finishing concrete sidewalk curbs

[calypso5]

I noticed that the forms on concrete curbs are pulled off fairly quickly. How do they finish these? Are they rubbed or brushed with a concrete slurry to get the bug holes out etc? Also, how soon after pouring can the curb forms be pulled..... Thanks John

[SteveBell]

It depends on the type of concrete used, and how moist it is. If you make it really soupy, it takes longer to get hard enough to stand on its own (and the resulting concrete is weak).

I watched a crew put in a driveway apron a couple of weeks ago. They did the pour, levelled, screeded, textured, and put in relief grooves, all in an hour or two. Then they took off the forms and went away. This was in Fort Worth, on a 100°F day, with clear skies.

I've used quick-setting concrete for patching, and it gets too hard to work in about ten minutes. Common bagged concrete for posts takes a few hours to dry hard enough that the post won't wiggle.

[DanG]

Much curb work is done with no forms on the front. It is typical to bring the truck at a 2" slump, and "stack" the curb areas, then wet the truck down to a 4 or 5 to pour the flat work. All concrete that is not left in forms is a timing game that requires knowledge of concrete, temperature, humidity, and wind. There is no single answer for stripping forms.

[jloomis]

[calypso5]

Thanks for the responses Steve and Dan. I have done slabs, foundation walls and retaining walls in the past. Pouring curbs without a form sounds interesting Dan. Has anyone here pulled a "formed" up curb after the concrete begins to set and if so how did you finish the face of the curb? John

[calypso5]

Thanks jloomis, that is the information I have been looking for. This site rocks. John

[Tom Cular]

John, Here in the northeast, with hand set forms ( about 400LF a day per crew) ( this time of the year) curb is normally placed with a 4-5" slump and 5-6% air entrainment.

Depending upon the weather, the face forms can be removed and the curb face (road side) hit lightly with a mag float and touched up with a whitewash brush dipped in water, not soaked within an hour of placement.

Slip- formed curb or curb & gutter requires a slip-form machine , and is suitable for long runs only.

[jloomis]

[DanG]

Wood float, flip water on a brush, curb brush for finish.

[calypso5]

Thanks for the information everyone. One more question. I was talking to the concrete truck driver on my last pour and he said contractors were starting to use the pea gravel mix for all their pours and not just "pumped" pours since it makes the floating and finishing way easier. Does anyone see any problems down the road with using the pea gravel instead of using 3/4" crushed rock?

[jloomis]

[calypso5]

Thanks jloomis.... John

[Tom Cular]

Sure do see issues with pea gravel!

1. It is a much softer stone than crushed stone.
2. It tends to be round and smooth as opposed to crushed stone that has been fractured and has an irregular surface that promotes a much better bond between the cement and the aggregate. The same reasom that beach sand is recommended to NOT be used in concrete, from the constamt motion, beach sand tends to become round and smooth. The same reason most states limit recycled glass content in hot mix asphalt mixes, it fractures fairlly smooth and does not present the bonding surface of mined or manufactured sand
3. In some areas, washed gravel is locally available and therefore much cheaper to use than crushed stone. Been to that movie.
4. The reference to pump mixes using pea gravel are bogus. With a properly designed pump mix, any pump can handle 3/4" crushed aggregate without significant problems.

I've been on projects where pea gravel was spec'd due to the amount and density of re-bar in a small area (column forms), but that was before the admixtures that are available today.

[calypso5]

Tom, that is interesting reading. Thanks for the information. What type of admixtures are you refering to in column forms? Is the admixture fibers that replace the aggregate in the concrete? John

[Tom Cular]

No, I was primarily referring to water reducers and super plasticizers. Super P can reduce the amount of water required by 12-30% , depending on the brand and dosage, and still produce concrete that flows and is as workable as concrete with an 8" slump. The effects of super p dissapear in 30-60 minutes. There are several others, accelerators, retarders, air entrainment additives, fly ash, silica fume etc., these products are generally specified by an engineer for a particular purpose.

[calypso5]

Thanks Tom John