This article is the first in a series of articles providing information on how to achieve a long lasting hot mix asphalt (HMA) parking lot or driveway. This article provides some basic information and an overview of the major factors—thickness, drainage, materials, construction and maintenance.
|This is the third article in a series that provides information on long lasting asphalt pavements for commercial parking lots and driveways. The second article provided an overview of the basics associated with these pavements. The third article covered foundation and structural design practices(subgrade preparation, drainage and thickness design). This article covers materials selection and construction practices.
If you are in the asphalt business, you probably have had a neighbor ask, “How do I get a good driveway?” Or, you may have been asked by a property manager, “What can I do about this problem with my parking lot?” The answers to these questions are not complicated, yet they remain a mystery to a lot of owners.
Good design, good materials, proper construction, along with timely maintenance will contribute to having a trouble-free driveway. A well-designed and properly constructed driveway should last for 20 years or more. A poorly designed and poorly constructed driveway can fall apart in as little time as one year. A similar life-span applies to commercial parking lots.
In addition to the engineering decisions (thickness, materials, construction details, etc.), there are some management choices that greatly affect the pavement performance. These include selecting a contractor and monitoring the quality of his work.
Selecting a Contractor
Finding a good contractor requires a little research, but it may be the most critical part of getting a good job. You need to find a contractor who is knowledgeable, experienced and pays attention to the details of the work. You may need to talk with several contractors with reliable reputations to feel comfortable about hiring one. After you have made a short list of potential contractors, check with the Better Business Bureau for any complaints. Ask the contractors on your list to refer you to three recent jobs. Call the owners of those pavements and ask if they are satisfied with the contractor’s work. If possible, visit these jobs and observe the quality of work.
Another way to start the process of finding a reliable contractor is to call the local hot mix plant and ask the operator to recommend one or two driveway/commercial contractors that have a good track record and a solid reputation. If they are reliable, the plant operator will know it and will be willing to give you a name and telephone number. Again, do your homework and check out the recommended contractor. The main thing is to ensure that the contractor does good work.
Another important consideration is to provide the paving contractor with written instructions on the scope and quality of work expected. Unfortunately, very few driveway projects have a set of specifications to follow. To ensure a good job, a simple specification or standard should be provided. It does not have to be extensive, but it should be accurate and clear. Sample specifications are available to follow. The Asphalt Institute’s publication, IS-91, Full Depth Asphalt Pavements for Parking Lots, Service Stations, and Driveways, contains a sample specification.
In order for a pavement to perform well, it must have sufficient structural capacity to carry the in-service loads. The pavement thickness required depends on the subgrade conditions and traffic loads.
Proper preparation of the subgrade is essential; the subgrade is the foundation of the pavement. It must be free of topsoil and vegetation. It should be shaped to match the final contour of the finished pavement. It has to be well compacted and free of soft spots. Any soft spots should be removed and replaced with properly compacted aggregate or good soil. An improperly compacted or soft subgrade will eventually cause structural problems in the pavement above.
For typical residential driveways, an aggregate base layer of 6 to 8 inches of compacted crushed aggregate is recommended on top of the subgrade. This course is followed by approximately 2 inches of dense-graded HMA base and 1.5 inches of HMA surface. The asphalt thicknesses are for the compacted layers, not the loose, behind-the-paver depths. (The compacted thickness is typically about three quarters of the loose depth. A loose layer of 4 inches compacts to about 3 inches.)
The thicknesses of the HMA layers for parking lots depend on the traffic loading and subgrade strength. Delivery trucks will routinely use the pavement and will require more structure than driveways. These more heavily loaded areas may require a much thicker pavement. For these applications, a simplified pavement design is needed.
Adequate drainage of the pavement structure is considered one of the most important factors in parking lot and driveway performance. Possibly no other feature is as important in determining the ability of a pavement to withstand the effects of weather and traffic. Once water initially enters the subgrade, it is usually slow to evaporate or drain. Even in dry weather, the subgrade may remain wet or damp indefinitely. Most subgrade soils contain some silt and clay, which lose strength when wet.
The subgrade should be properly shaped and sloped for drainage. Sodded areas must be graded to drain water away from the pavement. The pavement should have a slope from crown-to-edge of ¼-inch per linear foot.
There are numerous HMA mix types available, and some thought needs to go into choosing the right mix. The contractor should work with the HMA mix producer to obtain a mixture that is suitable for the lift thickness to be placed. A stony mix may not be the best choice for these applications. Likewise, mixes containing specialty materials may not be the best choices.
Most conventional dense-graded mixes are acceptable for driveways and parking lots. A 19mm Superpave mix makes a good base course. For the surface course, a finer blend provides a denser, more appealing appearance. Rather than using the classic Superpave blends for surface mixes, a mix containing more sand will provide a mat that sheds water better and is more compactable.
Make sure that your contractor purchases the HMA from a reputable local producer. He will likely have favorite recipes for commercial work mixes that have performed well. Unless you are familiar with the quality of the work, it may be a good idea to ask to see some completed jobs to assure that the mix and mat texture meet your expectations.
Paving and Compaction
Before paving, make certain that the subgrade is stable. For backfilled areas, it is important that the soil is well compacted and that any settlement has ended. It is a good idea to drive a loaded dump truck or similar vehicle over the subgrade to ensure the working platform is stable. After finishing the subgrade, the aggregate base is placed, shaped and compacted.
Paving should be done only when weather conditions are good. Avoid 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) but the thicker pavements of some parking lots will need heavier (and more) rollers.
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.
After investing in a well designed and constructed pavement, it makes sense to protect the investment. Maintenance of parking lots and driveways consists primarily of taking care of drainage issues, maintaining edge support, and sealing the pavement.
Water is the enemy of pavements. Any low spot found to be ponding water needs to be filled with a fine patching mix. Ditches, gutters and drain inlets need to be clear and free-flowing.
The edge of the pavement should be supported by an aggregate or soil wedge. Curb blocks can be used to keep traffic from driving too close to the end of the pavement.
To seal or not to seal? This is the most frequently asked question. The easy answer is to seal if it is needed. A tight, water-resistant surface probably does not need a seal. A mat that is beginning to lose some fine material should be sealed. Sealing provides a waterproof cover and gives a uniform appearance.
Following the technical details listed in this overview, along with careful attention to workmanship details, should result in long lasting commercial and driveway pavements.