concrete countertop questions(again!)

[RichC]

Hi,
just shuttering form for my concrete countertop with will be about 45mm thick at the thinnest point and have a 50mm overhang around the structure. I have some questions.

1. All the videos I see seem to use a very dry mix. Why is this? I always tend to mix concrete wet to make it easy to expel air and generally work with. Do I need a particularly dry mix for this?

2. I have a steel mesh for reinforcement. I also see people recommending use of a fibre or hair in the mix as reinforcement. Is this required and will it be exposed when I polish the surface back?

3. Im going for white. Is it just white sand and white cement or does it use a 'gravel' mix like a mormal concrete deck with soem pebble or stone in it? I.e. is sand and cement a strong enough mix for this?--no gravel?

4. I also intend putting some powdered/smashed glass into the mix for some blues/greens when I polish it back. My intention is to pour the form and trowel it and then just sprinkle on the glass and trowel it in when the concrete is starting to go off. Is this the right approach?

5. I usually mix concrete fairly strong, 1/2 bag or even a full bag per mixer but I never use ratios(laziness!!). What ratio should I mix my sand/cement for maximum strength?

Thanks

R

[jeeppiper]

Re: concrete countertop questions(again!)

You're in luck, I now consider myself an expert in concrete countertops having successfully completed my first countertop. (it was my summer project).

1. Using a dry mix supposedly develops a stronger bond during the curing process. I would use a high-strength concrete mix at a consistency you are comfortable working with....I wouldn't make it too wet. You will get micro-bubbles no matter what. These can easily be filled when you go to polish it. You can try to vibrate them out, but I didn't have any luck with that. You can rent a vibrating wand (like they use in some videos), but I didn't. I couldn't figure out where to stick the vibrator with all of the rebar/mesh etc.

2. Adding fiber cant hurt...will only make it stronger and you wont see it in your finished product.

3. I wouldn't rely on just sand as your binding aggregate. I think the fiber can be used instead of gravel. Gravel will show through in your finished top. I used a run-of-the-mill high-strength concrete that had gravel in it. It looked great when I first poured it, but when I started to polish it, all of the gravel came through. And, the gravel was all different colors (black, gray, brown, etc). Quikrete actually makes a concrete specially designed for countertops. It is expensive (about 3x regular concrete)My local home depot carries it....if yours doesn't, they can order it for you. I think this mix is simply cement with fibers added (no gravel). I chose not to use this mix because of the price ($15/50 pound bag) But, I needed 20 bags for my monster countertop!...see attached picture of my countertop. This was 12 feet x 4 feet x 4 inches thick.

4. I don't see why that wouldn't work. I think if I were doing that, I would pour the concrete and immediately "sprinkle" your glass (I wouldn't wait)..I think I would also trowel it over and cover the glass. When you polish the concrete, all of that glass will come though.

Do you have a polishing wet-grinder? I bought one with the seven progressive polishing wheels. It is amazing the shine you can get if you do it right.

5. Cant help you with the sand/cement mixture. I got pre-mixed.

6. Don't forget about sealing the concrete...this is super-important! It is also somewhat confusing because there are all different types of sealers.

[UtahBeehiver]

Re: concrete countertop questions(again!)

RC,

Under the Newbie Section look for a thread called Forno Bravo Treasure Achives by Lburou. There is a link to several concrete counter threads on the FB Forum that I hyperlinked to. There you can see at least a dozen or so threads on concrete counters.

[stonecutter]

Re: concrete countertop questions(again!)

Here is my .02.....

Hi,
just shuttering form for my concrete countertop with will be about 45mm thick at the thinnest point and have a 50mm overhang around the structure. I have some questions.

1. All the videos I see seem to use a very dry mix. Why is this? I always tend to mix concrete wet to make it easy to expel air and generally work with. Do I need a particularly dry mix for this?

A troweled surface needs more water than one that is hand pressed into a form. Water content will vary depending on what type of finish you want, the desired consistency you want the concrete to be. Too much water is as bad as too little. So, how much water volume to add depends on your mix design...there is no 'one fits all' answer.


2. I have a steel mesh for reinforcement. I also see people recommending use of a fibre or hair in the mix as reinforcement. Is this required and will it be exposed when I polish the surface back?

If you are using mesh, adding fiber makes it difficult to fill the mesh completely with the concrete mixture. Do one or the other, there is no real need to do both, and it complicates the process.


3. Im going for white. Is it just white sand and white cement or does it use a 'gravel' mix like a mormal concrete deck with soem pebble or stone in it? I.e. is sand and cement a strong enough mix for this?--no gravel?

Yes, you can build with a sand only concrete mix, it is commonly done and is plenty strong.

4. I also intend putting some powdered/smashed glass into the mix for some blues/greens when I polish it back. My intention is to pour the form and trowel it and then just sprinkle on the glass and trowel it in when the concrete is starting to go off. Is this the right approach?

5. I usually mix concrete fairly strong, 1/2 bag or even a full bag per mixer but I never use ratios(laziness!!). What ratio should I mix my sand/cement for maximum strength?
This is a basic mix design I have used for many pieces and had very good success.....by volume measuring - 1.5:2:3, H2O 1/2----Portland, sand, sharp graded aggregate. Substitute 10% of the water volume with a good polymer, which will greatly reduce the chances of shrink cracks. For sand only mixes - 1:2 Portland, sand. H2O 1/2 with 20% polymer. What you use for your volume measuring doesn't matter, as long as you stay consistent.

There was a good thread discussing mix design here. Homebrew concrete

[RichC]

Re: concrete countertop questions(again!)

Thanks guys!
@Stonecutter, whats the polymer? What do I shop for? Is that plasticiser?
I'll be making a dry-ish mix because I need the finished surface to slop slightly downwards towards the outer edge to carry water away from the oven

[stonecutter]

Re: concrete countertop questions(again!)

Thanks guys!
@Stonecutter, whats the polymer? What do I shop for? Is that plasticiser?
I'll be making a dry-ish mix because I need the finished surface to slop slightly downwards towards the outer edge to carry water away from the oven

Yes, it will work as a plasticiser and water reducer. If you can't find it, look for a latex or acrylic additive...it basically does the same thing.

A suggestion : Do the bulk of your concrete work with a wooden float, then finish it with a magnesium trowel...otherwise you could get 'burn' marks, which is especially easy to do with white concrete.

[RichC]

Re: concrete countertop questions(again!)

then finish it with a magnesium trowel...otherwise you could get 'burn' marks, which is especially easy to do with white concrete.

Thanks stonecutter. But ive another question, whats with the 'magnesium' trowel? I'm used to a normal plasterers trowel but I think theyre made from steel? I assume thats OK?

Thanks!

R

[stonecutter]

Re: concrete countertop questions(again!)

Thanks stonecutter. But ive another question, whats with the 'magnesium' trowel? I'm used to a normal plasterers trowel but I think theyre made from steel? I assume thats OK?

Thanks!

R

Steel works too.

[the_dr_masuess]

Re: concrete countertop questions(again!)

The Mag will help bring the water to the surface so it has a smoother finish but steel of course works.

[RichC]

Re: concrete countertop questions(again!)

Thx guys, need to buy a trowel so I'll check this out

[RichC]

Re: concrete countertop questions(again!)

Made 2 small slabs for my wife to see. 1st one is pure white sand/cement second uses 50% granite sand. I'm leaning towards the pure white but it's a bit chalky looking so I'm considering adding some coloured aggregate or glass(had given up on this idea for a while). If I do add something does it need to be visible in the concrete surface to come up when polished? How much does some polishing take back the surface? Eg does it just remove the outer millimeter or more?

[stonecutter]

Re: concrete countertop questions(again!)

Made 2 small slabs for my wife to see. 1st one is pure white sand/cement second uses 50% granite sand. I'm leaning towards the pure white but it's a bit chalky looking so I'm considering adding some coloured aggregate or glass(had given up on this idea for a while). If I do add something does it need to be visible in the concrete surface to come up when polished? How much does some polishing take back the surface? Eg does it just remove the outer millimeter or more?

Exposing aggregate depends on how aggressively you sand...how it looks depends on what you use in the mix design. If you want minimal sanding, you can seed the form before you pour ( of you are casting upside down) or after you fill the mould.

There are variables to your question. You need to first ask yourself, what do I want to see as a final finish? Then go from there.

[RichC]

Re: concrete countertop questions(again!)

Thanks Stonecutter. Casting in situ and whether I use glass or coloured aggregate ill seed afterwards so it's exposed. I assume I still need to trowel it in to obtain a decent finish to grind?

[stonecutter]

Re: concrete countertop questions(again!)

Yes... Mag or steel

[RichC]

Re: concrete countertop questions(again!)

@Stonecutter, jet trying to get this right re your recommendations on mix "This is a basic mix design I have used for many pieces and had very good success.....by volume measuring - 1.5:2:3, H2O 1/2----Portland, sand, sharp graded aggregate. Substitute 10% of the water volume with a good polymer, which will greatly reduce the chances of shrink cracks. For sand only mixes - 1:2 Portland, sand. H2O 1/2 with 20% polymer. What you use for your volume measuring doesn't matter, as long as you stay consistent. "
So for a sand cement mix I should use 25kg Portland:50kg sand:10l water and 2.5l plasticiser? I've never seen this much plasticiser used in a mix and I'm wondering have I the right product:
http://www.goodwins.ie/p-355-mortar-...iser-25lt.aspx
Thanks
Richard