scope:

This guide is based on the current knowledge and best practices for the design, construction, and maintenance of concrete parking lots placed on the ground. Some of these practices differ from those used in the design and construction of streets, highways, floors and other types of pavements and flatwork. This guide is not a standard or a specification, and it is not intended to be included by reference in construction contract documents; ACI 330.1 can be used for these purposes.

Parking lots have most loads imposed on interior slabs surrounded by other pavement. Highway and street pavements carry heavy loads along and across free edges and are subjected to greater deflections and stresses. Streets and pavements are usually designed to drain toward an edge where the water can be carried away from the pavement. Parking lots are usually designed so some of the water is collected internally and is conveyed away through underground systems. In urban areas where rainfall runoff from large impervious surfaces is regulated, parking lots often serve as detention basins (not addressed in this guide). This means that the pavement should store water for a period of time without incurring any damage due to loss of support from a saturated subgrade. Parking lots often accommodate appurtenances, such as lighting standards, drainage structures, traffic islands, and landscaped planting areas. Provisions for these appurtenances should be considered in the design of the jointing system and the layout for construction.

Some design methods for concrete parking lot pavements have been based on methods developed for the design of highway pavements such as the Portland Cement Association (1984a,b) method and the AASHTO (1993) design method. These methods are primarily concerned with limiting stresses in the slab (PCA) and the reductions in serviceability caused by mixed traffic (PCA and AASHTO), including heavy trucks, while parking lots usually serve fewer vehicles either parked or traveling at slow speeds. Additionally, AASHTO (1993) is a purely empirical method where the relevant range of vehicle types, subbase and subgrade materials, and pavement repetitions are generally outside of the relevant range for parking lots. Figure 1.2 presents a typical section through a concrete pavement showing subbase and subgrade layers. For many parking lots intended for only light traffic loads, the need for an extensive design process may be less critical. For such projects, a designer can rely on past experience to select conservative values for the design criteria of subgrade soil support and imposed vehicle loads. In these cases, a conservative selection of pavement thickness is prudent practice.

The widely accepted PCA method has been incorporated and extended into the web based Uniform Pavement Design software (PavementDesigner.org) and is the principal design tool used in this guide. Its ability to model traffic spectra typically encountered in parking lots, and its stress/fatiguebased mechanistic-empirical algorithm make it an appropriate procedure for parking lot design. Conversely, the AASHTO (1993) procedure is based on limited testing and materials used over a half-century ago for high-volume highway traffic at high speeds. It is not a stress- or fatiguebased method and its applicability is outside the relevant range of vehicle types, base and subgrade materials, and pavement repetitions typically encountered in parking lots. It is not used in this guide and is not recommended for parking lot design.

Although thickness design is important, other aspects of parking lot design are equally critical. Often, if a concrete parking lot has performed poorly, it is due to non-thicknessrelated design and construction issues. These may include:

a) Unsuitable subgrade support

b) Improper joint spacing and patterns

c) Improper placement and detailing of isolation and construction joints

d) Inappropriate use of deformed steel reinforcing bars for odd-shaped panels, tying certain panels together, and other specific functions

e) Poor construction aspects such as concrete mixture proportioning, placement and finishing methods, curing, and saw-cutting

f) Insufficient maintenance for joints and pavement, as needed

These layout and construction issues are addressed in this guide.

Determining and specifying practical thickness tolerances for pavements is important. Reduction of the pavement thickness beyond tolerance recommendations can unacceptably increase pavement stresses, reduce pavement structural capacity, and reduce pavement life. Although construction smoothness tolerances are not critical for parking areas for low-speed traffic, smoothness is important where concrete surfaces are expected to drain well and carry water long distances across pavements with minimal slope.

Aesthetic considerations of surface texture and crack control in parking lots can be important because of close scrutiny from pedestrians and the owner's desire to project a quality image. In large parking lots, it is important to direct traffic into designated driving lanes and deter heavy vehicles from crossing thin pavements. The future expansion of a parking lot and the facility it serves should also be considered during initial design so that light-vehicle pavements are not required to accommodate future heavy loads. Industries and shopping centers served by public transportation, and schools served by buses are examples where expansion can transform auto parking areas into more robust truck or bus driveways.

Previous versions of this document were intended to address a relatively broad scope of traffic types and loads, including truck traffic up to 700 trucks per day and concrete thickness of up to 9 in. (225 mm). In 2017, however, ACI Committee 330 published ACI 330.2R, "Guide for Design and Construction of Concrete Site Paving for Heavy Industrial and Trucking Facilities," which covers the design and construction of pavements for higher volumes of over-theroad trucks, and some special vehicles and applications. Because 330.2R more completely addresses pavements for heavier loadings, including more detailed design of subgrades and subbases, load transfer at joints, and other challenges, the scope of 330R has been reduced relative to design for higher truck volumes and heavier vehicle loads. Therefore, this guide is now written for light-duty, unreinforced, undoweled concrete parking lots. It is applicable for:

a) Passenger vehicle parking areas, where vehicles other than cars and light-duty trucks are channeled to other areas

b) Driveways and drive lanes that can include some trucks, busses, or fire trucks

c) Delivery and trash pickup areas that may see moderate numbers of loaded tractor trailers, delivery trucks, and/or garbage trucks

While there will naturally be some overlap between the two documents, the user is encouraged to consider referring to 330.2R whenever designs approach or exceed the upper limits of loading scenarios, and/or when the designer chooses to incorporate dowels or reinforcing for load transfer.