Architectural slabs and concrete


  #1  
Old 09-07-05, 05:58 PM
Hawkeye
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Architectural slabs and concrete

We've built a raised patio. It consists of 2'x2' (87lbs) architectural slabs approximately 1 and 5/8ths inches thick. This is layed on 4-6 inches of gravel and an inch of sand. The entire patio is surrounded by Pisa II stack stones (2 high). The slabs are spaced so there is 4" between each one. We'd like to put concrete between each one. Originally, we were going to use black pavers but the pavers are thicker than the slabs. The cement we want to put in is to be colored black - we already have a commercial dye.

I am now doing my due diligence and researching the concrete to make sure I do everything right. I have a feeling I might be in a little over my head, but I am such a handlyman my wife thinks I can do no wrong.

Anyway, I'll be renting a mixer so I can get a consistent (as possible) mix and I'll be pouring in sections per squared off area according to the slabs.

What I need to know is:

1) What kind of concrete to buy because the thickness of our concrete will be just under 2". Some sort of fiber reinforced or do I need worry? Is there a particular brand that would best for this sort of job?

2) I don't want any "slopped" concrete to wick dye into the slabs and stain them. Someone suggested spraying the area with sugar water first. Will that be sufficient?

3) Where can I find some detailed information about finishing concrete? Waiting times? Smoothing techniques? etc.

4) How do I estimate how many bags of concrete I'll need?
 
  #2  
Old 09-07-05, 07:13 PM
Join Date: Mar 2005
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Architectural slabs and concrete

You say you already have the "commercial dye". You do not use dye to get a permanent color in concrete. Black is the most difficult color to acheive.

If you have carbon black or any carbon material just toss it. Products like carbon black are just tiny black paticles that are mechanically tied into the concrete. In time they will wear off and the concrete will lighten.

You need synthetic iron oxide pigment. Iron oxide is the most permanent pigment for concrete. Iron oxide is available in many colors ranging from yellow to red, brown and black. It "stains" the concrete. Unfortunately, black does not seem to have the tinting strength of the other colors. Normally, the maximum amout of an iron oxide recommended is 10% of the weight of the cement, and some people recommend a maximum of 7%. Excessive amounts can have negative effect on the concrete.

The concrete will be its darkest when it is fresh. It will lighten as the concrete cures. Keep this in mind if you make samples to determine the desired amount.

Good luck!

Dick
 
  #3  
Old 09-07-05, 10:22 PM
Hawkeye
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It's iron oxide. We got it from my wife's brother-in-law. He does cement for a living. I've been unable to catch him to talk more about our project so I searched out these forums. When I do talk with him I'd like to know what he's talking about so I am trying to educate myself.

You say 10% - if so, then I'm not sure we have enough. He said a couple of tablespoons per cement bag. He used the same thing on my mother-in-laws patio (which they also stamped) and it's pretty dark.

We do have a test area which will be covered and won't be seen. I'll have to give that a try first and see how it goes.

How about the other questions? The one I am most concerned about is the thickness. Comments?
 
  #4  
Old 09-08-05, 11:56 AM
Join Date: Mar 2005
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Architectural slabs and concrete

The 7% or 10% figures are MAXIMUMS. Synthetic iron oxide is a very strong and permanent material (it does not wash off you hands - it wears off). It is used in many commercial applications ranging from electronic tapes to industrial coloring (concrete, plastics, etc.) My detailed experience is in the use of pigments in "zero-slump" concrete with accelerated curing methods. Wet or "slump" concrete is slightly different, but the maximums are probably the same. Your dosage will depend on the color you want.

The color does change appreciably in the first few weeks, so you might want to make some small prototype batches early if you want to get as close as possible to the desired color.

Since your brother-in-law has worked with pigments for wet concrete in stamped applications, he will give you good advice. He can also help with the selection and the use of sealers that are applied to give colored concrete a darker appearing surface.

It looks you are on your way. Good luck!

Dick
 
 

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