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.