The Basics of Curbs & Gutters
March 2012This three-part series will provide a general reference for the use, design, and construction of concrete curbs and curb and gutter sections along the edges of streets, parking lots, and other pavements on grade.
A curb, by definition, is something that restrains; an enclosing border or edging; a raised edge or margin; a wall; or as a verb, to strengthen or confine something. As most people think of curbs, they are raised strips of concrete along the edges of streets or parking lots. The benefits of curbs have been recognized since ancient times, and stone curbs were placed along the edges of traveled ways by early civilizations. Today’s concrete curbs still provide many of the same benefits, and more.
Concrete curbs or combined curbs and gutters serve several important functions. Curbs collect water from crowned pavements and convey it to points of collection, thus reducing the amount of water that gets under the pavement. They outline the edges of pavements and provide easily definable borders between traveled and untraveled surfaces. They confine pavement structures, especially if the pavements are composed of layers of materials that must be compacted in-place. Curbs help contain low speed traffic within the edges of pavements.
ADVANTAGES OF CURBS
Besides serving the purposes listed above, curbs provide several other advantages. The neat, straight lines of curbs add to the attractiveness of parking lots and streets, and the commonly used expression “curb appeal” implies that attractiveness of adjacent properties are also enhanced by sharp demarcations between streets and lawns.
Curbs strengthen pavements. The confining of flexible pavements by concrete curbs improves compaction during construction and helps maintain the integrity of edges under traffic. The added thickness given to the edges of concrete pavements by integral curbs increases strength and stiffness, reduces deflections induced by traffic loads, and therefore extends pavement life.
Curbs reduce the amount of space or right-of-way required for a street by eliminating drainage swales and their flat side slopes. Curbs also reduce the lengths of driveways built from streets to homes or businesses. In some jurisdictions where both curbed and uncurbed streets are allowed by subdivision ordinances, streets with curbed sections require less dedicated right-of-way than streets without curbs, for example, 50 feet for local streets with curbs, or 60 feet for local streets without curbs. The elimination of drainage swales also reduces maintenance by eliminating the cleaning of ditches, the mowing of ditch banks, and the care of culverts and their end sections that carry water under driveways.
The light reflective surfaces of concrete curbs delineate pavement edges and improve visibility for drivers at night, thus promoting safety. Where there are no concrete curbs to outline the edges of roads and streets, it is now common practice to mark the pavement edges with stripes of white paint.
Curbs improve the efficiency of street sweepers by concentrating debris for easy, mechanical pickup, as opposed to having it scattered along shoulders and drainage swales where it must be picked up by hand. Concrete curbs have the integrity to withstand the impact of snowplows.
TYPES OF CURBS
Concrete curbs are generally classified as barrier curbs or mountable curbs. Either type can be constructed in many different shapes, depending on regional preferences, purposes, and construction costs.
Barrier curbs, also known as straight curbs, resemble the stone slabs used originally for curbs, and form abrupt obstacles to vehicles leaving pavements. Mountable curbs, sometimes referred to as roll curbs, have sloping faces that allow vehicles to encroach on them without damaging tires and wheels; and if the slopes are gentle enough, cars can cross them to access driveways. Curbs that cannot be crossed without damage or discomfort must have sections where the heights of the curbs are reduced for vehicular entrances. The low portions are usually referred to as depressed curbs. When curbs are constructed in areas where buildings have already been erected and driveways established, the depressed portions can be easily designated, but in developing areas where the driveways have not been located, mountable curbs are usually preferred.
Either type of curb can have an apron or gutter section attached and become a combined curb and gutter. Combined curb and gutter sections are commonly used along streets and parking lots in urban areas, especially with asphalt pavements, to provide the advantages of stable concrete gutters with sustainable flow lines along the curbs.
Because concrete can be readily shaped to transition between crosssections, curbs can be tapered to meet ramps for pedestrian crossings where these are preferred or to meet requirements for the disabled.
Curbs built monolithically with concrete pavements project above the pavement at the edges. These are referred to as monolithic curbs or integral curbs, as opposed to separate curbs. As the edges of concrete pavements with the added thickness of curbs are stronger and stiffer, deflections caused by heavy wheels close to the outside edges are reduced. Where curbs are cast on hardened concrete slabs, resulting in cold joints between the curbs and slabs, there are opportunities for planes of weakness and water penetration, which can result in shortened service life.
A separate curb and gutter must be tied to the pavement slab with deformed steel bars if there is to be effective load transfer. If a curb is separate from the pavement the joint between the pavement and the curb may require maintenance. ■
Part two of this series will focus on requirements and the design of curbs and gutters.
For More Information:
This information was taken from general online sources. For detailed information, visit www.ehow.com and search for curbs and gutters and concrete curb types.
Download a pdf version of this article. | Adobe Reader is required.
