Asphalt Pavement Laydown and Compaction

1. Asphalt Pavement Laydown and Compaction Picture of plant

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

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

4. Fatigue Cracking

5. Rutting

6. Asphalt Binder Oxidation - Hardening

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

8. Thermal Cracking

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

10. 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

11. Number of Roller Passes Roller Passes Number Passes to Cover Paver Width Minimum Passes on Critical Longitudinal Section

12. 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?

13. 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

14. What are the Recommended Roller Speeds ? From Chapter 18 of Hot Mix Paving Handbook Range of Roller Speeds (mph) Operating Position Type of Roller Breakdown Intermediate Finish

15. You Need More Than 1 Roller !!!

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

17. Vibratory Breakdown

18. Pneumatic Breakdown

19. Amplitude (A) Up Movement Time Down Time (T) For Full Cycle 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.

20. Amplitude

21. High Frequency Low Frequency Impact Spacing Frequency DIRECTION OF TRAVEL

22. 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

23. 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

24. Factors Affecting Rolling Time allows LESS time allows MORE time FACTORS Mat Thickness THICK THIN Mix Temperature HIGH LOW Base Temperature HIGH LOW

25. Time for Compaction Exercise

26. Paver Operations

27. 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.

28. Mix Placement Concerns • Even and steady flow of mixture into the paver hopper • Even and steady flow of mixture to the paver • 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

29. Flow of Mixture to the Paver

30. Flow of Mixture into the Paver Hopper

31. Paver Hopper and Flow Gates

32. Flow Gates set to Allow Uniform Material Flow to the Auger

33. Operating the Screed

34. 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.

37. Paver Schematic

42. TOW POINT

43. 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.

44. 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.

47. 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.

48. Checking the Mat Thickness

49. Adjusting the Thickness

50. 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.