that one can be operated
forward and the other in reverse to send all the
received HMA to one side of the paver.
• The auger gearbox can either be located in the
middle or on the outside edge of each auger.
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15. Auger
• If an inadequate amount of HMA is
distributed under a middle-located gearbox
the result can be a thin longitudinal strip of
mat aligned with the gearbox that exhibits
lower densities from aggregate segregation
and/or temperature differentials
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Augers Distributing HMA

16. PLACEMENT
• Operation of the tractor, and specifically the
material feed system, can have significant
effects on overall construction quality and thus
long-term pavement performance.
• Although there are many detailed operational
concerns, the two broad statements below
encompass most of the detailed concerns:
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17. PLACEMENT
• HMA must be delivered to maintain a relatively
constant head of material in front of the screed.
• This involves maintaining a minimum amount of
HMA in the hopper, regulating HMA feed rate by
controlling conveyor/auger speed and flow gate
openings (if present), and maintaining a
constant paving speed.
• A fluctuating HMA head in front of the screed
will affect the screed angle of attack and
produce bumps and waves in the finished mat.
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18. PLACEMENT
• The hopper should never be allowed to empty during
paving.
• This results in the leftover cold, large aggregate in the
hopper sliding onto the conveyor in a concentrated mass
and then being placed on the mat without mixing with any
hot or fine aggregate.
• This can produce aggregate segregation or temperature
differentials, which will cause isolated low mat densities.
• If there are no transport vehicles immediately available to
refill the hopper it is better to stop the paving machine
than to continue operating and empty the hopper.
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19. Eng. S.A.S.T Salawavidana

20. Eng. S.A.S.T Salawavidana