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
PREFABRICATION REBAR PLACEMENT
If you're prefabricating your worktops, i.e. making a mould and pouring the concrete into it so that the concrete in the BOTTOM of the mould is the finished face (see How To Make A GFRC Worktop Guide) then the rebar will need to be towards the upper surface of the concrete in the mould. 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. See Image 3 below.
Image 3 - View through a section of the concrete containing the Rebar
Just bear in mind that rebar is 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.
CAST-IN-PLACE REBAR PLACEMENT
For a Cast-In-Place project where the UPPER surface of the poured concrete is the FINISHED face its important the bury the rebar further into the concrete so that it doesn't "ghost" through to the finished face.
Have a look at Image 4 below showing the placement of rebar in a Cast-In-Place project.
Image 4 - location of Rebars around the opening in a Cast-In-Place Worktop.
Finally to give some guidance on placement from above take a look at the image below. This is the ideal placement but may not always be possible in practice.
Image 5 - Closeup of Rebars around the opening.
Earlier in the article I mentioned "ghosting" a few times. Its not something you want to happen because this only appears once you've cast your concrete and it would be a case of starting again or going over your work with microcementbecause it won't generally disappear.
Here is an example;
Admittedly this is a steel mesh and not rebar but the effect is the same.
So in conclusion, I hope this will answer most of the questions you may have or nudged your thinking from the unknown into the known! But as ever if you have any questions contact me or the team through the usual channels!
