I am designing and building a concrete bath. I've found many interesting sites about this.

1. Outdoor Bathtub
2. Solar Heated Outdoor Bath
3. Concrete Recipes
4. Hinoki Wood Japanese Bathtubs

I called Wairau Stone who were exceedingly helpful and friendly. They provided the following recipe.

Using a 40L bucket:

1. 2 buckets of Pumice
2. 2 buckets of clean fine sand
3. 3/4 buckets of portland cement
4. 1 cup Febtite
5. 1/3 cup oxide color

Canterbury Landscape Supplies Ltd is able to supply pumice and sand. There are a number of different grades available. I will probably be using 3-5mm air-dryed.

I am planning to use a technique called Ferrocement where you build the shape out of chicken wire and galvanised steel mesh. Then with the concrete you “plaster” it on to build the shape you are after.

Here is a video about applying ferrocement.

On another note, I found this great video about building houses using concrete using a very interesting polystyrene technique.