How to Build a GFRC Concrete Sink

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About: By day I work in an office, but at the weekends I like to build stuff!!! and it can be anything; concrete to 3dprinting as long as I am making I am happy.

I have been wanting to build some furniture out of concrete for a while and whilst doing a home renovation I got the opportunity to build a small sink for a bedroom ensuite. After lots of research online this is the process I came up with including the lessons learned, lowlights and highlights.

The requirements for the sink are:

  1. it should fit an IKEA Godmorgan sink cabinet.
  2. there is no requirement for a hole for the tap as the tap is mounted on the wall. (A very hard feature to find on small sinks)
  3. the shape of the basin is a geometric design which magnifies elements of our house but also reinforces the fact that this is a custom made sink.

There are 2 options to consider when designing your sink. Most IKEA sinks for the Godmorgan unit sit ontop of the unit. This means the sides are quite deep. I noticed that more expensive designs have the sink recessed into the unit. In my opinon this creates a sleaker design but you will have to fix the top draw as there will be little room under the sink. I wanted this design as I wanted the sides of the sink to be as thin as possible.

For my project I used a Glass-fibre reinforced concrete (GFRC) as I wanted to keep the weight of the overall solution as low as possible and felt that the GFRC solution would give me more strength for such a small size pour. I used a premix solution that just required me to add water to the mix. I chose this as I wanted to save time on my project and did not feel ready to do a mix myself. My mix is designed for countertops and is self compacting, which eliminates as many airbubbles in the finished product as possible. Self compacting mixes are very runny which means that you have to cater for this in the design of your mould.

I split the process into 3 steps.

Step 1: Building the Mould

For GFRC concrete pours are done in reverse. My sink shape has many geometric angles so I have to build the mould with both a male and female part. The male part creates the top surface, side walls and the basin, the female part creates the bottom surfaces and prevents the concrete from slumping. Depending on your mix and design you may not need a female part.

Different concrete mixes and shapes will have different needs so it’s important to check with your concrete supplier that you have the right product/mix for your application.

For every hour you put into finishing your mould you will save 2 hours in finishing your concrete sink

I built my mould from a single large sheet of melamine. I used a track saw to rip down the sheet to create all the parts of the build. I don’t have a workshop so I did the rip down outside. I used a large foam insulation board as a cutting mat for the track saw.
The first part of the mould is the male part. Its important to remember that the concrete will take on all the surface elements you have on your melamine mould. For a crisp clean finish you need to make sure your mould is a good as it can be. For evey hour you put into finishing your mould you will save 2 hours in finishing your concrete sink so dont rush it!!.

I built the base first with the side walls. I glued these together with a hot glue gun. I built up the basin mould separately. I fixed the box together using pre drilled holes and wood screws, which meant I had to use a 2 part resin wood filler to fill in any gaps and the counter sunk screw heads.

The filler was sanded down to make as smooth a finish as possible. I used a cut down piece of 4inch pipe to make the hole for the waste system (aka Plug) to fit. I then used a hot glue gun to glue the waste pipe to the base of the sink basin.

Next step was to caulk all the corners with silicon. I used a white silicon to fill in the edges and used a cap from a pen to create the final angle in the silicon, I ended up doing this a couple of times as the mould was small and I struggled to get a decent finish on the silicon.
I had seen a number of other people use some form of releasing agent on their mould like carnuba wax. I did not use anything for this pour.

Step 2: The Pour

This was the least complex stage of the project but the most physical and stressful part!!! My mix came in a large plastic bucket which only required me to add water. I used an electric drill and mixing head that is normally used for mixing paint. During travel a lot of the product had compacted for form hard sections at the bottom of the bucket so I first mixed the powders to break up all the solid/compacted areas and get a ligh mix. I then started to slowly add water continuing to mix the concrete. Once it got to a consistency I was happy with I poured in the glass fibre, gave it a final mix and then stopped. I did not want the glass fibers breaking up with too much mixing.

I stared filling the mould with just the male part. Once I was happy that I was getting the mix into all the corners I fitted and screwed into place the female part. I finished filling the mould, but realised that the mix was leaking from some of the seams so I quickly had to drill and screw along the flange between the 2 moulds at regular intervals to ensure there was enough pressure to keep the leaks at bay!!

I used a small orbital sander running at very low speed to shake the mould to help get rid of as many air bubbles as possible.

I placed a plastic bag over the top and left it for a few days to cure.

Step 3: The De-mould and Finish

It is at this point where you find out how much effort you now have to expend on getting your sink to the level of finish that you want!!!

The demould process was relatively easy, except for the sink insert which had stuck firmly inside the sink. I think this was more due to the concrete expanding than not using a releasing agent. I ended up having to destroy the mould with a hammer and chisel which also resulted in me scratched the sink a couple of times!! I sanded down the surface with some wet & dry paper to get rid of the scratches.

Its a lot easier to shape your mould than the finished concrete

The size of the sink is perfect and it fits the IKEA cabinet really well. The depth is nice and it creates a nice sized sink for such a small space. The raw finish is good, but I have a few issues that I will factor into my next design.

  • I did not use a mould releasing fluid, so the 2 part filler stuck to the concrete and came away with it.
  • Strands of the white silicon caulk came away with the concrete and can be seen in the finish.
  • The missing flange on the sink waste knock out meant that I had to grind a recess in the concrete to get the sink waste to sit flush.
  • Some of the rough edges of the cut melamine imprinted into the concrete even though I tried to fill and sand them.

For my project I want it to look sleek and modern which means I now have to do a lot of finishing with grinding pads, where as if you were after a more rustic look you would probably be happy with the finish I have.

Next steps or this sink. I have now purchased a set of diamond grinding pads for my small angle grinder and will try and clean up the surfaces to get a better finish before I seal the the sink. For my next sink I will change the design of the mould to get a better finish. Here are my list of takeaways.

  • I need to grind and polish the sink to get a better finish
  • next time I would try and use a small diameter object to get a decent caulk line. My edges are too rounded for the style of the sink. It was suggested that I could use the end of a drill bit to get a nice small rounded caulk line.
  • a finishing sealer is needed to protect the sink from wear and tear
  • For my next sink design will spend a lot more time on the mould design, using mitre joints to keep sharp edges and try to put a small fall in the sides of the basin so that it can be easily removed.
  • i will use a light oil on the inside of the mould to help the mould release.

Step 4: Conclusion

You don’t need a lot of skill to make a pretty decent sink, but it does take time which is something I did not cater for. This was a prototype for me as I have a bigger and more complex project to try next, but I have learnt a lot and have a unique piece that will stand out which I am pleased with.

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    29 Discussions

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    seamster

    2 months ago

    Very impressive results! Nice to see how you did it, and how it turned out. Thanks!

    3 replies
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    doing2muchseamster

    Reply 6 weeks ago

    Hmm, I've never heard of it being able to float within the concrete - unless the mix is pretty watery. Is it supposed to be watery?

    So I guess you can't do caste in place with GFRC, then? Thinking out loud, couldn't you just put an additional layer - without the fiber - on the very surface? I ask because I am planning on making some GFRC caste in place (small) counters as part of an outdoor kitchen. Could someone please tell me whether what I want to do will work out or not - and why?

    I'd also like to know whether I can use recycled glass aggregate with GFRC. I have not yet been able to find anything definitive on the success of this combination being used by DIYers.

    Sorry, this is becoming long but 'an expert' told me that GFRC has to be installed by a professional or it has to be pumped in at the very least, for it to really be strong. Anyone know anything about this?

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    darkin100doing2much

    Reply 5 weeks ago

    The mix includes a number of additives to make the product move when its in the mould. You cant really describe it as watery because when you can grab it by hand but when you apply small moment or vibration it flows very quickly.
    As you suggested you could do a separate layer of concrete on the top of the GFRC if you were to do a cast in place and then float the surface to get your finish. I would suggest that you would need to experiment and build a sample.

    You are correct that the GFRC method typically uses a spray system to apply a finish coat and then either hand lay up or the spray system for applying the backing coat, but for a small counter top I do not see this being a requirement for you.
    If you are based in the UK you can try the pre-mix product that used.

    http://concretelab.co.uk/GFRC-Countertops-and-Worktops/Premix-25

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    darkin100seamster

    Reply 2 months ago

    Thanks. I have already started my next sink and will publish the process and findings. Hopefully the adjustments to the process should solve some of the challenges I had with the final article.

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    Heptode

    2 months ago

    You can buy a set of plastic radiused wiping tools made specially for creating a finish in a corner along an edge. I have a set and they are super easy to use. Screwfix sell them.

    3 replies
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    doing2muchHeptode

    Reply 6 weeks ago

    Could you post a link or the name of the item? I went to the website you mentioned but don't know what to look for...

    Thanks!

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    darkin100Heptode

    Reply 2 months ago

    Hi Heptode, thanks for the feedback. I will look into them.

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    FlorinJ

    2 months ago

    Don't use oil of any kind for making the mold easier to extract. Oil (or any other fat) reacts with the bases in the concrete and turns to soap, but in the process those bases no longer contribute to the reactions which make the concrete strong. What you'll get is crumbly concrete on the surfaces. Instead you could try some synthetic wax - animal waxes, such as beeswax or vegetable waxes, are not that inert.

    In order to be able to remove the mold easier you can also shape it from multiple pieces - one wedge-like/trapezoidal section in the middle of each surface, for example. True, that would most likely leave marks on each surface, after demolding, and that would need sanding, but on one hand with anything concrete if you want it polished you need to sand, on the other hand the seam traces left in the middle of each surface should be easy to sand mechanically, nothing like a scratch.

    2 replies
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    darkin100FlorinJ

    Reply 2 months ago

    Hi FlorinJ, thanks for the feedback. Its really helpful. I was looking into using a mould release silicon spray which I think would help but its pretty hard to find where I am at the moment. For the next pour which I am doing this weekend I will take your advice and use a wax. I tried going for a trapezoidal mould but quickly came unstack with all the angles!!! Instead I have gone for a similar design but mitred every joint so hopefully it will collapse easier!! I will know in 48hrs!!

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    jwilliamsen

    2 months ago

    Good stuff. Projects are all about learning - making mistakes is part of the mix (no pun intended).

    For a parting compound, wax would work better (like a Johnson's Paste Wax or even Paraffin). Also, whenever you have a deep mold like that, you're going to want to add at least a *little* draft angle to make removing the mold a lot easier and less risky. You did a good job getting the mold out without breaking the sink - but it didn't look like fun at all ;) Chances are that the moisture in the concrete caused the melamine to swell, possibly combined with minor shrinkage of the concrete. Adding a bit of draft angle would have had the parts come out with a gentle bit of pursuasion.

    1 reply
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    doing2muchjwilliamsen

    Reply 6 weeks ago

    Thank you for mentioning this. Despite being aware of the concept, I was not familiar with the term so I just googled 'draft angle.' The more you know!!

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    DuncanMJA

    2 months ago

    Nice work - love the results! Do you happen to know if you could use the same method for a fire pit? Or do you think the concrete would crack from the heat?

    5 replies
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    DuncanMJADuncanMJA

    Reply 2 months ago

    Great, thanks for the tips everyone. I'll do some googling.

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    nabojDuncanMJA

    Reply 2 months ago

    When doing with high temperature or fire, there exists special concrete for this.

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    darkin100DuncanMJA

    Reply 2 months ago

    Hi DuncanMJA, you would certainly be able to use this process to make the shape of the pit, but I dont know how well the concrete would perform with the heat. I imagine it would be ok.

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    DuncanMJAdarkin100

    Reply 2 months ago

    Ok, thanks for the help. I'll do some research.

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    Yonatan24DuncanMJA

    Reply 2 months ago

    Do some research, I have no experience working with concrete but I've heard pretty bad things about the humidity inside concrete causing it to explode or crack...

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    stephenfitton

    2 months ago

    For those with tiny houses or small rooms that echo a concrete sink eliminates those sounds and allows for washing when others are sleeping.

    Stephen Fitton.