By Tim Murphy, P.E. and Dwight Walker, P.E.
Part 3: Materials and Construction Practices for Commercial Pavements and Driveways
|This is the third article in a series that provides information on long lasting asphalt pavements for commercial parking lots and driveways. The first article provided an overview of the basics associated with these pavements. The second article covered foundation and structural design practices(subgrade preparation, drainage and thickness design). This article covers materials selection and construction practices.
Selecting the appropriate mixes and following good construction practices is critical to achieving long-term performance of commercial pavements and driveways. The appropriate asphalt mixtures for these applications are not the same as those used for high traffic installations.
The construction details for commercial pavements and driveways are similar to those used for highways. Careful workmanship, attention to detail, good compaction procedures, etc., all influence pavement performance, regardless of traffic level.
Definition of Terms
It is important that all parties understand what is expected of the paving job. Some of the commonly used terms are defined as follows:
- Base Course. The base course is the lower asphalt course (or courses), below the surface and any leveling course. The base course provides the strength of the pavement and typically has an aggregate top size of 3/4 to 1 inch.
- Tack / Prime Coat. The tack or prime coat is a spray applied application of emulsified asphalt. A prime coat is applied to an aggregate base to coat and bind particles on the surface of the aggregate layer and to promote bonding between the aggregate and asphalt base layers. A tack coat is used to create a bond between asphalt layers.
- Leveling Course. A leveling course is a thin layer of an asphalt mixture that is applied over an existing pavement to correct surface irregularities. The gradation of this mix varies depending on the thickness needed. The aggregate top size should not exceed the largest particles of the surface mix.
- Surface Course. The surface or wearing course is the top course of the pavement that acts as the riding surface and protects the underlying pavement structure.
- Pavement Thickness. The pavement thickness is the final, compacted thickness of the particular course, rather than the loose thickness of the mat prior to compaction.
Materials for Driveways
The asphalt mixtures used for paving driveways do not typically have to carry the loads associated with more heavily traveled pavements. For driveways, the base mix provides the support, and the surface mix should be sufficiently workable to provide a finished surface that is smooth and uniform.
The asphalt mixes typically consist of a blend of uniformly graded aggregate and an asphalt binder suited for local weather conditions. For driveways, mixes containing modified binders are not needed and should generally be avoided because these mixes may be difficult to hand-work and may result in open textured pavement surfaces. The coarse aggregate should be sound, angular crushed gravel, stone, or slag and the fine aggregate should be a well graded, moderately sharp to sharp sand.
Driveway mixes should be based on a mix design having about 3.5 percent air voids using a laboratory compactive effort suited for low traffic pavements. The base mix should be a well graded blend of aggregates that has a proven performance history. The selected surface mix should result in a pavement with a fine graded appearance and thus have reduced potential for segregation and water penetration from low in-place densities. The surface mix should contain some stone (rather than being an all sand mix) to minimize scuffing and punching depressions.
The base for driveways should use a 3/4- to 1-inch top size aggregate, and the surface mix should have a top size of 3/8 inch. Typical thicknesses for base courses are 3 to 4 inches; the surface course should be 1 to 1.5 inches. It is critical to recognize that all pavement thicknesses are in terms of the final, compacted thicknesses, rather than the loose thickness behind the paver.
Materials for Parking Lots
The materials for commercial parking lots are generally similar to those for driveways. There may be thicker, or more, base courses for selected areas of the commercial development that experience concentrated traffic. Examples of these locations include loading/delivery areas, dumpster pads, etc. For worst-case loading applications, such as dumpster pads, it is advisable to consult an engineer for advice concerning materials containing mixes with a high degree of stone-to-stone contact and/or modified binders.
Where thicker courses are used, an aggregate with a top size of 1 inch can be used. (For guidance on thickness/structural design, see Part 2 of this series, published in Spring 2008.) Surface mixes for commercial pavements should be placed at 1.5 inches and contain up to 1/2-inch top size stone.
Construction of Driveways
It is important that the subgrade is stable before beginning paving operations. For any backfilled areas, make sure that the soil is well compacted and that any settlement has ended. A loaded dump truck or similar vehicle can be driven over the subgrade as a proof roller to ensure that the foundation is stable. After finishing the subgrade, the aggregate base is placed, shaped and compacted.
Paving should be done only when weather conditions are good—not in cold or rainy weather. Compaction is critical to pavement performance, and the time to compact the mix is greatly reduced during cool or wet conditions. The compaction equipment should be sized to the job. Driveway jobs can be compacted with relatively small rollers (3 to 5 tons).
The contractor needs to work closely with the HMA producer to manage the temperature of the HMA. To avoid damaging the mix, it should not be overheated. But the mix temperature should be maintained at a level that allows thorough compaction. Similarly, during the entire operation, the contractor must manage the material handling and placement to avoid segregation. Hand-work should be minimized. As much as possible, let the paving machine do the work. Use extra care with construction of all joints.
Construction Practices for Commercial Pavements
For large commercial jobs, it may be worthwhile to name a “Person-in-Charge” who will oversee the project. This individual should know asphalt design, production, and installation.
All of the construction details described for driveways applies for commercial applications. Additionally, it is recommended that the surface course be placed all in one day, wherever possible. If it is not possible to complete the surface paving, it will be necessary to make a construction joint. Other important points include good handling practices such as managing segregation, minimizing raking and avoiding back-scattering of collected coarse particles.
For sizable commercial pavements, density testing may be appropriate. Generally, the pavement should be compacted to 92 percent or more of maximum density. Longitudinal joint density should exceed 90 percent. The thicker pavements of some parking lots may need heavier (and more) rollers.
Asphalt parking lots and driveways that are built with good materials and construction practices offer good performance and value to their owners. The process is not difficult; it just takes an understanding of the principles and a commitment to doing things right.
|Tim Murphy is the principal of Murphy Pavement Technology.
Dwight Walker is the editor of Asphalt Magazine.