Part3_Chapter6_AC Rehab

3.6.4

An HMA overlay is a candidate rehabilitation strategy for existing pavements with either HMA or PCC surfaces. The design methods described in this chapter can address HMA overlays of the following types of existing pavements:

• Conventional flexible pavements – thin HMA layer over granular base and subbase.
• Deep strength HMA pavements – thick HMA layers over granular base and subbase.
• Full-depth HMA pavements – flexible pavement consisting only of HMA layers.
• Semi-rigid pavements – HMA surfaced sections having some type of chemically stabilized layer.
• Composite pavements – HMA surface over PCC. These include previous HMA overlays of original PCC pavements.
• PCC pavements – Jointed plain concrete pavements (JPCP), jointed reinforced concrete pavements (JRCP), and continuously reinforced concrete pavements (CRCP).

The procedures provide powerful tools for analysis of a variety of overlay options. The overlay may consist of up to four layers, including three HMA layers and one layer of unbound granular or chemically stabilized material. The procedure can also assess the effects of various types of pre-overlay treatments such as cold milling of existing HMA layers, fracture/rubblizing of existing PCC layers, and in-place recycling of HMA and granular base layers.

Figure 3.6.2 presents a general flow chart for pavement rehabilitation with HMA overlays. As shown in figure 3.6.2, the various combinations of existing pavements and pre-overlay treatments reduce the overlay analyses to three general types:

• HMA overlay of HMA surfaced pavement
• HMA overlay of fractured PCC pavement
• HMA overlay of intact PCC pavement

For existing flexible or semi rigid pavements, the pre-overlay treatments may include nothing, a combination of milling, full or partial depth repairs, or in-place recycling. In either case, the resulting analysis is an HMA overlay of an existing HMA surfaced pavement. The analysis for existing PCC pavements may be either an HMA over PCC analysis or an HMA over fractured slab analysis depending on whether or not crack and seat, break and seat, or rubblization techniques are applied to the existing PCC pavement. Similarly, existing composite pavements may result in either an HMA over PCC analysis or an HMA over fractured slab analysis depending on whether or not the existing HMA surface is removed and the underlying PCC pavement is fractured.

The three overlay analyses predict the same distresses included in PART 3, Chapter 3 for the analysis of new and reconstructed flexible pavements:

• Load associated fatigue of the HMA layers, both top-down and bottom-up cracking.
• Load associated fatigue fracture of any chemically stabilized layer.
• Permanent deformation in HMA layers.
• Permanent deformation in unbound layers.
• Thermal fracture in HMA surface layers.

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ACKNOWLEDGMENT OF SPONSORSHIP

This work was sponsored by the American Association of State Highway and Transportation Officials, in cooperation with the Federal Highway Administration, and was conducted in the National Cooperative Highway Research Program, which is administered by the Transportation Research Board of the National Research Council.

DISCLAIMER

This is the final draft as submitted by the research agency. The opinions and conclusions expressed or implied in the report are those of the research agency. They are not necessarily those of the Transportation Research Board, the National Research Council, the Federal Highway Administration, the American Association of State Highway and Transportation Officials, or the individual states participating in the National Cooperative Highway Research Program.

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PART 3—DESIGN ANALYSIS

CHAPTER 6

HMA REHABILITATION OF EXISTING PAVEMENTS

3.6.1 INTRODUCTION

This chapter describes the mechanistic-empirical design procedures for rehabilitation of existing flexible, rigid, and composite pavements with hot-mix asphalt (HMA). Because many aspects of rehabilitation design are similar to new design, PART 3, Chapter 3 is referenced often to avoid duplication.

3.6.1.1 Scope

HMA can be used to remedy functional or structural deficiencies of existing pavements. It is important for the designer to consider several aspects, including the type of deterioration present, before determining the appropriate rehabilitation strategy to adopt. Several different rehabilitation options using HMA overlays can be applied to existing pavements to extend their useful service life. These range from thin surface treatments to structural overlays of existing flexible, composite, or rigid pavements and from in-place recycling of existing pavement layers followed by placement of a HMA surface, to reconstruction with HMA as described in PART 3, Chapter 5 of this Guide. These strategies are commonly used to remedy functional, structural, or other inadequacies.

This chapter presents detailed mechanistic-empirical design procedures for HMA overlays of flexible, semi-rigid, composite and rigid pavements. This chapter first provides an overview of the HMA overlay design process, and then describes in detail the design procedures for:

• HMA overlay of existing HMA surfaced pavements, both flexible and semi-rigid.
• HMA overlay of existing PCC pavement that has received fractured slab treatments; crack and seat, break and seat, and rubblization.
• HMA overlay of existing intact PCC pavement (JCPC and CRCP), including composite pavements or second overlays of original PCC pavements. Note that this Design Guide does not provide for design of JRCP and thus there is no specific overlay design procedure for JRCP. However, some recommendations are provided for approximate overlay design of JRCP considering reflection cracking and distress in the HMA overlay.

The mechanistic-empirical design of rehabilitated pavements using HMA overlays requires an iterative, hands-on approach. The designer must select a proposed trial rehabilitation design and then analyze the design in detail to determine whether it meets the applicable performance criteria (i.e., rutting, fatigue cracking, thermal cracking, and smoothness) established by the designer. If a particular trial rehabilitation design does not meet the performance criteria, the design is modified and reanalyzed until it meets the criteria. The designs that meet the

3.6.1.2 Organization

applicable performance criteria are then considered feasible from a structural and functional viewpoint and can be further considered for other evaluations, such as life cycle cost analysis.

This chapter is organized into the following six sections, following this introduction:

• Section 3.6.2—Overview of the HMA overlay design process.
• Section 3.6.3—Overlay design requirements.
• Section 3.6.4—Overlay design for existing HMA surfaced pavements.
• Section 3.6.5—Overlay design for existing PCC pavements that have received fractured slab treatments prior to overlay placement.
• Section 3.6.6—Overlay design for existing PCC and composite pavements.
• Section 3.6.7—Additional consideration for rehabilitation with HMA overlays.

While considering the information presented in sections 3.6.4 through 3.6.6, it should be noted that, although the methodology described is generally applicable for a wide variety HMA overlay design scenarios, the number of pavement sections used to develop the calibrated performance models for these various configurations were limited due to lack or unavailability of data. This is particularly true for HMA overlays of fractured PCC sections (section 3.6.5). The greatest amount of data were available for HMA overlays of existing HMA pavements (section 3.6.4). Even within this category more data were available for conventional and deep strength HMA pavements and very little data, if any, were available for HMA overlays of existing semi-rigid pavements. In addition, although the methodology described in section 3.6.4 can also be used for the structural design of rehabilitation alternatives that incorporate in-place recycling, the performance models have not been calibrated for these types of pavements. Finally, the design of thin surface treatments is not included in this Guide, while reconstruction is treated as the design of a new flexible pavement and is discussed in detail in PART 3, Chapter 3.

3.6.2 OVERVIEW OF REHABILITATION DESIGN PROCESS

Figure 3.6.1 shows the flow of the HMA rehabilitation design process. Actual structural design of feasible rehabilitation strategies is step 6 of the following procedure:

• Steps 1-4: Evaluation of the existing pavement (PART 2, Chapter 5).
  • Step 1: Determine existing pavement condition.
  • Step 2: Determine causes and mechanism of distress.
  • Step 3: Define problems and inadequacies of existing pavement.
  • Step 4: Identify possible constraints.
• Step 5: Rehabilitation strategy selection (PART 3, Chapter 5).
• Step 6: Rehabilitation design (PART 3, Chapter 6 for HMA rehabilitation).
• Step 7: Perform life cycle cost analysis (as desired).
• Step 8: Determine non-monetary factors that influence rehabilitation (as desired).
• Step 9: Determine preferred rehabilitation strategy (as desired).

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5

3.6.4

An HMA overlay is a candidate rehabilitation strategy for existing pavements with either HMA or PCC surfaces. The design methods described in this chapter can address HMA overlays of the following types of existing pavements:

• Conventional flexible pavements – thin HMA layer over granular base and subbase.
• Deep strength HMA pavements – thick HMA layers over granular base and subbase.
• Full-depth HMA pavements – flexible pavement consisting only of HMA layers.
• Semi-rigid pavements – HMA surfaced sections having some type of chemically stabilized layer.
• Composite pavements – HMA surface over PCC. These include previous HMA overlays of original PCC pavements.
• PCC pavements – Jointed plain concrete pavements (JPCP), jointed reinforced concrete pavements (JRCP), and continuously reinforced concrete pavements (CRCP).

The procedures provide powerful tools for analysis of a variety of overlay options. The overlay may consist of up to four layers, including three HMA layers and one layer of unbound granular or chemically stabilized material. The procedure can also assess the effects of various types of pre-overlay treatments such as cold milling of existing HMA layers, fracture/rubblizing of existing PCC layers, and in-place recycling of HMA and granular base layers.

Figure 3.6.2 presents a general flow chart for pavement rehabilitation with HMA overlays. As shown in figure 3.6.2, the various combinations of existing pavements and pre-overlay treatments reduce the overlay analyses to three general types:

• HMA overlay of HMA surfaced pavement
• HMA overlay of fractured PCC pavement
• HMA overlay of intact PCC pavement

For existing flexible or semi rigid pavements, the pre-overlay treatments may include nothing, a combination of milling, full or partial depth repairs, or in-place recycling. In either case, the resulting analysis is an HMA overlay of an existing HMA surfaced pavement. The analysis for existing PCC pavements may be either an HMA over PCC analysis or an HMA over fractured slab analysis depending on whether or not crack and seat, break and seat, or rubblization techniques are applied to the existing PCC pavement. Similarly, existing composite pavements may result in either an HMA over PCC analysis or an HMA over fractured slab analysis depending on whether or not the existing HMA surface is removed and the underlying PCC pavement is fractured.

The three overlay analyses predict the same distresses included in PART 3, Chapter 3 for the analysis of new and reconstructed flexible pavements:

• Load associated fatigue of the HMA layers, both top-down and bottom-up cracking.
• Load associated fatigue fracture of any chemically stabilized layer.
• Permanent deformation in HMA layers.
• Permanent deformation in unbound layers.
• Thermal fracture in HMA surface layers.

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