Laying a concrete slab

[Araucaria]

I plan to lay a concrete slab for an outside terrace. I'd like to mix the concrete myself rather than buy it ready-mixed, as there isn't any sensible access for a heavy lorry that won't wreck the garden. We've had it wrecked several times before - funny how the delivery always arrives during a week of heavy rain.

It's not an enormous area - about 5 meters each way - but as the ground slopes it will need be fairly thick, maybe 35cm at one edge, to provide a reasonable thickness at the upslope end. I'll be doing it single-handed and it would be more than I could mix in one day, so I hope to be able to do it a bit at a time. That's where I need advice.

I'd considered three possibilities, but if anyone could suggest a "best way" I'd appreciate it.

One option was to lay it in strips, about half a meter wide (and 5 meters long). Wait till each one is reasonably solid, and then put the next one in, just moving the shuttering along each time. This has the advantage of avoiding doing my back in, as it would spread the work over a couple of weeks.

The second was to cast the three outer edges first (the fourth edge is against the building), let that harden, infill the centre with a quantity of hardcore, and just pour (all in one go) a 10cm slab over the centre. This has the advantage of saving on concrete, but the shuttering for the outside edges is a bit complicated.

The third was to cast it all once, having dumped a mound of hardcore in the centre, thus reducing the amount of concrete to be mixed.

In all three options, the three outer edges have got to be concrete all the way down - the slab will eventually be tiled both on top and on the edges where they show above ground level. I've laid concrete before so I'm not a complete beginner, but I haven't tried a large area previously (or not without help, anyway).

Any suggestions/advice?

[Théière]

You need extra help!

I like the sound of your shuttering the outside edges and using hardcore (compacted and blinded) are you using trellis soude?

I dont like the idea of letting it harden and then do the next section of half a metre, it will not be one slab but several and could suffer from movement.

 

[PaulT]

I would buy a cement mixer and do it in one hit. The strip idea will possibly cause problems, i.e. cracked tiles etc in the future.

[Araucaria]

Thanks guys -

I'll have some help later in the year when the visitors start arriving, but this is one job I'd like to get done earlier rather than later. I do already have a mixer, but from past experience and at my age (60 but my back says it's a lot more) it's still too much to do all at once. We're talking a good 3 or 4 cubic meters of concrete (or concrete and hardcore). I have some reinforcement I could use in it, but the ground is very solid - bits of it are bedrock - and even if there's some movement eventually it won't be the end of the world. It's only an outside terrace!

Is the accepted view that if I lay it in sections, each section is pretty sure to move separately from the others? I'd hoped the separate bits of concrete would bond to each other, at least to some degree. If not, would it maybe be an idea to try to make the joins in the concrete coincide with joints in the tiling (30cm square tiles)?

I can't think of a way of using reinforcing mesh and doing it in sections, though with the second and third options that should be just about possible.

[Benjamin]

I'm an amateur and I did a similar thing for a pool terrace (115 sq mtrs) at our former home in Spain. To try to get around the problem that each strip could move independetly I cut reinforcing mesh to the width of the strip but laid it down the middle of the strip I was about to concrete so that half of it protruded into the next strip to be done.

Still with me? Obviously the first reinforcing mesh was cut one and a half times as wide as the strip to be concreted.

I had, I think, a 140 litre mixer. Would I do it again? Never, never, never.   [:D]

Of course laying 115 sq mtrs of tiling was another mammoth task.

[powerdesal]

[quote user="teapot"]

I dont like the idea of letting it harden and then do the next section of half a metre, it will not be one slab but several and could suffer from movement.

 

[/quote]

Funny that, its how the pros do it, but in bays not strips. Its certainly how I did an outside area in Wales many years ago, and when I left after 11 years there had been no problem. Well compacted ground is needed. The easy way is with a wacker but it can be done the hard way by ramming.

I suggest the area be segregated into multiple bays, half a meter is way to narrow. You could have 10 bays of one meter x 2.5 m doing the outer bays first, then the centre bays then the intermediate ones. You do need a mixer though for consistency and ease.

Put the mixer hard up against the bay to be done, mix by loading from the non bay side then when ready turn the mixer barrel completely over and drop the load down a pre arranged guide chute into the bay, repeat as necessary, spread with a rake and level with a plank edge-on across the shuttering top in the usual manner. Float or helicopter to finish. If you are tiling over you will not need a float finish of course.

Its a matter of choice, but you may want to put a 1200 gauge membrane down, possibly with blinding to stop the measured water content of the mix from soaking into the virgin ground below and hence altering the design strength of the mix.

[Nearly Retired (I am now)]

I reckon it'll be fine done in strips if:

a) You have the right material proportions each time to achieve the same colour match - oops, not relevant as you plan to cover it.

b) Lay mesh at about mid-slab depth across the proposed joint and bend the mesh up against your shuttering. When strip one has gone off, remove the shutter and bend the bars down straight. Repeat for each strip.You probably want at least 300mm length of bar in each strip, i.e. 60mm total mesh width.

or,

c) Drill reinforcing bar sized holes in your shuttering, poke half a bar length through each hole (say 3 per metre length of strip )and cast your first strip. Later, remove the drilled shutter (albeit with a bit of difficulty) and repeat for strip no.2.  To make removal of the shutter easier you could drill over size holes, insert the bars and plug the remaining gaps around the bars with foam, polythene or anything similar that comes to hand. The shutter should come off much easier then.

Edit: This was typed before seeing the two posts above - 3 similar tips in 8 minutes !

[clarksinfrance]

You must be super human.

5m x 5m x 0.35m is 8.75m3 of concrete, or to put another way, a fraction over a full lorry load of RMC. I can almost certainly predict, that by the time you have bought the aggregates to mix yourself, and paid for delivery, it will cost almost exactly the same as having RMC delivered. If you buy RMC you may be able to make use of the lorry with a tapis (conveyor belt to shift the concrete over a distance), it will be easier to lay in a day, and have fibres put in it to stop the concrete cracking.

8.75m3 of concrete will need:

2800Kgs of Cement

5500 Kgs of sand

10250 Kgs of gravel

2625 litres of water

That is nearly 23 tonnes of materials.

If you are going to mix and lay yourself, firstly get some help in. Secondly, split your slab into 5 strips a metre wide, high to low not across the slope. Do the outside strips, then the middle strip, then the other two strips last. As has already been pointed out, compacted hardcore is vital, but even this will not stop the slabs moving at slightly different rates.

Hope its useful info.

[Araucaria]

Thanks very much everyone - this is all very practical advice, encouraging too, and I should be able to start tackling it with confidence as soon as the sun starts shining (that'll be mid-May, then).

[Araucaria]

Clarks - it's not quite that much concrete, as it'll only be 35cm thick at one point on the downhill end - it should taper off to about a 10cm depth both at the uphill end, and towards the building.

This is difficult to explain, but the ground slopes away from the building both at right angles to the walls, and also along the wall from one end to the other. As a result there's only one external corner of the slab where the thickness will need to be as much as 35cm. The uphill corner will be at ground level. My own arithmetic was an average depth of somewhere between 15cm and 20cm, making it about the 4 cu meters I said.

But you can certainly see why I can't do it all at once! And as I find it hard to estimate the quantity with any degree of accuracy, getting it as a readymix and getting the quantity wrong would be a problem - particularly if it was too little.

Thanks again.

[Théière]

[quote user="powerdesal"][quote user="teapot"]

I dont like the idea of letting it harden and then do the next section of half a metre, it will not be one slab but several and could suffer from movement.

 

[/quote]

Funny that, its how the pros do it, but in bays not strips. Its certainly how I did an outside area in Wales many years ago, and when I left after 11 years there had been no problem. Well compacted ground is needed. The easy way is with a wacker but it can be done the hard way by ramming.

I suggest the area be segregated into multiple bays, half a meter is way to narrow. You could have 10 bays of one meter x 2.5 m doing the outer bays first, then the centre bays then the intermediate ones. You do need a mixer though for consistency and ease.

[/quote]

Oh Steve,

It's not how the pros do it. You have already stated that half a metre is too narow and its done in bays so you do agree with me. So you know how the pros do it [:D]

You don't let it harden if at all possible, you work in sections and you can allow a partial set as it is a chemical reaction a total set would mean almost separate slab, bays are used so as not to disturb the partially set mix.

It is a big area for one man and one back and then it needs tamping. On the second day the back will only do 50% of the first day.

Get help I say.

 

[powerdesal]

I certainly agree with the help comment, the more the better.

As a point of professional interest, I am presently installing 4t units on bays alongside the adjacent bays that were only poured today. The  bays being loaded have already been left for the 28 day full cure period. there are no expansion joints fitted.

The bays are approx 3.5 x 3.5 m and the strength achieved is 60+ against a spec of 44.!!!!!

The slab on grade between the rows of bays will be poured in 5 bays with 24 / 48 hours between pours, you could say that it is a partial set given the normal 28 day full cure I suppose.

Edit 11/2/09 20.54. I lied, there are expansion joints. I checked today

[BIG MAC]

Why not simply get readymix pumped?

[oldgit72]

Hi just picked up on this thread as I'm about to lay a floor using mesh to reinforce. In the case of a 100mm concrete floor, when you say lay the mesh at about half the depth (ie 50mm) then I assume that the section would be laid first to a depth of 50mm, then the mesh, then the final 50mm of concrete?. I will be laying in sections of around 1.75mx6m, would the lower 50mm be ok to kneel/step on to lay the mesh and the top level of cement or is there a better way ie cut the mesh into small sections of around 1-1.5metres and join them together with heavy duty wire to enable easier working?

[AnOther]

Not an expert just speaking as someone who's done their share of laying concrete slabs !

Depending on substrate and depth I'd say with 2 or 3 pairs of hands 5m x 5m for a patio is perfectly possible to lay in one go. If you could level off and well compact the site so that you could limit the thickness to 100mm or less you're now only talking about 2.5m3 max, definitely not an economical quantity for readymix.

In UK I laid a 6m x 3m slab with one central expansion joint for parking a car. Filled with about 100mm of builders rubble almost to the finished level the average thickness of concrete was probably no more than 50 mm so no more than around 1m3 in total. Did it on my own in a day easily, including floating, not just tamping down. 5 years on with cars parked on it on a daily basis there was no sign of movement or cracking.

[oldgit72]

Thanks Ernie. It is only me and Mrs OG i'm afraid hence doing the job in manageable sections. I was and am still a bit confusedf as to how to insert the steel  mesh. If I lay the mesh on top of the bottom 50mm of cement, there is a chance that it might have started to go off and I will cause a problem when laying the top half by having to stand/kneel on it. The job is indoors and will neccesitate working inside the shuttering to gain access for levelling off the surface. If I cut the mesh into 1mx1.75m sections, I can lay this amount of concrete in 1 go to the finished depth and insert the mesh as I go on but I'm not sure if this will weaken the overall result.

[powerdesal]

[quote user="oldgit72"]

Thanks Ernie. It is only me and Mrs OG i'm afraid hence doing the job in manageable sections. I was and am still a bit confusedf as to how to insert the steel  mesh. If I lay the mesh on top of the bottom 50mm of cement, there is a chance that it might have started to go off and I will cause a problem when laying the top half by having to stand/kneel on it. The job is indoors and will neccesitate working inside the shuttering to gain access for levelling off the surface. If I cut the mesh into 1mx1.75m sections, I can lay this amount of concrete in 1 go to the finished depth and insert the mesh as I go on but I'm not sure if this will weaken the overall result.

[/quote]

OG72,

Its normal to lay the mesh inside the excavation, spaced off the base level by small concrete 'stools', generally wired to the mesh to keep it in place at (say) mid point of the depth (for a single layer of reinforcing). Then, when the concrete is poured, the mix flows through and around the mesh making a full depth with the mesh in the middle.

edit.

You can make the stools/spacers well in advance by casting (say) 50mm x 50mm x50mm blocks with wire inserted, the wire being used to fasten the spacers to the mesh when it is in place. The spacers can be done in a lean mix or even in straight mortar as they are not load bearing but are only there to keep the mesh in the right place for pouring the concrete. The spacing of the blocks is entirely up to you, dependent on the thickness / rigidity of the reinforcing mesh.