Hot Mix Asphalt Production and Placement

1. Texas A&M University, CVEN 342 Thursday, November 27, 2014 Hot Mix AsphaltProduction and Placement Gary L. Fitts, P.E. Sr. Field Engineer Asphalt Institute

2. ASPHALT INSTITUTE • International association of petroleum asphalt producers, manufacturers, and affiliated businesses, established in 1919 • Promotes the use, benefits and quality performance of petroleum asphalt through engineering, research and educational activities. • HQ office-Lexington, KY

4. References • Asphalt Institute • www.asphaltinstitute.org • 859-288-4960 • Texas Asphalt Pavement Association • www.txhotmix.org • 512-312-2099 • National Asphalt Pavement Association • www.hotmix.org • 888-468-6499 • Asphalt Pavement Alliance • www.AsphaltAlliance.com

5. HMA Plant Functions • Aggregate and asphalt storage • Aggregate drying • Dust collection, air pollution control • Aggregate and asphalt proportioning • Mixing • Mixture discharge/storage

6. Topics • Basic information on: • HMA Production • HMA Placement • HMA Compaction • Considerations for developing HMA specifications

7. Batch Plant

8. Batch Plants-Features • Aggregates dried, separated by size • Aggregates recombined by weight in weigh hopper • Aggregates introduced into pugmill, briefly mixed • Asphalt introduced by weight, mixed with aggregates • Completed HMA discharged or stored

9. Batch Plant Layout Dust Collector Cold Feed Bins Batch Tower Dryer Asphalt Cold Elevator Hot Elevator Storage Silo (optional) Asphalt Trucking, Inc

11. Drum Plants • Aggregates are dried, mixed with asphalt in a continuous operation • Quality control entirely dependent on: • stockpile management • plant calibration • Mixture must be stored in surge bin or silo

12. Drum Plant Layout

13. Multipav, S.A., El Salvador

14. Drum Plant-Knippa, Texas

15. Drum Plant Configurations • Parallel flow • Counter flow • “Coater” (continuous dryer/pugmill) • Drum in a drum • double • triple

16. Weigh Bridge

17. HMA Production • Objective is to produce a mixture meeting the specified design requirements • Volumetric • Mechanical • Samples are normally obtained from trucks at the plant, taken to the laboratory, and tested to confirm these qualities

18. Surface Preparation Prime Coats • Applied to unbound surface • Moderate, uniform application (0.15-0.35 gal/sy) of low viscosity liquid asphalt • Example materials • MC-30 • MC-70 • AEP

19. Surface Preparation Tack Coats • Applied to bound surface • Light, uniform application of liquid asphalt (0.03-0.05 gal/sy residual) • Example materials • Asphalt emulsions • SS-1h, CSS-1h, MS-2 • “Special Tack Emulsions” • Paving grade asphalt • PG 64-22

20. Tack Coat Application

22. Paving Equipment Paving Machine Components • Tractor unit • Screed • Electronic grade controls

24. HMA Delivery • Paver pulls up to meet the truck • DON’T BUMP THE PAVER! • Break the load before opening tailgate • Charge the hopper before it’s empty

25. Tractor and Screed Units MS-22, Fig. 5.11 (Courtesy of Blaw-Knox)

26. Forces Acting on Screed

27. Automatic Screed Controls • Electronic adjustment to screed height using sensing and reference system • Sensor detects elevation changes, adjusts height of tow point • Slope (transverse) controls

29. Paving Operations • Maintain uniform resistance to face of screed! • Keep uniform head of material at the face of the screed • Operate paver within a narrow range of forward speed • Coordinate mixture delivery, paver speed and compaction operations

30. Uniform Head of Material

32. Compaction The process of compressing a material into a smaller volume while maintaining the same mass.

33. Compaction • Essential to good performance! • Need to compact to desirable air voids level • Fine graded mixtures: 4-8% • Coarse or gap-graded mixtures: 3-6% • Compaction can only achieved if: • Mixture is confined • Mixture is hot (workable)

34. Factors Affecting Compaction • Mixture properties • Base/subgrade support (confinement) • Ambient conditions • Lift thickness

35. Compaction-Lift/Layer Thickness • Coarse-graded mixtures, mixtures using modified asphalts • Minimum 4X nominal maximum size • ½ in NMS – minimum 2” lift thickness • Fine-graded mixtures • Minimum 3X nominal maximum size • Thicker lifts also conserve heat, providing more time to complete compaction

36. Aggregate Gradation 100 max density line restricted zone nom max size max size Percent Passing control point 0 .075 .3 2.36 12.5 19.0 Sieve Size, mm, raised to 0.45 power

37. Fine graded Coarse graded Design Aggregate Structure 100 nom max size max size Percent Passing 0 .075 .3 2.36 12.5 19.0 Sieve Size, mm, raised to 0.45 power

38. 30 30 20 20 10 10 0 0 Mix Temp. = 275F 90F 60F 30F Time avail. for Compaction, min ~ 6 min 1 2 3 4 Compacted Thickness, in

39. Rolling Phases • Breakdown • Intermediate • Finish • Different equipment and different techniques for each phase

40. Compaction Equipment • Screed unit • weight of screed • external force applied to screed • vibratory unit • 35 Hz (2100 VPM) • tamper bar • Rollers • vibratory steel-wheeled • pneumatic • static steel-wheeled • combination

41. Vibratory Rollers • Commonly used for initial (breakdown) rolling • 8-18.5 tons, 57-84 in wide (“heavy” rollers) • 50-200 lbs/linear inch (PLI) • Frequency: 2700-4200 impacts/min. • Amplitude: 0.016-0.032 in. • For thin overlays (≤ 2 in.) use low amplitude or static mode • Operate to attain at least 10 impacts/ft • 2-4 mph

42. Amplitude & Frequency Time between blows, t Frequency = 1/t Amplitude

43. Roller Eccentrics Low amplitude High amplitude Example from Dynapac CC 501

44. Amplitude vs. Frequency • High amplitude generates most force • At same frequency, high amplitude does more work • Is it logical to use high amplitude with high frequency?

45. Vibratory Frequency • Frequency is drum impacts per minute • Working speed must match frequency • Best results when impact spacing is 10-14 per foot

46. Frequency Impacts/ft = 1/Impact Spacing

47. Impacts per Foot of TravelVibratory Rollers Reed Tachometer-used to check frequency of vibratory rollers

48. Static Steel-Wheeled Rollers • 10-14 ton rollers normally used for HMA compaction • Commonly use vibratory rollers operated in static mode • Lighter rollers used for finish rolling • Drums must be smooth and clean • For initial compaction, drive wheel must face paver AI MS-22, Figure 6.05

49. Static Steel-Wheeled Rollers

50. Pneumatic Tire Manipulation • Overlap manipulates mat under and between tire • Tight finish resists moisture penetration • Manipulation increased by lowering tire pressure • Static force increased by high tire pressure