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type of cement for building concrete benches? Answered

I recently bought a bench mold so I could build a cement garden bench   The mold has some delicate designs in it.   I tried using quikrite 5000 and other cements with basic aggregates already premixed, but it did not come out crisp. The designs were pitted by aggregates I think.   I am wondering if a simple Portland cement with only sand would work? I also incorporate rebar as well?

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CrLz

2 years ago

I volunteer at a 0playground and interface with the local Parks Department. Recently, there is a technique to add long (2" to 4") strands of fiberglass into concrete for applications like table tops and benches. This is replacing internal rebar, although fiberglass "rebar" is also being used for long spans. https://en.wikipedia.org/wiki/Glass_fiber_reinforc... The fiber additives provide tensile strength, allowing bridging spans without long tern cracking.

Instructables search of "GFRC" has a bunch:

https://www.instructables.com/id/Fabric-Formed-Tea-...

A google search for GFRC turned up a make magazine blog post, among other links

http://makezine.com/2012/04/11/how-to-mix-and-mold...

Love to see an ible of your project!

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Downunder35m

2 years ago

1. You want a mix that is really fine, so no big gravel or such.
2. If you want something to show through, like bigger pebbles you add them to the mold before pouring the concrete - just looks better.
3. Lets include one vital step ;) :

You need to work the concrete once your mold is full!
That means first to use some tools to punch into stuff to get it all evenly mixed.
And for something that requires a really good surface finnish you should consider a vibrator.
Either make the form vibrate or where the form rests on - this will force all these fine air bubble up that might still be stuck to the mold.

Another trick that I like to use is to pour a very thin concrete slushi into the mold first.
This way all the air is gone and water will be driven out by the concrete anyway.
But this only works if the detail work is not too deep otherwise you might end up with just a sand mix on the highlights.

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steveastrouk

2 years ago

Lots of advice on line, pre-mixed is not generally recommended.

This site has a working recipe.

http://www.ornamental-concrete-molds.com/index.php...

I quote

Cement Mixtures:
I'm going to keep it simple on mixtures. I use portland cement, mixing
sand and pea gravel to the consistency of a pancake batter. 1 part portland cement, 1 part sand, 2 parts pea gravel.
Pea gravel will add strength and may be used more with statues. A
little less gravel and a little more sand may be used for smaller
detailed pieces such as 1 part portland cement to 1 1/2 part sand and 1 1/2 partpea gravel. If your mixture is too dry it may be hard to get air bubbles out. Too much water your item can become sandy or worse.

...and use SHARP sand, not builder's sand for concrete.