1. Session 3-6
HMA Overlays

2. Learning Objectives
Describe the characteristics of typical Hot-Mix Asphalt (HMA) overlays
Identify best applications
Describe preoverlay repair need and feasibility
Describe thickness design approach
Describe key construction issues

3. Introduction Most popular method
Relatively fast and cost-effective means for:
Correcting deficiencies
Restoring user satisfaction
Adding structural capacity
Poor performance is NOT uncommon

4. Definitions
Functional performance - Ability to provide a safe, smooth riding surface
Structural performance - Ability to carry traffic without distress
Empirical - Design based on past experience or observation
Mechanistic - Design based upon engineering mechanics

5. Purpose and Applications
Improve functional and/or structural characteristics
Wide range of applications
Road surface categories
Climate and support conditions
Both HMA and PCC overlays correct structural deficiencies and increase load carrying capacity. There is a major difference in their initial cost, traffic control requirements, and potential service lives.

6. Characteristics of Typical HMA Overlay
Dense graded HMA
Flexible or rigid surface
25 to 200 mm (1 to 8 in) thickness
Mill and Fill
One to eight inches are typical thicknesses of HMA overlays.

7. What is the best application for HMA overlays?
Best Applications
What is the best application for HMA overlays?
Use this slide as segue to next section. HMA overlays can fit many different needs from correcting functional to structural deficiencies.

8. Limitations and Effectiveness
Why do we have premature failures?
Improper selection
Wrong type
Inadequate design
Insufficient preoverlay repair
Lack of consideration of reflection cracking

9. Limitations and Effectiveness
What limits the effectiveness of HMA overlays?
Distress exhibited in HMA
Intended design life of the overlay
Availability of quality materials
Many times we accept certain distresses, rutting, reflective cracking, thermal cracking as givens.
How long are we really expecting this overlay to last? We should then design it accordingly.

10. Limitations and Effectiveness
How can we improve our overlays?
Preoverlay treatments
Better materials and practices
Sound engineering judgment

11. Overlay Selection to Correct Deficiencies
Thin Overlay
Thick Overlay
Surface Defects
Structural Defects
Thin overlays – Functional
Polishing.
Roughness.
Correct cross slope or surface drainage issues.
Thick overlays – Structural
Fatigue, rutting, patching, reflection cracks, etc.

12. What Are Considerations in Overlay Selection?
Construction feasibility
Traffic control
Constructibility
Vertical clearances
Utilities
Performance period
Funding
There are many factors that have to be balanced when selecting overlay options and determining its thickness.

13. Preoverlay Treatment and Repair
Dependent upon:
Type of overlay
Structural adequacy of existing pavement
Existing types of distress
Future traffic
Physical constraints
Cost
This is the single most important factor in determining the success of the overlay.
If you are doing a thin overlay to restore ride and you have several areas with structural distress are they likely to be able to bridge those weak areas?

14. To Repair or Not to Repair?
Would you normally repair an area like this or pave over it?

15. Types of Preoverlay Treatments
Localized repair (patching)
Surface leveling
Controlling reflection cracking
Drainage improvements

16. Localized Repair
Repair
cost
Overlay
% Area repaired

17. Localized Repair Total Cost Optimum Minimum Total Cost
% of Area Repaired
Total
Cost
Minimum Total Cost
Optimum
There should be an optimum point for balancing overlay thickness and patching quantities.
You also need to consider performance when you are trying to select this optimization point.

18. Surface Leveling Purpose Rut filling Restore cross slope
Improve longitudinal profile
Method
Cold milling
Leveling course
Either a leveling course or milling should be used if surface profile defects are evident in the existing pavement.

19. Controlling Reflection Cracking
What do you use to control reflective cracking?
Geotextiles or fabrics
Stress relieving or stress absorbing membrane interlayers
“Band aid” type crack sealants
There are various methods of reflective crack control that have various reviews.
The rule of thumb is that it takes one year for a crack to reflect through one inch of HMA. Geotextile is often thought to replace one inch of HMA.
Large stone mixes are very effective but result in very thick overlays.
Products such as Glassgrid and PavePrep actually bridge the crack but are generally very expensive to apply.

20. Reflective Crack Treatments
Geotextile: varied results on its effectiveness
SAMI: varied results also
Aggregate Interlayers: effective when designed correctly
Route and seal: does not prevent but effective at limiting deterioration

21. Drainage Corrections Drainage survey
Identify moisture / drainage related distresses
Develop solutions that address moisture problems
The designer should check that the pavement and overlay will not be influenced by drainage conditions that can be corrected.

22. Thickness Design
Make it as thick as you need, but no thicker. Asphalt doesn’t grow on trees you know.
How many designers feel that they are hoop jumpers? h

23. Two Aspects of Overlay Design
Asphalt mixture
Fatigue cracking
Permanent deformation
Thermal cracking
Moisture susceptibility
Overlay Thickness
Engineering judgment
Deflection approach
Structural deficiency
Mechanistic approach
There is no one mix design nor structural design approach.

24. Mix Design Superpave Goal – integrate mixture/structural design
Performance Graded (PG) asphalt cements
Mix design
Materials
Aggregate structure
Binder content
Moisture susceptibility
Reference:
From October 1987 through March 1993, the Strategic Highway Research Program (SHRP) conducted a $50 million research effort to develop new ways to specify, test, and design asphalt materials. The final product of the SHRP asphalt research program is a new system referred to as SuperpaveTM, which stands for Superior Performing Asphalt Pavements. It represents an improved system for specifying the components of asphalt concrete, asphalt mixture design and analysis, and asphalt pavement performance prediction. Superpave mix design is a structured approach consisting of the following four steps:
Selection of materials.
Selection of design aggregate structure.
Selection of design asphalt binder content.
Evaluation of moisture susceptibility.
Selection of Materials
This step is accomplished by first selecting a Performance Grade asphalt binder for the project climate and traffic conditions. Superpave binders are designated with a high and low temperature grade, such as PG For this binder, "64" is the high temperature grade and is the 7-day maximum pavement design temperature in degrees centigrade for the project. The low temperature grade, "-22," is the minimum pavement design temperature in degrees centigrade. Both high and low temperature grades are established in 6-degree increments. Thus, the binder grade is an indication of the project-specific temperature extremes for which the asphalt mixture is being designed. In addition to climate, traffic speed and traffic level may also influence Superpave binder selection. A project with slow moving or stationary traffic would require a binder with one or two higher temperature grades than would otherwise be selected on the basis of climate alone. Projects with very high traffic levels in excess of 30 million 80 kN (18 kip) equivalent single axle loads would also require an increase in high temperature binder grade.
Five asphalt mixture types are specified in Superpave according to nominal maximum aggregate size: 9.5 mm, 12.5 mm, 19 mm, 25 mm, and 37.5 mm (0.4 in, 0.5 in, 0.75 in, 1 in, and 1.5 in). To specify mineral aggregate, Superpave uses two approaches. First, it places restrictions on aggregate gradation by means of broad control points and a restricted zone. Second, it places consensus requirements on coarse and fine aggregate angularity, flat and elongated particles, and clay content.
Selection of Design Aggregate Structure
Once binder and aggregate materials have been selected, various combinations of these materials are evaluated using the Superpave gyratory compactor. Three, and sometimes more, trial blends are evaluated. For example, assume that four aggregate stockpiles have been selected for use. The following table shows possible blend percentages for each of three trial blends. Blend Percentages Stockpile Trial Blend 1 Trial Blend 2 Trial Blend 3
Coarse Aggr Course Aggr Manufactured Sand Natural Sand Total Once the trial blends are established, a trial asphalt binder content is selected for each blend. The trial asphalt binder content is selected using an estimation procedure contained in Superpave or on the basis of the designer's experience. Two specimens of each trial blend are batched and compacted in the Superpave gyratory compactor. In addition, two loose specimens of each trial blend are produced and used to measure maximum theoretical specific gravity. The volumetric and densification characteristics of the trial blends are analyzed and compared with Superpave mix design criteria. Any trial blend that meets these criteria can be selected as the design aggregate structure.
Selection of Design Asphalt Binder Content
The next step involves selection of the design asphalt binder content for the design aggregate structure. This step is necessary to verify the approximate binder content used in the preceding step. The Superpave gyratory compactor is used to fabricate test specimens composed of the selected design aggregate structure, but at four different asphalt contents. The asphalt content that results in 4 percent air voids at the design number of gyrations is the design asphalt binder content. The design aggregate structure containing the design asphalt binder content becomes the design asphalt mixture.
Evaluation of Moisture Susceptibility
This final step requires that the design asphalt mixture be evaluated using a test procedure called, AASHTO T283, "Resistance of Compacted Bituminous Mixture to Moisture Induced Damage." This test method was already in wide use prior to the development of Superpave. Six test specimens are fabricated using the Superpave gyratory compactor. Three of the six are moisture conditioned. The remaining three specimens remain unconditioned. All of the test specimens are evaluated for their indirect tensile strength. The ratio of conditioned to unconditioned tensile strength is called tensile strength ratio or TSR. The design asphalt mixture is judged to be non-moisture susceptible if it has a TSR greater than 80 percent.

25. Structural Design Deflection Approach50
100
150
200
250
0.0
0.4
0.8
1.2
1.6
2.0
Overlay Thickness, THov (mm)
Deflection, (mm)
Limiting
Deflection
Original Surface
HMA Overlay
HMA Layer
Granular Layer
Natural Soil
Simulated 80-kN
Axle Load
THov
150 mm
75 mm
The larger the deflections the weaker the pavement structure and subgrade soil is. In this case overlays are designed to limit the deflections of the total pavement structure. The theory was that the more a pavement deflects/bends the more fatigue the pavement undergoes. This was the basis of many of the early overlay design methods which used the Benkleman Beam to measure deflection.
0.9 mm (35 mil) deflection yields a 100 mm (4 in) overlay.
See figure in the text.

26. Structural Design Structural Deficiency
Traffic
PSI
2.5
1.5
Overlay SC
eff oL f
Figure in text.
SC is the structural capacity of the original pavement.
SCeff is the effective structural capacity of the existing pavement.
The effective structural capacity of the pavement can be determined by
Deflection.
Observation of pavement condition.
Prediction of past load application.
SCf is the required future structural capacity of the pavement.
Scol is the required structural capacity of the overlay.

27. Structural Design AASHTO Approach
SCOL = SCf – SCeff
Problem: Determining SCeff
Distress survey
Remaining life
Deflection testing

28. Structural Design Mechanistic-Empirical
Natural Soil
Subbase
Base
Original
HMA Surface
Layer
HMA Overlay TH E 1 2 3 4
Thickness
Young’s
Modulus ov
HMA
Simulated
Design Axle Load
Mechanistic approach.
Limit tensile strain in the existing HMA and the overlay to limit fatigue cracking.
Limit vertical stress on the subgrade to limit rutting. σV

29. Structural Design Mechanistic-Empirical
Allowable
Traffic
Rutting
Fatigue
In this case the design overlay thickness is the one that limits fatigue to an acceptable level.
Critical Stress or Strain

30. Structural Design Mechanistic-Empirical
HMA Overlay Thickness, THov (mm)
Allowable Axle Load Applications
(80 - kN ESALs, millions)
Simulated 80 - kN
Axle Load
THov
THhma
THbg
ov HMA overlay
Base
Natural soil
Design HMA
Overlay
Thickness 25 50 75
125
100
150
175 1 2 3 4
Design 80-kN ESALS, W
Original HMA layer (fatigued) 80
Graphical method for determining overlay thickness. Simply plot a few points of assumed overlay thickness and the calculating the allowable traffic for each of those overlay thicknesses. In this simplified case the existing HMA is considered fully fatigued and is treated as another granular layer. If this layer were in better condition an approach similar to figure could be applied.
80 kN = 18,000 lb axle load.
125 mm overlay = 5 in.

31. Structural Requirement Varies Along Roadway1 2 3
Distance Along Roadway
Design Overlay Thickness
Which overlay thickness are you going to choose for this roadway.
Example of a profile of overlay thickness along a roadway. If you designed for the worst case the whole overlay would be at thickness 2.

32. Key Construction Issues

33. Lift Thickness Old – 2X maximum aggregate size
New – 3X nominal maximum aggregate size
Lift thickness versus mix design
Need to assure that you have room for roll-down. It is not desirable to crush aggregate due to insufficient lift thickness.
Up to 10 percent of the aggregate particles may be larger than the nominal maximum size.
Need to be careful when developing overlay thickness, mix design and lift thickness and make sure these are all coordinated.

34. Compaction Newer mixes more difficult to achieve density Consequences
Rutting
Raveling
Oxidation
Stripping
Keys
Proper equipment
Roller immediately behind paver
One of the key complaints of the use of Superpave mixes has been difficulty in achieving density.
Huber (1999) reports that 17% of rehabilitation is driven by rutting. This is an increase from a similar survey conducted in It is more likely that this is due to lower tolerance for rutting on pavements rather than drastic increases in rutting itself.
Use of a test strip to test compaction on each project.
Proper lift thickness for mixture.
Proper rolling methods.
Roller size.
Roller speed.
Number of rollers matches production rate.
Breakdown roller close to paver.

35. Ride Quality Public’s No. 1 issue Ride specification
Method of measurement
Incentive/disincentive
Increased pavement performance
Key – continuous, steady operation
Key to achieving a satisfactory ride quality is a good ride specification. These customarily include bonus/penalties for achieving the target smoothness.
Research has shown the smoother pavements have increased performance and tend to stay smoother longer.
Use of milling and number of lifts that are used provide the contractor with more opportunity to achieve smooth pavements.

36. Review What are the characteristics of an HMA overlay?
Where are HMA overlays applicable?
What types of pre-overlay repair should be considered?
Name three structural design approaches?
What are some of the key construction issues?
What are the characteristics of an HMA overlay?
Dense graded HMA.
Over a flexible or rigid surface.
25 to 200 mm (1 to 4 in) thickness.
Mill and Fill is also used.
Where are HMA overlays applicable?
PCC or HMA pavements in almost any conditions. Serviceability and life of the overlay will be dependent upon the existing condition, pre-overlay repair, and the thickness of the overlay. Can be used to improve ride (functional) or load carrying capacity of the pavement (structural).
What types of pre-overlay repair should be considered?
Localized repair (patching).
Surface leveling.
Controlling reflection cracking.
Drainage improvements.
Name three structural design approaches?
Deflection, structural deficiency, mechanistic-empirical, Superpave, and remaining life.
What are some of the key construction issues?
Lift thickness, compaction, and ride quality.

37. Key References
Daleiden, J. F., A. Simpson, and J. B. Rahut Rehabilitation Performance Trends: Early Observation from Long-Term Pavement Performance (LTPP) Specific Pavement Studies (SPS). FHWA-RD Federal Highway Administration, Washington, DC.
Brown, E. R Superpave Construction Guidelines. Special Report National Asphalt Pavement Association, Lanham, MD.

38. Key References (continued)
Huber, G. A Methods to Achieve Rut-Resistant Durable Pavements. Synthesis of Highway Practice Transportation Research Board, Washington, DC.
Collura, J., T. El-Korchi, K. Black, M. Chase, and L. Jin Guidelines for Ride Quality Acceptance of Pavements Final Report. New England Transportation Consortium, University of Connecticut, Storrs, CT.

39. Key References (continued)
American Association of State Highway and Transportation Officials (AASHTO) a. AASHTO Guide for Design of Pavement Structures. American Association of State Highway and Transportation Officials, Washington, DC.