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| Figure 10.30: Dowel Bar Slots | Figure 10.31: Dowel Bars in Slots |
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| Figure 10.32: Filling Slots with Grout | Figure 10.33: Slots After Diamond Grinding |
| Major Topics on this Page | ||
| 5.1 | Dowel Bar Retrofit | |
| 5.2 | Structural HMA Overlays | |
| 5.3 | Structural PCC Overlays | |
| 5.4 | 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 (ITS, 2000):
"Measures to improve, strengthen or salvage existing deficient pavements to continue service with only routine maintenance. Deficient pavements exhibit distress in excess of what can be handled through routine maintenance."
A wholesale replacement of the entire pavement structure is considered reconstruction rather than rehabilitation since it follows new pavement construction methods. Rigid pavement rehabilitation options depend upon local conditions and pavement distress types but typically include:
This section will concentrate on structural overlays by describing several typical overlay design methods. Discussion of rubblization or break-and-seat surface preparation techniques used to prepare existing rigid pavements for either rigid or flexible overlays can be found in Section 7.2.2.4, Flexible Overlays on Rigid Pavement.
| 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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Dowel bar retrofitting is a method used to restore or provide better load transfer across transverse joints or cracks using dowel bars. Usually, dowel bar retrofits are necessitated by excessive faulting due to a loss of aggregate interlock over time. It is interesting to note that much of the rigid pavement built in the U.S. during the 1950s and later included dowel bars for load transfer - except on the west coast where dowel bars did not become common practice in some states until the late 1990s. Thus, large-scale dowel bar retrofit projects are largely confined to western states. The basic procedure is as follows:
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| Figure 10.30: Dowel Bar Slots | Figure 10.31: Dowel Bars in Slots |
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| Figure 10.32: Filling Slots with Grout | Figure 10.33: Slots After Diamond Grinding |
Retrofitting dowel bars has grown in popularity over the last 10 years and has resulted in good pavement performance.
HMA structural overlays are used to increase rigid pavement structural capacity. Therefore, they are considered rehabilitation, although they typically have some maintenance-type benefits as well. Although the specific constants used may be different, flexible overlays of rigid pavement are designed in the same basic manner as flexible overlays of flexible pavement. For a discussion of these design methods, see Section 10.3, Flexible - Rehabilitation.
PCC overlays of existing rigid pavements have been used for years to restore pavement structural capacity. All types of rigid pavement designs (JPCP, JRCP, CRCP) are appropriate to be overlaid and to be used as overlays. This section briefly describes the AASHTO design procedure for the two major types of PCC overlays: unbonded and bonded.
An unbonded PCC overlay consists of a relatively thick PCC layer (typically 125 - 305 mm (5 - 12 inches) thick) over an existing rigid pavement. Bonding between the existing pavement and overlay is intentionally prevented by using a slurry seal, BST, or HMA bond breaking interlayer. This intentional separation allows the original pavement and overlay to act independently of each other and helps prevent distresses in the existing pavement from reflecting through into the overlay (ACPA, 2001). Unbonded overlays are generally used as an alternative to rubblization when the existing rigid pavement is badly deteriorated. Their primary advantages are that they (1) can be applied over a badly deteriorated pavement without much surface preparation and (2) they do not require the existing pavement to be removed. Their primary disadvantages are (1) because they are relatively thick and placed directly over the existing pavement, they add substantially to roadway elevation, which could pose overhead clearance problems, and (2) they are relatively expensive.
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 pavement resilient modulus. Although perfectly acceptable, this method gives little credit to the existing pavement's remaining strength. The basic equation in the 1993 AASHTO Guide is:
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where: |
Dol |
= |
required thickness of unbonded rigid overlay |
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Df |
= | slab thickness required to carry all future traffic | |
|
Deff |
= |
effective thickness of the existing slab |
The 1993 AASHTO Guide design procedure is summarized below.
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where: |
Deff |
= |
effective thickness of the existing slab |
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Fjcu |
= | joints and cracks adjustment factor - accounts for the extra loss of serviceable life caused by deteriorated transverse joints and cracks in the existing pavement. | |
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D |
= |
existing slab thickness, maximum value is 250 mm (10 inches) even if the existing slab is thicker |
Joints are typically designed to be mismatched with underlying existing joints. The assumption is that the thick overlay and bond breaking interlayer will prevent reflective cracking, therefore mismatching the joints will improve load transfer. AASHTO recommends that "the placement of joints in the overlay should be mismatched from existing joints and working cracks by at least 0.9 m (3 ft.) where possible" (AASHTO, 1990).
A bonded PCC overlay consists of a relatively thin PCC layer (typically less than 100 mm (4 inches) thick) over an existing rigid pavement. The overlay is intentionally bonded to the existing pavement with a PCC slurry or grout in order to create a composite pavement section (McGhee, 1994). Bonded overlays are generally used to add structural capacity to existing rigid pavements that have little deterioration (e.g., no faulting or spalling and cracked slabs should be replaced before overlay). Their primary advantages are that they (1) are thinner than unbonded overlays and (2) their structural design accounts for the strength of the underlying pavement. Their primary disadvantages are (1) they should not be applied over badly distressed pavements because the distress may affect bond quality, and (2) they are dependent upon good bond development - if for some reason this does not occur, the pavement could be structurally inadequate.
The design procedure in the 1993 AASHTO Guide is somewhat similar to that for unbonded overlays, however, it is more dependent on existing pavement characteristics. Specifically, there are three key differences:
The basic equation in the 1993 AASHTO Guide is:
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where: |
Dol |
= |
required thickness of unbonded rigid overlay |
|
Df |
= | slab thickness required to carry all future traffic | |
|
Deff |
= |
effective thickness of the existing slab |
The 1993 AASHTO Guide design procedure is summarized below.
|
where: |
Deff |
= |
effective thickness of the existing slab |
|
Fjc |
= | joints and cracks adjustment factor - accounts for the extra loss of serviceable life caused by deteriorated reflection cracks in the overlay that will result from any unrepaired distresses in the existing pavement. | |
| Fdur | = | durability adjustment factor - accounts for the extra loss in serviceability caused by any durability problems (such as "D" cracking) in the existing pavement. | |
| Ffat | = | fatigue damage adjustment factor - accounts for past fatigue damage in the existing pavement. | |
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D |
= |
existing slab thickness |
Joints are typically designed to coincide with underlying existing joints, otherwise uncontrolled reflective cracking may occur and substantially affect overlay performance (McGhee, 1994). Dowels and reinforcing steel are generally not placed in the overlay joints.
Rehabilitation essentially reverses the effects of deterioration by adding or replacing material in the existing pavement structure. This section has concentrated on structural overlays of existing rigid pavements. These overlays, which can be either flexible or rigid, are used to increase a pavement's structural capacity. In order to do this, they must be structurally designed using one of several methods. Flexible overlays tend to be less expensive, thinner and quicker to construct, while rigid overlays are more expensive, thicker and take longer to construct, but may offer longer life. The choice of overlay type and method is highly dependent upon local practice and conditions.
Finally, 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.
