Concrete shed base over existing patio slabs

[djaychela]

I've got an existing 6mx4m patio at the bottom of my garden, which has been in place for 30 years+.

In the same place I want to build a large shed/garage which will be for vehicle mechanics (my own stuff, just a restoration of a camper, not heavy daily vehicle traffic), and want to put in a 4" thick concrete slab for the base. The building itself will be a wooden one of shed-type construction.

I've read the (very informative) stuff on the site, but have a couple of questions.

Firstly, is pouring the slab directly on top of this feasible? I appreciate that laying a patio over it isn't the thing to do, but wondered if pouring a slab is (I've read a number of people saying they've done similar, and most have said they have put a barrier between on top of the old surface so that they didn't bond together)?

Secondly, there's a 2" or so change in level between the rear of the existing patio and the front - if was to go ahead with the slab, should I just make my formwork true level and end up with a slab that's thicker at one end than the other?

Any advice is appreciated.

Darren

[Tony McC]

If the existing patio is sound and none of the paving units are moving, then you can pour a new slab over it, as long as the concrete is at least 100mm thick.

Not sure why some are saying they wouldn't want the new concrete to bond to the old paving. Personally, I'd use a DPM beneath the new concrete but that would be with the intention of keeping the water/cement ratio at the required level rather than to prevent bonding.

When it comes to the level difference, as new slab of uniform thickness is preferable, but, to be honest, a diff of 50mm is not that big an issue. If we were being all professional and accurate, the old patio would be regulated with coarse sand and/or crushed rock so that you get an even 100mm slab throughout , but is it worth all that effort, and then what is going to keep the regulating in place at the free edges?

For a home DIY job, I'd be very tempted to just erect the formwork to give me a base with a slight fall towards whichever end will be the entrance (1:100 is OK) and then pour it as one, with fibre reinforcement rather than steel mesh. Just make sure your shuttering is absolutelysound and relkaible - there's nowt worse than a shutter springing as you pour the concrete!

[djaychela]

Thanks Tony, that's great, and judging by the site, you're a man who knows what he's talking about!

So, you think I should put a DPM in? I'll take your advice as gospel!

Darren

[Tony McC]

For what it costs, a DPM is the best 'upgrade' you can have for a concrete slab.

[TheRockConcreting]

pour it as one, with fibre reinforcement rather than steel mesh.

I've said it before and i'll say it again, when you order fibres in your concrete from the plant it is not for the purpose of replacing steel mesh.

The mix you get from the plant is purely for plastic shrinkage, yes there are engineers that specify fibre for steel replacement but this is at a much higher dose than what the plant will supply you with.

The plant only deal with a plastic shrinkage dose, if they where to start messing around with a steel replacement dose they open them selfs up to loads of clams.

[msh paving]

When you order fibres in ready mix, the batching office will always as the the KG/M3 rate, they don't just guess a hand full to the cube. MSH

[Brucieboy]

Just to clarify, the common monofilament fibres used by the ready mixed concrete industry, e.g. Fibermesh, Adfil, etc, are suitable as a replacement for crack control mesh reinforcement in ground floor slabs – refer to the respective manufacturers data sheets. Dosage rates are typically in the range 0.6 to 0.9 kg/m3 depending on type (supplied in pre-weighed water-soluble bags).

They are designed, as you say, to control the occurrence of plastic shrinkage (and sometimes plastic settlement) and not, under any circumstances, to be used to replace structural steel reinforcement.

The problem with crack control mesh, typically A142, is that all too often it’s found closer to the bottom of a slab, generally trodden in as an after-thought with little care. To do its job effectively, it should be closer to the top of the slab with say 50mm cover properly supported by chairs.

Macro synthetic and steel fibres are becoming more and more popular in higher loaded ground floor slabs such as warehouses, commercial buildings etc, predominantly for their fast-track benefits. Dosage rates are much higher than monofilament (4 to 6 kg/m3 for macro synthetic and 20 to 40 kg/m3 for steel). The inclusion of both types of fibres are by specific design via the manufacturer’s in-house engineer depending on loadings, slab thickness, concrete strength etc.

[TheRockConcreting]

When you order fibres in ready mix, the batching office will always as the the KG/M3 rate, they don't just guess a hand full to the cube. MSH

They add a pre-packed bag to the mix, 1 bag per m3.

Im down the plant tomorrow so i'll grab a bag, i can't remember how much is in a bag.

[Tony McC]

The reason for suggesting fibres in place of steel mesh is precisely because this is a ground-supported, light-use residential slab, where steel mesh would not be structural, but purely a shrinkage/crack control measure. I'd never advocate replacing steel mesh in a structural pour.

The experience of the better end of the decorative concrete industry shows that, when carried out properly, a fibre-reinforced slab performs adequately for such applications, and as Brucieboy pointed out, mesh is often mis-positioned to the point where it offers more of a threat to shallow pours (100m) than it does protection.

The only caveat would be if the poposed shed/garage turned out to be a brick or block built garage in which case a thicker steel reinforced keeled slab might be more appropriate as shown here

[TheRockConcreting]

IMO based on my experience in the concreting trade, fibres do nothing for concrete once its cracked, unlike steel which when used as crack control will hold the slab together, preventing an enlarged crack once the slab rebounds after being overloaded.

In the real world all concrete slabs get overloaded at some point, its not cost effective to design every slab to take the load of a 10ton axel, so by using steel mesh prevents a wide messy crack that i see loads of on PIC drives that have been overloaded when the extension was being build out the back. The drives done with steel mesh still over load but the crack is held together and hardily noticeable.