# Presentation: 572255

## HMA PLACEMENT 

## To Be Covered

**Hauling**
**Placement**
**Compaction**

**Notes:**
This slide shows the four topics that will be covered in this block.

## Tandem Axle End Dump Truck

**Notes:**
Trucks for use in hauling HMA come in many sizes.  The key is that they have metal beds, be cleaned of foreign material and hardened asphalt.  It is a practice within the industry to coat the beds with a release agent.  For many years this release agent was diesel fuel - the diesel fuel will soften the asphalt binder and can result in soft spots in the pavement.

The number of trucks required depends on many factors:  the production of the plant, the length of the haul, the type of traffic encountered and the expected time needed for unloading.

This is an end dump truck that loads directly into the paver.  During delivery the driver must direct the truck squarely against the paver, and should stop the truck a few inches in front of the paver.  The truck bed is slowly raised.  When the mix is dumped to quickly segregation occurs.

## High Dump Semi-truck

## Belly Dump

**Notes:**
Bottom dump trucks can be used when a pick-up device is used to feed the windrow left by the truck into the paver hopper.  It is essential that the windrow deposited by the truck be as uniform as possible.  Windrowed material can cool  below spreading and compacting temperatures in cool weather.  This can be a major problem is the paver has a malfunction.  To prevent excessive cooling of the mix in cool weather; the limit of windrow should be no more than one truck load ahead of the pick up machine.

## Multi-WHEELEDConveyor Feed

**Notes:**
This a truck used in Michigan.  The material is feed into the paver from a conveyor belt at the bottom of the truck.  NOTE- the number of wheels.

## Self Propelled Paver

**Notes:**
The mix is placed with a self-propelled paver.  It can be done with a motor grader; but, that is seldom done.   The purpose of the paver is to place the HMA to the desired width and thickness and to produce a satisfactory mat texture.   The mix is usually placed at 3 times the thickness of the nominal aggregate size.

## Self Propelled Paving Machine

**THE BASIC PRINCIPLE HAS NOT CHANGED MUCH**

**Notes:**
The basic principle behind the pavers has not changed much since the 1930s.

## Self Propelled Paving Machine

**Self Propelled Paving Machine**

**HMA Flow **

**Notes:**
This picture identifies each of the components of the paver:
1.     The tractor unit receives the mix from the haul trucks, carries the material back to the spreading screws and distributes the mix across the width of the screed.
2.     The hopper is used to temporarily hold the mix.  

## Screed Unit

**Screed Unit**

**Notes:**
The screed unit is towed by the tractor unit.  It establishes the thickness of the asphalt layer and provides the initial texture to the new surface.  In addition, the screed imparts some level of density to the material being placed through the vibratory action of the screed or tamping bars.
The principle of the free-floating paver screed was developed in the early 1930s.  That concept allow the paver sceed, which is attached to the tractor unit at only one point on each side of the machine to average out  changes in grade that are experienced by the wheelbase of the tractor unit.

## Forces On Screed

**Forces On Screed**

**Notes:**
     The screed unit is attached to the tractor unit at only one point on each side of the paver.  This point is called the tow point.  It is a pin-type connection that allows the leveling arms of the screed to rotate or pivot around that point.  The position of the tow points can be altered by raising or lowering the rods on which the pull points are mounted. 
     The screed assembly is shown here.  There are two primary forces acting on the paver screed as the paver places the mix.  The first force is the F3 - it varies as the speed of the tractor increases or decreases.
    The second force on the screed is the head of the material pushing against the screed.  As the amount of material in the augar chamber that pushes against the screed changes, the net forces acting on the screed also changes.   As the forces acting on the screed change the screed unit must come to a new angle of attack in order to compensate for the change in force acting on it.
     The forces on the screed unit must be in equilibrium for the screed to remain at a constant angle of attack as it is towed by the tractor unit.  When a change occurs in one force, the screed will rise or fall and will change the thickness of the mat.

## ONE OF THE ESSENTIALS FOR A CONSISTENT AND HIGH QUALITY HOT MIX ASPHALT PAVEMENT IS TO PROVIDE FOR A CONTINUOUS OPERATION

**ONE OF THE ESSENTIALS FOR A CONSISTENT AND HIGH QUALITY HOT MIX ASPHALT PAVEMENT IS TO PROVIDE FOR A CONTINUOUS OPERATION**

## Slide 13

**Notes:**
Bottom dump trucks can be used when a pick-up device is used to feed the windrow left by the truck into the paver hopper.  It is essential that the windrow deposited by the truck be as uniform as possible.  Windrowed material can cool  below spreading and compacting temperatures in cool weather.  This can be a major problem is the paver has a malfunction.  To prevent excessive cooling of the mix in cool weather; the limit of windrow should be no more than one truck load ahead of the pick up machine.

## Material Transfer Device 

**Notes:**
This is a material transfer device.  It’s primary purpose is to keep the operation flowing smoothly.

## COMPACTION

**COMPACTION**

**GOOD COMPACTION LEADS TO GOOD PERFORMANCE**

**Notes:**
Compaction is the single most important factor that affects the ultimate performance of an HMA pavement.

## Reason For Compaction

- To prevent further compaction
- To provide shear strength or resistance to rutting
- To ensure the mixture is waterproof
- To prevent excessive oxidation of the asphalt binder

## Durability vs Air Voids

**% Air Voids**

**Stability**

**Cohesion**

**Zone of Highest Durability**

**0**

**Flushing**

**Raveling**

**Notes:**
The goal is 4 to 7% air voids on the roadway.

## Factors Affecting Compaction

- Mix Properties
- Aggregate
- Asphalt
- Mix Temperature
- Layer Thickness
- Environmental Factors
- Rollers

**Notes:**
A coarse harsh mix is difficult to compact - a sandy mix is easy to compact.  
A stiff modified asphalt is tougher to compact than a soft low viscosity asphalt.  Also a increase in the asphalt content makes the mix easier to compact.
The temperature of the mix directly relates to the viscosity of the binder.  Thus, the higher temperature the softer the asphalt.
The old rule of thumb was twice the maximum aggregate size for layer thickness.  The new rule of thumb is that the layer thickness should be three times the nominal aggregate size.
It is harder to compact a mix on a cold wet day than on a warm sunny day.
The type, size and number of  rollers significantly affects the compaction operation.

## Temperature is critical

**Temperature is critical**

**Notes:**
One of the most important factors is the mat temperature.  Too cold and the mix cannot be compacted - too hot and the mix will not support the rollers.

## Rollers

- Vibratory
- Pneumatic-tired
- Steel wheeled tandem rollers

## Vibratory Roller

**Vibratory Roller**

**Notes:**
Vibratory rollers come in a variety of configurations.  Single drum and double drum as shown here.  These rollers can be operated in any one of three modes:  static (with the vibrators off), with one drum vibrating and one static, and with both drums vibrating.
These rollers provide two types of compactive force to the the HMA - static force and dynamic (impact) force.  The static from is caused by the weight of the roller and frame.  The dynamic force is caused by the impacte produced by a rotating eccentric weight located inside the drum or drums.  As the eccentric weight rotates about the shaft inside the drum, a dynamic force is produced.

## Pnuematic Roller

**Pnuematic Roller**

**Notes:**
Most pneumatic rollers are operated as intermediate rollers - behind a vibratory or static steel wheel breakdown roller and in front of a static wheel finish roller.
The compactive force is a function of the wheel load of the machine, the tire pressure, the tire design and the depth of penetration of the tires into the mix.
The tire pressures range from 60 to 120 psi and the minimum weight is 15 tons.  For a tender mix a lower tire pressure should be used for an stiff mix a higher tire pressure should be used.

## Steel Wheeled Roller

**Notes:**
Steel wheel rollers range from 3 to 14 tons.    They can either be three wheel like this one or tandem.  

## Checking Density With Nuclear Gauge

**Notes:**
The density can be checked using a nuclear density gauge.  It needs to be correlated to core samples.   The results are affected by the thickness of the layer being tested and the properties of the underlying layer.  

## Extracting A Core

**Notes:**
Cores are taken for density determination and thickness.  They are usually cut to obtain a sample of the layer being placed.  The bulk specific gravity of the core is determined and using that with the maximum specific gravity it is possible to determine the percent compaction.

## Questions - ?