1. PCC Overlays of HMA Pavements
Module 4-13 : PCC Overlays
4/17/2017
MODULE 3-7
PCC Overlays of HMA Pavements
(Whitetopping)
This module presents PCC overlays of HMA pavements, a process sometimes referred to as “whitetopping.” Although whitetopping has been in used since 1918, its use in the last 10 years has grown considerably.

2. Module 4-13 : PCC Overlays
4/17/2017
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
These are the learning objectives for this module.

3. Types of Whitetopping Overlays
Module 4-13 : PCC Overlays
4/17/2017
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 load-carrying capacity
We first want to define the two types of whitetopping overlays. Conventional whitetopping overlays are greater than 100 mm (4 in) thick and are placed directly on a deteriorated HMA pavement with little preoverlay repair or surface preparation. UTW are a relatively new technology that has generated considerable interest in the last few years. UTW are thin slabs with short joint spacings that are bonded to an existing HMA pavement to increase the load-carrying capacity.
Conversions: 50 to 100 mm = 2 to 4 in; 0.6 to 1.8 m = 2 to 6 ft.

4. Conventional Whitetopping
Module 4-13 : PCC Overlays
4/17/2017
Conventional Whitetopping
PCC Overlay
Existing
HMA Pavement
Subbase
Interface
Here is a schematic of a conventional whitetopping overlay. Again, these are placed on HMA with little overlay repair. Some surface preparation such as milling to remove severe rutting may be required. An unbonded condition is assumed although some degree of bonding does occur.

5. Ultra-Thin Whitetopping
Module 4-13 : PCC Overlays
4/17/2017
Ultra-Thin Whitetopping
Thin Slabs
(50 to 100 mm)
Short Square Slabs
(0.6 to 1.8 m)
This is a schematic of UTW. The slabs are very thin (50 to 100 mm [2 to 4 in]) and the slabs are short and typically square. The existing HMA surface is milled to enhance the bond between the two surfaces.
Conversion: 0.6 to 1.8 m = 2 to 6 ft.
Existing HMA Pavement
Milled Surface

6. Effect of Joint Spacing on UTW
Module 4-13 : PCC Overlays
4/17/2017
Effect of Joint Spacing on UTW
0.6 m
3.0 m
Short Joint Spacings
(slabs vertically deflect under loading)
Conventional Joint Spacings
(slabs bend under loading)
Stresses
The effect of the short joint spacing on UTW behavior is shown here. The shorter joint spacing allows the slabs to deflect vertically under loading (much like a paver block), so the much of the load is transmitted to the underlying HMA. Shorter joint spacings also reduce the curling and warping stresses.
Conversion: 0.6 m = 2 ft; 3.0 m = 10 ft.

7. Applicability Conventional Whitetopping
Module 4-13 : PCC Overlays
4/17/2017
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
When are these techniques applicable? Conventional whitetopping is appropriate for use on badly deteriorated HMA pavements under most any traffic level. UTW are appropriate on low volume roads exhibiting some structural distress but the HMA must still have some load carrying capacity. UTW have been widely used at urban intersections where recurrent rutting and washboarding has been a problem.

8. Conventional Whitetopping —Use and Performance—
Module 4-13 : PCC Overlays
4/17/2017
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.
Conventional whitetopping has been widely used in several states, perhaps most notably in Iowa on their county road system. They have shown to provide good to excellent performance, perhaps due to some partial bonding between the PCC and HMA. However, as with conventional PCC pavements, effective joint design is required.

9. Ultra-Thin Whitetopping —Use and Performance—
Module 4-13 : PCC Overlays
4/17/2017
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
UTW has seen tremendous growth in the last 10 years, with over 200 projects constructed in 35 states since Tennessee, Kentucky, and Kansas have been big users of UTW. Generally the performance of UTW has been good, although some cracking has occurred on a few projects shortly after construction, perhaps due to either debonding or late sawing. Also some cracking has occurred on panels adjacent to HMA pavements, perhaps due to impact forces of vehicles.

10. Overlay Selection Detailed pavement evaluation (distress, FWD, coring)
Module 4-13 : PCC Overlays
4/17/2017
Overlay Selection
Detailed pavement evaluation (distress, FWD, coring)
Construction feasibility
Performance period
Cost effectiveness
What factors should be considered in selecting a PCC whitetopping overlay as a potential rehabilitation option? Pavement condition is always a key factor, and supplemental FWD and coring data are often needed to fully assess the structural condition of the pavement. For UTW especially, some load-carrying capacity of the existing HMA is required. Other factors include the construction feasibility, desired performance period, and overall cost effectiveness, each of which will be discussed in the following slides.

11. Whitetopping Overlay Feasibility—Constructibility
Module 4-13 : PCC Overlays
4/17/2017
Whitetopping Overlay Feasibility—Constructibility
Conventional
UTW
Vertical
Clearance
Traffic
Control
Construction
Generally not a problem
May be difficult to construct under traffic
No special equipment
Can be a problem
May be difficult to construct under traffic
No special equipment
When evaluating the construction feasibility of whitetopping overlays, some of the main factors to consider are constructibility, performance period, and cost effectiveness. This table, taken from page in the reference manual, summarizes some of the constructibility characteristics of whitetopping overlays.

12. Whitetopping Overlay Feasibility—Performance Period
Module 4-13 : PCC Overlays
4/17/2017
Whitetopping Overlay Feasibility—Performance Period
Conventional
UTW
Existing Condition
Extent of Repair
Future Traffic
Historical Reliability
Very deteriorated HMA pavements
Limited to very severe areas
Any traffic level
Very good
Deteriorated HMA pavements
Severe areas and milling
Lower traffic volumes
Good but not well established
This summarizes some of the performance characteristics of whitetopping overlays. Both can be placed on deteriorated HMA pavements, but UTW still require an HMA pavement with load-carrying capacity. Conventional whitetopping can be used for any traffic level, but UTW are limited to lower traffic roadways. The reliability of conventional whitetopping overlays has been good, whereas the reliability for UTW is good but not well established (limited long-term performance data).

13. Whitetopping Overlay Feasibility—Cost Effectiveness
Module 4-13 : PCC Overlays
4/17/2017
Whitetopping Overlay Feasibility—Cost Effectiveness
Conventional
UTW
Higher than conventional HMA overlay
Competitive
Higher than conventional HMA overlay
Competitive
Initial Cost
Life-Cycle Cost
Although the initial costs of whitetopping overlays are higher than conventional HMA overlays, they are cost effective when the overall life-cycle costs are considered. This is particularly true for UTW placed in an urban area where frequent maintenance and rehabilitation may have been required for rutted and washboarded intersections.

14. Design Considerations
Module 4-13 : PCC Overlays
4/17/2017
Design Considerations
Slab thickness
Joint design
Drainage design
Reinforcement design
PCC mix design
Preoverlay repair and surface preparation
Moving on now to the design of whitetopping overlays, the most critical design considerations are shown here. Although the thickness design of whitetopping overlays generally receives the most attention, other factors such as an appropriate joint design, considerations of drainage, and performing adequate preoverlay repair or surface preparation all have a significant effect on the performance of whitetopping overlays. Some of these key items are discussed in the following slides.

15. Slab Thickness Design —Whitetopping—
Module 4-13 : PCC Overlays
4/17/2017
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)
The 1993 AASHTO Guide, which is the most commonly used overlay design procedure, designs a conventional whitetopping overlay as a new PCC pavement and treats the existing HMA as a stiff base. Thus, an unbonded condition is assumed, although some partial bonding does occur. A minimum thickness of 150 mm (6 in) is recommended for primary roadways and 100 mm (4 in) for secondary roadways.

16. Slab Thickness Design —UTW—
Module 4-13 : PCC Overlays
4/17/2017
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
The PCA recently developed an interim procedure for determining the load carrying capacity of a UTW overlay. Specifically, it provides the allowable truck traffic for a UTW of specified thickness, strength, joint spacing, and k-value. It does account for the bonding between the new PCC and the existing HMA pavement.

17. Joint Design —Whitetopping Overlays—
Module 4-13 : PCC Overlays
4/17/2017
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 for conventional whitetopping overlays generally follows conventional PCC pavement joint practices. The joint spacing for JPCP should be a maximum of 21 * D, and for JRCP about 9 to 12 m (30 to 40 ft). Dowels should be used for all JRCP and for JPCP if the design ESALs is greater than about 4 or 5 million.

18. Joint Design —UTW Overlays—
Module 4-13 : PCC Overlays
4/17/2017
Joint Design —UTW Overlays—
Joint spacing
Maximum spacing of 12 to 15 * D
Load transfer
Dowels not used (aggregate interlock across closely spaced joints)
For UTW, the joint spacing should be a maximum of (12-15) * D in order to minimize slab bending effects and thermal curling effects. Note that if the PCA design procedure is used, a joint spacing will have already been selected (the design charts are for a fixed combinations of slab thickness and joint spacing). Dowels are not used in UTW because of the thinness of the slabs and the reliance upon the aggregate interlock across the joints of the short slabs.

19. PCC Mix Design Whitetopping Overlays Conventional mixtures
Module 4-13 : PCC Overlays
4/17/2017
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
Synthetic fibers for ductility
PCC mix design for conventional whitetopping overlays uses conventional mixtures, although the use of fast track, high-early strength mixes may be needed if stringent opening times exist for a project. UTW are generally high strength mixes, obtained by using a low w/c ratio (~0.42) and high cement contents (often greater than 386 kg/m3 [650 lb/yd3]). Again, high-early strength mixes may be needed if early opening times are specified. Many UTW projects have employed synthetic fibers (generally polypropylene at about 0.1 to 0.5 percent) for toughness, ductility, and improved post-cracking behavior. Note that these synthetic fibers do not increase the PCC flexural strength (but steel fibers do).

20. Preoverlay Repairs —Whitetopping Overlays—
Module 4-13 : PCC Overlays
4/17/2017
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
Preoverlay repairs for conventional whitetopping overlays is not that critical. It usually requires repair of severely deteriorated areas (due to subgrade or base failure), filling of potholes, milling (or surface leveling) if rutting is greater than 50 mm (2 in), and repair of severe alligator cracking if poor support would otherwise result. Good uniform support is the goal.
Goal: Uniform support

21. Preoverlay Repairs —UTW Overlays—
Module 4-13 : PCC Overlays
4/17/2017
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
Preoverlay repairs for UTW consists of localized repair of failed areas, filling of potholes, repair of alligator cracking if it will detract from the load-carrying capacity of the HMA, and milling of the HMA surface. The milling is particularly critical to the performance of the UTW, as it not only removes rutting and restores the surface profile, it also enhances the bond between the layers.

22. Construction —Whitetopping Overlays—
Module 4-13 : PCC Overlays
4/17/2017
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
The construction of conventional whitetopping uses conventional PCC paving equipment and construction practices. The PCC is placed either directly on the HMA or on a milled or leveled surface. Whitewashing of the HMA surface may be required on hot days to reduce the temperature of the surface and prevent the buildup of excessive heat in the PCC that could lead to cracking.

23. Whitetopping Overlays —Milling HMA Surface—
Module 4-13 : PCC Overlays
4/17/2017
Whitetopping Overlays —Milling HMA Surface—
This shows a photo of milling an existing HMA pavement on a whitetopping project.

24. Whitetopping Overlays —PCC Paving—
Module 4-13 : PCC Overlays
4/17/2017
Whitetopping Overlays —PCC Paving—
Photo of PCC placement on HMA pavement. Note the whitewashed HMA surface in the photo on the right. (Note also that this is poor practice using the dump truck instead of a side dump. The dump truck is likely to jar or move the dowel baskets. There is also the problem of staking baskets into concrete!)

25. Whitetopping Overlays —New Pavement—
Module 4-13 : PCC Overlays
4/17/2017
Whitetopping Overlays —New Pavement—
Photo showing a new whitetopping overlay. Note the thickened edge design (from an Iowa whitetopping project).

26. Whitetopping Overlays —Joint Sawing—
Module 4-13 : PCC Overlays
4/17/2017
Whitetopping Overlays —Joint Sawing—
Consider increased saw depth over major distortions D
Sawcut Depth D/3
+ 50 mm
PCC Overlay
HMA Pavement
One critical part of whitetopping construction is the sawing of joints in areas where major distortions (rutting greater than 50 mm [2 in]) exists. Consideration should be given to increased sawcut depths in these areas to ensure the establishment of the weakened plane joints.

27. Construction —UTW Overlays—
Module 4-13 : PCC Overlays
4/17/2017
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
As with conventional whitetopping, UTW also uses conventional paving equipment and practices. Three critical factors in the construction of UTW are 1) milling of the HMA to enhance bond (must have between 75 and 150 mm [3 and 6 in] of HMA thickness after milling for load-carrying), 2) timely joint sawing to prevent development of random cracking, and 3) effective curing is required to help maintain moisture. The latter is very important because the high surface-area-to-volume ratio of UTW slabs means that water can be lost very rapidly. The use of a double application of curing compound is recommended.

28. UTW Overlays —Milling HMA Surface—
Module 4-13 : PCC Overlays
4/17/2017
UTW Overlays —Milling HMA Surface—
The UTW construction process begins by first milling the HMA surface

29. UTW Overlays —PCC Paving—
Module 4-13 : PCC Overlays
4/17/2017
UTW Overlays —PCC Paving—
After milling, the UTW is placed. Either fixed form or slip form paving can be done. Many urban intersection projects have used fixed form paving.

30. UTW Overlays —Finishing and Curing—
Module 4-13 : PCC Overlays
4/17/2017
UTW Overlays —Finishing and Curing—
After placement, the PCC pavement is finished and cured. Again, double applications of curing compound is recommended.

31. UTW Overlays —New Pavement—
Module 4-13 : PCC Overlays
4/17/2017
UTW Overlays —New Pavement—
Photo of completed pavements on Route 21 near Belle Plain, Iowa. Sealed and unsealed joints shown here with short 2x2 joints.

32. UTW Overlays —Thickened Edge at Transition Areas—
Module 4-13 : PCC Overlays
4/17/2017
UTW Overlays —Thickened Edge at Transition Areas—
PCC Overlay
Existing HMA Pavement h
h + 75 mm
2 m
Base or Subgrade
One performance problem of UTW has been cracking of the transition slabs adjacent to a left-in-place HMA pavement. This is believed to be due to impact loading of vehicles. It is recommended that a thickened edge design be adopted in these locations.

33. Module 4-13 : PCC Overlays
4/17/2017
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?
Ask the participants these review questions.

34. Module 4-13 : PCC Overlays
4/17/2017
Key References
American Concrete Pavement Association (ACPA) Whitetopping—State of the Practice. Engineering Bulletin EB210P. American Concrete Pavement Association, Skokie, IL.
McGhee, K. H Portland Cement Concrete Resurfacing. NCHRP Synthesis of Highway Practice Transportation Research Board, Washington, DC.
Key references on whitetopping overlays.