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Mix laydown involves everything used to place the delivered HMA on the desired surface at the desired thickness. The asphalt paver is the principal machine involved in mix laydown and is assisted by the material transfer vehicle (MTV) in some instances.

asphalt paver
close-up of an asphalt paver
Figure 1: Asphalt paver. Figure 2: HMA placement.

Video 1: SMA Placement at Hilo Harbor

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. This subsection presents a few of the basic considerations that apply in virtually all situations:

Lift thickness plays an important role in both HMA compaction and final mat smoothness.  Lifts thinner than three times the nominal maximum aggregate size are difficult to compact because there is simply not enough room for the aggregate particles to rearrange within the lift as it is compacted.  HDOT uses a method specification rather than an end-result specification on lift thicknesses less than 1.5 inches for this reason.

Overly thick final lifts have a tendency to shove or displace during compaction making it difficult to achieve a smooth finish. 

Generally, for a State mix IV a good final lift thickness that is compactable but will not shove excessively is about 2 inches.

Asphalt Paver

The asphalt paver is a self-propelled formless (does not require side forms) laydown machine with a floating screed (see Figures 1, 2, 3 and 4). 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:

paver track

Figure 3: Tracked paver.

Figure 4: Paver schematic animation.


Figure 5: Screed components and forces.


Factors Affecting Mat Thickness and Smoothness

Since the screed is free floating it will slide across the HMA at an angle and height that will place the six forces shown in Figure 5 in equilibrium. When any one of these forces is changed, the screed angle and elevation will change (which will change the mat thickness) to bring these forces back into equilibrium. Therefore, changing the following paver characteristics will affect these forces, and thus mat thickness, in the described manner:

Screed reaction

Figure 6: Screed Reaction to a Manual Decrease in Screed Angle (after TRB, 2000)

Automatic Screed Control

Since it is not practical to manually control tow point elevation, pavers usually operate using an automatic screed control, which controls tow point elevation using a reference other than the tractor body. Since these references assist in controlling HMA pavement grade, they are called “grade reference systems” and are listed below (Roberts et al., 1996): 

Stringline picture of a grade control system
Figure 7: Stringline Figure 8: Mobile Reference System (Circled)


Material Transfer Vehicles (MTVs)

HDOT requires the use of an MTV when placing any main traffic lane HMA surface course. Hawai'i essentially does not use bottom dump trucks or windrows for paving.

Material transfer vehicles (MTVs) are used to assist the paver in accepting HMA. Most pavers are equipped to receive HMA directly from end dump or live bottom trucks, however in certain situations it can be necessary or advantageous to use an MTV. Paving using bottom dump trucks and windrows requires a windrow elevator MTV, while other MTVs are used to provide additional surge volume, which is advantageous because it allows the paver to operate continuously without stopping, minimizes truck waiting time at the paving site and may minimize aggregate segregation and temperature differentials (see Figures 9 and 10).

picture of an MTV
picture of an MTV
Figures 9 and 10: Roadtec Shuttlebuggy™ MTV.

Video 2: HMA Placement Using a Shuttlebuggy™ MTV.

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