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slip formed curb

A concrete curb usually serves 4 main purposes:

-provide a uniform structural limit to the roadway edge

-provide constraint and control of storm water drainage within the road way

-provide an aesthetic ‘finished’ look

-control vehicles from driving on and disturbing the right of way (grasses, sods, sidewalks, plantings, trees, etc)

Curbs are typically found in urban and suburban areas and can rarely be found in rural areas except when particularly necessary. There are numerous styles of concrete curb profiles ranging from ‘straight’ to ‘curb and gutter’ to ‘slope faced’ curb.

Standard 6" x 18" 'straight' curb, battered down to 8" at bottom

Curb and gutter refers to a monolithic curb used where drainage is planned to be contained along the gutter channel into a drainage structure or other outlet location.

Slope faced curb is commonly seen in suburban areas where the curb can be mounted at low speeds and passed over.

Machine formed curb refers to a curb ‘slip form’ machine which receives concrete into a hopper and carefully sends it through a forming mold which shapes the curb in the exact section shape specified. Machine formed curb is more efficient, less resource intensive, and often more economical than hand formed curb.

Some of the benefits of slip formed curbs versus hand formed curbs are straighter edges, reduced wood/steel form use, better production, reduces concrete waste.

Prior to installation of slip formed curb, a ‘string-line’ is set up with highly visible string and stakes specifically made for holding the string in position. The string layout must be an accurate representation of where the curb will be placed, radii and all.

This string line is then used as a guide for the curb machine operator by lining up the string guide which places the slip mold in the exact vertical and horizontal position for correct curb placement.

Concrete curb machines are most commonly track mounted and self-propelled, but rubber tire units are used as well. Also, trailer units are available.

Unlike asphalt pavers which tow the asphalt truck, concrete curb machines typically do not tow the concrete truck. The concrete ready mix truck will carefully accelerate and brake as needed to move with the pace of the curb machine. The concrete mix design is specially formulated to assure the concrete is just plastic/wet enough to pass smoothly through the slip form but with a low enough slump (typically 1-2″)  to maintain it’s own weight on the ground. The slip mold is kept continuously wet through the machines water tank (or by direct water spray from the operator) which helps the concrete pass smoothly through the mold and provides a smooth appearance. The operator will also typically spray the top of the concrete trucks chute to maintain a lubricated path for the concrete. The mold is typically stainless steel and seals the concrete curb.

After the machine extrudes the curb, the curb may have pinholes and bugholes which need to be floated/darbied to smooth out any imperfections. Then it is struck with a broom finish very soon after extrusion and sprayed with a membrane curing compound shortly thereafter. The use of accelerated set concrete for slip formed curbs can be a challenge as the water/cement ratio of such a mix is often too low to release the concrete from the slip mold smoothly.

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