Section 6: HMA Overlays

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Structural Overlays

For flexible pavements, structural hot mix overlay thicknesses are designed using FPS-19 design, option 5. Currently, the only department-approved rational method for design of structural HMAC overlays on rigid pavements is by using the appropriate overlay option in DARWinTM (AASHTO 93). In considering the actual overlay thickness, guidelines established for lift thickness based on the type of mix / nominal maximum aggregate size must be followed. In addition, the type of mix selected should complement the overall structure in terms of resilience, durability, permeability, texture, etc.

A reasonable investigation of the condition of the existing substructure and hot mix on the project must be made to insure the desired performance of the structural overlay. In addition to deflection measurements, ground penetrating radar (GPR), when combined with selective coring, is a rapid method of determining the depth and extent of delamination or stripping problems. Where rutting-susceptible mixes exist in the old structure, if these mixes are within 5 in.of the newly overlaid surface, the chance of renewed rutting originating in the old mix will still exist (zone of high shear and compression). Evaluation of road cores for rutting and stripping susceptibility using Tex-242-E (Hamburg) should be used if in doubt. Even if there is no evidence of current stripping, it is advisable to evaluate the existing material for stripping susceptibility; experience has shown that stripping problems often start only after the new overlay is placed. There are also certain surface materials that should not be overlaid, including plant-mix seal or permeable friction courses (open-graded friction courses). Where poor substructure is located, full-depth repair should be accomplished prior to the overlay. In the case of Portland cement concrete (PCC) pavements, a determination must be made into the uniformity of the underlying support. The rolling dynamic deflectometer (RDD) has been a useful tool in identifying areas of low support. Slabs must be prevented from moving by stabilizing the material beneath them. This involves drilling holes in an unstable PCC slab or section and injecting an asphaltic or cementitious material to fill any underlying voids. Typically, this method is only an option for isolated instances of instability. It does not work well as a general roadway treatment. Application of a stress absorbing membrane interlayer (SAMI) may be useful in retarding reflective cracking when overlaying jointed concrete pavements. SAMI is a fine mix that is designed for cracking resistance using the overlay tester or flexural beam fatigue. The mix is typically placed 1 in. thick and should be covered with an adequate overlay thickness to provide adequate resistance to rutting (3.0 in. minimum is recommended).

Other reasons for removing a portion of the existing HMA surface include leveling because of rutting, reducing crack width caused by spalling, and eliminating raveling.

As a minimum, a higher rate of tack coat application will be needed on a milled surface prior to overlaying. For planning purposes, a seal coat may be applied to the surface of the milled structure, especially if there are visible or latent cracks. The designer should also consider other measures to thwart reflective cracking through the new overlay. Geotextiles have been used successfully for this purpose, but require increased vigilance on the part of the contractor to insure manufacturer’s guidelines are strictly followed. Mix design, selecting a mix that incorporates increased resilience, low permeability, and overall mat thickness are also important considerations.

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Non-structural Overlays

These overlays are designed using a combination of experience and guidelines established herein. Generally, this type of overlay is used to improve ride, texture, cross-slope drainage, and weatherproofing, and is often categorized as “preventive maintenance.” Often, the process is combined with milling of a like amount of existing surface, essentially keeping the same profile while improving functional characteristics (mill and fill).

Condition surveys (including deflection testing and GPR) should be undertaken to insure that a non-structural overlay is appropriate. This would also include evaluating the existing HMAC or PCC slabs for stability (and HMA stripping potential) if necessary. As with overlays designed for structural purposes, corrective action to the substructure should be accomplished prior to the overlay. The condition survey should also reveal whether minor milling, leveling, and undersealing (or grout injection beneath PCC slabs) are necessary.

The type of mix selected should complement the overall structure in terms of durability, permeability, and texture. The designer should consider appropriate lift thickness based on the desired mix type and nominal maximum aggregate size. Where permeable friction courses (PFC) are applied, it is imperative that the underlying structure is water-tight, hence these overlays are almost always applied with an underseal. Where active cracks exist (especially jointed PCC structures), thin overlays will rarely perform well without the use of geotextiles and careful consideration of the resilient properties of the mix. Some success has been realized by saw-cutting and sealing thin overlays over active joints on jointed PCC pavements.

Two thin overlay options are available by special specification. The ultrathin bonded wearing course (SS 3001) and thin bonded PFC (SS 3000) both use a warm polymer-modified asphalt emulsion membrane followed immediately by the application of a hot plant mixed paving mixture. The ultrathin bonded wearing course is placed in thicknesses from 1/2 - 3/4 in. The thin bonded PFC is placed in thicknesses of 3/4 to 2 in., so some minor improvement in ride is possible. The PFC mixture will allow for the rapid removal of surface water, improving splash/spray characteristics; the open void structure will also reduce tire noise. These treatments should be considered on higher volume highways where average speeds exceed 45 mph, and where chip pickup and road noise from the alternate surface treatment are more objectionable to the traveling public. The application of the warm polymer-modified asphalt emulsion membrane is designed to seal the existing surfaces where minor cracking (< 1/4 in.) is the most severe distress, and is seen as a potential remedy for pavements that have leaky joints and/or segregation problems. Existing rutting or more severe cracking must be addressed separately before using these options.

There are several construction-related concerns in placing these non-structural HMA overlays, including:

  • Thin lifts require less HMA per foot of road length than thick lifts. This can result in high paver speeds (in excess of 70 ft./min.). Compaction may not be able to keep pace with these high speeds.
  • Thin lifts will cool quicker than thick lifts. This can result in little time available for compaction before the thin overlay reaches cessation temperature (sometimes as little as 3 to 5 min.). Therefore, laydown and roller variables should be set to account for this (e.g., slower laydown machine speed, enough rollers and an adequate roller pattern to compact the material before it reaches cessation temperature).
  • Thin lift construction produces greater screed wear. If the lift depth is less than about twice the maximum aggregate size, the HMA may tear under the paver screed. Very thin lifts (less than 25 mm [1 in.]) can be damaged by the screed dragging large articles.
  • Thin lifts are more sensitive to vibratory rolling. Incorrectly chosen amplitude, frequency or roller speed can result in aggregate degradation (i.e. breaking) and damage of the bond between the overlay and the existing pavement.
  • Density control is difficult. Thin lifts provide fewer options for aggregate particles to rearrange under compaction. Thus, mat densities will tend to be less uniform than those associated with a thicker lift. This should be recognized if pay is in any way tied to mat density.

In general, compaction is more difficult and more variable on thin lifts.

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