Free Floating Screed Screed Weight Forward Motion Material Stability Head of Material Angle of Attack
Stability of the Mix Stability of Mix is Critical. Temperature Changes in Mix Can Effect Mat Being Laid. Consistent Temperature = Smooth Mat
Free Floating Screed Screed Weight Forward Motion Material Stability Head of Material Angle of Attack
Screed Weight Surface Area of Screed in Contact With Mat (PSI)
Screed Weight Watch When Extending & Retracting Screed Changes PSI When Extending and Retracting Variable Width Screed
Screed Adjustments Lead Crown Correct v Even texture across full width v Zero crown for most mixes
Screed Adjustments Lead Crown Low v Open texture in center v Tight on sides v Put in crown
Screed Adjustments Lead Crown High v Tight, shiny strip in center v Open texture on sides v Reduce lead crown
Paving Speed v Maintaining a consistent paver speed is key l material feed system is set by paver speed
Paver - Segregation v Stopping & starting paver v Paving speed
Paver - Segregation v Truck v Keep End drag slats covered
Paver - Segregation v Cycling hopper wings
Don t Run the Hopper Empty! Don t Expose Conveyor Chains
Don t Allow HMA to Dribble When Raising the Bed! Segregation
Don t Bump the Paver!
Don t Disturb the Paver Steering
Don t Clean & Leave Piles of HMA on the Grade!
Don t Dump Material on Grade!
Alabama Fire Alarm
Texas Horseshoes
v Compaction basics v Asphalt compaction
Importance of Compaction v Improve Mechanical Stability v Improve Resistance to Permanent Deformation v Reduce Moisture Penetration v Improve Fatigue Resistance v Reduce Low-Temperature Cracking Potential
Hot Mix Compaction v Asphalt binder holds particles together l Provides lubrication at high temperatures l Provides cohesion at in-service temperatures v Prevents air and water intrusion into mat
Factors Affecting Compaction v Properties of the Materials v Environmental Variables v Laydown Site Conditions
Properties of the Materials v Aggregate v Asphalt Cement v Mix Properties
Environmental Variables v Layer thickness v Air and base temperature v Mix laydown temperature v Wind velocity v Effect of sun
Laydown Site Conditions v Lift thickness versus aggregate size v Lift thickness uniformity v Base Conditions
Compaction Forces Static Pressure Manipulation Impact Vibration
Static Pressure Weight at Drum Ground Contact Area Drum Width
Manipulation
Vibration v Eccentric weights cause drum to move up and down v Vibration increases compactive effort
Frequency & Impact Spacing
Drum Impacts per foot (10/ft min.) Frequency 2 MPH 3 MPH 4 MPH 5 MPH 2000 vpm 11.36 7.58 5.68 4.55 2200 vpm 12.50 8.33 6.25 5.00 2400 vpm 13.64 9.09 6.82 5.45 2600 vpm 14.77 9.84 7.39 5.91 2800 vpm 15.91 10.61 7.95 6.36 3000 vpm 17.05 11.36 8.52 6.82 3200 vpm 18.18 12.12 9.09 7.27 3400 vpm 19.32 12.88 9.66 7.72 3600 vpm 20.45 13.64 10.22 8.18 3800 vpm 21.59 14.39 10.80 8.63
Compaction Rules Compaction must be achieved while viscosity of binder and stiffness of mix are low enough to allow reorientation of aggregate particles under action of rollers
Compaction Rules Initial compaction should occur directly behind paver in order to reach required density quickest
Drum Movement Large Amplitude Small Amplitude At same frequency large amplitude generates greater force
Amplitude vs Centrifugal Force Machine 1 Machine 2 0.02" Amplitude (lift height) 0.078" 3,000 vpm Frequency 1,500 vpm (number of hits) 40,000 lb Centrifugal Force 40,000 lb
Asphalt Compaction
Keys to Compaction v Roll mix as hot as possible l Keeps asphalt viscosity low l Able to re-orient aggregate particles v Keep rollers close to paver
Density Growth n Primary compaction n Aggregate movement v Some additional compaction v Binder movement n Minimal additional compaction n Smooth surface
Amplitude *For very thin lifts, vibration is not recommended, especially on rigid base supports
Double Drum Vibratory v Keep close behind paver v Breakdown should be completed before mix falls below 250 o F v Operated at highest frequency level v Amplitude dependent on thickness of HMA being placed
Pneumatic Compaction
Pneumatic Roller v Keep tires at same temperature as mat being compacted v Don t let rollers sit v Difficult to avoid pickup with some PMA s v Compaction should be completed before mix temperature drops to about 210 o F
Asphalt Sticking Possible causes: v Tires too cool v Modifiers in asphalt v Nonstick emulsion ineffective
Static Compaction
Static Compaction v Get last little bit of density v Remove any roller marks v Complete before about 175 o F
Time Available for Compaction (TAC)
TAC Example # Mix Temp, o F Air/Base Temp, o F Mat Thk, in 1 300 60/60 3 2 250 60/60 3 3 300 40/40 3 4 250 40/40 3 THE Answer, minutes
TAC Example # Mix Temp, o F Air/Base Temp, o F Mat Thk, in THE Answer, minutes 1 300 60/60 3 64 2 250 60/60 3 36 3 300 40/40 3 52 4 250 40/40 3 29
TAC Example # Mix Temp, o F Air/Base Temp, o F Mat Thk, in 5 300 60/60 2 6 250 60/60 2 7 300 40/40 2 8 250 40/40 2 THE Answer, minutes
TAC Example # Mix Temp, o F Air/Base Temp, o F Mat Thk, in THE Answer, minutes 5 300 60/60 2 33 6 250 60/60 2 18 7 300 40/40 2 26 8 250 40/40 2 15
Time Available for Compaction www.asphaltpavement.org Search for MultiCool