This is the second part of a two-part post. See part one here.
Step 3 - Place it
Since the steel inside the concrete is there to provide tensile strength, it makes sense to ensure it is in the place where the tension is greatest. This location differs depending on the application.
When a span of concrete (or other material) is bent for example, the inside of the bend is typically experiencing compression while the outside is under tension.
Our two retaining wall types differ in design and as a result the location of the area under tension also differs. The pool walls have the retained side opposite the footing side whereas the rear retaining wall has the retained side on the same side as the footing. This leads to different sides of the footing being in compression and tension:
So the tension location differs, but we also need to maintain adequate cover. Cover is the minimum distance we need to keep between the steel and the outside of the concrete. It's needed to protect the steel from corrosion and ensure adequate connection to the concrete. For our footings, it's 50mm. So the main reinforcing bars needs to lie 50mm from the top of the footing for the rear retaining wall, and 50mm from the bottom of the footing for the pool retaining walls.
Chairs and ties
How do we position the steel accurately and ensure it stays in place when the concrete is being poured and we are walking all over it? With chairs and ties.
Chairs sit under the steel and ensure it sits at the right height. Chairs can be wire or plastic or as simple as a small rock. The concrete is poured with the chairs in place and they remain in the concrete after it cures.
The chairs we'll be using are plastic look like this:
The bar resting in the low channel (as shown) sits 50mm above the ground while any perpendicular bar sitting on the higher cutout it at 65mm. Unsurprisingly, this is known as 50/65 chairs.
The second piece of the puzzle for keeping the steel in place is tying it together with wire. The ties are not structural and are just there to keep the steel in place until the concrete cures.
There are many different ways to tie rebar, but I'll be using a twister.
The twister ties a short length of wire like this:
How to keep it vertical?
Tying the steel together at the base works well to hold the structure together, but doe little to keep the starter bars vertical. Some of the starter bars extend 1 meter up from the footing and must be kept in position during the concrete pour.
After considering a few different options, I decided to mount some guides on the edge of the footing and tie the starter bars on each side like so:
The rear retaining wall starter bars required support on all sides and so I drilled out support blocks and attached them to the footing form:
These supports will definitely make screeding (leveling) the concrete more challenging.
The finished product
After a great deal of work cutting, bending and tying, the completed reinforcement looks like this: