APAI Article Archive

(APAI Web Site, August 2005)

HMA NOTES

By: Bob Nady, P.E.

This NOTE compares some aspects of materials used for HMA with the same considerations for PCC. A few construction items are also reviewed.

HMA pavements are built in layers. A page from the Perpetual Pavement book shows that by this layered system, the layers in a Hot Mix pavement can be specifically designed for the stresses acting on that layer. The critical stress in the bottom-most layer is fatigue. A mixture can be designed to accommodate those fatigue stresses. The next zone above the fatigue zone is the zone of a strong base. The stresses in this zone are the result of the applied traffic loads using the pavement. This zone must be designed to resist those stresses and to spread the loads (reduce the stresses) so that lower layers including the subgrade are not overloaded. Finally, the surface zone must resist the concentrated stresses of the applied loads (trucks, concentrated since this zone is in contact with the wheel load). Also, the pavement layer at this level must resist the wide seasonal temperature changes as well as wet/dry, freeze/thaw stresses of the environment. Do not forget that the grinding, damaging effects of ice control chemicals and abrasives must be resisted by this layer. Again, the HMA material for this exposure can be specifically selected, and built, to do this. The layers in the HMA pavement can be composed of materials specifically selected and designed for the job each layer must perform to be characterized as a Perpetual Pavement.

Now consider the PCC pavement. The full depth of the slab is cast in one gob. The mix design for that concrete can only be a compromise, since different stresses occur at different depths in the pavement. PCC strength can be controlled in the mix design. But what about the other damaging stresses? If strength is controlled, other stresses may not be considered, or at least, the consideration must be compromised since the full depth of the slab is cast at one time. There is no possibility of tailoring the mix for stresses occurring at different depths.

Consider for a moment the construction operations. A precisely controlled asphalt paver is used to construct each layer in the HMA pavement. Each lift of the pavement not only addresses the stresses which must be resisted, but the accuracy of the surface of each layer improves as the paving operations progress. When the top layer is constructed the result is a very smooth pavement which rides well and will resist the pounding of heavy trucks. By contrast, the PCC pavement is constructed as a single layer. There is only one chance to remove any roughness in the finished slab. Any residual roughness is literally “cast in concrete”.

There is only one Perpetual Pavement.

Hot Mix Asphalt - Smooth, Quiet, Perpetual