Concrete casting question, part deux

Good morning.
This is a follow-up to my original concrete casting question, detailing what I am actually up to. I would appreciate any advice you can offer.
Thanks -Mark
http://www.canleyworks.com/temp/pier_caps /
My "Pisa" block piers need caps!
The standard caps are too small. . .I would need to assemble caps out of at least seven mitered pieces, which is not the look I want. So my plan is to pour caps large enough to complete the piers. My plan is as follows:
1. Build a form. Use masonite or melamine to create a smooth top surface, and wood or rubber trim to create the beveled edges. The bevels would actually form a channel where a brick chisel will be used on the finished product to strike off the outer two inches or so, creating a broken edge to match the standard caps. The mould would include bevels on the upper side of the mould as well, aligned with the bevels on the bottom. This forms the "pinch point" where the edges will eventually be stuck off.
Should the block be reinforced? Rebar, or screen?
2. Through experimentation, arrive at a color mix that reasonably approximates the factory-made blocks. A separate color mix would be used to create the streaks of contrasting color that run through the blocks.
3. Mix the concrete. I plan to use a cement-heavy mix (formula suggestions?), and screen the pea gravel to remove aggregate larger than 1/4" or so. Larger aggregate doesn't look good when the edge is broken off. Include the coloring in the mix.
4. When the mix is ready, sprinkle the accent color over the concrete and fold it in. Repeat once or twice. Don't over-mix.
5. Treat mould with release compound (suggestions?). Pour the concrete into the mould and vibrate the mould with my rotary hammer.
6. Remove finished block from mould. Allow to harden for a week or so, then chisel off the edges. Install on piers. Celebrate.
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Mark wrote: ...

What are the proposed dimensions? What are they sitting on? How much overhang? Not knowing any of the above, I'd hazard a guess bar is overkill, a little wire wouldn't hurt, nothing would probably be all right...

This _could_ be an extended experiement, but have to admit I have no experience whatsoever on coloring concrete so will defer other than anything to do with colors and experimentation can be frustrating w/o some guidelines to use...
...

See previous thread--at least two had quite good suggestions. Don't over-vibrate and separate water out excessively.
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You had shown us examples of super smooth concrete caps earlier. These are a bit rougher with a split face look on the outside edges to boot.
Are you just looking for basically a flat top square/rectangle with fractured look edges about 2" thick? You can go for the fractured edge as you describe, but I would cast a few extras. Concrete often doesn't like to break where you intend. You might consider creating the rough edge in your form work. The other cap stones you show, do not look slick finished to me; I think if you cast them on a masonite surface they will be way too smooth to blend with what you have going. I think I would consider using an oversized plywood base, create the rough edge with fractured Styrofoam or eat the edges with a solvent. Cast and vibrate as described. The squares ( they look like you headed for about 30"x30") are large enough to warrant some hog wire reinforcement. They are not thick enough for rebar and I don't think 6/6 x 10/10 mesh will help enough. The finish looks like something you could get by acid etching the formed surfaces and power washing. Your existing caps don't have the shiny look of cast concrete. ______________________________ Keep the whole world singing . . . . DanG (remove the sevens) snipped-for-privacy@7cox.net

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Yep. I was curious about the glassy-smooth caps and I wanted to know how they were made. But they are different from the look that I need.

The finished caps that I require would be 23" x 30" x 3" thick, with a bevel and a fractured edge all around. The commercially-available blocks are cast with two blocks face-to-face and a channel down the center where you use a chisel to split them into two. This creates two blocks, each with a fractured edge and a bevel.

You are correct. The top surfaces are level, although some exhibit a very slight crown when checked with my carpenter's square. So you think I could cast and vibrate them, then acid etch the cast surface to get the slightly pitted texture of the factory blocks?
Thanks -Mark
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I wasn't going to go into the methods of getting a controlled broken face, but you already are using most of it.
You will probably need at least 6" extra to break off and throw away on all four sides. If you truly want a bevel, tack down chamfer strip to establish the bevel at all four finish dimensions. Form and pour, at 3" you can probably eliminate the reinforcement, or use the hog wire making sure it does not extend into or close to your fracture planes. I would still cast the units upside down. Run an edger or jointer at your fracture lines. Use a fairly deep finned edger/jointer. The edges should break off cleanly from the bevel of the chamfer strip to the jointer track.
Yes, I still think your cap stones look acid washed. There is a retarder that can be painted onto forms that will keep the concrete soft just at the surface allowing water and brush cleaning after stripping the forms. One I have used is by Sika called Rugasol. See here: <http://www.google.com/search?hl=en&q=rugasol&btnG=Google+Search
______________________________ Keep the whole world singing . . . . DanG (remove the sevens) snipped-for-privacy@7cox.net
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Dan, can you elaborate on this a bit?
- How long should the concrete cure before acid etching?
- How long should the acid solution be applied before washing it off?
- I don't have a power washer, is this critical to obtaining the desired surface? Would mechanical means (like a heavy brush) suffice?
Thanks -Mark
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These are fairly big sections of concrete. You will be casting them upside down. Are you planning to mix your own concrete? and if so about what recipe? How were you planning on handling these 200# blocks?
If you think you can turn them over and strip them at 24 hours, have at. I think I would let them get at least 3 days old to manhandle them. The greener they are, the faster the acid will work, though you can acid wash 20 year old concrete to clean/etch/and change the finish. Muriatic acid, rubber gloves, old clothes, safety goggles, stiff fiber or plastic bristle brush, no steel or tools anywhere close or rinse them well, garden hose with spray nozzle, work outdoors with a breeze or a fan. Don't try to save the left over Muratic. It is cheap. The fumes that will get out of a used bottle will rust every piece of steel you own in the shed or garage where you try to keep it. Factory sealed jugs are OK. Baking soda or some other base will neutralize the acid, though thorough rinsing should take care of the concrete. No, a high pressure washer is not necessary.
You may want to make one or two miniatures to test your form work, the chamfer and edger work, the finish from the acid etch so the big blocks go well. You might also consider making the bottom form (top of the cap stone) with a piece of 1/4" ply and holding the 4 edges off the ground with scrap 2x material so the face will bow with the weight of the concrete. This should give your caps a slight crown to help shed water.
______________________________ Keep the whole world singing . . . . DanG (remove the sevens) snipped-for-privacy@7cox.net
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Yep. I'm a masochist, so I mix my own in a heavy-duty wheelbarrow. I start mixing with a short flat-blade shovel, then switch to a mixing hoe.
I'm planning another test cast this afternoon with the following mix:
2-1/2 parts sand 2-1/2 parts aggregate 2 parts cement water as needed
If you have a more appropriate mix to suggest, please let me know.
My very first color experiment (a mix of "brown," "buff" and "red" coloring powder) came out quite well. I'm going to use a little more buff and a little less brown on this next test and see how it compares.

By lifting with my knees, not with my back. :-)
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