By Phil Blankenship, P.E.
Research and history have shown that important factors in improving density in asphalt pavements involve compaction specifications, lift thickness and mix design.
Of the three factors, compaction specifications are often considered the most important. Yet proper compaction is strongly related to proper lift thickness and mix design.
State DOTs create specifications so the motorist receives the best value from the road, while the contractor follows the specifications in the most competitive way possible. After the job is completed and the pavement is down, state DOT officials and the contractor may even look at the job and agree that it could have been done better. Their question to one another is, “How can we improve and extend the life of our pavements?”
The intent of this article is to explore the use of improved specifications to provide better density.
Compaction specifications
Improved density makes the mixture less permeable and more uniform. This in turn produces an improved pavement.
There is general agreement among many state DOTs that increasing the density of a pavement will increase its fatigue life, rutting resistance and overall durability. Laboratory testing based on a joint Asphalt Institute and Kentucky Transportation Center study found that increasing the pavement density from 92 to 93 percent of the maximum theoretical density (or 7-8 percent air voids) could improve the pavement fatigue life by 10 percent.
A few states have already moved to increase compaction standards to 93 percent of Gmm (Gmm = density with no air voids) or higher. However, many state DOTs still use 92 percent of Gmm.
A lower compaction specification of 91 or 92 percent of Gmm in-place density allows continuation of the status quo and lowers the durability of the road. If the compaction specifications require density of 93 percent of Gmm or higher, the contractor must improve the mix design for compaction and/or increase the lift thickness to achieve that density.
Mike Anderson, Director of Research for the Asphalt Institute, estimates “on a typical six-lane interstate with a ten-year rehabilitation cycle, compaction specification changes can yield a 3 to 5 percent decrease in the life-cycle cost of pavement rehabilitation.”
Here is an illustration from an Asphalt Institute lab compaction study:
1.0-inch lift, 5.4% asphalt binder = potential 91 to 92 density range = average pavement density
1.5-inch lift, 5.4% asphalt binder = potential 93 to 94 density range = improved pavement density = improved pavement durability
Maintenance projects
Maintenance projects and less-inspected paving projects may be facing an even more difficult challenge, because they may specify thinner lifts.
There is a desire to decrease the surface layer thickness on some projects to save initial construction dollars. But this is really a short-term savings because thinner-than-optimal lifts of dense-graded Superpave mixtures can rarely be compacted properly.
Lift thickness
Superpave guidelines specify that a coarse-graded mix should be four times the nominal maximum aggregate size (NMAS), or 4 stones thick, to compact properly and achieve density. A finegraded mix should be at least three times the nominal maximum aggregate size (NMAS), or 3 stones thick, to do the same.
Here is a question using a hypothetical example: Will a 9.5-mm NMAS Superpave coarse-graded mix that is one-stone thick provide adequate density?
The answer is, of course, “no” because it won’t compact. The roller will be resting on top of the rock. The rock will be touching the existing pavement. The roller will crush the stone or grind it into the existing pavement so a one-stone lift won’t compact.
There is only a slight improvement at two stones thick. The laboratory prediction of a one-stone lift (9.5 mm) is only 83 percent of Gmm (17 percent air voids). A two-stone, or 19 mm lift is about 88 percent of Gmm (12 percent air voids). In other words, a two-stone lift will compact, but not much.
A four-stone thickness lift of a 9.5-mm aggregate equates to a 38 mm (1.5-inch) lift and has room for sufficient compaction. In fact, based on information found in the Asphalt Institute’s “MS-4 Asphalt Handbook”, a 38 mm lift can be properly compacted assuming a 9.5-mm NMAS mixture is used. This lift thickness would be four times the NMAS of the mixture being compacted.
How, then, can we consistently construct pavements with an in-place density of 93 to 94 percent Gmm (6-7 percent air voids)? The answer is to increase the layer thickness to a four (4 x NMAS) or more aggregate thickness.
Improved compaction costs
Improving compaction may cost more because:
» You may need more rollers.
» You may need improved compaction equipment using intelligent compaction.
» You may need thicker lifts.
» Mix designs may require more asphalt binder.
By requiring increased pavement compaction through specifications, a thicker lift with a more compactable design is preferred and the pavement will last longer—a minimum of 10 percent longer. Ten percent will add one year to a ten-year surface course.
Good mix design
Theoretically, a good mix design starts in the lab, but because mix design is driven by VMA (Voids in Mineral Aggregate), it really starts at the quarry.
A mix design is most often, if not always, optimized around the VMA parameter. Therefore, it becomes critical to have an accurate bulk gravity (Gsb) of the combined aggregate.
For example, for a given mixture assume that the combined Gsb decreases from 2.680 to 2.670 – a reduction of only 0.01. If this reduced Gsb is used for the Asphalt Institute’s laboratory standard mix the resulting VMA will drop from 15.1 percent to 14.7 percent – a decrease of 0.4 percent. A contractor’s design usually is optimized on VMA, so a 0.4 percent loss in VMA could result in an approximate 0.4 air void change, which is equal to a 0.1 percent decrease in the design asphalt binder content to maintain appropriate volumetric properties.
“The amount of voids in an asphalt mixture is probably the single most important factor that affects performance throughout the life of an asphalt pavement,” says Ray Brown, National Center for Asphalt Technology (NCAT) Director Emeritus. “The voids are primarily controlled by asphalt content, compactive effort during construction and additional compaction under traffic.”
Since asphalt binder acts as a lubricant and aids in densification, it is critical that we have the right amount of asphalt binder in the mix. All too often, we can have good lab design, then move to the field for a test strip, or begin paving, only to find that the construction specifications allow the contractor to reduce the asphalt binder content by 0.3 to 0.5 percent and allow an increase in air voids as a field adjustment. As a result, the mix has less asphalt binder in it and is more difficult to compact.
There’s no question that good mix design and sufficient lift thickness are essential, but they are not alone. Proper density starts with proper compaction specs, which are especially critical for our interstate and primary roads. As critical arteries in the transportation system, it could be easily argued that long-lasting interstate and primary roads need in-place density at the time of construction of 94 percent Gmm or more to produce longer-lasting roads.
To conclude, a sufficient density goal is driven by proper compaction specifications, but is accomplished by three essential elements—proper compaction specifications, proper lift thickness and good mix design. That’s how to improve pavement density.
And remember, good density means a durable pavement.
Blankenship is an Asphalt Institute Senior Research Engineer.