How to pour in place concrete countertops?

Contertop Inspo

I'm planning to make pour in place concrete countertops for a new kitchen and I was wondering if anyone had done this and had some feedback to share.

The tutorials/youtubes I've found tend to be american liquidcrete products and forms, but I'll be making my own. So my questions relate to the materials, reinforcing and thickness of pour.

I plan to make up my cabinet carcasses out of standard melamine particle board as per normal, but reinfoce the tops with 90x35 pine. The base will be hardieboard, overhanging the carcasses by 30mm. The forms will be L shaped of melamine cut to 50mm strips. These will be screwed to the hardiboard overhang through the bottom, spaced out 20mm out so that the concrete edge will overlap the edge of hardiboard so it wont be visible from the side. Here's a profile of what I mean.

countertop profile.jpg

As for materials, I plan on using MaxPRO 20kg Concrete Mix as it has "crack resistant fibre reinforcement", tinted black with Dulux Avista Oxide (1 bag of oxide per bag of concrete.) I'm assuming I still need some sort of reinforcement so using rapicmesh galvanised 25mm mesh panels, fixed in place with plastic spacers so it doesnt float during the pour.

For thickness of pour I'm going for 40mm - unfortunately the height of the dishwasher and the height of the window sills means I need to have it no more than 45mm thick.

It'll be 7 linear metres of benchtop, which works out at about 17 bags of concrete or about 350kg finished, so I think the reinforced cabinetry will be strong enough.

Has anyone got any thoughts or experiences on this plan?

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JacobZ

Hi @jase2,

This sounds like a fantastic project and I'm excited to see it come to fruition in the near future.

Your plans look solid, and the choice of materials will surely produce a great result. Have you decided on a colour for the oxide?

The only thing I'd advise you to consider is that the concrete will weigh approximately 50kg per lineal metre of benchtop and at present the only lateral bracing at the top of the carcasses is the fibre cement sheet. Creating a frame with the 90x35 that laterally braces the carcasses is likely going to be worthwhile to prevent the melamine from folding inwards.

Allow me to tag @markw whose Concrete tabletop project is similar. Hopefully, he can offer some advice.

Also, allow me to tag some of our experienced members to see if they have any advice, @Dave-1, @Nailbag, @Jewelleryrescue.

Let me know if you have any further questions.

Jacob

Nailbag

Hi @jase2

a few years back I helped a mate do the same project. We basically made the frame out of Form-ply, and poured the concrete straight in via buckets. Once allowed to dry and harden for a week, he then actually used a hired concrete polisher and used it standing on the top. The sides were then down by hand. It came out amazing.

My only concern would be to take @JacobZ advice on ensuring the structure can handle the weight. Cabinetry carcass's on their own have poor structural so anything you can do to reinforce that is a worthwhile investment. I am no concreting expert, so my only question would be does 40mm depth need any form of basic reinforcement (steel mesh) to prevent cracking over time. We didn't use it, but his house was sold as soon a the reno was completed. So, I have no idea of long term durability.

Look forward to the finished project.

Nailbag

Dave-1

Evening @jase2

I agree with @JacobZ and @Nailbag about the bracing as I am not sure if the carcuses would hold the concrete bench without some warping over time, especially if someone comes along and sits or stands on it for any reason (350kgs plus 100kgs and thats a half ton...)

You say 40mm thick, I have seen a few of the benchtops in videos and also in here. I havnt seen one as thin as 40mm thick. Not a huge concern but something to facor in if you dont have a lot of experience with concreteing (I fully admit I dont. I have wanted a benchtop like what you are suggesting but these things hang in mind)

Dave

Jewelleryrescue

Hi @jase2

Concrete pathes can be 50 mm thick to walk as a min as a guide. So a cement top at 40 mm is very strong and the Max pro cement with extra fibre is 2.5 x stronger than General purpose cement . You might not need the extra gal mesh as the cement has fibre reenforcing the concrete edges are the weakest point . But concrete isnt weak so no damage should occur. Keep mesh 20 mm from inside edges of the mold as a thinner layer of concrete might be more prone to impacts and chipping as a worse case possibility. If the concrete breaks or chips later no dramas simply repair the crack or chip with the same color oxide (keep some spare for repairs) GP concrete.

Where ever possible make sure your cupboard frames are solid uprights from the floor to the slab . If melamine particle board is the only upright I might consider double or tripple wall thickness in key places ie between cabinets and on the ends.

But the cement slab across several cabinets is nicely weight distrubuted and you original design might be good to go. Dont forget to strengthen the under cupboard zone is it just sitting on a melamine frame? Add more supports.

-Do use a measuring cup re oxide to try get consistany in colour into a consistant mix measure your water input to for more even results.

-Do make extra sure you aggitate the concrete mixture all the way along the bench top mold edges with a thin flat stick like a ruler so there are no bubbles left on the edges.

-Use a large 400mm steel float to get a smooth surface. The biggest mistake (which isnt the end of the world is over working the concrete finish) It is so easy to try get a mark out with the trowel but at the same time put another 3 marks in you end up chasing your tail all the time working the cement the water floats up and cement mixture down leaving a watery top (If this happens leave for 30 mins and water should reabsorb some what.

Post pour results if there are marks and un eveness and you are not happy with the results there is on problems with hiring a concrete hand polisher

The best one to get is a three disk planetary have unit with reducing grade sand papers from 80Gt to 3000gt for a ultra smooth finish.

I have even used a angle grinder 125mm with dry sanding pads to great effect but not for a beginner. Also for the edges. You need a good eye for flatness/ level

@jase2 You are definatly on the right track enjoy your project.

jase2

@JacobZ @Dave-1 @Nailbag @Jewelleryrescue
Thanks so much for the feedback and advice fellas - all has been taken to heart. I've attached a few mockups of the layout so you'll see how the bench is spread over the cabinets - there is plenty of scope for beefing up the carcasses with additional timber supports so I'll be doing that. Especially over the breakfast bench section!

I'll post a followup with lessons learned after its all done, but I feel much more confident knowing I'm on the right track. Screenshot (247).png Screenshot (248).png

Nailbag

Wow awesome design software @jase2 it really clearly shows the outcome.

Good luck and look forward to the process pics!

Nailbag

Jewelleryrescue

Hi @jase2

Last min ideas tips.

Re oxide go easy on its use a little goes far. I might suggest making little test bench blocks100mm x 100mm as a guide to test how much oxide to use for your desired results. Plus if you use melamine scrap for the test form it will give you a chance to put in rounded corners using a rounding trowel. let concrete set 1 day before reslease as its just a test block.

Normal out door cement work will rarly or never need a release agent and never recomend it ( typically cement form work boards in industry are per treated for fast release and are re usable no release spray required eliminating one step). But for your concrete bench top frame to get the best possible smooth results use a silicon spray or if you molding more complex shapes or statues etc. Melamine is already ultra slick but for the price of some silicone spray from bunnings IN 0400395 $7.20 as a fail safe/

I would leave the frame on the poured concrete for 7 days minimum up to two weeks for long cure time fresh poured concrete chips and marks very easy. I just poured a slab with general purpose cement at (highest strength )two days later I can easily mark the surface with a wood float i was rubbing down some extra high towel makes left when poured. So let your frame work protect the concrete edge for a good period.

I think your U shape kitchen and pour will be supported by your bench top well as it is fixed into the room. Ie not and island bench where the cupboard could be leaned on and collapse side ways.

This is not essential

Design weak points possibly needing reo rod as re enforcing ideally to my mind.

Four areas I would run 12 mm reo bar is in the thin cement edging in front of and behind the sink and stove top insert points.

Run the reo bar 400 mm longer each end past the sink and bend the bars with 400mm tags in towards the middle about 30 degrees to 45 degrees at both ends if theres room. This will great a torsion bar effect and resist concrete breaking in the thinner stove and sink fronts. If the concrete ever breaks it will crack and not fall out and easy to repair.

I/n 1060313 $14.13 per 3meters

I am with nailbag nice drawing software.

jase2

Thanks for the additional info @Jewelleryrescue , really appreciate you taking the time to share your extra thoughts.

I have a couple of pieces of james hardie cement flooring - 19mm thick. Do you think it would be better to use these as the base rather than 6mm cement board? It would provide more support for the longer gaps, but would mean the cement on top could only be 21mm thick (for a total thickness of 40mm)

https://www.bunnings.com.au/james-hardie-2700-x-600-x-19mm-scyon-secura-interior-flooring_p0710445

@Nailbag I used the IKEA kitchen planner for the design - its pretty good for basic layout etc. You are limited to the IKEA catalogue but since that covers standard carcass/cabinet sizes its a good way to get the general layout/spacing down.

https://kitchen.planner.ikea.com/gb/en/

EricL

Hello @jase2

Let me tag @Jewelleryrescue to make them aware of your questions. I believe that using the JH cement flooring will provide better support than the 6mm cement board. Plus, you won't have to use as much concrete. My only suggestions are to make sure that the cement flooring is totally level and secure before you begin your pour. Please make sure to double check your measurements before you begin.

If you need further assistance, please let us know.

Eric

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