Kercher Engineering, Inc.

Asphalt concrete (sometimes referred to as hot mix asphalt or simply HMA) is a paving material that consists of asphalt binder and aggregate. The asphalt binder glues the aggregate particles together to form a stable, relatively stiff material. When properly bound together by the asphalt binder, the aggregate particles form the structural framework that provides the HMA strength and toughness. Since HMA is a mixture of two materials, the long-term performance of the mixture is dictated by the characteristics of the individual components as well as how they react with each other in the mixture.

There are several characteristics of asphalt binders that can greatly affect the performance of HMA. Two of the most important characteristics which play a major role in the Superpave HMA system are:

Asphalt binder is a visco-elastic material that can act both as a viscous (fluid-like) material and as an elastic solid (rubber band-like) material. At high temperatures (greater than 100^o C), asphalt binder behaves basically as a viscous fluid similar to motor oil. This allows the asphalt binder to act as a lubricant that coats the aggregates and provides “workability” to the mixture during construction. At very low temperatures (less than 0^o C), it behaves like an elastic solid that will stretch or compress when loaded and return to its original shape when unloaded. At intermediate temperatures, which also happens to be those in which the pavement is expected to function most of the time, asphalt binder has characteristics of both a viscous fluid and an elastic solid.

Asphalt cement reacts with oxygen in the environment. This reaction, which is called oxidation, changes the structure and composition of the asphalt molecules. The result of this change in structure over time is that the asphalt binder becomes harder and more brittle (sometimes referred to as age hardening). Oxidation occurs more rapidly at higher temperatures. That is why a significant amount of hardening occurs during the production of HMA when the asphalt binder is heated to facilitate mixing and compaction.

The film thickness of the asphalt binder surrounding the aggregate influences the rate of oxidation. The outer layer of all asphalt binders will oxidize. Therefore, the performance of the asphalt binder will be determined by the effective film thickness which is the “non-oxidized” portion of the asphalt film. If the film thickness is inadequate, there will be an insufficient amount of “non-oxidized” asphalt available to bind the aggregates together. As shown in the illustration below, the layer of non-oxidized binder in Figure 2 will not be sufficient to adequately withstand the stresses created by repeated vehicular loadings and environmental conditions. Hence, cracks due to binder aging may develop at the pavement surface and propagate downwards.

Figure 3 shows that the “non-oxidized” layer is much thicker and would more likely be able to adequately withstand the stresses created by repeated vehicular loadings and environmental conditions. However, excess binder may cause bleeding, shoving, slippage, and rutting.

Note: The film thickness shown in Figures 2 and 3 have been exaggerated for clarity.

Aggregates are generally defined as granular mineral particles. Although there are many sources of aggregate including natural sand and gravel deposits, synthetic aggregate and crushed rock from recycled pavements, the majority of aggregate used in HMA is from quarried rock that is processed through crushing and screening equipment.

Regardless of the source, aggregate must be able to provide a strong, stone skeleton that is able to withstand the applied loads from vehicles. Aggregates have many properties that can determine performance including absorption, abrasion and clay content. However, there are two properties that will usually have the most dramatic effect on a pavement’s ability to withstand applied loads from vehicles:

Gradation, which is determined by a mechanical sieve analysis, refers to the distribution of the different particle sizes found in a sample of aggregate. Aggregate that contains mostly larger sized particles (coarse gradation) will typically be stronger than material consisting of smaller sized aggregate (fine gradation).

Particle shape refers to whether the particles are cubical or rounded and texture refers smooth or rough. Cubical, rough-textured aggregate that have been processed through crushing and screening equipment tend to lock together better than rounded, smooth textured aggregate, thereby resulting in a mass of material that is more resistant to applied loads.

Aggregates and asphalt binder are blended together and uniformly mixed in a HMA plant. The proportioning of these components, referred to as the volumetric proportions, will play a very large part in whether or not the HMA will be both strong and durable (assuming that the raw materials are of good quality). Although there are many elements that are involved in volumetric proportioning, two of the most important elements are the following:

Asphalt binder content is critical to the durability and stability of HMA. As stated above, if the binder content is too low, the asphalt film thickness surrounding the aggregate will be inadequate, thereby resulting in raveling. On the other hand, if the binder content is too high, stability problems such as rutting and shoving may occur. Also, excess binder may flush up to the surface causing safety problems (bleeding) by reducing skid resistance.

Intergranular spaces that exist between the aggregate particles in compacted HMA, which are occupied by either asphalt binder or air, are referred to as voids in the mineral aggregate (VMA). A durable HMA requires a sufficient amount of voids to allow for an adequate asphalt binder film thickness and air void. Additional air voids are necessary to allow for post- construction consolidation that occurs as a result of heavy vehicular loadings.