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Benefits
Applications
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Curb Construction Testing for Quality | Curing | Joint Cutting | Backfilling
Like concrete pavements, curbs are constructed with traditional forms or slipform equipment, separately or along with the construction of concrete pavements. The procedures for efficient and economical construction of concrete curbs are practically parallel to those for concrete streets and roads. These include the preparation of the subgrade to provide uniform support, the accurate alignment of forms or slipform equipment, the placing and consolidating of durable concrete, jointing for crack control, finishing and texturing, curing, and protecting until ready for traffic loads.
The subgrade for curbs may be prepared separately, but it is preferable to prepare the subgrade for streets and curbs simultaneously in order to take advantage of the efficiency and effectiveness of larger equipment and to establish uniform support. After any underground utilities and sub-surface drainage features are installed and properly backfilled, subgrades should be carefully compacted to specified densities with controlled moisture. 95% of standard Proctor density is usually specified. In some cases, separate curbs are designed to extend to the bottom of the pavement structure, but where there are granular drainage courses or subbases, they should carry through under the curbs. Where there are compacted aggregate bases at lower elevations than curb bottoms, the bases should extend under the curbs and far enough beyond to support the curb construction equipment. Besides the curbs themselves, curb forms and curb slipform machines need firm support to produce straight, stable curbs at the specified elevations.
Curb forms should be straight, clean, and adequately interlocked and braced to hold to designed lines and grades. This is especially critical for combined curbs and gutters in order to assure the gutters will drain efficiently. It is also critical for tall straight curbs whose forms must be stable to maintain lines and elevations. Steel forms are preferred to wood forms because of their greater stability, but wood forms can be used for small projects and for curved sections if enough stakes and braces are employed. Forms should be sprayed with a release agent prior to placing the concrete, but the release agent should not be allowed on the reinforcing steel, if used. While the backs of formed curbs are usually vertical, exposed portions of front faces are often sloped or battered. Sloping formed faces require spacers or templates to maintain correct cross sections and are therefore more difficult to form. Slipformed curbs, on the other hand, are preferably constructed with battered or sloping back and front faces for greater stability, faster placement and more efficiency. It is not uncommon to have gaps or short sections in slipformed curbs filled in with formed sections. In these instances, it is important that the formed sections match the slipformed sections in cross-sections and textures.
Reinforcing steel in concrete curbs is not usually necessary or recommended if correct jointing is used. However, if formed curbs are to be reinforced with steel bars, the bars should be positioned and secured within the forms so that they will not be displaced during placement and consolidation of the concrete. The deformed bars should not extend through expansion joint material placed at tangents of radii, but should extend through any spacer plates used to hold the forms in position. In reinforced slipformed curbs, the reinforcing bars are laid out in front of the slipform machines and fed through positioning plates in the fronts of the machines as they advance. Reinforcing bars must be lap spliced appropriate lengths to develop continuity and the laps should be tied to keep them in place, unless ties would interfere in slipform operations.
Concrete can be placed directly from ready mixed concrete trucks into forms or into hoppers of slipform curb machines. The ready mixed concrete trucks usually operate on the subgrades of the streets being constructed, but if compacted aggregate base is already in place, precautions should be taken to avoid having the delivery trucks disturbing the compacted surface or tracking soil on to the base. If delivering ready mixed concrete trucks are to operate behind the curbs, it is essential that any underground utilities or sub-surface drainage trenches behind the curbs have been properly backfilled. As the low slump concrete required for curb and gutter construction is susceptible to stiffening rapidly, it is important to schedule the concrete delivery so that there is no delay in discharging the concrete from the ready mixed concrete trucks when they arrive at the site. Concrete placed in forms can be struck off manually or by form riding machines. Concrete must be consolidated to avoid honeycomb and voids and it must be shaped to the proper cross sections. When finished manually, curb and gutter sections are shaped by drawing straightedges across templates or by mules shaped to be pulled along the tops of the forms. Curbs and gutters can be checked for drainage by covering them with burlap and carefully pouring water into the gutters to be sure there are no flat spots. Curbs and gutters should be textured with light broom finishes or by dragging wet burlap or artificial turf along the surfaces.
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