General Question

LuckyGuy's avatar

After pouring concrete, what material should be used to fill the gap left by the forms?

Asked by LuckyGuy (29614 points ) September 15th, 2012

I am pouring a 10 ft x 10 ft concrete slab that is grade level with the surrounding grass. I am using well anchored pine 2×4s as the forms. After the slab sets up and the forms are removed, is it better to fill the gap with top soil or should I fill it with something porous like pea gravel. I live in a cold climate that ‘enjoys’ winter ice and snow for several months of the year.
Which is best for the life of the slab?

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9 Answers

gailcalled's avatar

What’s the purpose? Car park?

I would vote for pea gravel. The ground will heave and swell and shrink, and the drainage might be better.

If it were my gravel and concrete would throw some wildflower seeds mixed with a little topsoil as a carrier under the gravel. Daisies,bluets, black-eyed Susans and Shirley poppies might pop up.

dabbler's avatar

That’s what I was thinkin’, pea gravel can help with drainage, and it’s pretty attractive.
You might even want to cut the channel a little wider than 2×4, and make an intentional decorative border out of it.

What’s the drainage from the slab going to be like? And what’s around the slab (that might be vulnerable to puddling/flooding)? Is it perfectly level or is there a set mild angle for drainage? If the slab will be exposed to the elements you could consider a “French drain” incorporated into the gravel border.

LuckyGuy's avatar

@gailcalled, @dabbler This as actually a patio and it is bigger than 10×10. I just used that as an example.

It is set on a slight angle away from the house. To be honest it is actually 35 feet along the edge of the house and 11 ft from the house wall. On one side, I left a garden that’s 3.5 ft wide and 11 ft long covered in pea gravel. The other two sides (35 ft and 11 ft) are just level with the lawn. I was thinking if I put top soil in there and made it match the sod it would not attract water and would have soft earth against the concrete. Pea gravel would drain nicely but might fill up with water, and ice and push against the edge of the concrete. I don’t know.
The concrete was just poured. The forms come out on Monday so there is some time to buy pea gravel if that is best. .

dabbler's avatar

Cool ! May you have many happy patio moments !

If the area where the runoff will go will have no problem with the water then matching the lawn seems like it would be fine. Doesn’t seem like soil would retain enough water to make freezing a problem.

LuckyGuy's avatar

The frost line here is 48 inches deep. Water lines need to be run 54 inches below the surface.

I was worried that the pea gravel would be directly connected to the 6 inches of gravel under the slab and water would get underneath easily and cause it to heave. The whole thing is sitting on a solid base of tamped crush and run.gravel. with reinforcing mesh and rerod. Let’s hope it does not rain for the rest of the day.

Adirondackwannabe's avatar

@LuckyGuy I’m thinking the run off from the patio would run right underneath the concrete with pea gravel, but soil or maybe even clay it would run over the ground and away from the patio.

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LuckyGuy's avatar

@Adirondackwannabe That is my feeling too. I don’t recall ever seeing pea gravel along the edge of sidewalks. Considering how much snow we get and how cold it is for months, I think I’ll just go with compacted topsoil to force the water to run off and away. I might even put in some kind of drain to direct some of the runoff toward my garden. But that’s a project for another day.

gailcalled's avatar

I would plant a border garden. Lay down some pea gravel for drainage and throw decent top soil over it. I have perennials growing along the edges of the paved part of my drive. Eventually the weather damages everything.

The water run would water your flowers.

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