More slab advice

Hi all,
I need some advice on the design of our slab. The pour date is set for June 20th so I have a little time to sort get organised. The slab will be approximately 13x7m split level with a 120mm step up at 9/4m mark. It will be coloured, polished, fully insulated with electric heating. I will use 32 mpa concrete. The first dilemma is the thickness of the slab. Originally we allowed for 120mm thickness throughout, however with 30mm insulation I can get it too 100mm easily and 120mm with a lot of messing around. Is it worth attempting to get the extra thickness? This leads on to the next question. It was recommended by an energy efficient home designer that we position the heating cables as low down in slab as possible. So I decided to use 2 layers of reo. A layer of SL 52 for the heating cable positioned about 30mm from base of slab and a layer of SL 72 positioned 30mm from top of slab. Will this work with a 100mm slab?

TIA,
Peter

Some more information would be helpful.

Is the slab for a house or shed.

I assume the slab has been designed by an engineer?

Generally speaking there should be no need for the slab to be greater than 100mm.

As the walls, posts etc should be supported by a far greater depth of concrete.

Do you have any concrete beams running through the slab?

You are going to have a little difficulty with bar chairs supporting the mesh at those suggested heights.

I'm a little lost as to why the electric coils should be so low in the slab.

How are you going to insulate the slab; waffle pods?

Finally I would recommend to look at hydronic heating my choice over electric coil.

Finally make sure you machine trowel the finish, it will make the polishing far less difficult

Good luck

Pulpo

Hi Pulpo,
a bit more info. The slab is for a house where we are replacing rotten wood floor. It is open plan so the slab is not supporting anything it is within the existing walls. The slab has not been designed by an engineer. However, there has been some input from one. He suggested a beam through thefull length of centre and 3 beams across at about 4m intervals. The slab will be insulated with isoboard which is 30mm extruded polystyrene. I haven't looked at reo supports yet. I think the theory of putting heating cables lower is to give off a lower heat over a longer period of time. The house is designed with passive solar heating in mind with the slab heating as a background heat. I did look seriously in to hydronic heating but coudn't justify the expense for something we may not use often. My main concern is that I am not over reinforcing with 2 layers of reo which I heard can be a problem. Why, I don't know.

Regards,
Peter

Talk to your engineer again. I think you are "under" reinforceing the slab if you are only using SL72 & SL52 for shrinkage control. Refer to previous posts re polished concrete. I think you should get some advice from an engineer with experience in polished concrete slab design. Also 2 layers of mesh + heating + sufficent top and bottom cover will be difficult to achieve using a 100mm slab.

All the information I have seen is that the cables should be close to the surface of the slab, not buried too deeply.

In an episode of Grand Designs (uk) I saw, they accidentally poured the slab too deep. They spent the next week chiselling back the entire slab by a few inches!

Anyway, that's all I know, I'm not a qualified "energy efficient home designer", but I'm also not aware of any such qualification. I would suss out what this guy's qualifications are, and also seek a second opinion, maybe from archicentre or a heating company.

I'm researching this myself, because we are currently designing our new home also. I'll be posting some more questions in a new thread.

Thanks all for the advice so far. A bit of an up date and a few more questions. We have decided to use SL 81 reo which cost an arm and a leg but hopefully worth while. Heating cables attached. Chairs will be 65mm with slab thickness at 115. The question I have here is how important is it to have the reo level. There will be some variation in the thickness of the slab and if we use the 65mm chairs throughout the reo will sit at different levels. Should we correct level or not? The other question I have is regards to the internal beams. We are insulating with 30mm polystyrene which will be difficult to place around beams because of all the angles involved. Can the beams be 300x300 square to the slab or is it important to minimise the angle and say come up 200mm from the bottom of beam then angle at say 30 degrees to base of slab? Hopefully you can understand what I am talking about.

TIA,
Peter

G'day Peter,
I was designing a slab for my garage, so I was looking through AS 2870 - Slabs and Footings.

I found a section on heating cables.

Clause 5.3.6 (a) states that electric cable can be embedded without increase in thickness of reinforcement.

Clause 5.3.6 (b) states that for hot water heating pipes thickness needs to increased by 25mm and reo needs to go up one level eg from SL72 to SL82.

With regards to reo location, cover is 40mm to unprotected ground, 30mm to a membrane and 20mm to the top, and should be placed as high as possible within these limits.

With regards to your last question, the diagrams in AS 2870 indicate that the the beams should be angled at the top to extend at east 50mm wider than the beam. ie for a 300mm wide beam, the top will be 400 mm wide with a 50mm flare to support the slab on each side.


Hope this helps
Pulse

Thanks pulse, will have to flare beams. Makes putting insulation down a nightmare.

John, it was decided to put the heating cables lower in the slab due to the fact it will be insulated and we wanted a low background heat that would be emitted for long periods. If the slab wasn't insulated and we wanted to get more heat from the slab more quickly we might place them nearer the surface. We have incorporated some passive solar design and will use a wood fire as our source of heat. After much thought this is what we are comfortable with.

Regards,
Peter