concrete question

[RobS.]

I want to make some tiles from concrete for a floor in a 12' x 24' screened building. They'll be dry set in a yellow sand base. I've made 3 boxes from 2x4 and plywood for molds. The plan is to buy bags of quikrete or similar product, mix it and scoop into the molds. I was thinking about lining the boxes with tarpaper/roofing paper to keep the tiles from sticking to the wood. Sizes are 16" x 16", 16" x 8" , and 8" x 8" x about 3" thickness. I might build one 16" x 32".
My question is will I be able to get the cured tile out of the box without tearing up the box? Will the tiles shrink slightly and pull away from the wood as they dry :icon_scra ? Is the tarpaper a good idea or bad idea :dontknow:? Anyone have experience with this?

I've checked prices on pre-made tiles at the big stores and I can do it myself for much less money and make a better tile too. 'Course it'll take a ton of concrete in 80 lb. bags...and my molds will have to work...and I'll need a new wheelbarrow..

 

[Mike Davis]

and your tiles will break up.

The tar paper will help, but the best thing is to make the mold with slightly slanted sides so the tile will pop out easily. If the walls are 90° the tile has a great grip on the mold all around. Slanted walls slide right off. Two degrees will probably work, but you need to experiment a little.

I would make as many molds as a batch of mix will fill up at one time. Mix a whole bag, fill up all molds, unmold next day, keep damp to promote hardening, repeat...

Second thing is chicken wire.

You need to cut chicken wire squares to fit in the mold, lay one in the bottom maybe suspended 1/3 height and a second piece at 2/3 height of mold. The wire adds a lot of strength to the tile.

 

[Mark Stewart]

Two things you can buy a bag of fiber fill for 5.00 - 10.00 thats enough for a full truckload on concrete. The second thins is when buying the bags ask if they have any ripped or torn bags. You will get these for 1/2 price . Just take a roll of trashbags with you to the big box store.

Thanks Mark

 

[McRabbet]

I would recommend lining your tapered forms (per Mike's suggestion) with 6 mil Polyethylene -- concrete won't stick to it, but it will stick to tar paper.

 

[Glennbear]

The only thing I would add is keep in mind the weight of your slabs if you are moving them after casting. 16x32x3 or even 16x16x3 is quite heavy and concrete is strong in compression but weak in tensile strength such as when being moved as a slab. :wsmile:

 

[FredP]

there is a form release agent that is sold. unfortunately I dont know what its called or where to get it.:dontknow: its an oil base stuff and you can spray it on with a pressure can like you use for weed killer. chicken wire will help with strength as mike suggested.

 

[RobS.]

and your tiles will break up.

Why? As long as I have no air pockets under them and the ground is flat and they're set in the sand I would think they would be fine. On the other hand, I did notice a lot of broken ones at Lowes, but then they were rather thin too.

The tar paper will help, but the best thing is to make the mold with slightly slanted sides so the tile will pop out easily. If the walls are 90° the tile has a great grip on the mold all around. Slanted walls slide right off. Two degrees will probably work, but you need to experiment a little.

Good idea.

I would make as many molds as a batch of mix will fill up at one time. Mix a whole bag, fill up all molds, unmold next day, keep damp to promote hardening, repeat...

That is my plan but I was hesitant to spend any more time making molds.

Second thing is chicken wire.

I had not thought about the chicken wire. Glad you mentioned it.

Also thanks McRabbet. I'll get polyethylene.

 

[timf67]

along with the chicken wire and keeping the concrete damp while it sets you can increase the strength by keeping air pockets and voids out of the concrete. I would build handles into your molds or some other means of enabling you to tamp the air out of the concrete.

 

[froglips]

Rob, I have the book "Making Concrete Countertops". You are welcome to borrow it. It covers in great detail forms, mixes, fillers and release agents doing it yourself. It also covers coloring and finishing.

One thing I haven't seen mentioned here is vibrating. There are several options, in your case, renting one from a rental place might be easier. They are long snake like tubes hooked up to a heavy drill.

You could also make a vibrating table. Think of a spindle on a lathe slightly off center. So each revolution causes the spindle to strick the underside of a table.

This settles the mix down and forces air bubbles to float out.

Not doing this can lead to cracking and voids.

Oh yeah, that is the voice of experience (for a change).

Jim

 

[Outa Square]

I've been looking about making my own cobblestones for a sidewalk project that i am going to have to tackle soon. What i have in mind is using 2 bys for the sides of the mold with plywood bottom and using old door/gate hinges to make a mold that will open... think spring form pan with removable bottom. I have read that a thin coating of vegatable oil can be used as a release agent, but don't over do it as it can cause bubbles. Some folks use used motor oil, i would shy away from that.

Good luck, let us know how it turns out.

 

[russellellis]

Ive made quite a few concrete tiles, stones, pavers, and countertops. The absolute best thing to use for the molds is a slick covered wood...ie melamine, skatelite. ive even used mdf that had about 4 layers of poly on it. I never had a problem with the concrete sticking in any combination.

My brothers good friend is working construction on a green "high rise" in carrboro. The builders used recycled veggie oil to coat their forms to pour all concrete there....footings, retaining walls, basement deck.

 

[RobS.]

So my project is underway. I took a lot of advice from you guys. I made my boxes from 2 x 4's and plywood - all old stuff laying around the shop (if I may quote Sawduster "it ain't scrap til I say it's scrap"). I put 4 coats of polyurethane on the interior side. I already had the poly. Then I cut some 6 mil plastic to line the boxes. The plastic was wrapped around a rug we bought my daughter several years ago (I knew I'd find a use for it one day). Instead of chicken wire I used a aliuminum window screen mainly because I already had it and it was free.
I mixed 1 bag of concrete at a time in the wheelbarrow, then scooped it into each mold about an inch or so. Then I placed the screen into the concrete and added more concrete to fill the mold. I added cabinet drawer pulls to 2 sides of each box so I could lift them. The pulls were saved from some past remodeling job - I always figured I'd find a use for them too. I set the filled molds on a small stoop outside the shop and set my air compressor on the stoop and turned it on. It vibrated enough to settle air bubbles. I pulled the release on the side which made the compressor run continuously. I ran it about 5 min. then off for a couple, then on for another 5 or so. Didn't want to burn it up (though I doubt that would happen).
The cured concrete slid out of the molds fairly easily. I think the poly in the molds was adequte by itself to prevent sticking. The plastic was probably overkill. I also misted the sides of the boxes with Pam cooking spray - the only time in my life I've ever used Pam. I've been able to reuse the plastic.
I was going to use a brush to give a slight texture to the exposed surface but when I took them out of the molds the plastic side looked great and had a little texture of it's own from the plastic.
I used the 8 bags of concrete to make 3 batches. The first batch did well but my thickness was inconsistant. The second batch was better but I realized that the boxes were not sitting level, or rather the stoop was not as level as i thought it was. The third batch I used wedges and a level to insure they cured level.
I had a truck deliver a load of sand yesterday and I worked til dark last night setting the tiles I made. Today I'm off to the building supply to get 12 more bags. Sometime this afternoon after my son gets up I'll try to get him to take some pictures. It's a BIG project which is going to take some time to finish but It'll be worth it

Thanks for all the advice!

 

[Mike Davis]

Aluminum screen won't add much strength to the concrete, it may even weaken it.

There was a study done on the effect of iron or steel in concrete. I don't know where to find the info but I'm sure you can search "ferro cement" and find several articles.

Anyway the iron reacts with the cement making it much stronger. It has to be evenly distributed about an inch apart, that is why most folks use chicken wire. It is not so much having the physical wire in the dried cement but the chemical reaction that takes place while the cement is curing.

Window screen has such small openings the cement may not adhere thoroughly. I can imagine the screen making a sheer plane that weakens the blocks you are making.

Try some each way and see which are stronger.

 

[Glennbear]

Aluminum screen won't add much strength to the concrete, it may even weaken it.

There was a study done on the effect of iron or steel in concrete. I don't know where to find the info but I'm sure you can search "ferro cement" and find several articles.

Anyway the iron reacts with the cement making it much stronger. It has to be evenly distributed about an inch apart, that is why most folks use chicken wire. It is not so much having the physical wire in the dried cement but the chemical reaction that takes place while the cement is curing.

Window screen has such small openings the cement may not adhere thoroughly. I can imagine the screen making a sheer plane that weakens the blocks you are making.

Try some each way and see which are stronger.

Mike's thoughts are similar to mine, I would be concerned about creating a shear line because larger aggregates would be held to each side of the screen. Another concern would be the lime in the concrete attacking the alumminum screen. :wsmile:

 

[RobS.]

Guys, it's set in a base of packed sand with no air pockets under it. I used some cloth my father picked up a Lowes to cover the ground to prevent grass or weds from popping up. The tiles are pressed into the sand and against each other. There is no room for them to slide around and no movement underneath. There's a roof over the shelter. I won't be driving the car over them and I'm not planning on doing any constuction projects on them. The shelter will be used for picnics maybe 3 or 4 times a year. I think they'd be plenty strong without the screen. It's not like this is a commercial job involving big $$.

Naysayers! :gar-La;

 

[Mike Davis]

It's you time.

If you don't want opinions... :dontknow:

 

[RobS.]

[QUOTE

Good luck, let us know how it turns out.[/QUOTE]


This was the part that threw me.

Seriously, I can see the potential for breakage along the screen seam if I were to beat on them. My options are to cut holes in the screen large enough to let the aggregate mix, use chicken wire, or leave it out altogether. I'm opting for the latter.
You can quite literally drive nails with my tiles. I did it last night. But I won't be smashing walnuts on them.


I was kinda proud of my little project...

 

[Bas]

Rob, I'm glad the tiles came out well! But, I can't believe we're up to post #18 in this thread without someone saying
:worthless: