Transcript HMA Module 03

MODULE 3-7
PCC Overlays of
HMA Pavements
(Whitetopping)
Learning Objectives
• Differentiate between the two types of
PCC overlays of HMA pavements
• List conditions appropriate for the use of
each type of PCC overlay
• Describe the factors contributing to the
performance of UTW overlays
• Describe preoverlay repairs and surface
preparation required for each type of
PCC overlay
Types of Whitetopping
Overlays
• Conventional Whitetopping
– Slabs greater than 100 mm thick
– Placed directly on HMA pavement (little
preoverlay repair)
• Ultra-Thin Whitetopping
– Thin slabs (50 to 100 mm thick)
– Short joint spacing (0.6 to 1.8 m)
– Bonded to existing HMA to increase loadcarrying capacity
Conventional Whitetopping
Interface
PCC Overlay
Existing
HMA Pavement
Subbase
Ultra-Thin Whitetopping
Short Square Slabs
(0.6 to 1.8 m)
Thin Slabs
(50 to 100 mm)
Milled Surface
Effect of Joint Spacing on
UTW
0.6 m
0.6 m
0.6 m
Stresses
Short Joint Spacings
(slabs vertically deflect under loading)
3.0 m
Stresses
Conventional Joint Spacings
(slabs bend under loading)
Applicability
• Conventional Whitetopping
– Badly deteriorated HMA pavements
– Most any traffic volume
• Ultra-Thin Whitetopping
– Low volume roads exhibiting rutting,
shoving, potholing
– Urban intersections where recurrent
rutting/washboarding has been a
problem
Conventional Whitetopping
—Use and Performance—
• Extensive use in Iowa, Nebraska, Utah,
Nevada, California, Texas
• Good to excellent performance
• Some partial bonding occurs which may
contribute to performance
• Effective joint design required.
Ultra-Thin Whitetopping
—Use and Performance—
• Since 1992, over 200
projects in 35 states
• Generally good
performance
• Some cracking on a few
projects shortly after
construction
• Some cracking observed
on panels adjacent to HMA
Overlay Selection
• Detailed pavement evaluation (distress,
FWD, coring)
• Construction feasibility
• Performance period
• Cost effectiveness
Whitetopping Overlay
Feasibility—Constructibility
Conventional
UTW
Vertical
Clearance
Generally not a
Can be a problem
problem
Traffic
Control
May be difficult to May be difficult to
construct under construct under
traffic
traffic
Construction
No special
equipment
No special
equipment
Whitetopping Overlay
Feasibility—Performance Period
Conventional
Existing
Condition
UTW
Deteriorated
Very deteriorated
HMA pavements HMA pavements
Extent of
Repair
Limited to very
severe areas
Severe areas
and milling
Future
Traffic
Any traffic level
Lower traffic
volumes
Very good
Good but not
well established
Historical
Reliability
Whitetopping Overlay
Feasibility—Cost Effectiveness
Conventional
Initial Cost
Life-Cycle
Cost
UTW
Higher than
Higher than
conventional HMA conventional HMA
overlay
overlay
Competitive
Competitive
Design Considerations
• Slab thickness
• Joint design
• Drainage design
• Reinforcement design
• PCC mix design
• Preoverlay repair and surface
preparation
Slab Thickness Design
—Whitetopping—
• New PCC pavement designed by
characterizing existing HMA as stiff base
• Unbonded condition assumed
• Minimum thicknesses:
– Primary/Interstate: 150 mm (6 in)
– Low Volume/Secondary: 100 mm (4 in)
Slab Thickness Design
—UTW—
• Interim procedure available for
determining load-carrying capacity
• Accounts for bonding of PCC to HMA
• Inputs:
– PCC strength
– k-value
– Thickness
– Joint spacing
Joint Design
—Whitetopping Overlays—
• Generally follows conventional PCC
pavement jointing practices
• Joint spacing
– JPCP: Maximum spacing of 21 * D
– JRCP: 9 to 12 m
• Load transfer
– JPCP: Dowel if > 4-5 million ESALs
– JRCP: Always use dowels
Joint Design
—UTW Overlays—
• Joint spacing
– Maximum spacing of 12 to 15 * D
• Load transfer
– Dowels not used (aggregate interlock
across closely spaced joints)
PCC Mix Design
• Whitetopping Overlays
– Conventional mixtures
– Possible fast track for early opening
• UTW Overlays
– Generally high strength mixes
 Low w/c
 High cement contents
– Possible fast track for early opening
– Synthetic fibers for ductility
Preoverlay Repairs
—Whitetopping Overlays—
• Localized repair of failed areas
• Filling of potholes
• Milling if rutting greater than 50 mm
• Repair of severe alligator cracking if
poor support would otherwise result
Goal: Uniform support
Preoverlay Repairs
—UTW Overlays—
• Localized repair of failed areas
• Filling of potholes
• Repair of severe alligator cracking or
other areas that will detract from the
load carrying capacity
• Milling of the HMA surface
– Remove rutting
– Restore profile
– Enhance bond
Construction
—Whitetopping Overlays—
• Conventional PCC paving equipment
and construction practices are used
• PCC may be placed directly on HMA or
on milled or leveled HMA surface
• Whitewashing of HMA surface may be
required on hot days
Whitetopping Overlays
—Milling HMA Surface—
Whitetopping Overlays
—PCC Paving—
Whitetopping Overlays
—New Pavement—
Whitetopping Overlays
—Joint Sawing—
Consider increased saw depth
over major distortions
D
+ 50 mm
PCC Overlay
HMA Pavement
Sawcut
Depth
D/3
Construction
—UTW Overlays—
• Conventional paving equipment and
practices
• Critical factors
– Milling of HMA (minimum HMA
thickness of 75 to 150 mm)
– Timely joint sawing
– Effective curing
UTW Overlays
—Milling HMA Surface—
UTW Overlays
—PCC Paving—
UTW Overlays
—Finishing and Curing—
UTW Overlays
—New Pavement—
UTW Overlays
—Thickened Edge at Transition
Areas—
h
PCC Overlay
h + 75 mm
Existing
HMA
Pavement
2m
Base or Subgrade
Review
• What are the two types of PCC overlays
used on HMA pavements? How are they
different?
• What conditions are favorable to the use
of each PCC overlay type?
• What factors contribute to the
performance of UTW overlays?
• What preoverlay repairs and surface
preparation are required for each PCC
overlay type?
Key References
• American Concrete Pavement Association
(ACPA). 1998. Whitetopping—State of the
Practice. Engineering Bulletin EB210P.
American Concrete Pavement Association,
Skokie, IL.
• McGhee, K. H. 1994. Portland Cement
Concrete Resurfacing. NCHRP Synthesis of
Highway Practice 204. Transportation
Research Board, Washington, DC.