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Section 6: HMA Overlays

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6.1 Structural Overlays

For flexible pavements, structural hot-mix overlay thicknesses are designed using FPS 21 design, option 6. Currently, the only department-approved rational method for design of structural HMA overlays on rigid pavements is by using the appropriate overlay option in DARWin® 3.1 (AASHTO 93). An M-E based program incorporating findings of the balanced mix design approach (research project 0-5123) is available in consultation with MNT – Pavement Asset Management. 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 ensure 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 4 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 total pavement acceptance device (TPAD) 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, cementitious, or high-density polymer 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 such as the crack attenuating mixture (CAM) may be useful in retarding reflective cracking when overlaying jointed concrete pavements. CAM 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 (2.0 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 ensure 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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6.2 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 “pavement preservation.” 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 ensure that a non-structural overlay is appropriate. This would also include evaluating the existing HMA 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 or by using the thin bonded friction course (Item 348) method. Due consideration must also be given to insuring a proper cross-slope by using a level-up course before applying the PFC. 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.

6.2.1 Thin Overlays

Two statewide thin overlay options are available by standard specification: The thin bonded friction courses (Item 348) and thin overlay mixtures (Item 347 [TOM]).

Item 348 covers PFC and wearing course varieties, both of which use a warm polymer-modified asphalt emulsion membrane followed immediately by the application of a hot plant mixed paving mixture. The bonded wearing course is placed in thicknesses from 1/2 to 3/4 in. The thin bonded PFC is placed in thicknesses of 3/4 to 1.5 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. Item 348 allows RAP/RAS and WMA, if desired. 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. The thin overlay mix (TOM, Item 347) uses conventional tacking procedures and allows for a WMA option. However, no RAP/RAS is allowed. The TOM-C is placed in thicknesses ranging from 0.75 to 1.25 in.; TOM-F is placed in thickness ranging from 0.50 to 0.75 in.

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

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  • 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. Use of two rollers in echelon may be necessary.
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  • 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).
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  • 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.
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  • 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.
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  • 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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