principles of paving

Principles of Paving rev 8-2009

Principles of Paving


Paver System Five

= The answer for perfect, smooth paving?

Grade Control Systems

Tremendous benefits to the paving contractor: Increase efficiency and productivity Control Material Quantities Improve smoothness and rideability Increase jobsite safety


Paver System Five Grade Control Good Paving Practice

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Decent paving

Fair paving

Great paving!



•Good principles of paving must come BEFORE Grade Control• Paving is a TEAM project• Grade control is a great tool to help the paving team:

be more efficient control material quantities provide a smoother surface

Train the entire TEAM!


Paver Components

Asphalt Paver

The paver has two primary components:

• Tractor• Screed


Paver Tractor

Tractor has two primary functions

• Material Handling• Forward Movement


Paver Tractor

Asphalt distribution

The paver typically receives the asphalt:

• directly from a truck• by a pickup machine (windrow elevator)


Paver Tractor


The asphalt is received into the paver’s hopper

• Hopper is a holding station for the asphalt• The operator tilts the hopper to deliver material into the feeder conveyor


Paver Tractor


•Feeder conveyors distribute the asphalt to the augers• The augers spread the asphalt in front of the screed• End gates contain the material at the outer edge of the screed


Screed Components

Paver Screed• Function of the screed is mat laydown• Screed has several components


Floating, Self-Leveling Screed

• The screed floats on the asphalt as it passes under the screed plate

• The screed will “self-level” and maintain a thickness based on the forces acting on the screed.

Screed concept


…controls mat thickness

…

Screed angle of attack…

Screed Concept

• The desired thickness is controlled by the screed angle of attack

• As the angle of attack is increased or decreased, the screed will float up or down changing the thickness


Changing Angle of Attack

The angle of attack can be changed by:

• The manual crank

• The hydraulic cylinder at the tow point


increased,

Mat thickness increases

decreased,

Mat thickness decreases

Screed Concept

If the angle of attack is…


Floating, Self-Leveling Screed• The paver tractor, and therefore the screed tow point, travel over the existing “rough” surface

• The floating screed doesn’t react immediately to the variations in the tow point path

• The floating screed seeks it’s own level, smoothing out small humps and bumps.


Fills in low spots

Cuts off high spots

Floating, self-leveling screed• The floating screed naturally cuts off high spots and fill in low spots.


Screed ReactionTow point “step disturbance” and screed reaction

• Two point is changed by a unit of one (1)

• Net effect = the angle of attack has been changed


Screed Reaction Example of tow point “step disturbance” and screed reaction

• Two point is changed by a unit of one (1)


Screed Reaction Example of change in angle of attach by turning manual crank and screed reaction.


Checking Mat Depth• Goal is to “average” a particular paving thickness

• Always check several places, horizontally and laterally


Adjusting Mat Depth

• Once an adjustment is made – GIVE THE SCREED TIME TO REACT!


• Determine average depth by measuring tons/mile

• Make small adjustments based on calculation

Adjusting Mat DepthGoal – Provide smoothness and compaction, while hitting target tonnage

for entire project!


Left alone, the screed will self-level and assume a thickness based on forces that are acting on the screed.

•What are these forces?

Floating, Self-Leveling Screed


Forces Acting on the Screed

There are four major forces acting on the screed


Forces Acting on the Screed If any one of these forces change, the mat thickness will change.

But in “real life” job applications, these forces cannot always remain constant.


Goal - Non-stop, continuous paving!

decreases;

mat thickness will increase

increases;

mat thickness will decrease

Tow Point Force

If the forward speed…


• Determine paving speed based on plant supply in tons/hour

• Regulate trucks to the paving supply and operation, not the paving operation to the trucks

Tow Point Force

Goal - Non-stop, continuous paving!

Plant supply = 210 tons/hour

Paving speed = 25’/minute


Bump in mat at every truck exchange

(probably “caused by faulty grade control!”)

Tow Point Force

Maintain smooth truck exchange!


• While stopped waiting for trucks, the screed will settle slightly

• Screed goes down, the tracker will also go down slightly

In Motion Switches


• Grade control would want to drive the tow cylinder up to compensate

• The paver’s In-Motion switch stops the valve electrical signal

• A timer initiates the electrical signal after the paver is in-motion

In Motion Switches

X


Maintain a CONSISTANT head of material

Head of Material Force


Head of Material Force

Maintain a CONSISTANT head of material


Mix Characteristics

• Gradation

• Segregation• Mix Temperature

Reaction of Material Force


Compaction Force = Pounds per square inchWeight of the Screed Force


Weight of the Screed ForceCompaction Force = Pounds per square inch

• How can the weight force change? (lbs per square inch)

• People climbing on and off the screed can have a small effect on overall screed weight


Weight of the Screed Force

Screed extension

• Changing screed width changes the compaction force - Lbs per square inch


Roller Compaction

• Good rolling practice is critical to the quality of the mat


Roller Compaction

• A freshly laid, smooth mat will reflect some of the original surface after rolling

• Quality of the base being paved affects smoothness

• Multiple lifts help to minimize these effects


Mechanical condition of the Paver

The paving machine itself needs to be in good shape and adjusted correctly

• Feeder system

• Hopper, flight chain operation and adjustment, augers, etc.

• Operation and adjustment of feeder controls

• No excessive wear on screed components

• Screed plates

• Thickness control screws, bearings, and bushings

• Screed pivot point bushings

• Screed adjustment

• Strike off plates

• Screed flatness

• Screed extensions adjusted properly, elevation and slope


Paver System Five Grade Control Good Paving Practice

=+

=+

=+

Decent paving

Fair paving

Great paving!


Paver System Five

Tow point is smoothly controlled to a desired path:

• Curb

• Stringline

• SAS smoothing out existing surface

Then let the floating, self-leveling screed do it’s job!


Best Paving Practices

Good mechanical condition of the paver

Minimize changes in the natural forces on the screed

Non-stop, continuous paving

Good, smooth truck exchange

Constant paving speed

Constant head of material

Consistent mix characteristics

Good rolling techniques

Good quality of sub-base

Understand proper setup and operation of Grade Control

principles of paving

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