Curbs must meet several basic requirements if they are to serve their intended purposes and have long service life. Curbs must have the required mass, stability and strength to withstand the impacts of traffic and the effects of their environments and to maintain their positions even when crossed by traffic or struck by snowplows. They must have the strength to bridge small areas where subgrade support is inadequate. Separate curb and gutter sections should be at least two feet wide with greater widths having more stability for a relatively small amount of construction costs. In severe environments, curbs must have adequate durability to resist freeze-thaw cycles while being buried in snow and ice plowed to the sides of streets and being saturated with brine from deicing salts that is carried to the sides. Another important requirement is visibility. Because of their light and reflective surfaces, concrete curbs can be easily seen, even at night when pavements are wet. The washings of rain and the removal of debris by street sweeping are sufficient to meet this requirement.

Curb Design
The design of curbs is more dependent on successful experience and regional preferences, and less on rigorous analyses compared to the design of other concrete structures. The review of a few available publications on concrete curbs reveals what types have been used but there is no specification regarding the forces acting on curbs or calculations on reducing stresses to acceptable limits. This is because experience has shown that curb sections proportioned to have adequate mass to provide the required stability are unlikely to fail from any imposed loads or impacts. Like other concrete members, curbs should be jointed or reinforced to accommodate the effects of volume changes due to shrinkage, temperature, or moisture changes. Jointing and reinforcing will be discussed in a later blog, so sit tight fot that. Besides meeting the basic requirements discussed above, good curb design should allow economical and efficient construction. Economical construction results from designs that reduce labor, permit the use of any of the efficient curb forming machines available today, and take advantage of standardized cross sections that provide the necessary properties. Minor variations in shapes or dimensions that add nothing to the strength or utility should be avoided. Templates or "mules" can be manufactured for any desired cross-sections form curb shapes, but they are costly. If the entire cost of a special mule must be amortized on a single project, the cost of the curb must necessarily be increased to cover that expense, even though the utility of the curb is not increased over that of a similar standard section.

Curb Drainage
For combined curb and gutter sections, the aprons (the portions between pavements and the faces of the curbs) should have adequate hydraulic capacity to carry runoff from most rainstorms. Making aprons wider reduces the opportunity for rainwater to move down through joints between curbs and pavements. Wider aprons may also discourage drivers from driving close to curbs. Since one important function of curbs is to collect runoff, provisions must be made periodically to drain water away before the roadways are flooded. In areas where there are storm sewers, the flow in gutters is diverted through inlets built into the curbs and/or gutters. In semi-arid regions where rains are infrequent, inlets are sometimes only gaps in curbs through which water can exit. In most other places, inlets are fitted with iron castings designed to match the shapes of the curbs (another good reason for using standard shapes), and with grates that extend one or two feet into the gutters. The spacing of inlets depends on the amounts of water that must be handled, and are calculated to avoid flooding of streets or roadways except on infrequent occasions, such as once every ten or twenty years. While inlet locations must be governed by rainfall and pavement elevations, designers should consider their effects on curb jointing. Inlets, which are more or less anchored in place, should be isolated from curbs and gutters. Because the sides of streets adjacent to curbs are often used by bicyclists, it is important that grates in the aprons of curbs have openings that will not be safety hazards to bicycle wheels. Grates are available that allow for the safe passage of bicycles.

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