# Asphalt Pavement Laydown and Compaction

## Presentation on theme: "Asphalt Pavement Laydown and Compaction"— Presentation transcript:

Asphalt Pavement Laydown and Compaction
Picture of plant

Compaction of the asphalt mat, typically, has more affect on the life of the pavement than any other item in asphalt construction.

Adequate Compaction Helps Reduce or Retard….
Fatigue Cracking Rutting Binder Oxidation Moisture Damage Thermal Cracking

Fatigue Cracking

Rutting

Asphalt Binder Oxidation - Hardening

Moisture Damage Adequate compaction will help minimize the infiltration of water.

Thermal Cracking

Mixture Items that Affect Compaction
Aggregate texture, hardness and shape Asphalt Binder Grade Mixture Properties Voids Asphalt Binder Content Mixture Temperature and Cooling Rate

Field Operations that Affect the Compaction of the Bituminous Mixture
Number of Roller Passes Types of Rollers and Order they are used Mode of the Steel Rollers Vibratory mode on or off High/Low Amplitude High/Low Frequency in most cases the roller should be set to high frequency and low amplitude Speed of Rollers

Number of Roller Passes
1 3 5 7 9 2 4 Number Passes to Cover Paver Width Minimum Passes on Critical Longitudinal Section

Roller Speed Example Given: Plant & Paver Specifics - 12 feet wide
- 500 tons per hour - 1.5 inch lift Roller Specifics - minimum 3 passes coverage - roller width is 54 inch Required What is the minimum roller speed you need to compact the mat?

Roller Speed Answer The speed of the paver is:
500 tons per hour = 500*2000/145 = 6900 cuft per hour Pavement Cross section = 12 feet * 1.5 inch/12 = 1.5 sqft Paver speed = (6900 cuft/hr) / 1.5 sqft = 4600 ft/hour The required roller speed is: Roller Speed = 4600 ft/hour * 9 passes /5280 ft/mi = 7.8 mph

What are the Recommended Roller Speeds ?
Range of Roller Speeds (mph) Operating Position Type of Roller Breakdown Intermediate Finish Static Steel Wheel 2 – 3.5 3 - 5 Pneumatic 4 - 7 Vibratory 2 - 3 2.5 – 3.5 -- From Chapter 18 of Hot Mix Paving Handbook

You Need More Than 1 Roller !!!

Roller Settings and Roller Patterns
Typical Rolling Pattern: Vibratory Steel Roller as break down Pneumatic Roller as intermediate Static Steel Roller as finish roller Vibratory Steel Roller generally works best in the high frequency and low amplitude mode Only Pneumatic rollers should be used when the temperature of the mat is within the bituminous mixtures “tender zone”

Vibratory Breakdown

Pneumatic Breakdown

Amplitude (A) Time (T) For Full Cycle
Up Amplitude (A) Movement Time Down Time (T) For Full Cycle The elements of comparison for the dynamic component of a vibratory roller are the magnitude of the centrifugal force, its vibrating frequency, the nominal amplitude, and the ratio of the vibrating and non-vibrating masses acting on the drum. Amplitude: How high does the drum jump? Frequency, f = the number of hertz (cycles/s)--a single cycle is one full rotation of the eccentric weight. Frequency = 1/T Amplitude, A = the maximum deviation from position at rest -- one-half the total movement.

Amplitude Normal values of nominal amplitude range from 0.25 to 1 mm (1/100 to 1/32 in). Some rollers can operate at only one fixed amplitude, while others have “high” and “low” amplitude positions. Typically as the layer thickness increases, it is often advantageous to increase the nominal amplitude applied to the asphalt mix. Unless ‘high’ amplitude is needed to achieve a particular density level, the vibratory roller should be operated in ‘low’ amplitude. Ask: Does changing frequency change the compaction effort? (Yes vibrations/ft. Never less than 10.)

Frequency Low Frequency High Frequency Impact Spacing
The frequency of vibration is the number of complete cycles that the eccentrics rotate per minute. The faster the rotation of the eccentrics, the greater the frequency of vibration. Some vibratory rollers can operate at only one frequency or have a very limited selection of frequencies. Other vibratory rollers can alter the frequency of the applied load between 1600 and 4000 vibrations per minute. Frequencies below 2000 vibrations per minute are not normally acceptable to compact asphalt mixtures. DIRECTION OF TRAVEL

Joint Compaction Compaction of the Longitudinal Joint is the Weak Link in the Paving Process Compaction of the Both the Confined and Unconfined Edges is a Challenge The Unconfined Edge Needs to be Compacted with the Edge of the Steel Roller Over-hanging the Mat by 6 Inches If a Pneumatic Roller is used in the Break Down Position, it Should not be Closer than 6 Inches to the Joint

Joint Compaction Compaction of the Confined Joint is Best Achieved by Rolling on the Hot Mat and Overlapping the Cold Mat by 6 Inches Rolling from the Cold Side is No Longer Recommended Steel Rollers may Tend to Bridge from the Cold Mat and this May Impede Compaction Right at the Joint. A Pneumatic Roller will be More Effective in Reducing this Bridging. Raking of the Joint is Not Recommended

Factors Affecting Rolling Time
allows MORE time allows LESS time FACTORS Mat Thickness THICK THIN Layer thickness is the thickness of an asphalt pavement layer, and it is the most important variable in the rate of cooling of asphalt mixtures. It is very difficult to obtain the desired density on thin lifts of mix in cool weather because of the rapid loss in temperature in the mix. A portion of the heat in the asphalt layer is lost to the air as well as the base as it is placed. Depending on the air and base temperature as well as moisture content, the loss of temperature could be large or small. Mix Temperature HIGH LOW Base Temperature HIGH LOW

Time for Compaction Exercise

Paver Operations

Paver Operations The primary objective of the paver is to place a bituminous mixture on the roadway, in a manner that results in mat that has an even distribution of temperature, thickness and pre-compaction, in both the longitudinal and transverse directions.

Mix Placement Concerns
Even and steady flow of mixture to the paver Even and steady flow of mixture into the paver hopper Even and steady flow of mixture through the paver and in front of the paver screed The appropriate angle of attack of the screed needs to be set and frequent adjustments minimized

Flow of Mixture to the Paver

Flow of Mixture into the Paver Hopper

Paver Hopper and Flow Gates

Flow Gates set to Allow Uniform Material Flow to the Auger

Operating the Screed

What Affects the Thickness of the Bituminous Being Paved
If the paver speed, the height of material behind the screed, and tow point height are held constant, there is only one screed angle of attack that will give you the desired mat thickness.

Paver Schematic

TOW POINT

Automatics Screed Controls
Primary purpose of automatic screed control is to produce a smooth asphalt pavement layer. Smoother than a screed operator can accomplish by continually changing the setting of the thickness control cranks.

Automatics Screed Controls
Automatic Screed Functions by: maintaining the elevation of the screed tow points in relation to a reference other than that of the wheelbase of the paver itself. Elevation of the tow point is kept at a constant elevation in relation to a given grade reference. The automatics don’t allow the relative position of the tow point to change even though the tractor unit is moving up and down.

Automatics of the Paver
The paver operator controls the angle of attack of the screed. The automatic controls on the paver control the tow point height.

Checking the Mat Thickness

Screed Reaction Time The screed reacts to change in angle of attack over 5 tow arm lengths. 65% of change occurs in the first tow arm length. 35% of change occurs in the last 4 tow arm lengths.

How Long Does it Take the Screed to React to an Adjustment of the Thickness Control Screw

What do the Automatics on a Paver do?

Contract Special Provisions

Bituminous Web Page

Common Errors When Using Spread Sheets
Most common error happens when the macros are disabled due to the security setting being set to high Set the security level to “low” Move cursor to the menu on top and under the “Tools” menu, pick Tools: Macro: Security In the security dialog box, set the security level to “low” and press “OK”

Common Errors When Using Spread Sheets
Problems with creating random numbers The two versions of the core locations spread sheets use random number functions to pick the core locations. The program must make a decision whether or not be make new random numbers. Read the instructions for the spread sheet to be sure that the random numbers are being treated properly.

Questions ?