2012-05-24

Concrete Casting Tips

The actual Blog-Statistics indicates strong interest for concrete casting. I assembled some main hints to help you for your own casts. I am not a professional concrete expert, these tips are just condensed info's of my total 5 days of concrete casting.

A Basic Recipe

This is the mixture we used for the models shown in this blog. The mixture has been provided by our concrete specialist Heinz Richner (D-Baug / IfB)

Mixture with Sand 0/1 "Fine"

Material               1 Liter
-------------------------------
Cement 52R         982 g 
Flue Ash              165 g
Silika Dust             92 g 
Sand 0/1 *            740 g
Fibers PVA 6mm    16 g
Water                    270 g
Glenium ACE 30   15 g

*For concrete mixture "Normal" use Sand 0/4

Building cast-forms

(In German. Provided by Heinz Richner (D-Baug / IfB))

Schalungsbau Manual

Getting ready

  • Be sure you prepare your project early and gather some experience with this material.
  • Calculate the volume of your project and produce 30-50% more concrete. On one side for covering fails and on the other side to be sure to have enough material.
  • Buy  vaseline, silicone spray, gloves and dust masks
  • Have a recipe ready to calculate your material.

 Preproduction

  • When designing your form, think early on how you remove your model when the concrete hardened out.
  • Concrete creates massive forces while hardening. Build your forms accordingly.
  • Stuff floats in fluid concrete. When using foam inserts, remember this.
  • Concrete surfaces reflect the form surface exactly. Use acrylic-glas or polished MDF to get mirror like surfaces. Plastic foils will be deformed because of the forces during hardening. Rough wood-planking result in nice wood structures.
  • Use Vaseline and silicone spray to lubricate your form. Badly lubricated forms are very hard to open after hardening.
  • Review your form. Imagine how you pour concrete in your form, how it distributes, what forces it cause and how you remove the result.

Production

  • Prepare your workspace. How do you transport your finished objects, which form has priority.
  • Prepare your mixture.
  • First dry-mix everything.
  • Slowly, very slowly add 3/4 of your water mixture. Let it stir 1-2 minutes. Then add very small amounts of water while constantly checking the fluidity of your mixture. Never add too much water or add it too fast. The moment, concrete fluidizes happens really fast.
  • Fill your forms and pay attention on how you pour the mixture in your form. If necessary angle your form to release air and be aware of air-traps.
  • Slightly bumping the form on the floor releases bigger air-bubbles.
  • After filling your forms store them in a room with high humidity. Concrete does not dry, it hardens by chemically processing water!
  • Store the forms for a minimum of 24 hours, depending on the mixture you used.

Postproduction

  • When removing your models, be gentle and patient. Never use brute force, you'll damage your model. When getting nervous, better go for a coffee and try again later.
  • To glue model parts together you may use Epoxy or Silicon glue.
  • Handle the model with care, it easily breaks depending on your mixture and thickness.

I hope this helps to get started with concrete casting. Leave a comment if you got success.

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