By James A. Scherocman, P.E. and Dwight Walker, P.E.
Compaction is the process of densifying, or reducing the volume of, a mass of material. Most practitioners consider achieving appropriate compaction critical to the performance of an asphalt pavement. For asphalt mixtures, compaction locks the asphalt-coated aggregate particles together to achieve stability and provide resistance to deformation (or rutting) while simultaneously reducing the permeability of the mixture and improving its durability.
There are many factors that affect the compactability of asphalt mixtures. Among those factors are the properties of the asphalt mixture, the type and density of the underlying base course material, the thickness of the asphalt layers, and the environmental conditions at the time of placement. If any of these factors change, the final modulus, stiffness or strength of the mix will be directly affected. Additionally, the final compactability of the mix is affected by the type of rollers, the number of rollers, and the rolling patterns used during the compaction process.
Asphalt Mixture and Pavement Variables
There are a wide variety of asphalt mixtures currently being used in North America. Among those mix types are dense graded mixes, open graded mixes, and stone matrix asphalt (SMA) mixes. Within the realm of dense graded mixes, some of the mixes are fine graded mixes, some are coarse graded mixes, and some are very densely (uniformly) graded mixes. Some of the open graded mixes are used for friction course layers while some are used as a permeable base course layer. SMA mixes, which are usually very dense graded, typically have a significantly different gradation than normal dense graded asphalt mixes and incorporate an increased amount of mineral filler as part of the gradation. Gap graded asphalt mixes are also used by some agencies.
There are also many types and grades of asphalt binder materials currently being used. A few of those binders are still graded using the penetration system, some are still viscosity (AC) graded, while most meet at least some of the criteria for a Performance Graded (PG) asphalt binder. Further, some of the asphalt binders are polymer modified, using either elastomeric or plastomeric type materials. Asphalt rubber binders are also used in some asphalt mixtures. Each of these types and grades of asphalt binder materials will affect the degree of stiffness obtained in the asphalt mixture, both at the time of construction and throughout the compaction process.
The effective asphalt binder content of the asphalt mixture also affects workability and compactability. As the asphalt binder content increases, the film thickness on the aggregate particles increases. At compaction temperatures, this increased film thickness enhances the lubricating effect of the asphalt binder, and, up to a point, makes mix compaction easier.
There are a wide variety of aggregate types currently being used in North America. Some of those materials are sedimentary, some are igneous, and some are of glacial origin (gravel). In addition, the absorption, soundness, angularity, surface texture, and degree of flat and/or elongated particles all affect the properties of the different aggregate materials and, therefore, the properties of the asphalt mixture into which they are incorporated. In particular, the amount of crushed coarse and fine aggregates in the mix has a direct affect on the stiffness and compactability of the resulting mixture.
Volumetric Property Considerations
The volumetric requirements for the various asphalt mixes vary widely. In many jurisdictions, voids in mineral aggregate (VMA), voids filled with asphalt (VFA), and air void (AV) content requirements are included as part of the mix specification specifications. In some places, Hveem mix design methods are still popular and, in general, the Hveem mix design methods do not require the calculation of the VMA or VFA values of the mix. It is well known that the volumetrics of the asphalt mixture have a direct affect on the performance of the mix under traffic. The volumetrics, however, also have a very dramatic affect on the stiffness of the mix during the compaction operation and on the ability of the contractor to achieve the desired level of compaction.
The environmental conditions at the time of mix placement can directly influence the amount of compaction obtained by affecting the time available to compact the mix—the cooling rate of the mix. Air temperature, base temperature, wind velocity, and solar flux or cloud cover (to a minor degree) all govern the cooling rate of the mix and the ability of the contractor to obtain the desired level of density in the asphalt mix. The environmental conditions are different on each project and will affect the level of density obtained with each pass of the compaction equipment.
The effect of the base type and condition is also a factor which affects the level of stiffness or compaction achieved in the new asphalt layer. The amount of compactive effort needed depends, in part, on whether the new asphalt layer is placed on top of the subgrade soil, an aggregate base course, a cold mix asphalt layer, a cracked asphalt pavement layer, a new asphalt concrete layer, or a Portland cement concrete pavement layer. In addition, the thickness of the asphalt layer being placed is also a factor to be considered when attempting to compact the mix. Thinner asphalt layers will cool faster than thicker layers.
Lift Thickness and Particle Size
In general, thick lifts of asphalt mix can be compacted more easily than thin lifts. The thicker the lift, the longer the heat is retained, and thus there is more time for the rolling to take place.
As a general guideline, for finer dense graded mixes (those plotting above the 0.45-power chart maximum density line), the minimum lift thickness should be three times the nominal maximum aggregate size. Similarly, for a coarse graded mix (plotting below the maximum density line), the lift thickness should be at least four times the nominal maximum aggregate size. These guidelines provide sufficient lift thickness for the aggregate particles to re-orient and pack together during the compaction process.
It has been said that the three most important factors that affect the ability of a contractor to achieve the desired level of density in an asphalt mix are, in order of importance, temperature, temperature and temperature.
Asphalt mixes can be divided into two primary categories as far as the resistance to compaction is concerned. Some mixes are stiff and are difficult to compact. Some mixes are tender and move excessively under the action of steel drum rollers. The tender mixes generally will check or crack in the “middle temperature zone.” Instead of gaining density when the mix is rolled in the middle temperature zone, density is often lost when the mix moves in front of the steel wheels on a double drum vibratory roller or a static steel wheel roller.
The roller pattern used to compact a stiff mix is typically significantly different than the roller pattern used to compact a tender mix. Because of the three temperature zones usually found when compacting a tender mix, the stiffness of the mix at a particular temperature may not be related at all to the final density of the mix.
The temperature of the asphalt mix is continually changing during the rolling process. The rate of cooling of the mix is related to a number of factors such as the thickness of the layer being compacted, the temperature of the mix at the time it is extruded from under the screed on the paver, the properties of the asphalt mix (dense graded or open graded), as well as the environmental conditions such as air temperature and wind velocity.
The temperature at which an asphalt mix is produced affects both the ease of compaction and the time available for compaction. The time available for compaction increases with mix temperature, but there are limits as to how high the production temperature can be in order to avoid damaging the asphalt binder.
The continually changing mix temperature is the primary factor that makes prediction of the ultimate density of the mix when rolling is completed so difficult to estimate during the actual rolling process itself.
The goal in compacting an asphalt mix is to produce a smooth, stable and durable asphalt pavement. Understanding the factors affecting compaction is an important step in achieving this goal.
Jim Scherocman is a consulting engineer specializing in asphalt pavement design and construction. He can be reached at (513) 489-3338. Dwight Walker is the editor of Asphalt magazine. Prior to working for the Asphalt Institute, he was an asphalt materials engineer for the Kentucky Department of Highways.