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So following on from my last blog post about fibreglass reinforcement bars, I thought I'd elaborate a touch on the finer points of the placement of the bars within the concrete. This applies to mesh also when used with our GFRC Worktop Premix and our Cast-In-Place. And specifically this relates to overhangs (otherwise known as cantilevers) and unsupported spans.
As you can see from the image above if there is no reinforcement the concrete will eventually sag. It may come as a shock to think that concrete can become bendy but believe me it will over time.
Preventing this is pretty simple. You reinforce the concrete either with mesh or fibreglass "rebar". Both of these will hold the concrete in tension like a taught nylon rope, but placement of them in the concrete is key.
Both of the images seem the same in that the concrete is sagging, but actually there are different things going on in terms of tension and compression. Bear with, bear with...
The highest tension for the unsupported span is on the bottom face and the highest tension in the cantilever (or overhang such as a breakfast bar) is on the top face.
Consequently when it comes to reinforcement the bars will have to be placed closer to the top face of the concrete for a cantilever or breakfast bar and then towards the bottom for an unsupported span.
Cantilever Diagram example - breakfast bar
Unsupported Span Diagram example - appliance space, log store.
So we've established;
- that you need to reinforce your unsupported sections of concrete and,
- where that reinforcement needs to be located
So now a word of warning! The rebar or mesh may "ghost" through to the surface if its TOO CLOSE to that surface.
So the bar or mesh needs to be at least 10mm away from the finished face. It should also come in 100mm from any outer edges. If you do use bars then the bars should be spaced no more than 250-300mm (except around openings - see previous blog post).
Also in relation to breakfast bars (cantilevers) the length of the bar embedded in the supported part of the concrete should be at lease equal to the length in the overhang. Sounds complex? Here's another drawing!
So hopefully that hasn't confused you too much! As ever if you have any questions just pick up the phone, send us an email or click on Chat and we'll help!
Bye for now.