Talking Asphalt: Density and compaction – February 2012

Talking AsphaltThis month, I’m back to my series of columns on factors affecting asphalt pavement performance. This column is about density and compaction. I believe density is the most critical construction property influencing pavement performance.

For asphalt pavements, compaction is the means of compressing the asphalt mix into a smaller volume. This process forces the aggregate particles together, which is necessary to increase the stability, rutting resistance and load-carrying capability. The strength and durability of a pavement is greatly dependent on achieving interlock, or stone-to-stone contact, of the aggregate particles. The compaction process decreases permeability, which in turn increases mix durability. Water and oxygen can damage asphalt materials. Achieving an impermeable condition is important for avoiding oxidation and stripping. Density is the measurement of the degree of compaction.

Mixture compaction is dependent on the mix properties, the environmental conditions, and the placement conditions. Obviously, mixes with rough-textured surfaces and stiff binders take more effort to compact. Cool air and paving surface temperatures and thin lifts reduce the time for compaction. Sufficient thickness of mix must be placed to allow for the aggregate particles to re-orient under the rollers. Typically, a lift thickness of four times the maximum particle size is recommended for coarse mixes and three times for fine mixes. The stiffness of the underlying materials also influences compaction. For most dense-graded mixes, the compaction operation should reduce the in-place air voids content to 7 percent or less to achieve a near-impermeable condition.

In recent years, some innovations have been introduced to help with compaction. Two of the more promising ones are Intelligent Compaction (IC) and the Bailey Method. IC uses high-tech rollers equipped with an integrated stiffness measuring system, GPS mapping capability and an on-board computer recording and reporting system. A color-coded graphic is displayed to show the number of roller passes and the mixture stiffness estimate. Most US roller manufacturers have, or are developing, IC machines. We’ll talk more about IC next month.

The Bailey Method was developed by Robert Bailey with Illinois DOT. It is a tool for developing and analyzing aggregate blend/gradations. The focus is on evaluating the way aggregate particles pack or fit together and how the resulting aggregate structure influences mix volumetrics and mixture compactability. It helps designers understand why some mixes are hard to compact or why small in-production gradation changes can make big differences in air voids and VMA and mixture compactabilities and density test results. More information on the Bailey Method is available on the Asphalt Institute?s website. Bailey training seminars are also available from AI.

As I said earlier, I think proper compaction and achieving good density is critical to pavement performance. If you have experienced problems with low densities, take another look at your mix designs. See if they can be fine-tuned. Make sure your lift thicknesses are adequate. Have your equipment in good working order. Optimize your rolling patterns. Make the effort to get good compaction and our pavements will last longer.