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    Default Castable Cement Like Material

    Hello all,
    I am making some outdoor furniture using some CNC foam molds. I have been using cement, and bracing it with a 'waffle' structure, but cement (at least the kind I have been using) really sucks. Its brittle, cracks, and has very little tensile strength. The reason I am using it is because it is completly weatherproof, I really want a material that does not age outside in my backyard.

    Is there something synthetic that can replace concrete? Non-porous, easily moldable, and relatively strong?
    Thanks!

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    cast iron is a traditional outdoor furniture material.
    Bill D.

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    Quote Originally Posted by oloneeye View Post
    Hello all,
    I am making some outdoor furniture using some CNC foam molds. I have been using cement, and bracing it with a 'waffle' structure, but cement (at least the kind I have been using) really sucks. Its brittle, cracks, and has very little tensile strength. The reason I am using it is because it is completly weatherproof, I really want a material that does not age outside in my backyard.

    Is there something synthetic that can replace concrete? Non-porous, easily moldable, and relatively strong?
    Thanks!
    .
    .
    2 part polyurethane you mix and it cures usually in 10 minutes to 24 hours depends on type. whenever over 1 or 2" thick fast curing will over heat and even start boiling. thus you use slower curing if you need to have thicker sections.
    .
    comes in different durometers some like plastic some like rubber

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    DIY CNC Epoxy Granite Part 1 of 2 - YouTube

    The link is both Machining, and an alternate castable weather proof concrete

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    Quote Originally Posted by oloneeye View Post
    Hello all,
    I am making some outdoor furniture using some CNC foam molds. I have been using cement, and bracing it with a 'waffle' structure, but cement (at least the kind I have been using) really sucks. Its brittle, cracks, and has very little tensile strength. The reason I am using it is because it is completly weatherproof, I really want a material that does not age outside in my backyard.

    Is there something synthetic that can replace concrete? Non-porous, easily moldable, and relatively strong?
    Thanks!
    Concrete is a very different material depending on the admixes, just the basic cement water sand aggregate mix is an art in balance. try some of the suggestions here http://makersgallery.com/concrete/index.html

    dee
    ;-D

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    The problem with any of the polymers is they degrade with UV exposure. I was thinking of a polymer concrete, something along the line of what is used for machine castings. Another problem with any of the above ideas, including mine, is the cost will be much higher than cement.

    Have you thought of adding a reinforcing material to your cement?

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    I guessing that the mix is the problem. have you ever noticed how many bridges, building and sidewalks are made of cement and last decades? get or make a good high portland recipe, maybe with fiber.

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    I just remembered the first reinforced concrete bridge is still standing in Golden Gate Park.
    Bill D.

    Alvord Lake Bridge - Wikipedia

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    you just aren't using the right mix, do some research on precast concrete
    adding reinforcing fiber will also help.
    you will probably find the ideal mix is fairly dry, and a good deal of vibration is needed

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    It hasn't been mentioned yet, but mortar and concrete typically are not very good when used in a tension application, they are best used when they are placed in compression.

    Personally I'd like to see some pictures of your work - it sounds interesting.

    -Jim

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    Several issues. Concrete re-enforced with fiberglass has be tried but over time the alkalinity of the concrete destroyed the silane coupling agent. The fiber glass became useless. I would try steel re-enforcing bars positioned in the mold. Note that concrete does not stand up well where salt is used for de-icing.

    Tom

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    Maybe just too much water. Fibermesh for reinforcing if you want. Shape may or may not be a problem.

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    Quote Originally Posted by oloneeye View Post
    Hello all,
    I am making some outdoor furniture using some CNC foam molds. I have been using cement,
    you are using cement? or concrete? huge difference, cement is a component of concrete along with aggregates. Concrete becomes quite strong when used with rebar. Bent up wire skeleton with concrete poured around it?

    check this out...and trying googling concrete sculptures or art etc.....lots of specialized knowledge out there http://www.makersgallery.com/concrete/howto1b.html

    Photo's would be interesting.....could range from "why didn't you buy something" to" that is friggin incredible"

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    Isn't the best concrete what the ancient Romans used and has lasted for 2000 years?
    Joe

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    Quote Originally Posted by Bill D View Post
    I just remembered the first reinforced concrete bridge is still standing in Golden Gate Park.
    Bill D.

    Alvord Lake Bridge - Wikipedia
    I think this came before:

    Roman bridge - Wikipedia

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    The strength of concrete is determined by the water/cement ratio. Less water means stronger concrete. Paradoxically, once the has set it is important to keep the concrete moist or even wet. Water is essential for the chemical reactions in curing concrete.

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    The dryer the mix the stronger it is.

    Tom

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    I absolutely would not use fiber on concrete for skin contact. The fibers will stick out of the concrete and it will be like working with fiberglass insulation every time you use the furniture.

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    ALL concretes are poor in tension relative to compression. This is the whole point of pre-stressed reinforced concrete structures: you have rebar or all-thread in the stuff, and stretch it before the concrete sets. Then you have bolts and washers (or cast in pieces) that allow the prestressed rod to compress the concrete. The rod keeps the stuff in compression and avoids the concrete from seeing much tension.

    That said, you see park benches made of concrete admixture. Cement, sand, and stone. Test your google fu.

    One spec sheet listed:
    1.1 MATERIALS

    1. Cement: Portland Type I or Type III white and/or grey meeting ASTM C 150.
    2. Fine Aggregate: Carefully graded and washed natural sands, or limestone sands meeting ASTM C 33 except that gradation may vary to achieve desired finish and texture.
    3. Coarse Aggregate: Carefully graded and washed limestone or other durable stone meeting ASTM C 33 except that gradation may vary to achieve desired finish and texture.
    4. Color: Colors added shall be inorganic (natural or synthetic) iron oxide pigments meeting ASTM C 979 excluding the use of a cement grade of carbon black pigment, and shall be guaranteed by the pigment manufacturer to be lime-proof. The amount of pigment shall not exceed 10 percent by weight of the cement used.
    5. Admixtures: ASTM C 494.

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