Floor Concrete Advice

[Aden30mm]

I'm just in the process of laying out a new shed.

I've dug out some strip footings and plan to float a new raft when the first brick course is laid.

I am using part of an old concrete raft that has been down for a 15 years, the raft is in good condition, and I have cleaned it up further by treating the area with brick acid and washing off. I will have to add to the raft with fresh concrete to bring it up to the planned level.

The question is is there any further treatment to the old raft that will ensure good adhesion with the new concrete?

Kind regards

Aden

 

[mindthatwhatouch]

As long as its clean, the brick acid will do a good job. You could always drill some holes and epoxy in some rebar. How thick will the concrete above the old slab be?

 

[Aden30mm]

About 2 to 3 inches, lower raft is 4 inches which has not moved.

 

[blackrodd]

As long as you are sure that the old part of existing concrete is solid under, as over time it will, by weight and time, all sink, and make sure you compact the new area by wacking down well with a compacter.

Just before the concrete goes in, Prime the existing, old concrete area, with cement powder and water, literally scatter cement and a bucket of water and scrub it in,wiv a broom, much as the floor screeders do before laying the finish screed.
A Problem you could experience here by building on a raft, or slab, is capillary action drawing in moisture under DPC, on the slab/raft when the walls are built and finished

I suggest you either lay a 4x2, on its flat to form a rebate,incorporated with the shuttering, using the top of the 4"x 2" as a finish level, use this as you're wall plate, and it's not wasted,
Or brick up to one brick above concrete finish height on you're footings first and then use a notched tamper, thereby ending up the same as above.
Do you have any provision for underfloor insulation and where you're polythene membrane is going?
HTH Regards Rodders

 

[Aden30mm]

Not bothering with ground insulation its just a garden shed. Membrane will go under raft. I have planned to go one brick above raft.

Do you lay the raft with the slurry still wet??

I plan to use a waterproof in the mortar mix for the bricks, but don't want to over engineer the build.

Thanks for the valuable advice.



Aden

 

[blackrodd]

Yes, lay right on top and just ensure that the old surface is wet/tacky.
Try and avoid "swimming" the new concrete in, the drivers love this as they can be off site quicker because it's so much faster laid, but you will find, again over time, the floor dusting up as the cement stays lower and the sand rises to the surface, and a weaker floor, and you're concrete floor paint keeps lifting.
Plenty of hands, 2 loaves of bread and 2 lbs of bacon will be very helpful, you'll find

The remedy for the floor dusting after being laid is to again scrub the floor, but with a solution of 50% pva to 50% water, this will seal the floor, a second coat is good and then, the concrete paint.
If you're old previous concrete surface is really smooth some pva in the water first, then scrub in to the cement, as this combination of water, pva (at about 25%pva to 75% water and cement will prime any previous, smooth concrete and help prevent "blowing" or separation of the second layer.
HTH Regards Rodders

 

[DMF]

Hi,

Excuse the chopped quotes please!

its just a garden shed... Membrane will go under raft. Aden

Is this between the bit you want good adhesion on? Not worth prepping something and then covering in plastic dpm if that's the membrane? As it's just a garden shed, without knowing ground levels what your doing with it etc is it not worth pouring at existing slab height and depth which sounds stable and then bothering with damp control and any height issues? Either though boarding out or a reinforced screed which would perform better at the thin end on the scale and would give a more consistent slab if you have too. If it's just garden junk then few piers and a bit of framework surely?

I plan to use a waterproof in the mortar mix for the bricks, but don't want to over engineer the build.
Aden

Maybe I have it wrong though, waterproof mortar for a shed, swanky!

Dean

 

[blackrodd]

Hi,

Excuse the chopped quotes please!

its just a garden shed... Membrane will go under raft. Aden

Is this between the bit you want good adhesion on? Not worth prepping something and then covering in plastic dpm if that's the membrane? As it's just a garden shed, without knowing ground levels what your doing with it etc is it not worth pouring at existing slab height and depth which sounds stable and then bothering with damp control and any height issues? Either though boarding out or a reinforced screed which would perform better at the thin end on the scale and would give a more consistent slab if you have too. If it's just garden junk then few piers and a bit of framework surely?

I plan to use a waterproof in the mortar mix for the bricks, but don't want to over engineer the build.
Aden

Maybe I have it wrong though, waterproof mortar for a shed, swanky!

Dean

Good thinking, but The poster said that the new level would be 2", or 3"on top of the old existing concrete area, I would have thought not enough cover for a membrane, even in a shed.
Regards Rodders

 

[DMF]

Good thinking, but The poster said that the new level would be 2", or 3"on top of the old existing concrete area, I would have thought not enough cover for a membrane, even in a shed.
Regards Rodders

Always great thinking Rodders maybe I'm just pants at writing it out loud :lol:

You have lost me though, didn't mention cover for a membrane? Message with membrane from me was not to spend money to try to make existing sticky and then cover it with plastic basically if that's the plan because that's not sticky, only checking! Others just alternative thoughts to having a thin bit of concrete that will crack especially if half of it is thicker one end.

Dean

Edit meant to say if membrane (of this yet to be determined type!) doesn't go over the old bit it's not worth putting under the new, I'm sure that makes sense!

 

[blackrodd]

Good thinking, but The poster said that the new level would be 2", or 3"on top of the old existing concrete area, I would have thought not enough cover for a membrane, even in a shed.
Regards Rodders

Always great thinking Rodders maybe I'm just pants at writing it out loud :lol:

You have lost me though, didn't mention cover for a membrane? Message with membrane from me was not to spend money to try to make existing sticky and then cover it with plastic basically if that's the plan because that's not sticky, only checking! Others just alternative thoughts to having a thin bit of concrete that will crack especially if half of it is thicker one end.

Dean

Edit meant to say if membrane (of this yet to be determined type!) doesn't go over the old bit it's not worth putting under the new, I'm sure that makes sense!

Dean, apologies! as you pointed out, there's no sense in making the concrete sticky with cement etc, to then cover with a membrane which was good thinking on you're part.
Then I have obviously had a senior moment and told you instead of the poster that 2" or 3" isn't enough cover for a polythene membrane really but, as it's a shed perhaps that will suffice his needs.
Maybe this is not a shed with machinery toys etc and therefore a good thick floor isn't necessary.
Regards Rodders

 

[Aden30mm]

Thanks for the valuable advice, I will just put down a raft and buy a garden shed to sit on it. Its only for gardening paraphernalia. My old shed sat on the original base (without membrane) for 15 years, and I had no trouble with that.

I was thinking about building one but looking at the cost of the timber etc there probably not much in it, and much less hassle.

Thanks again for your kind advice.

Kind regards

Aden

 

[DMF]

Rodders none needed

I will just put down a raft and buy a garden shed to sit on it. Its only for gardening paraphernalia. My old shed sat on the original base (without membrane) for 15 years, and I had no trouble with that.
I was thinking about building one but looking at the cost of the timber etc there probably not much in it, and much less hassle.

Sounds like a plan! Put one up for my old man in a similar fashion recently, extended the existing base to match same height, i had some rebar to drill into the side of the existing and chucked a bit of mesh in so that didn't cost him but base done. From there I got some 3x2 pressure treated timber and I think 24 block pavers, 2 bags of sand and a bag of dust for about £40 I think and bedded a frame on top of that with some dpc between top of block and bottom of frame which I had along with some gravel boards to trim so rubbish doesn't blow under there, maybe something similar would suit especially if you would prefer it a bit higher? Not including mesh and dpc I had but including a 8x6 pressure treated shed all in it was just under £250 and a bacon and egg sarnie. For chucking his old spades and plastic plant pots and the like he's well happy.

Dean