Concrete w/ embedded slate

bobby769September 6, 2010

I have a walkway beteween the house and detached garage w/ old slate tile just laying on the ground.

I'm thinking of making 2'x2' blocks of cement and embedding the slate into it.

I imagine I'd make some wood forms, place the slate tile in the form and then pour concrete over it. When the concrete dries it would be flipped out of the mold so the slate now appears on the walking surface of the concrete blocks.

The cement blocks would be 3" to 4" high w/ galvanized mesh for rebar. The slate that I currently have ranges ion thickness from about 1" to 2" and some are not very flat on the top. This uneven slate would probably result in a rustic looking block but I'm ok with that.

Your thoughts on this would be appreciated.

Some of the questions I have are:

- How heavy will these blockes be? Since I have to flip them over.

- The plan is to use regular plywood. Will I have a hard time freeing the cement blocks after curing? I was hoping I could simply flip them out.

- Do I have to use special wood? Will the cement cause the plywood to start falling apart?

The plan is to counter sink the concrete blocks so the are realtively flush w/ the ground. Can I just dig out enough depth for the blocks or would i have to dig our more and build it up w/ sand for drainage?

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missingtheobvious(Blue Ridge 7a)

- How heavy will these blockes be? Since I have to flip them over.

Concrete seems to be about 145 lbs. per cubic foot.

A 3" thick, 2'x2' square of concrete would be 145 lbs. (1 cubic ft.)

A 4" thick, 2'x2' square of concrete would be 193 lbs. (1.33 cubic ft.)

Slate seems to be 168 lbs. per cubic foot.

So the concrete with slate would weigh more, but it would depend how large and thick the slate is and how thick the square is.

Then there's the weight of the reinforcing mesh to factor in. That would weigh more than the same volume of concrete or slate, but we don't know how exactly what kind you'd be using, how much you'd be using, or what metal it's made of.

There's also lighter-weight concrete. Hypertufa is one type. I don't know if it's suitable for what you're doing.

Not a pro, but I can look things up on the Net, and my PC has a calculator 8-)

    Bookmark   September 6, 2010 at 4:46PM
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I wouldn't expect the blocks to simply flip out once cured - you might have to treat the plywood with some oil based product; Pam or maybe even everyday vegetable oil would probably work. I also think you would still have to hammer (or at least pry) the wood away from the cement slabs.

    Bookmark   September 6, 2010 at 5:36PM
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150+ lbs is heaver than I expected but might be doable if I limit how much the blocks have to be handled.

I checked out Hyperufa and it's strength is a 10th of concrete so that's out.

The mesh I had in mind is something like this:

I'm reading confliciting info online regarding remesh.
Some say that if the subground is properly prepared remesh isn't needed, others disagree. I'll probably wind up using it anyway.

I was hoping to make only 2 forms and reuse them. I'll probably need about 10 2'x2' blocks so hopefully, the forms won't get damaged after the first use if I use a release agent.

I've also been reading about people making latex/rubber molds for concrete. Anyone have any experience w/ this?

    Bookmark   September 6, 2010 at 7:42PM
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missingtheobvious(Blue Ridge 7a)

You can buy molds for making decorative stepping stones. Apparently they don't last through very many uses -- and have to be treated very carefully. Here's a site I had bookmarked; these molds are polyurethane:

If you could find a local vendor, you could look at the molds; perhaps it might give you ideas, or perhaps the vendor might have useful supplies. Apparently some crafts stores stock molds and supplies for making decorative stepping stones and other small items, so that's a place you could try if you want to see ready-made molds (Michael's, Hobby Lobby, etc. -- probably best to call ahead and ask if they stock such things).

Here's a Google search which brings up interesting results. It looks like most people who make stepping stones are making very small ones. But I think it's always helpful to read how different people do things. I looked at the first five sites, and I think they all had useful info, particularly how some of the wooden molds were made and what they used to keep the mold from sticking. They mostly seemed to use hardware cloth for reinforcement -- but then, their projects weren't as large as yours.

    Bookmark   September 6, 2010 at 9:26PM
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hosenemesis(SoCal Sunset 19 USDA 8b)

It might look just as good if you formed your concrete where you want it, cure it and remove your forms, then use thinset to attach your slate to the top.

    Bookmark   September 6, 2010 at 11:29PM
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formed concrete in place is something i'm considering.
What I had in mind was to make forms, pour concrete and before it dries to push the slate so it's flush w/ the rest of the concrete and then point around the slate to make it neat.

If I use thinset wouldn't I need to build up the thinset so it's flush w/ the slate?

    Bookmark   September 7, 2010 at 3:03PM
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1" of concrete thicker than the slate should suffice. The mesh can be used but should not be necessary provided you keep the concrete wet while it cures and don't start with a wet soupy mix. I would do a mix of one part portland cement, one and one half parts sand and one and one half parts 1/4" stone mixed with one part water. It will be thick so let it sit for a bit during mixing if you are doing it by hand. Too much water in the mix will make them weak and they will most likely crack. Wear rubber gloves and "massage" the concrete into the forms with your fingers. Concrete is caustic and unlike acid, peroxide, chlorine and other caustic materials it does not burn or hurt until it is too late. Cement is only considered an occupational hazard but you should take proper precautions while using it.

To do what you want to do correctly you should line the bottom of the forms with sand (about 1/8-1/4"), place the slate and get it wet before you pour the concrete. This way you will not have concrete on the top of the slate and a nice looking finish. While the concrete will usually not stick to plywood forms a vegetable oil or other type of release agent would make it easier to handle. I would wait at least 36 hours before trying to remove them from the forms. If you do have one that breaks you can break up the concrete and reuse it.

    Bookmark   September 7, 2010 at 7:37PM
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Thanks jey.

Just a couple of quick questions.
So, brush/roll on the release agent/veg oil,
sprinkle 1/8 - 1/4" sand, lay the slate tile, then massage the concrete into the forms?

I guess it's expected that the layer of sand would only be on the bottom of the form and not on the sides?

I've seen people tap or hammer the sides of forms like this. Is that done to get out air bubbles or help the block release easily?

Would the consistency of the mixed concrete be crumbly?

I may have additional questions about the ratios you mentioned but am researching this and may answer my own questions.

Thanks again. I really appreciate everyone's input.

    Bookmark   September 8, 2010 at 4:40PM
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I don't think it is worth the effort, but if you did want 2'x2' concrete blocks with your flagstone attached, why would you not just buy 2'x2'blocks and mortar the stone to them? It would be cleaner, easier, probably cheaper, and would look a lot better. Maybe not good, but better.

Concrete mix bags are not high in cement content and getting flag stone to bond to a quickcrete mix might be asking for a lot.

    Bookmark   September 8, 2010 at 9:33PM
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The release agent only needs to be on the sides as the (wet) sand and slate on the bottom will act as the release agent there. The sand is only on the bottom to hold the slate in place and prevent the concrete from getting underneath. It should come at least 1/16" above the bottom face of the slate to keep the concrete between the slate and prevent it from sticking above it.

The mix will seem crumbly if you mix it by hand and don't let it rest for a few minutes. If you start with moist sand and stone it makes it a bit easier.

Excessive water makes the concrete weak. The ratio of sand and stone can be altered but the cement to water should be whatever the manufacturer recommends (usually one to one by volume) and no more. 3/8" stone would be the largest you'd want to use and avoid round sand and stone.

I wouldn't bang on the forms with a hammer because it will knock the slate around and also creates air pockets and voids. Pressing it in with your hands will give you the best results. If you place the forms on a level bed of sand you can use a rubber mallet on a piece of wood to tamp the concrete but really shouldn't need to.

    Bookmark   September 9, 2010 at 12:10AM
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It is easy to get hung up on using things we already have, but think this through as if you don't already have the flagstone. First, are these stepping stones actually something that you really want? Second, is the time, effort and expense really worth the product that you are going to get vs. just going out and buying a higher quality finished product?

    Bookmark   September 9, 2010 at 7:25AM
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It's a walkway that currently has slate tile just laying there. At this time there's a lot of grass growing between the slates.

If I didn't have this slate, I'd still want to place some kind of stone material in the location.

I suppose some of my options would be simply a concrete walkway (very much like a regular sidewalk), patterned brick, or anyone of the other usual walkway materials.

In the end Slate would proabably be my choice anyway because the house is white w/ blue (smokey blue to be exact) shutters and other trim. The slate that's there matches the trim of the house very nicely.

I know I can dig out the dirt below the slate, lay down gravel and then locking sand then the slate. What I'm imagining (embedded slate) I think is just going to give an overall cleaner look.

Can you give me examples of finished products?
The only thing I can think of is brick pavers. I did a search online for slate embedded in concrete and found nothing.

    Bookmark   September 9, 2010 at 3:10PM
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Personally, I would rather see a nice slate pathway than some slate embedded in the concrete pathway. Will the grass still grow up between the concrete stepping stones?

For all the work you're going to do, you may as well just make the path.

    Bookmark   September 9, 2010 at 4:48PM
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If you like 2'x2' slabs, there is bluestone, sandstone, granite, and quartzite to name a few natural stones that are avaialable in that size and in thick enough pieces to behave themselves. These and others are available in irregular shaped slabs in most parts of the country as well.

If you want to save money, you can get concrete stepping stones already made. You can get them with exposed aggregate finish, or made with pigmented mix.

I don't know if you have 2'x2' square cut flagstone, but if you don't it is going to be a pretty odd looking stepper unless you saw cut the stones to fit the block. A square slab of concrete with an irregular shaped piece of flagging embedded in it?

    Bookmark   September 9, 2010 at 10:38PM
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Slate is INSANELY slippery when wet. The kids are always falling on the neighbors' slate walkway and scraping themselves quite a bit.

    Bookmark   September 10, 2010 at 9:50AM
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I expect that the joints between the concrete slabs may get some grass come up through it, but it will be nowhere near as much as what you normally see in a slate pathway.

In the end it will look almost like a sidewalk so what ever grass occasionally grows in sidewalk seams is what I'd get here.

    Bookmark   September 19, 2010 at 9:19AM
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It will be irregular shapes of slate embedded in concrete.
I'm figuring the ratio of slate to concrete that you would see when it's all complete is about 1:1 to 2:1 per slab.

This difference in slate to concrete should also take care of any slippery when wet concerns.

    Bookmark   September 19, 2010 at 9:24AM
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