Making a mold

After completing work on the compacted base for the retaining wall footings, it's time to make preparations to pour the concrete footings themselves.

Footings for the retaining walls

Footings for the retaining walls

I use the word pour because concrete is poured as a fluid (more like honey than water) and hardens over time. In order to create the concrete shapes that we want, it's necessary to make a mold to pour the concrete into. The materials used to make molds for concrete are called concrete forms. A large variety of materials can be used for formwork. Professional forms are expensive, but almost endlessly reusable and so cheap on a per-use basis.

Our forms are likely to be single use and we're on a tight budget so we're using melamine covered particleboard. This is the type of wood you typically find in Ikea furniture and kitchen cabinets. It's crap. It's made from wood flakes glued together with semi-toxic adhesives and is anything but waterproof*. It is however very cheap and will suffice for one-time formwork. The melamine coated surfaces are waterproof and to protect the sawn edges against rain during the construction period we're painting them with an acrylic paint.

On the plus side, the smooth melamine surface should make removal of the forms easier.

* Water resistant versions are available

The first in a trinity

There are three concrete pours required for this project:

  • The retaining wall footings
  • The retaining wall block fill
  • The pool to retaining wall bond beam

Pool and retaining walls

The three separate concrete pours

Reasonably extensive formwork is required for the footings, none for the block fill (the hollow block wall itself forms the mold), and simpler forms for the edges of the bond beam, although I'm yet to determine exactly how they will be held in place atop the walls.

Preparing the forms

All of our footings are 200mm deep and so most of our formwork will be 200mm wide strips. There are several level changes ranging from 400 to 600mm and so we'll also need larger pieces to manage the level changes.

Melamine covered particleboard sheets ready for cutting

Formwork wood

Wood cut into 200mm strips, awaiting paint to for water resistance

Building the forms themselves is just a question of cutting the right pieces and screwing it all together. Concrete is crazy heavy so we'll also need to brace it and put in a bunch of pegs to stop it moving when the concrete is being poured and pushed around. The more challenging aspect is ensuring that the pieces are in the correct place as the concrete footing needs to match up exactly with the pool shell.

Built formwork

Construction partner inspecting my partially built form

Built formwork

Completed (aside from bracing) formwork for one side trench

The next step will be to set up all of the steel reinforcement that will go inside the concrete.