Need professional advice for concrete floor

First: I am not a professional, therefore my question to you guys (even non profs, but experienced)

I need to get a concrete slab for my shed.
The shed will be 18m x 6m :)

The shed builder suggested a 75mm floor, with a F53 mesh reinforcement.
I started reading on te internet, and found out that a F53 is not standard for floors over 12m. ( http://www.standards.com.au/PDFTemp/.../3600/3610.pdf )

This is what I mentioned to the builder, and he went to the grano contractor to ask for advice. Now he gets back to me and says: "he advised that most commonly required for your area is 100mm slab with F53 mesh for your given size"

"My area" is the hills area near Chittering in WA, site is classified as a sand/clay/rocks site.

Do you guys agree with the 100mm floor? Or with the F53 mesh?

Any advice welcome.

Thanks,
WoodEd

Woodyed,

You might like to give your local building inspector a call, most are helpfull and will give you recommendations for your area. Does the slab support the shed? or is the shed erected and then the floor poured. It does make a difference, you may need to stiffen the edge of the slab to support the structure. I'm not a professional but have done a few slabs for what it's worth and would probably not go under F72 with black plastic underneath as a moisture barrier. That said it depends on your area and a number of large prefab shed builders do provide minimum recommendations for the slabs under their sheds, but in the end you should have a permit and it needs to conform to local regs.

John

The difference between f53 and f72

F53= 5mm at 300mm intervals

F72= 7mm at 200mm intervals

The previous questions are important.

Will the slab support vehicles or heavy machinery?

I would personally go with at least 100mm slab and F72 and the plastic membrane.

I also may even order 30mpa concrete rather than 25mpa small additional cost.

Getting a good concrete slab that will last for years is worth every cent.

I would also be inclined to have a machine trowel to finish off.

However I'm no expert or professional but I have done a few slabs, to know what I want.

Good Luck

Pulpo

I have put in two shed floors, workshop 6x 9metre and timber storage 9 x 12 metre. Both sheds are minimum 100mm thick, F72 mesh compacted base and edge thickening at doorway. Underneath the ground is basically clay for workshop and rock for timber storage shed. Workshop has been down ten years no cracking whatsoever, timber storage shed, 18 months again no cracks evident in slab.

Kev M

hi guys,

I will now and then drive my car and trailer in the shed, and want to use us to store a trailer and a caravan as well. Further, it's just a shed. Might do some woodworking in in, maybe some wood chopping etc.

Thanks for the advice!

I agree F72 100mm. The last shed I put up I dug a 300x300 mm trench around the edge where the shed bolted down so I had 400mm of concreat there.

100mm and f72 for sure, edge beams are probably required if the shed is supported by the slab. If the shed is constructed first and the posts are concreted into the ground then beams are not necessary, however I would recommend some thickening and possibly a 1/2" rebar under the mesh across the opening you will be driving across.

Depending on your car's ability to drop oil, or any of your other shed activities, have a look at Diamite from cementaid, http://www.cementaid.com.au/diam.htm It is not the cheapest stuff around but the results are amazing, 250MPa+ surface hardness and oil and water will just sit on the surface without soaking in.

Cheers
Ben

Quote:

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Originally Posted by Wildman

a 1/2" rebar under the mesh across the opening you will be driving across

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Ben could you tell me what this is please? I'm laying a 900mm wide strip at the doorway to my shed which I'll also be driving across sometimes. Should I also pin/dowel this section into the (already laid) main slab?

Inside the shed I have to lay 350mm side strips to fill a gap between the existing slab and wall. (It's a strange shed). Do I need to place a membrane of some sort between the old and the new? An old handyman book I have suggests this to allow for movement, but I figure the 350mm wide strip is 'free' to do this.

Does the black plastic come up the sides of the new slab at all?

Sorry to hijack the thread, hope this is relevant.

Quote:

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Originally Posted by JB

Ben could you tell me what this is please? I'm laying a 900mm wide strip at the doorway to my shed which I'll also be driving across sometimes. Should I also pin/dowel this section into the (already laid) main slab?

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I would. A small strip like that doesn't have much bearing area, so is likely to end up moving quite a lot.

Quote:

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Originally Posted by JB

Inside the shed I have to lay 350mm side strips to fill a gap between the existing slab and wall. (It's a strange shed). Do I need to place a membrane of some sort between the old and the new? An old handyman book I have suggests this to allow for movement, but I figure the 350mm wide strip is 'free' to do this.

Does the black plastic come up the sides of the new slab at all?

Sorry to hijack the thread, hope this is relevant.

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A "bond breaker" of some sort is certainly desirable, mostly because the new concrete will not behave the same as the old either thermally or mechanically for some time. It isn't essential though. An alternative would be to use a wax bond-breaker or even some log-end sealer if you have it would do the job.

To the OP:

Whereabouts are you in Perth? if you're on the sand belt, you can get away with much lighter construction than if you're in the hills. The Perth sand is about the best foundation material around. Many of the early homes in Perth used limestone block footings (blocks of about 400 square x 500 or so to support brick and tile structures. The only time there's been trouble with them in 100 years is when the dewatering was done for the Northbridge Tunnel, which caused settlement over an extended area. Make sure your subgrade is well compacted and you'll have very little trouble.

Quote:

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Originally Posted by JB

Ben could you tell me what this is please? I'm laying a 900mm wide strip at the doorway to my shed which I'll also be driving across sometimes. Should I also pin/dowel this section into the (already laid) main slab?.

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A rebar is a solid 1/2" or 3/4" (or bigger commerically) steel bar with a texture rolled into it to allow a better bond with the concrete. I would reinforce a garage slab edge that is being driven over with one unless there is extra thickening of the edge of the slab. You simply tie it to the mesh with twist ties. You would want to ensure the slab is at least 125mm thick at that edge to prevent over reinforcing but a car is not too heavy so it wont really matter but you definitely need to ensure proper coverage of the steel, 40-50mm min.

Yes you should dowel the strip in, drill 3/4" holes into the existing slab approx 300mm apart at least 2-300mm deep near (try to miss the existing mesh) the middle of the slab and insert 3/4" SMOOTH pins with at least 200-300mm sticking out (must overlap with the mesh). grease the end of the bar that is going in the holes with a bit of lithium grease to allow it to move and not corrode but keep the free end clean to allow it to bond with the new concrete. Push on some 10mm foam expansion joint material onto the bars then form up the new concrete and pour it. The pins will allow expansion movement but will provide the shear reinforcement required. The foam will fill the gap.

Quote:

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Inside the shed I have to lay 350mm side strips to fill a gap between the existing slab and wall. (It's a strange shed). Do I need to place a membrane of some sort between the old and the new? An old handyman book I have suggests this to allow for movement, but I figure the 350mm wide strip is 'free' to do this.

Does the black plastic come up the sides of the new slab at all?

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For a strip that narrow, I would be bonding to the old slab rather than making a movement joint. There will not be much mevement at all. If the edge of the old slab is rough, then I would bond the new to it, rough it up some more, wet with water then paint with bondcrete just before you pour the new bit. If it will be taking more load then a pinned solution may be better but will be difficult with 300mm clearance. The best and most expensive solution is to cut back approx 150-200mm into your old slab with a concrete saw down to the existing mesh (but not through it), jack hammer the top off the old slab and overlap some mesh for the new section with the old mesh so you have a stepped join. Still make sure it is wet and paint with bondcrete to ensure a more watertight join. If the new section of slab butts up against the shed walls (tin/colourbond etc), just pour the slab right up to the tin, no plastic up the sides, no bond breaker, get it to stick to the tin for a good seal. Put some plastic underneath though.

Cheers
Ben

Quote:

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Originally Posted by Wildman

A rebar is a solid 1/2" or 3/4" (or bigger commerically) steel bar with a texture rolled into it to allow a better bond with the concrete. I would reinforce a garage slab edge that is being driven over with one unless there is extra thickening of the edge of the slab. You simply tie it to the mesh with twist ties. You would want to ensure the slab is at least 125mm thick at that edge to prevent over reinforcing but a car is not too heavy so it wont really matter but you definitely need to ensure proper coverage of the steel, 40-50mm min.

Yes you should dowel the strip in, drill 3/4" holes into the existing slab approx 300mm apart at least 2-300mm deep near (try to miss the existing mesh) the middle of the slab and insert 3/4" SMOOTH pins with at least 200-300mm sticking out (must overlap with the mesh). grease the end of the bar that is going in the holes with a bit of lithium grease to allow it to move and not corrode but keep the free end clean to allow it to bond with the new concrete. Push on some 10mm foam expansion joint material onto the bars then form up the new concrete and pour it. The pins will allow expansion movement but will provide the shear reinforcement required. The foam will fill the gap.





For a strip that narrow, I would be bonding to the old slab rather than making a movement joint. There will not be much mevement at all. If the edge of the old slab is rough, then I would bond the new to it, rough it up some more, wet with water then paint with bondcrete just before you pour the new bit. If it will be taking more load then a pinned solution may be better but will be difficult with 300mm clearance. The best and most expensive solution is to cut back approx 150-200mm into your old slab with a concrete saw down to the existing mesh (but not through it), jack hammer the top off the old slab and overlap some mesh for the new section with the old mesh so you have a stepped join. Still make sure it is wet and paint with bondcrete to ensure a more watertight join. If the new section of slab butts up against the shed walls (tin/colourbond etc), just pour the slab right up to the tin, no plastic up the sides, no bond breaker, get it to stick to the tin for a good seal. Put some plastic underneath though.

Cheers
Ben

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Good advice, although if I understand the question correctly, the side strips are essentially "free", so a tied-in solution is not required. As for movement, if the new bits are not placed on properly compacted subgrade (a fair chance with a small strip like these), movement may well be an issue, especially given they are on the extremity of the building and thus subject to larger subgrade moisture variations than the main slab. Also, the cost of tying-in properly would be excessive in the circumstances, I would think.

Thanks fellahs for your very helpful and thorough responses; somewhat contradictory advice regarding strip joint but I see the logic in both suggestions.

Wildman, you say rebar needed if no edge thicknessing of concrete. Would it be simpler to just edge thickness? If so, how wide is the thicker section? I take it that it's about 25 mm thicker.

Also you mention rust prevention. Why is this needed for something buried in concrete, presumably isolated from oxygen? I notice on the concrete bridge the council are building down the road that all the inserted reinforcing is quite rusty.

Regarding 3/4" dowel holes by a total of 600mm deep...well I hate to reveal my laziness, but I even if I had to drill holes that size and that long in a hard wood I would be looking for alternatives, let alone in concrete. The 900mm strip is on stony clay soil, pretty hard and compact. Can I be excused the dowels?:D

Wildman, you say rebar needed if no edge thicknessing of concrete. Would it be simpler to just edge thickness? If so, how wide is the thicker section? I take it that it's about 25 mm thicker.

Quote:

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Originally Posted by JB

Also you mention rust prevention. Why is this needed for something buried in concrete, presumably isolated from oxygen? I notice on the concrete bridge the council are building down the road that all the inserted reinforcing is quite rusty.

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One of the biggest long-term killers of exposed concrete structures is "spalling", in which the re-bar rusts and hence expands, thereby breaking a chunk of concrete off the mass. Even in the absence of spalling or serious rust, surface rust during placement will prevent the concrete from bonding with the rebar, thus reducing the tensile performance of the structure (concrete is great in compression, lousy in tension or bending). As to concrete preventing the ingress of oxygen, all I can say is "ha". You can achieve a waterproof mix, but N20/25/32 as supplied by your friendly local concrete dispenser and laid by your local grano man ain't it. Portland cement is a mixture of chemicals, some of which are highly corrosive (slaked lime, alumina, water) to steel. It continues to react for years after placement and any steel in the structure is a candidate for attack. If the rebar in the bridge is rusty, it should be cleaned before placement. They may also be using cathodic protection which will extend the life of the steel.

Quote:

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Originally Posted by JB

Regarding 3/4" dowel holes by a total of 600mm deep...well I hate to reveal my laziness, but I even if I had to drill holes that size and that long in a hard wood I would be looking for alternatives, let alone in concrete. The 900mm strip is on stony clay soil, pretty hard and compact. Can I be excused the dowels?:D

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Not if you want the slab to be properly supported. I don't know that 600 is strictly necessary, though I'd recommend at least 450, depending on the quality of the existing structure. If you have a larger masonry drill, go for larger rebar;).

Quote:

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Wildman, you say rebar needed if no edge thicknessing of concrete. Would it be simpler to just edge thickness? If so, how wide is the thicker section? I take it that it's about 25 mm thicker.

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The amount of thickening will depend on the foundation below the concrete as well as the load you will be driving across it. If you are moving a tractor or other heavy equipment across it, then an extra 50%-100% thickness would be required toward the edge, at least the last 300mm. If it is just a car or caravan, then an extra 25-50% will be heaps. A rebar would reduce the amount of thickening required to some extent but it obviously needs to retain its level of cover. If the base you are pouring the slab on is less stable, then more thickening is required but the slab itself should also be thicker in that situation and will more than likely have a footing around the perimeter anyway.

Quote:

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Even in the absence of spalling or serious rust, surface rust during placement will prevent the concrete from bonding with the rebar, thus reducing the tensile performance of the structure (concrete is great in compression, lousy in tension or bending).

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This is not technically correct, a small amount of surface rust, ie discolouration actually aids the bond with the concrete by micro texturing the surface of the steel and further corrosion is basically stopped by the alkaline nature of concrete as long as there is sufficient coverage of the steel from any ingress of water and salts (very important, there are coverage requirements for different areas of Australia, coastal areas usually require at least an extra 15-25mm coverage to the steel due to the high salt environment). Any rust on the rebar that can come off is very bad, however a light coating is not a problem. It is very difficult to even buy mesh that is completely rust free, this is not an issue in practice.

Cheers
Ben