Commercial and Residential Paving
Part I: An Overview
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.
Part II: Subgrade Preparation, Drainage and Pavement Thickness
By John Davis
Three of the essential elements of constructing a long-lasting asphalt driveway or parking lot are subgrade preparation, drainage and pavement thickness design. If any of these steps are flawed, the driveway or parking lot will probably not meet expectations.
Subgrade Preparation for Driveways
Buddy Prather, president of Prather Paving in Lexington, Kentucky, a driveway and parking lot contractor, always makes a careful evaluation of the subgrade before doing any work.
“If there are severe structural problems or subgrade problems, we remove the old asphalt, dig out the soft part of the subgrade and fill it back up with 2-inch rock, plus Dense Graded Aggregate on top,” says Prather. (Dense Graded Aggregate is a Kentucky DOT specification material consisting of a well graded, 0.75-inch top-size aggregate with high dust content, which is commonly used as an aggregate base.)
“When we do prep work, like cutting out humps or soft spots, we level them or remove the bad soil and replace it with 12 inches of 2-inch rock plus 2 inches of Dense Graded stone,” says Prather.
Leveling and contouring the subgrade is important so that there is proper drainage and no low spots. Prather adds that on a driveway that is 20 or 30 years old, he uses a stringline to ensure leveling.
As part of the prep work, Prather cuts the grass close to the side of the driveway and thoroughly cleans the surface of the old asphalt. “Then we mill along the sidewalk so the new mat properly abuts the concrete sidewalk,” he says.
“What’s good to know about new driveways for most homes or townhouses or condos is that practically every worker on the job eventually drives his truck on the subgrade where the asphalt will be placed,” says Tim Murphy, President of Murphy Pavement Technology of Chicago. “This means that plenty of continuous compaction occurs on the subgrade and aggregate base, and if weak spots do exist they will be discovered by default and removed and replaced because no driver wants to get his work truck stuck.”
Subgrade for Commercial Parking Lots
When preparing subgrades for parking lots, Prather Paving cuts out soft spots and replaces them with 12 inches of 2-inch uniformly-sized rock. “We also put fabric under the 2-inch rock to strengthen it,” says Prather. “Then we place 8 inches of Dense Graded Aggregate on top of the big rock, then pave it with 2 inches of hot mix base course, and 1.5 inches of surface course.”
Murphy says that soils conditions in the subgrade must be evaluated and addressed before a good parking lot can be constructed. “Because of the various soils types around the country, it is imperative to hire an experienced geotechnical engineer with a working knowledge of local soils conditions when determining subgrade treatments,” says Murphy. “Some areas require lime stabilization or under-cutting one foot of soil and replacing it with high quality aggregate material.”
Murphy says that driveway drainage concerns can almost always be mitigated by simply sloping the asphalt pavement one way or the other, and by avoiding a built-in “bird-bath” depression. “In southern climates the bird-bath is a nuisance, while in northern climates it can become a litigious situation if someone slips and falls on the ice,” says Murphy. “Often times the biggest problem with drainage is with improper surface course installation, as well as the aggressive use of a sprinkling system.”
For driveways, Prather says his company uses a drainable subbase. “We use 4 inches of 2-inch rock as the subbase, then add 2 inches of Dense Graded Aggregate. We pave it with 2 inches of an intermediate-sized, binder mix and 1.5 inches of surface course.” On small parking lots and long residential driveways, Prather checks that there is a 2 percent (or more) slope to ensure drainage.
For parking lot drainage, a proper asphalt pavement slope, adequate spacing of catch basins, and locating catch basins to avoid trafficked areas will create an effective drainage system. Also, contractors can talk to the property manager about the excessive use of sprinkling systems that may saturate the pavement structure.
Pavement Thickness Design for Driveways
The Asphalt Institute has informational brochures that provide guidelines for thickness design. Murphy’s experience and recommendations generally agree with the Institute guidelines. In general, Murphy recommends 4 inches of compacted hot mix asphalt on a full-depth driveway (hot mix asphalt placed directly on subgrade). When using a combination aggregate subbase and hot mix mat, Murphy recommends 3 inches of asphalt placed over 4 inches of aggregate subbase.
“What is important,” says Murphy, “is that the maximum particle size for the aggregate base be well-graded—including 1-inch top size through fine material—and the hot mix surface course contain 1/2-inch top size aggregate.”
For upgrading longer, existing driveways that carry a mix of traffic, Prather Paving puts down two inches of hot mix asphalt binder, composed of 1/2- or 3/4-inch top size rock and 1.5 inches of a dense graded surface mix. “On small driveways where the subgrade is good, we put down 2 inches of compacted surface mix,” says Prather.
Parking Lot Thickness Design
Many parking lots fail not because of aging or alligator cracking, says Tim Murphy, but because heavily-loaded trucks or heavily-used locations in the parking lot do not have adequate pavement thickness. Entrances, approaches, exits, drive-throughs and dumpster-pad areas break down because there is not enough structural capacity (thickness) to support the heavy use and heavy loads.
Parking lot pavement designers need to specify increased thicknesses for heavily used areas and areas where heavy vehicles move. They should also require traffic control devices that will direct heavy vehicles to use the areas where the pavement is thicker.
“Tens of thousands of dollars can be saved in pavement repairs by instructing drivers of delivery and disposal trucks where to drive, with signs or physical barriers, speed bumps and traffic control patterns,” says Murphy.
Light Duty and Heavy Duty
For full-depth, light-duty commercial parking lots, Murphy recommends 4.5 inches compacted thickness of hot mix asphalt on the subgrade. For full-depth, heavy-duty parking lots, he recommends 7.5 inches of hot mix asphalt on subgrade.
For light-duty parking lots with an aggregate base, Murphy recommends 3 inches of hot mix asphalt on 6 inches of aggregate base. For heavy-duty parking lots, he recommends 6 inches of hot mix asphalt on 3 inches of aggregate base.
“It is important to recognize that both the light-duty and heavy-duty pavements total 9 inches,” says Murphy. This allows for constructing the subgrade soils to one continuous elevation with a slight slope away from the structure. If the thicknesses vary, then an underground accumulation of water will most certainly occur. This would require underdrains, which increases the cost of the parking lot with no inherent benefit to the structure.”
Prather Paving’s recommendation for the aggregate base is somewhat different. “On commercial jobs,” says Prather, “most engineers will specify 8 inches of dense-graded aggregate. If I can talk with the engineer, I recommend 6 inches of 2-inch size rock plus 2 inches of dense-graded aggregate. That makes a strong subbase. Then we lay down 2 inches (compacted thickness) of binder mix and 1.5 inches of surface course.”
Assuring Adequate Pavement Support
Asphalt pavements are versatile and long lasting. But careful planning, design and construction is needed in order to achieve these characteristics. The process includes following proper guidelines for subgrade preparation, drainage and thickness design. The Asphalt Institute has several informational brochures on these subjects, which can be obtained from their website,www.asphaltinstitute.org. The website also has basic information and answers to frequently asked questions about driveways and commercial pavements.
Part III: Materials and Construction Practices for Commercial Pavements and Driveways
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. 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.