A PCC overlay of an existing flexible pavement, called “whitetopping”, is a newer, viable rehabilitation alternative for flexible pavements. The overlayed rigid layer offers a reasonably thin, highly durable wearing course with a significant structural capacity. Although there are risks, whitetopping can be effective for almost all applications. They have been successfully used on interstate highways, state primary and secondary roads, intersections, etc. as well as major airport and general aviation runways, taxiways, and aprons (Mack, Hawbaker and Cole, 1998). This subsection covers:
- Unbonded PCC overlays, often called “classical whitetopping”
- Bonded PCC overlays, often called “thin composite whitetopping”
Unbonded – Classical Whitetopping
Classical whitetopping is an unbonded PCC overlay of an existing flexible pavement. Because there is no bond, the existing flexible pavement is assumed to function only as a base for the new PCC overlay. Most often, the PCC overlay is placed directly on the flexible pavement surface after sweeping to remove loose debris. Generally, classical whitetopping works well as long as rut and pothole depths in the existing flexible pavement are less than 50 mm (2 inches). If rut or pothole depths are deeper, the potholes are filled or the surface is milled. All three types of rigid pavement (JPCP, JRCP and CRCP) have been successfully used as classical whitetopping (McGhee, 1994).
The chief advantage of classical whitetopping is that it requires minimal surface preparation. However, minimum overlay thicknesses tend to be in the 125 – 175 mm (5 – 7 inch) range, which is quite thick and possibly unsuitable in situations where a specific elevation must be maintained such as in curbed areas or under bridges.
The design procedure contained in the 1993 AASHTO Guide is virtually identical to the AASHTO empirical design for new rigid pavements with one exception: The effective modulus of subgrade reaction (k) is determined based on the existing flexible pavement resilient modulus. Although perfectly acceptable, this method gives little credit to the existing pavement’s remaining strength.
Bonded – Thin Composite Whitetopping
Thin composite whitetopping (Figure 1) is a PCC overlay intentionally bonded to an existing flexible pavement with a PCC slurry or grout in order to create a composite pavement section (Mack, Hawbaker and Cole, 1998). This composite section, acting as a single layer, is thicker than just the PCC overlay and thus, results in substantially reduced maximum slab tensile stresses (on the order of 1/2 for edge stresses and 1/4 for corner stresses) (Mack, Hawbaker and Cole, 1998). Overlay thicknesses tend to be 50 – 175 mm (2 – 7 inches) thick but can be thicker for high volume roads; overlays in the 50 – 100 mm (2 – 4 inches) range are often referred to as “ultra-thin whitetopping” (UTW). Thin white topping (i.e., bonded PCC overlay greater than 100 mm (4 inches) thick) is considered appropriate for all situations and traffic levels. UTW as conceived and developed in the early 1990’s is intended more for lower-volume roads, vehicular parking areas and light duty airports (Mack, Hawbaker and Cole, 1998).
The chief advantage of thin composite whitetopping is that it can be made thinner than classical whitetopping because of the composite layer action. However, issues with slab size, joint location and bonding effectiveness can complicate its use. This subsection covers:
The 1993 AASHTO Guide design procedure does not account for the bonded composite action of the combined pavement-plus-overlay. Therefore, it treats the bonded overlay design exactly the same as the unbonded one and does not credit the existing flexible pavement with any structural capacity. In reality, if the bond between layers is adequate then the structural support capacity of the underlying flexible pavement should be considered. Although multiple studies have shown this bonding to be adequate (Mack, Hawbaker and Cole, 1998), the assumption of adequate bond performance is still a significant risk. If, for some reason, the bond does not perform as intended then the pavement will most likely fail prematurely. Surface preparation is critical.
The http://www.pavement.com/ American Concrete Pavement Association (ACPA) has a web page that will calculate the load-carrying capacity of an ultra-thin whitetopping (UTW) pavement during its service life. The calculations are based on a comprehensive mechanistic analysis and correlation to UTW performance data. This web page can be found at: http://www.pavement.com/Concrete_Pavement/Technical/UTW_Calculator/index.asp http://www.pavement.com/Concrete_Pavement/Technical/UTW_Calculator/index.asp.
Joints are typically design much closer than for typical new-construction rigid pavement. The closer joint spacing, on the order of 1 – 4 m (3.3 – 13.1 ft.), does the following (Mack, Hawbaker and Cole, 1998):
- Reduces the moment arm of the applied wheel load and minimizes the stresses due to bending.
- Reduces the curling and warping stresses by reducing the size of the slab that can curl or warp.
Because of the short joint spacing, the overlaid PCC slabs transfer load to the underlying flexible pavement by deflecting downward as a unit rather than bending (Mack, Hawbaker and Cole, 1998). Figures 2 and 3 show two different joint spacings.
Some criteria for deciding when to consider thin composite whitetopping as a rehabilitation alternative are (Vandenbossche and Fagerness, 2001):
- The existing flexible pavement must be more than 100 mm (4 inches) thick. This provides a reasonably strong structural layer to which the PCC overlay can bond. Mack, Hawbaker and Cole (1998) suggest a minimum thickness of 75 mm (3 inches).
- No raveling on the existing flexible pavement. Raveling will adversely affect bonding.
- Little to no bottom-up fatigue cracking in the existing flexible pavement. Bottom up fatigue cracking will continue to progress and weaken the flexible pavement structure even after the PCC overlay.
- Joints should not be in the wheel paths (Figure 4). Joints in the wheel paths will lead to edge loading, which induces high slab edge and corner stresses that can lead to cracking. Thus, shorter joint spacing is not always better.
- Proper PCC curing is critical. The thin slabs have a high surface-to-volume ratio and can lose water to evaporation quite rapidly. Mack, Hawbaker and Cole (1998) recommend applying a curing compound at twice the normal rate to help avoid shrinkage cracking and debonding between the PCC and flexible pavement layers.
- PCC – flexible pavement bonding is critical to performance. If the bond is inadequate, the PCC overlay will, in essence, function alone. This will substantially increase maximum slab tensile stresses, increasing the potential for cracking.
- Mack, J.W.; Hawbaker, L.D. and Cole, L.W. (1998). Ultrathin Whitetopping: State-of-the-Practice for Thin Concrete Overlays of Asphalt. Transportation Research Record 1610. Transportation Research Board, National Research Council, Washington, D.C. pp. 39-43. A discussion based on this paper is available on the American Concrete Pavement Association (ACPA) web site at: http://www.pavement.com/PavTech/Tech/Fundamentals/fundutw.html.↵
- McGhee, K.H. (1994). National Cooperative Highway Research Program Synthesis of Highway Practice 204: Portland Cement Concrete Resurfacing. Transportation Research Board, National Research Council. Washington, D.C.↵
- Vandenbossche. J.M. and Fagerness, A.J. (2001). Performance and Repair of Ultra-Thin Whitetopping: The Minnesota Experience. Paper presented at the 2002 annual Transportation Research Board meeting. Minnesota Department of Transportation. St. Paul, MN. Available on the TRB 81st Annual Meeting CD-ROM.↵