Concrete Slab for Workshop, Advice Needed.

[jamesmerrix]

Hi,
I am looking for a little advice regarding pouring a slab for a workshop floor.
I have cleared the top soil and put down about 100mm of stone as a sub base, and now am deciding how best to make the shuttering.
2 sides of the shuttering could be formed by a sleeper wall, and the concrete base of next doors drive.
Is is ok to simply shutter off the two 'open' sides and use the sleepers and drive for the other two?
Or should i build a normal square shuttering wall.
Below is a photo to help show what i am talking about.

The slab would be about 8x3.6 to fit into an area of about 8.1x3.8
thanks for your help.

 

[carlb40]

Has the sub base been thoroughly compacted? If not it will need to be.

Over that you will need a couple of inches of sand, and compacted.

Then i would add a membrane to stop damp. Plus i would take the membrane up the neighbours wall and add some insulation to keep the concrete off their wall and causing future issues.

How are you insulating the floor, if at all?

Sleepers should be ok for shuttering as long as they are securely pegged / staked and level.

 

[John15]

Hi Paul,

To ensure the finished levels are correct it is important to have sturdy formwork that will not move while poring the slab. Also I would put a timber through the middle of the pour to help with screeding the concrete to level.
It is also wise to cure the concrete for the first few days in order to allow the slab to gain sufficient strength and so reduce the risk of cracking.
One other point. Don't be tempted to water down the mix to the consistantcy of self-levelling soup - the slab will crack.

Cheers,

John

 

[Random Orbital Bob]

Having ready mix do the pour will save a ton of time and backache, but will cost more of course.

 

[Grahamshed]

A polystyrene slab under the concrete will do a lot to keep the heat in.

 

[jamesmerrix]

Thank you for your replies, and to answer some questions asked of me:

No the hardcore has not been compacted down, yet. I will be hiring a plate compactor at some point.
I will be putting about 50mm of sand over the top, followed by a dpm and then around 100mm of concrete.

Having moved the (6 tonnes) of stone from the drive to the base i can assure you will be using ready mix for the base!

I think i will get one of the 'mix on site' trucks in.

I think i am leaning towards 'normal' shuttering so that i can use the formwork as a level when tamping down.

I will probably insulate over the slab, but under a screed.

Looking forward to getting it done - cant wait to have a workshop again!

 

[blackrodd]

I imagine this slab is for a timber or similar light weight shed/workshop? Building off the slab edge?
Don't forget to fit the polythene up the edge of the slab and up the stud or workshop wall to stop damp or water ingress by capillery action, and cover the concrete slab edge for frost protection, with the wall finish material.
Should you be contemplating masonry walls, you would need to dig a foundation for brick or blockwork wall, the foundation which the top would need to be 500mm below ground for frost protection, then build on to above the slab height and use a notched tamper to tamp the concrete down.
As you can appreciate, You can't build off 100mm of stone and 100mm of concrete.
Compact the base stone and sand well or you could be a unhappy bunny 2 years down the line.
How about 2 x 2 timbers at 400mm centres on the floor and celotex between, green chipboard on top?
Don't let the mixer driver talk you into "swimming" the concrete in, to make laying easier, as you will have a constant dusting floor as the cement sinks driving the sand up to the surface.
When you ring the concrete dispatcher, tell him what you are up to and he will give you the mix and price.
Looks good access on yer pic, plenty of men is the answer, with rakes and shovels!
HTH Regards Rodders

PS For concrete levels on the block wall, fit masonry nails at the concrete height, and lay to them.

 

[jamesmerrix]

Hi Rodders,
Yes, Timber frame is what i have planned - maybe a course or two of bricks to keep the timber off the slab?

Thanks for the tips about the poly sheeting.
I also like the idea for the floor insulation.
looks like it is going to be a weekend of compacting stone!

 

[blackrodd]

There's no advantage to the brickwork if it's going direct on the slab as you will get loads of damp and water ingress by way of capillary action through mortar bed under the first course, that's why the polythene is best passed up over the wall/studwork, keeping the damp out.
A couple of courses of solid brickwork is a good idea, but the only way around the capillary action is to fix a 4x2 timber on you're shuttering, or 2- 2"x2" screwed, one after the other,(easier to fix) the top of the 4x2 section is the top of the concrete this will form a nice rebate which, the 6" dpc will be laid in (LAYING INSIDE OF THE REBATE. FORMING AN "L"
shape and there for protecting the bed and abutment.
"Podge" or poke the concrete well into the rebated lengths and get the biggest hammer drill you can borrow, buy, or whatever and fix a 12mm coach bolt in the chuck and use it up and down the shuttering on full hammer, or if you have plenty of luchre, go get a honda driven poker.
Do not use the compactor you will destroy the shutter and levels! HTH Regards Rodders

 

[Farmer Giles]

I would ask the concrete co to add some fibres to the mix, only costs a couple of quid and strengthens the slab and stops cracking. if your only doing a 4" slab I would also be tempted to put some A142 mesh in too, its much easier to put it in now than later Lift it off the membrane with some small bricks or you can buy little plastic doo dahs.

 

[johnf]

Put some A 142 mesh in the slab

 

[Farmer Giles]

Annoying in-laws can also strengthen the slab if you already have a patio

 

[blackrodd]

Put some A 142 mesh in the slab

I don't think 2" of concrete is enough "cover" for weld mesh, you need at least 3".
I would think the polymer strands mix would be more cost effective and cheaper than the extra 2" of concrete, if you see a need for re- enforcing 4" of concrete
As i said tell the concrete dispatcher what you are doing and he will advise as to the mix etc
Regards Rodders

 

[DMF]

I would ask the concrete co to add some fibres to the mix, only costs a couple of quid and strengthens the slab and stops cracking.

The fibre mixes were designed foe power floated floors originally I was told when they were introduced, we were doing loads of factory floors for BOC Edwards at the time. Because you bring more fat to the surface with power floating and hence a weaker top surface, the fibres were designed to bind and strengthen that only. The bags of stringier nonsense for people to add to mixes themselves would be a rant proclaiming ten types of poo.

Concrete is reinforced with steel for me. For a slab that size I wouldn't leave it out and would thicken up the concrete, doesn't make sense to spend the money and risk it for the sakes of some mesh and bit more concrete. Also why insulate then screed for a workshop? You would need to thicken up the screed and reinforce that anyway I would of thought?

Other stuff of course sand dpm insulate etc but looking at the pics looks out the ground a bit? I don't know what structure you have planned but I would be concerned too about there being no trench around it with a couple of courses to stop it sliding and anchor it in and also cover up the slab edges, gives the loadbearing part more strength and gives you your shuttering at the same time. This is from the view point of it looks bigger than a shed, if your just chucking a big shed up then whatever really but if your thinking proper workshop then no, at the very least thicken the edges and reinforce otherwise you stand a good chance of the slab failing I'd have thought. Apologies but I try to imagine what I would of thought if I'd been asked to turn up to look at a job to take over from someone else and that always makes me grumpy :lol:

Dean

 

[Farmer Giles]

I use this site for most of my groundwork, very good site with calculators for hardcore, sand etc.

The link shows the difference between fibres and steel reinforcement, although there is some overlap depending on what fibres you use, typically fibres are used for crack control and steel reinforcement for structural strength. A 4" slab is on the limit in terms of depth for reinforcement, you need at least 2" above it however as its a shed base rather than the Apollo rocket launchpad you may just get away with it

Cheers
Andy

 

[hanser]

Useful site Andy, now bookmarked - thanks

 

[jamesmerrix]

Thanks for all of the replies guys, i really appreciate it and have got some useful information.

I think i will up the thickness a litre - perhaps another inch or so; and i think i will put some steel in as well.

Looking forward to getting it started now.
james

 

[DMF]

I use this site for most of my groundwork, very good site with calculators for hardcore, sand etc.

The link shows the difference between fibres and steel reinforcement, although there is some overlap depending on what fibres you use, typically fibres are used for crack control and steel reinforcement for structural strength. A 4" slab is on the limit in terms of depth for reinforcement, you need at least 2" above it however as its a shed base rather than the Apollo rocket launchpad you may just get away with it

Cheers
Andy

AArrrrggghhhh! Just typed out a big reply only for it to disappear! Brief summary I use my time on site as a ground worker using drawings provided via calculators prodded by engineer's which also happen to have the benefit of knowing the loading the slab will take, the lucky ....! It was the mix of fibres and strength in a sentence and a OP not considering mesh in the first place which made me want to ensure the was no chance he would substitute fibres for reinforcing.

"typically fibres are used for crack control and steel reinforcement for structural strength" Always for domestic I would say not typically, in a commercial setting fibres can have there uses as said a power floated floor but that has a industrial coating as well, typically the main reason given is that it stops micro cracking of the slab surface, I prefer proper working methods in this case predominantly correct curing and less money spent on things we don't need.

Other concrete things deleted but the main point is the margin of error you create if you have a slab thick enough to mesh, its the main goals of roof stays up and floor stays put for any self build shed / shop and a big mess can result!

The link states various caveats throughout its text if you read between the lines, there is some good general information but none to be taken gospel.

Op glad to hear i'll stop worrying :lol:

Dean

 

[blackrodd]

I worked for a joinery firm that also specialised in form work and steel re-enforcing, I have shuttered, and tied quite a few tons of steel, and formed/shuttered for concrete piles, ground beams, stairs, column&beam and floor slabs, retaining walls, factory floors etc.
Water, apparently, is the key to controlled curing of concrete, plenty of it!
Fast uneven drying causes major stresses from drying/curing from the outside faster than inside, especially the sun!
Inconsistant mixing, poor compaction and weak or overly strong ratio's are also problematic.
If laying more than 4" get or hire a poker
This is why it's a good idea to speak to the dispatcher and get the mixes correct, don't "swim" the concrete in place and keep it as wet as possible after initial set for 24hours, or longer, if possible.
Also, protect well from frost and keep any foundation top, 500mm below ground or finished paths etc, or unprotected slab ends upstands, or "kickers etc well covered and protected, from Jack F.
Damp concrete and then a frost are a bad combination.
Regards Rodders

 

[DMF]

Rodders lots of good points to think about for folks and thanks for counter balancing my rant about fibres (something I fell strongly about as you can tell, its a product optimised to confuse!) consistency is a great word for getting the basics right, from mixing to controlling the moister leaving the slab to correct vibration. There's lots of info by lots of people on here offering good advice on a strong well designed slab for home workshop builds but due to the individual project demands It is the responsibility of folks to consider the info they get and how it applies to their situation and ask questions if they want, I hope I have not upset anyone but concrete moves from a know proven product to quite a complex material very quickly, driven by the demand for high performance products for specialised uses.

Dean.