Figure 10.16: Thin Composite Whitetopping at the Mn/ROAD Test Facility
| Major Topics on this Page | ||
| 3.1 | Structural HMA Overlays | |
| 3.2 | Structural PCC Overlays | |
| 3.3 | Summary | |
The combined effects of traffic loading and the environment will cause pavements to deteriorate over time. Although maintenance can slow the rate of deterioration, it cannot stop it. Therefore eventually the effects of deterioration need to be reversed by adding or replacing material in the existing pavement structure. This is called rehabilitation. Formally, rehabilitation can be defined as (Hall et al., 2001):
"...a structural or functional enhancement of a pavement which produces a substantial extension in service life, by substantially improving pavement condition and ride quality."
A wholesale replacement of the entire pavement structure is considered reconstruction rather than rehabilitation since it follows new pavement construction methods. Flexible pavement rehabilitation options depend upon local conditions and pavement distress types but typically include:
NCHRP Web document 35 (Project C1-38): Rehabilitation Strategies for Highway Pavements (http://gulliver.trb.org/publications/nchrp/nchrp_w35-a.pdf) provides some good guidelines for collecting data, evaluating pavement, selecting rehabilitation techniques and forming rehabilitation strategies.
This section will concentrate on structural overlays by describing several typical structural overlay design methods.
| WSDOT Structural Design Policy |
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Specific WSDOT structural design policy is contained in the WSDOT Pavement Guide, Volume 1. In general, WSDOT uses the following structural design procedures:
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Structural overlays are used to increase pavement structural capacity. Therefore, they are considered rehabilitation, although they typically have some maintenance-type benefits as well. Asphalt concrete structural overlay design can be broadly categorized into the following (modified after Monismith and Finn, 1984):
Each of the above categories will be briefly described.
This classification of overlay design is the most subjective of the four listed and can be heavily influenced by political and budget constraints. Selection of overlay thickness and the associated materials is often based on local knowledge of existing conditions, which can result in cost effective solutions; however, local expertise is fragile and subject to retirements, agency reorganizations, etc. Currently, more agencies appear to be relying on quantifiable overlay design approaches but tempered with local expertise.
This approach to overlay design essentially requires that the total pavement structure be developed as a new design for the specified service conditions and then compared to the existing pavement structure (taking into account pavement condition, type, and thickness of the pavement layers). Current component design procedures require substantial judgment to effectively use them. This judgment is mainly associated with selection of "weighting factors" to use in evaluating the structural adequacy of the existing pavement layers (i.e., each layer of the pavement structure is assigned a layer coefficient often on the basis of experience).
Pavement surface deflection measurements can be used to determine pavement structural properties, which can then be used to determine the required amount of additional pavement structure. Basically, a pavement's surface deflection in response to a known loading is used as a measure of effective strength. This "effective strength" is influenced by a variety of factors including material properties (including subgrade), thickness of pavement layers, and environmental effects. Most currently used deflection based overlay design procedures do not attempt to isolate material properties of individual pavement layers.
Mechanistic-empirical based design methods are useful in overlay design as well as new pavement design. Their greatest advantage is the versatility provided in evaluating different materials under various environments and pavement conditions. Mechanistic-empirical procedures provide a basis for rationally modeling pavement systems. As these models improve, better correlations can be expected between design and performance parameters. In many places these procedures have replaced limiting deflection overlay methods, since the latter do not account for subsurface material properties. Mechanistic-empirical overlay design is essentially the same as mechanistic-empirical structural design for new pavements but with the addition of more evaluation locations. Module 6, Section 4, Mechanistic-Empirical covers this design method.
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:
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.
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Figure 10.16: Thin Composite Whitetopping at the Mn/ROAD Test Facility |
Thin composite whitetopping (see Figure 10.16) 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 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/pavtech/tech/utwcalc/main.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):
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 10.17 and 10.18 show two different joint spacings.
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| Figure 10.17: 3.7 x 3.7 m (12 x 12 ft.) UTW Slabs at the Mn/ROAD Test Facility |
Figure 10.18: 1.2 x 1.2 m (4 x 4 ft.) UTW Slabs at the Mn/ROAD Test Facility |
Some criteria for deciding when to consider thin composite whitetopping as a rehabilitation alternative are (Vandenbossche and Fagerness, 2001):
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Figure 10.19: Typical Corner Breaks Resulting from Joints Placed in the Wheelpath (the White Dashed Lines Represent the Approximate Wheelpaths). |
Other considerations are (Mack, Hawbaker and Cole, 1998; Vandenbossche and Fagerness, 2001):
Rehabilitation essentially reverses the effects of deterioration by adding or replacing material in the existing pavement structure. Although there are several common methods of rehabilitation (HIPR, CIR and overlays) this section has concentrated on structural overlays - those used to increase a pavement's structural capacity. Non-structural overlays are treated in Section 2, Flexible - Maintenance.
New road construction in the U.S. is not nearly as prolific as it has been in previous generations. Urban areas have filled out greatly and the ratio of existing roads to new roads is now quite high. Consequently, rehabilitation (and not new construction) has become the dominant force in today's pavement design and construction arenas.
