You may or may not have seen that we sell fibreglass reinforcement bars. The purpose of these bars as you might expect is to give the concrete more strength typically around weaker points such as openings. Openings may include hob cutouts, sink cutouts, apertures for BBQs etc. etc.
Now traditionally steel has been used to reinforce concrete but steel has a quite a few drawbacks. Its newer fibreglass cousin is lightweight, suffers no-corrosion, has superior tensile strength, and has a higher mechanical performance.
Incidentally we often get asked whether our mixes require reinforcement and the answer generally is no, because both our Cast-in-Place and GFRC Mixes comes with glass fibres which are a lightweight direct replacement for tradition steel mesh. Apologies for the side-track!
Anyway so back to the bars.
So when you are designing your worktop and you have an opening we recommend that you use bars to strengthen the weak points. Take a look at the image here;
Image 1 - Hairline Cracks
The hairline cracks shown on the sketch are those that will definitely occur without any reinforcement. That's if you're lucky. We've had worktops snap where we've left the our the "rebar" (ok, forgotten!).
So rebar is required. But how do you locate them and how many are required. Well we recommend 4 bars around each opening generally, with an overlap of roughly 300mm beyond the opening.
Image 2 - Plan View of Rebar Placement
It's not infallible and if the concrete isn't cured sufficiently, too thin or isn't handled correctly then hairlines cracks may still occur, however at worst these will be cosmetic and can be filled with Smoothing Paste
and then sanded back and sealed.
Here are a few more images of the placement of the bars in detail;
Image 3 - View through a section of the concrete containing the Rebar
Image 4 - Closeup of Rebars around the opening.
Image 3 shows the rebar being close to the upper surface of the concrete in a GFRC worktop. This is the location of the rebar during the manufacturing process and not once it's been flipped over and installed. Hopefully that makes sense!
The reason that the rebar is offset and positioned in the back coat is because you don't want it too close to the finished face of the concrete because the rebar may "ghost" through to the surface.