4. Rehab & Maint. HMA Overlays 4
PavementCondition
Alt.-A
Alt.-B
Terminal Serviceability Index
Analysis Period
Analysis Period
RSL
5. Rehab & Maint. HMA Overlays 5
Covered in this module
• HMA Overlays over HMA pavements
– Functional
– Structural
• HMA Overlays over PCC pavements
6. Rehab & Maint. HMA Overlays 6
Overlay Types
• Functional overlays
– Typically used to address surface
distresses or improve ride quality
– Generally not designed and rely on past
experience
• Structural overlays
– Correct structural deficiencies and are
designed using the methods presented
(or other methods)
7. Rehab & Maint. HMA Overlays 7
Thin Overlay Thick Overlay
Surface Defects Structural Defects
Selection of an Overlay to
Correct Deficiencies
8. Rehab & Maint. HMA Overlays 8
Functional Overlays
• Remedy functional deficiencies
• Minimum constructable thickness
• May involve surface milling and repair with
overlay
10. Rehab & Maint. HMA Overlays 10
Surface Friction
• Force developed at tire-pavement
interface that prevents sliding
• Factor influencing
– Microtexture
– Macrotexture
– Transverse slope
• Poor friction can be caused by polishing
or bleeding
12. Rehab & Maint. HMA Overlays 12
Hydroplaning
• Tires ride on thin film of water and lose
contact with the pavement
• Hydroplaning is worsened by water standing
on the pavement and is affected by the
following
– Wheelpath rutting
– Lack of surface texture
– Lack of cross slope
13. Rehab & Maint. HMA Overlays 13
Photo of rutting in wheelpaths
The steps in the pavement rehabilitation process apply to both HMA and PCC pavements.
These roads will need repair. Timing is everything. A surface treatment put on the road early is much less expensive and will extend the life of the pavement. The goal is to put together a maintenance and rehabilitation program that will be the most cost effective over the life of the pavement. An overlay placed before failure can result in a savings of over $200000 per mile.
Engineering economics procedures are used to analysis various alternatives. There are many alternatives to the maintenance and repair of a HMA pavement. The concept is to come up with a procedure that keeps the pavement condition high, the traffic disruption at a minimum and at the lowest life cycle cost.
HMA overlays are the most popular method of pavement rehabilitation. They provide a relatively fast, cost-effective means of correcting existing pavement deficiencies, restoring user satisfaction and adding structural capacity to the pavement structure. However, it is not uncommon for an HMA overlay to perform badly. The module should cover approximately three lectures. The first lecture would be introduction to HMA overlays, the second lecture would be structural overlays and the third would be HMA overlays over PCC pavements.
The purpose of a functional overlay is to provide a safe smooth riding surface to the traveling public. They are usually thin, (1 to 2 inch) and do not require extensive repair work prior to the overlay. The preparation prior to the overlay is minimal. It will consist of some crack sealing and maybe milling. A structural overlay is generally very thick and the preparation prior to the placement of the overlay can be extensive.
Where you would have only surface defects – a thin overlay or some type of surface treatment is appropriate – this would be a functional overlay. If there are structural problems with the pavement a thick overlay is required or possibly reconstruction.
A functional overlay is designed to deal with skid problems, ride deficiencies, etc. It is not used to correct a structural problem.
This slide shows a listing of the various functional problems that a thin functional overlay can correct.
If you have a friction problem it can be corrected by a thin overlay.
This photo shows polishing aggregate. The result is a very smooth, slippery surface.
The pavement show will present a hydroplanning problem. Note the water in the wheel paths. It appears that action is about to be taken to correct the problem. Note the milling machine. Typically the solution is to mill out the rutting layer and replace it with a new high stability HMA mixture.
Roughness can be caused by many things. It can be big bone jarring problems such as heaving or settlements or it can be raveling of the surface.
This shows a raveled pavement.
Generally functional overlays are thin layers and can consist of any or all of the items shown in this slide.
For many functional overlays the first step is to mill out the pavement.
There are a number of approaches to the development of the asphalt pavement thickness: 1. Engineering judgment – This approach us based on the experience of the design engineer. The engineer has been working in an area for many years and based on experience with past projects and the surface defects that are seen a thickness is chosen. 2. Deflection – The general concept is that the larger the deflection – the weaker the pavement is. Overlays can be used to strengthen the pavement structure to reduce the deflection to a desired level. Critical deflections can be determined below which the pavement is expected to perform satisfactory. 3. Structural deficiency – This approach uses the AASHTO design manual and makes assumptions about the structural coefficients for the layers of the existing pavement. The structural capacity of the existing pavement is determined and the needed structural capacity is determined. The difference between the two is the deficiency. 3. Mechanistic – Uses engineering mechanics and fundamental engineering properties . For overlay design it uses an engineering mechanics based simulation of the state of the stress and strain in the pavement to develop the design.
One of the most common ways of determining the properties of the asphalt pavement is to use deflection testing. A higher deflection results from a weaker pavement structure. The two distressess that are being addressed are the tensile stresses in the asphalt layers and the compressive stresses in the subgrade.
The typical surface deflection measuring equipment places an nondestructive test (NDT) load on the pavement and then sensors are used to measure the deflection. This data is used to compute the layer strengths.
A strong pavement will deflect less than a weak pavement. There are two things of interest – the amount of deflection and the steepness of the deflection basin.
This picture shows one model of deflection testing equipment.
If you have a structural problem – these are options to its correction. They range from a straight overlay to complete reconstruction.
As a part of the design process a number of things must be considered.
The amount of repair work done will affect the overlay thickness needed.
The next two slides shows different pavement distresses that may require preoverlay treatment. This slide shows rutting. It is necessary to consider the cause of the rutting problem and then correct it.
These cracks will reflect up through any overlay placed on the road. They need to be corrected – either by sealing or the section may need to be removed and replaced.
