Parking Lots

General

Nearly all building structures will incorporate some parking features to accommodate their workers, customers, and deliveries. Office buildings and retail stores across America are opening their doors and trying to attract new customers. First impressions are critical and the parking lot serves as a crucial gateway to what that first impression might be.

Today's asphalt parking lots are engineered with the latest advances in road science to meet the needs of motorists as well as the demands of traffic. As a result, the industry utilizes the technology and advancements to improve the quality of the pavement continuously. Ultimately, the goal is to provide a finished product that remains durable, smooth, safe, and sustainable for a long period of time.

Typical Sections

Asphalt pavements are typically characterized as a layered system where different materials are utilized and each layer contributes to the overall strength and function of the pavement structure. Most parking lots in Illinois are built on a foundation of native subgrade soils and an aggregate layer (typically dense graded aggregate) to provide load-carrying structure and to improve the working platform for the asphalt paving materials. Following the placement and compaction of the soil and aggregate layers, two or more layers of asphalt pavement are added to complete the pavement structure. The most common approach is to utilize a base asphalt mixture over the aggregate layer and then to utilize a surface mixture as the final riding course. Full-depth asphalt may also be used.

Terminology

In order to properly identify and specify asphalt mixtures in Illinois, you must first know the nomenclature and understand a few key parameters. This information is critical in specifying or reviewing asphalt mixtures for parking lot and roadway applications. Please review the Terminology section of this web site.

Drainage

Adequate drainage of the pavement structure is considered one of the most important factors in parking lot. 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 and aggregate layer should be properly shaped to ensure the pavement surface is sloped to drain stormwater from the surface. The pavement should have a slope from crown-to-edge of 2% (¼-inch per linear foot). Sodded areas must be graded to drain water away from the pavement.

It is absolutely critical that any pavement be well designed for proper drainage. Drainage problems are frequently a major cause of parking area pavement failures and should be given special consideration during the design and construction phases. Without proper drainage, stormwater may cause premature deterioration of the surface layers. In addition, moisture will penetrate into the subgrade layers, which could weaken the entire pavement structure resulting in severe distress.

Subgrade & Aggregate Base

Proper preparation of the subgrade is essential since the subgrade must serve both as a working platform to support construction equipment and as the foundation for the final pavement structure. During construction, the native soils may be evaluated by proof-rolling the area using heavy construction equipment. This is done to identify any unsuitable or soft areas that need to be removed or improved prior to placing subsequent layers. Unsuitable soils can be improved by blending aggregates with soil or by chemical stabilization using cement, kiln dust, or hydrated lime. All debris, topsoil, vegetation, or unsuitable materials should be removed and replaced with quality materials. It should be shaped to match the final contour of the finished surface.

All fill materials should be placed in thin lifts (12 inch maximum) at the proper moisture content and compacted prior to placement of the next lift. A properly prepared subgrade will not deflect excessively under the weight of a loaded truck. Prior to the start of paving operations, the subgrade soils should be checked for stability, moisture content, and proper grade. For projects designed with a layer of stone between the soil subgrade and the asphalt pavement, that layer must also be placed and compacted to proper moisture content, density and grade.

Asphalt Base Layer

The asphalt base course should be placed directly on the soil subgrade (full-depth design) or on the prepared aggregate base (aggregate base design). Asphalt mixtures used in base applications are characterized by larger aggregates and are typically placed in thicker layers. The base layer should be placed and compacted to the thickness indicated on the plans. The thicknesses shown on the plans represent the finished and compacted pavement thickness – not the loose thickness prior to compaction. Compaction of the asphalt base layers is critical to the performance of the pavement because it provides the structural foundation to support the weight of the traffic. In order to achieve compaction of a base mixture, research and experience indicates that the thickness of the layer must be at least three times the size of the largest aggregate in the mixture. See minimum lift thicknesses for Illinois mixes.

Asphalt Surface Layer

The asphalt surface layer is typically placed in one layer and compacted to the finish grade shown on the plans. The surface should not vary from the established grade by more than ¼ inch in 10 feet when measured in any direction. Rolling and compaction should start as soon as the asphalt material can be compacted without displacement and continue until it is thoroughly compacted and all the roller marks disappear. . In order to achieve compaction of a surface mixture, research and experience indicates that the thickness of the layer must be at least three times the size of the largest aggregate in the mixture. See minimum lift thicknesses for Illinois mixes.

Tack Coat

The purpose of a tack coat is to promote the bond between pavement layers. A tack coat may be used between the aggregate base layer and/or asphalt layers. A tack coat may not be required if the asphalt layers are placed in subsequent days and the surface remains clean and free of dust. Older pavement surfaces that will receive an overlay and milled surfaces will often utilize a tack coat.

The tack coat material is typically placed just prior to paving and must be applied to a surface that is clean and free of debris or loose materials. Most tack coat products are asphalt emulsions which need some time to "break" or cure. After the tack coat cures, the pavement is ready for the next layer of asphalt. The time necessary for the tack coat to cure is dependent on the type of tack coat, condition of existing material, and weather conditions at the time of placement.

Pavement Thickness

Subgrade condition and traffic vehicle type and volume will impact the overall design of your parking lot.

For light duty traffic (less than 1,500 vehicles per day and primarily passenger vehicles with less than 2% single unit trucks) and poor subgrade (typical in Illinois):

the typical minimum aggregate base design will require a 6-inch aggregate layer, 2.5-inch asphalt base layer, and 1.5-inch asphalt surface layer;
the typical minimum full-depth design will require a 5-inch asphalt layer.

For moderate duty traffic (average daily traffic from 700 to 3,000 and primarily passenger vehicles with less than 5% single unit trucks and less than 3% combination trucks) and poor subgrade (typical in Illinois):

the typical minimum aggregate base design will require a 6-inch aggregate layer, 2.5-inch asphalt base layer, 1.5-inch base layer, and 1.5-inch asphalt surface layer;
the typical full-depth design will require a 6.5-inch asphalt layer.

For heavy duty traffic (average daily traffic greater than 3,000 and less than 24,000 and primarily passenger vehicles with less than 5% single unit trucks and less than 3% combination trucks) and poor subgrade (typical in Illinois):the typical minimum aggregate base design will require a 6-inch aggregate layer, two 2.5-inch asphalt base layers, and 1.5-inch asphalt surface layer;

the typical minimum full-depth design will require an 8-inch asphalt layer.

Entrances, approaches, exits, drive-throughs, and areas in and around truck loading docks and trash dumpsters represent severe loading conditions for the pavement and should be carefully considered when designing the parking lot. Light duty pavement sections utilized in these areas can be prone to premature failures. The designer should take two factors into consideration: (1) the location of these special areas and (2) pavement thickness in that area.

If possible, locate the dumpster and/or loading dock in such a way to minimize the route that trucks must travel through the parking lot to and from the dumpster pad and/or loading dock. Routine truck traffic in an otherwise light duty section will result in a much thicker pavement design than is necessary and will increase costs. It is typically more cost effective to isolate the dumpster area and truck traffic to a small portion of the parking lot and address this area with a separate pavement design. Once the truck route to and from the dumpster pad and/or loading dock has been established, consider a moderate duty or heavy duty pavement section to accommodate this rather severe loading condition.

Once the thickness is known for each section of the parking lot, the designer may want to consider adjusting the aggregate layer to provide a uniform pavement thickness. This will allow a uniform subgrade with one continuous elevation to be constructed which may help eliminate the need for underdrains. For example, if the heavy duty section is composed of a 6-inch aggregate layer and 6.5-inch asphalt layer and the light duty section is composed of a 6-inch aggregate layer and a 4-inch asphalt layer, the designer may consider increasing the light duty area's aggregate layer to 8.5-inches.

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