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Concrete machine bench?

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John Hill:

--- Quote --- You also want to design it to avoid wild cross section variation, different thicknesses will dry/cure different speed AND shrink different speed AND that generates the cracks. Therefore, you probably want to make base and top separate.

--- End quote ---

Thanks Pekka, a very good point.

PekkaNF:

--- Quote from: John Hill on July 02, 2011, 03:33:11 AM ---
--- Quote --- You also want to design it to avoid wild cross section variation, different thicknesses will dry/cure different speed AND shrink different speed AND that generates the cracks. Therefore, you probably want to make base and top separate.

--- End quote ---

Thanks Pekka, a very good point.

--- End quote ---

It's an great honor for a junior member to help a senior practioner of applied science!  :wave:

For a well seasoned metal worker another difficult concept of composite materials is to understand that they are COMPLETELY different from homogenous materials. E.G. Concrete is really good in compression, but sucs in loading of any other direction. You need to load it in compression. Therefore you need rebar or other type of reinforcement. Like table top. You place a load on top of it. Compression on top = good for top. Bottom not so good....therefore you need a mesh or rebar. On bridges and such they go even further and tension the tiebars to keep the whole contraption on compression.

Pekka

John Hill:
Yes Pekka the intention is to have steel reinforcement in the slab but I had not thought of tensioning it but nothing is impossible. :scratch:

David Jupp:
Post tensioning might be a bit simpler to arrange than pre-tensioning.  Either would be significantly more complex than simple reinforcing.

PekkaNF:

--- Quote from: John Hill on July 02, 2011, 06:28:49 AM ---Yes Pekka the intention is to have steel reinforcement in the slab but I had not thought of tensioning it but nothing is impossible. :scratch:

--- End quote ---

Sorry I did't mean that table top needs to be prestressed, just trying to explain the need for rebar. Prestressed concrete would not be my first attempt with concrete. See this article:
http://en.wikipedia.org/wiki/Prestressed_concrete

I was thinking more like this on basics:
http://en.wikipedia.org/wiki/Reinforced_cement_concrete

There must be better than this instruction somewhere, just could not find it fast:
http://www.diynetwork.com/how-to/how-to-build-a-concrete-countertop/index.html

That has all the basic stuff.

Here it would be cheaper and easier to buy concrete on the bags, just add water, you can choose right size agregate for the required thickness. I also would use somewhat thicker mesh than hog fence. You want the reinforcement close to the surface, but not too close. This will have something to do thickness. Do you have an access to local building codes? Most countries have typical cross section drawings of floors and such, they probably have lot of useful instructions and I'm pretty sure you'll find those materials locally easier.

Just tell me to shut up if I get too longwinded :coffee:

Pekka

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