Saturday, June 1, 2013

Securing Reinforcing Mesh When Concreting

When I started this blog I intended it to be about electronics and technical things, I also said don't hold me to that.  So today's post is a little different.  Recently my sister moved into a new house and had her heart set on keeping chickens, therefore a coop was needed.  The ones available on the market are nice but expensive, not only that, from what I can tell they're all made in China out of wood that isn't going to last long when exposed to the elements.  The best solution was to build it ourselves.  Due to this, my time for the aforementioned "technical things" has been limited, but to keep the flow of information happening here I'll write about problems encountered during the build.

The design of the chicken coop is basic, it's pretty much a 1.8 m cube, with a corrugated iron roof.  The original plan was to build it so it could be moved, but as it's in an exposed location, wind became a problem.  From what I could determine from available information, a worst case uplift force of 2 kPa could be exerted on the roof.  For a roof that's 1.8 m square, that means a lifting force of 6480 N, or 660 kg.  With this information we decided to build it on a concrete pad.  It means it's not really movable any more, but at least it'll be secure in strong winds.

I've concreted before, but it's been a while, so I wanted to use this job as practice for future projects.  We may have been able to cut some corners in places, but because I'm not an experienced concreter I did everything by the book.  To start with I levelled and compacted the site by hand, 3.5 inch wooden boxing was then constructed.  As the ground contains a lot of clay it expands and contracts depending on how wet it is.  This means that the level of the ground can change considerably, in some parts it looks like a golf course. That combined with the largish size of the block, 2 by 2 meters, reinforcing mesh was pretty much mandatory.

Concrete Formwork / Boxing
Concrete Formwork / Boxing with Reinforcing Mesh

The above image shows the boxing with the reinforcing mesh in place.  One piece of mesh wasn't large enough to cover the area, I had to use two and overlap them.  If you look closely you can see the three rows in the middle that were tied together with wire.  The mesh is held 40mm above the ground by plastic supports called bar chairs.  Just a note, googling "bar chairs" is not the way to find out more about them, you will however find plenty information about bar stools.  The supports aren't entirely necessary for a block this size, you could just lay half your concrete, throw the mesh in and put more concrete on top.

Placing the mesh on the supports is easy except for a small problem I came across, one corner of my mesh was slightly bent upwards.  Not too much, but enough to make it come above the level of the finished concrete.  To correct this I needed to pull it towards the ground.  Initially I planned to just put some tent pegs in the ground and wire them to the mesh, but this breaks one important rule of using reinforcing mesh.  The mesh should be completely encapsulated to prevent rust penetrating the concrete slab.  Rust could have made its way to the mesh from the external tent peg via the tie down wire.  It takes time, but it does happen.

Concrete Reinforcing Bar Chair
Bar Chair Pinned to the Ground

The solution turned out to be simple.  By pinning the bar chair to the ground with tent pegs, and cable tying the mesh to it, the mesh remains isolated and completely encapsulated in the concrete.  Although it's probably not necessary on a job like this, I like to do things properly on jobs that don't matter so when it comes to jobs that are important I know what I'm doing.

After this, the concreting went smoothly and turned out pretty well if I don't say so myself.

Concrete Pad
Concrete Pad


  1. I've read your content Securing Reinforcing Mesh When Concreting...simply good job Grant!