Garage floor

[47p2]

I am in the process of building a new garage and am almost ready to install the concrete floor. Never having done this before I am looking for advice and tips and I have a few questions that I am looking for answers to.

The floor is approx 10.2 x 6.7 metres and 200mm deep. I have been told this size of slab can be put down in one without any expansion joints?
I have also been told I will need to leave a 'gap' (non technical term) around the perimiter to allow for expansion, how wide should this gap be?

I would like a smooth non-dusty surface. The person who advised me on the above also said I might consider laying a 150mm slab and then laying a 50mm granite slab on top which would be easier to smooth out (polish I believe is the technical term).

The concrete I have been advised to use is C30 with added fibres.

I am open to all sorts of suggestions, tips etc and look forward to your replies

Cheers

John

[GB_Groundworks]

concrete will be fine just trowl it off then leave it to fully set for a few months, sweep or hoover with old hoover then paint with industrial floor paint.

you can get addatives that reduce the amount of dust.

are you building the walls first and pouring the slab into them or are you doing a ring beam round the outside to carry your brick work, or is it pre fabricated garage?

[47p2]

Thanks for your replyGB_Groundworks

The walls are built, roof on, doors on, only the floor to do then I start on the outside paving

22 tons of hardcore has been laid down for the past 4 or 5 months and been driven on, so should be well compacted by now.

If you click the link you can see the stage I am at Garage floor ready to be concreted

I don't want to paint the floor if I can help it as I have done this in the past and it's a never ending annual task which I could do without.

[Tony McC]

I don't think you can lay that slab as a single entity: I think it needs at least one movement joint splitting the slab into two, as well as the perimeter expansion joints. Personally, I'd look to split it into four, with dowelled joints.

The rule of thumb for concrete is that the maximum span of any slab is 30 times the thickness. At 200mm thick (that's *some* chuffing slab!) you could span up to 6 metres, but you have a span of 10.2m.

There's absolutely nothing to be gained by using a 50mm granolithic topping. If you look at any of the concrete floors used in modern warehouses and DIY Sheds, the finish is power-floated concrete. As GB says, you get the right mix, with the right additives, and a smooth, dust-free finish is relatively easy....if you know what you're doing.

And this is what worries me: You have a slab of almost 70m², you'll be placing 14m³ of concrete, but you don't seem too familiar with the sheer amount of hard graft that's involved in such an undertaking. Concrete is a harsh and unforgiving mistress. While a 14m³ pour probably wouldn't intimidate most of the contractors that come into The Brew Cabin, I think it's too big a job for DIY, especially if you've no previous experience with the kit that would be required.

14m³ of concrete is helluva lot to jack-hammer out if you don't get it right first time - concrete doesn't give you a second chance!

Why not get a price from a professional concrete finishing contractor, even if only to gain some idea of the costs involved?

[GB_Groundworks]

As the gaffer has said its not really a diy job if you want the finish you desire, has it been spec'd by a structural engineer with steel mesh in it. and hes worked out the lateral loadings on the wall from the concrete, hence as the gaffer says you'll need a perimeter expansion joint to stop it pushing on the walls.

i take it the walls are sat on a ring footing?

power floating is an art form knowing when the concrete is at the right point to start working it, you can make a real mess if you start when its too wet.

have a read of http://www.pavingexpert.com/concjnt1.htm and the other concrete sections, if you are determined to go ahead then i'd pour it in 2 pours. split it into 4 and pour the opposite slabs across the diagonal.

if it does go wrong i can bring our 3 ton in there and hammer it out in a day for you haha

[47p2]

Thanks for the information so far. Do I look up yellow pages to find a concrete finishing contractor in my area (Glasgow) if so what heading do I look for as I cannot find anything under 'concrete finishing contractor'.

Yes it has to have steel mesh in it and the walls are sat on block laid on there side for extra strength.

I would like one bay which is 6.6 x 3.9m to be laid in one slab as I wish to have a 4 post car ramp fitted here, the other two bays can be laid separate.

Can anyone give me an indication of the cost involved to have a contractor do this job?

Thanks

[GB_Groundworks]

based on assumption such as perfect conditions meaning 6m3 wagon can back right upto pour, levels and sub base are correct

just working it out roughly i'd say between £3k - £4k depending on how much steel is in the slab.

but without seeing the job i can't say so don't use that as a yard stick to beat anyone with haha

get 3 quotes, look for groundworkers etc with experience of concrete finishing don't accept a tamped finished

but others may disagree, i can get c30 concrete for £58+vat/m3 where as others might pay £100/m3+vat. but i buy over 300m3 a year so you wont get that rate.



Edited By GB_Groundworks on 1237936693

[lutonlagerlout]

prices prices prices
we buy over 200m3 a year giles and the cheapest on a full load is £80 per M plus vat
a mate of mine had a similar floor and was convinced he could lay it him self with a couple of other desk jockeys on the shovels
to use a politically incorrect phrase its an "abortion" and has ruined his garage
as the gaffer says ,there is no second chance with concrete
LLL

[GB_Groundworks]

speak to your cemex rep they are selling hard up here, last pour the other week 52m3 at £58+m/3 for c35

but i worked that rough price out allowing 100+ vat for the concrete but every contractor is different

get 3 detailed quotes and specifications and don't go on price go on quality previous work etc



Edited By GB_Groundworks on 1237936753

[lutonlagerlout]

its probably more difficult for us because we are mainly on domestic jobs,the cemex drivers all seem to have an attitude problem i.e. if its not a straight pour they start whining even though there is 40 minutes standard drop time
so i use custom concrete a lot,drivers are more amenable,they are ultra reliable and barrowing is no problem
cheers
LLL

[GB_Groundworks]

yeah the cemex drivers were putting 45 minute turn arounds on their tickets even though they were discharging straight into pump taking maybe 15 minutes with wash out to get going again. one driver even managed to get back to us before he had left according to his ticket. for smaller jobs or difficult access i use a local mini mix and they charge about 90 /m3 in 3.5 cube wagons. those lads even get on the rake haha

[TheVictorianCobbleCo]

I'm not raising these questions because I think I know better, and don't want you to redesign your floor, but am just raising these points for consideration and comment by those more qualified.
So - under normal conditions wouldn't a 150mm thick floor would be ample for any reasonable use. Next, why use steel and added fibre, at 30mPa the added fibre cannot contribute a hell of a lot (assuming steel included)
Lastly, would it not be an option to cast footings for the car lift that are much thicker than the floor, with steel rebars and holding down pockets for the bolts to tie down the lift upstands, and then have the rest of the floor 150mm thick? Still cast in 4 sections with expansion joints as mentioned by others.

[worldofpaving]

John,

The chaps have given you lots of excellent advice particularly the part about not doing it yourself.

Leaving skills aside for the moment, it's easy to forget how heavy a barrow of concrete is - a cubic metre weighs 2.3 tonnes so the 12 cube you will need weighs in at approaching 30 tonnes!

To get a quality result, concrete needs to be poured at a slump that, in turn, makes it a real skill to strike off to level and finish. What tends to happen if those skills are not around is that it gets watered up to virtually self levelling consistency leaving you with a crack prone, rutted and weak, dusting surface that produces a substance known as laitence, something that you will get to know well as you spend the next years sweeping up!

A C30/35 mix is fine, with polypropylene monofilament fibres and some steel mesh or steel fibres.

Good luck,
WOP

[47p2]

Thanks once again for all the advice.

The problem I have is locating the right guys to do the job and do it correctly as there are so many cowboys here who don't know a spade from a shovel.

I have trawled through the website here and it has at least allowed me to gain a certain amount of knowledge which I can ask when I am getting quotes.

The reason the floor will be 200mm thick is mainly for the ramp, not only has it got to withstand the weight of the vehicle but it also has to withstand a lot of lateral force when cars are going on and off, 200mm is the minimum recommended thickness and a single slab of 4.5 metres long, hence my thought of doing 3 sections.
Yes I could put down another 50mm of hardcore where the ramp will not be, but by the time I have that delivered, shift it from the road to the garage I wouldn't be saving very much, so it is just as easy for me to have an extra couple of cube of concrete.

Another few questions if you don't mind which will allow me to assess who best to do the job when it goes ahead.

If it is decided to lay 3 slabs, what would you recommend in the way of joints. I have looked at the website and there are various options i.e. dowelled joints, non-dowelled joints, dowelled expansion joints etc. but I would like to hear your opinions.

Also what sort of size should the expansion joint be between the slab and the walls, again the website gives different sizes of Flexcell which are available.

For your information the 3 bays will be 1 bay at 3.9 x 6.7m and 2 bays at 3.1 x 6.7m

Cheers

John

[GB_Groundworks]

vic cobble co i thought the fibres were to stop plastic cracking at the surface and not to replace the steel, as it sounds like it will see a lot of car use?

200mm is not really excessive for car ramp and workshop use, our workshop is 300mm thick the saving on concrete wouldn't justify the hassle of perfectly aligning the bolts for the ramp, even if set in polystyrene then burn out to allow some play. we did the the bases for a rugby stand, 30 bases all with 4 bolts set in each one was a real pain getting them aligned within tolerance for the steel.