# Forum Home Renovation Concreting  garage slab concrete - subbase

## XT4V

I am pouring a 100mm slab on M class soil which is "moderately reactive" from memory with clay and all that other crap in there  :Cool:   
I am going for 25mpa as my father in law owns a concrete truck and drives for [will not plug name here for certain reasons  :Smilie: ].  I guess it will be a bit stronger in the long run.  I think the reo he recommended was F52 or something? 
Question for you smart blokes: how much subbase (crushed rock) should I be putting under the slab if any?  The ground is very firm, I kid you not, it is like concrete at the moment!  How do you concretors compact it, with a basher or just stamp on it?

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## Exador

> I am pouring a 100mm slab on M class soil which is "moderately reactive" from memory with clay and all that other crap in there   
> I am going for 25mpa as my father in law owns a concrete truck and drives for [will not plug name here for certain reasons ].  I guess it will be a bit stronger in the long run.  I think the reo he recommended was F52 or something? 
> Question for you smart blokes: how much subbase (crushed rock) should I be putting under the slab if any? The ground is very firm, I kid you not, it is like concrete at the moment! How do you concretors compact it, with a basher or just stamp on it?

  What's going on the slab? Is it going to have vehicle traffic? How big is it? My advice is to be more concerned with footings than the subbase, just 100mm of clean sand or crusher dust will be fine for that. Yes, it should be compacted, although this is much easier with sand as you only need a hose and a vibrating plate compactor. The crusher dust should be done with a Wacker Packer or a roller, preferably vibrating.  If the underlying soil is the natural material (not controlled fill compacted in layers), you may need some thickening beams to prevent the slab from cracking with soil movement. There is also the issue of subsurface drainage, to make sure that the soil under the slab doesn't get constantly wetted and dried.

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## XT4V

crap. not the answer i was looking for :eek:  
I have 100mm clearance now which is enough for the concrete, which means I need to go down another 100mm. 
The size is approx 7.5 x 3.5 meters. It will have vehicles up to 2.5 tonne on it.

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## Exador

> crap. not the answer i was looking for :eek:  
> I have 100mm clearance now which is enough for the concrete, which means I need to go down another 100mm. 
> The size is approx 7.5 x 3.5 meters. It will have vehicles up to 2.5 tonne on it.

  Make sure you use 25MPa and you might like to consider 2 layers of mesh, top and bottom, depending on how concerned you are about cracks. Is there a building going on it or is it just a driveway?

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## XT4V

It is just the slab in the garage, no buildings on it.

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## johnc

100mm is a bit thin for double mesh if looking for strength just go up a bit in mesh size. Check your local council for their regs on footings and slab thickness, that will be the best place to start. Plenty of depth at 100mm to support 2.5 tonne but shed loads will have some bearing on slab design as will soil type. The local building inspector is really the best place to start, at least that gives you the minimum standard which will be sufficient for most purposes. 
John.

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## DanM

Most carpark pavement are 150mm thick with 82 mesh,  62 or 72 mesh is heaps.  !00mm thick is fine on good ground,  i would have a thicknessing beam around the perimeter and where a dummy joint is to be trowelled eg 200mm thick and 300 mm wide.

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## boban

This answer is based on having worked with my father (a concretor for over 30 years).  
Never have we put 100mm of sand or gravel dust (what we generally use) below a slab. About 50mm is enough. Its generally used to level off as sand/gravel is cheaper than concrete. Do not be concerned if you have a 30mm cover somewhere and 60 elsewhere. Just get is close to level. Use a string line and measure the clearance you have.  
Then the plastic and then use F72 mesh. F82 if you want to be sure. If its one of these colourbond sheds then just thicken the edges with a 200mm deep trench (overall). If its brick go to 450. Dont forget the trench mesh or bars. 
A really good idea would also be a 900mm path right around the garage.  It helps keep the water away from the garage

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## Exador

> This answer is based on having worked with my father (a concretor for over 30 years).  
> Never have we put 100mm of sand or gravel dust (what we generally use) below a slab. About 50mm is enough.

  Depends on the subsoil and the moisture likely to get under. The subbase is there to keep moisture away as well as act as a cushion over the clay.   

> Its generally used to level off as sand/gravel is cheaper than concrete. Do not be concerned if you have a 30mm cover somewhere and 60 elsewhere. Just get is close to level. Use a string line and measure the clearance you have.  
> Then the plastic and then use F72 mesh. F82 if you want to be sure. If its one of these colourbond sheds then just thicken the edges with a 200mm deep trench (overall). If its brick go to 450. Dont forget the trench mesh or bars. 
> A really good idea would also be a 900mm path right around the garage.  It helps keep the water away from the garage

  All good advice, but I strongly disagree with you about the importance of a good sand/crusher dust layer.

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## DanM

I agree exador!  U normaly put sand down to stop the plastic membrane from being pierced from the subgrade, which u only need for a dwelling not pavement.  U can wet the subgrade before pouring concrete to stop it drying out to much.  u normaly put crushed rock down when the ground is no good or you have dug to deep.

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## XT4V

how important is the plastic membrane? my concretor hasnt mentioned anything about it, so i am presuming he isnt going to be using it?

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## Exador

> how important is the plastic membrane? my concretor hasnt mentioned anything about it, so i am presuming he isnt going to be using it?

  It's pretty important. He is going to use it, as pouring directly onto sand would be nearly unworkable due to the water in the concrete draining away, leaving dry cemented aggregate - not pretty.

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## XT4V

> It's pretty important. He is going to use it, as pouring directly onto sand would be nearly unworkable due to the water in the concrete draining away, leaving dry cemented aggregate - not pretty.

  got my subbase down, approx 60mm of crushed rock. 
with regards to the plastic membrane, on the concrete.net.au website, it states it is 'optional' and is only to reduce friction under the slab. Doesnt really state that the concrete will be unworkable or leaving a dry cemented aggregate.  However, what you have stated Exador makes a little sense. 
Here is a link to the referenced information, http://www.concrete.net.au/viewpdf.p.../Driveways.pdf have a read of it and let me know what you think?

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## Exador

> got my subbase down, approx 60mm of crushed rock. 
> with regards to the plastic membrane, on the concrete.net.au website, it states it is 'optional' and is only to reduce friction under the slab. Doesnt really state that the concrete will be unworkable or leaving a dry cemented aggregate. However, what you have stated Exador makes a little sense. 
> Here is a link to the referenced information, http://www.concrete.net.au/viewpdf.p.../Driveways.pdf have a read of it and let me know what you think?

  All good information, except that they're suggesting a very thin sand layer as it is only a driveway, therefore not much water drains away, therefore the concrete remains wokable. In your case, use the plastic - it's cheap and you'll have a better pour and a better slab for longer. It's always down to that cost tradeoff in the end.

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## johnc

The plastic membrane is simply a barrier to prevent ground moisture carrying up through the slab. When pouring the plastic serves as a barrier to prevent moisture getting away thus slowing down the rate of cure. In summer this is a help in winter a pain. Not sure about concrete becoming too dry for lack of a barrier, that would mean the ground beneath was capable of wicking away moisture very quickly and could be aleviated by drenching the ground before the truck arrived. I'd only use plastic if the area is covered such as a garage, for outside driveways and paths I have never used it myself nor ever seen it used by others. Incidently sub base when dug below ground level can become a sump holding water every bit as much as an aide to drainage when the surrounding ground acts to absorb or carry away moisture. 
John.

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## Exador

> The plastic membrane is simply a barrier to prevent ground moisture carrying up through the slab. When pouring the plastic serves as a barrier to prevent moisture getting away thus slowing down the rate of cure. In summer this is a help in winter a pain. Not sure about concrete becoming too dry for lack of a barrier, that would mean the ground beneath was capable of wicking away moisture very quickly and could be aleviated by drenching the ground before the truck arrived. I'd only use plastic if the area is covered such as a garage, for outside driveways and paths I have never used it myself nor ever seen it used by others. Incidently sub base when dug below ground level can become a sump holding water every bit as much as an aide to drainage when the surrounding ground acts to absorb or carry away moisture. 
> John.

  As I said, John, it's a cost/quality tradeoff. The recommendations from the CCA are for minimum cost/minimum acceptable quality. I do agree with you that plastic is not essential, but it is extremely beneficial. Having, on many occasions watched 80 slump concrete become 60 slump in a matter of a couple of minutes after being placed on the ground, I wouldn't accept a pour without it, especially considering the minimal cost. It's the sand layer which wicks it away if no plastic is in place. Under a building it is essential.  Your comment about excavated subgrade holding moisture is also very true, which is why I usually recommend(ed) a subsurface drain (agg line) along the high side of any such excavation. This, together with the plastic membrane, stops any moisture changes in the subgrade, and thus any shrinkage/swelling. 
The bottom line for me is that the concrete and steel are expensive, whilst all of the preparation measures are cheap. It doesn't take an Einstein to work out that spending a little to protect a lot is a good idea. Don't skimp and don't necessarily accept what the concretors do, that will always be the minimum and they don't care what it looks like in 5 years as long as it's OK when they get paid. I've seen some absolutely horrible concreting that looked great on the surface...

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## XT4V

> All good information, except that they're suggesting a very thin sand layer as it is only a driveway, therefore not much water drains away, therefore the concrete remains wokable. In your case, use the plastic - it's cheap and you'll have a better pour and a better slab for longer. It's always down to that cost tradeoff in the end.

  whats the cost of the plastic, do bunnings supply it?

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## vsquizz

> whats the cost of the plastic, do bunnings supply it?

  HD Black polythene for grano workers is $66.00 per 100 metre roll and 2 metres wide....Do not buy from Bunnings..any good builders hardware will sell you it by the metre for a coupla bucks more.  Its also available in 4 metre wide rolls of a lesser length for around the same price.  Its generally folded in half so a 2 metre wide roll will only be 1 metre wide...if that makes sense:confused: ...theres a roll in the boot of my car..amongst other things....:eek:

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## johnc

Craig and Untamed, 
The cost of black plastic is nothing, probably less than the cost of a counter meal even at Bunnings rates. You would not leave it out on the basis of cost. I would never have considered using it on a driveway in the past but after reading the arguements for it's use would consider that the use of plastic has merit. The speed of cure has a lot to do with final strength and minimisation of cracking. Cracking will also occur with sand with to high a clay content or organic matter present in the aggregate. The old concretors would slow down the cure by covering with hessian and hosing down, a practice you no longer see around here. My experience is garage slabs, driveways and like Boban learnt the skills from my father which was a mix of hand mixed small pours and solid plastering but small areas. You can't beat a truck mix for consistancy and if chasing a set slump and MPa rate only a truck mix will achieve it. 
You have nothing to loose using plastic, coming into winter the concretor may want to add some dryers to the mix to get it to set before dark, which does involve a bit of a strength trade off I believe. However one of our relatives used to use a brickies trowel to punch holes in the membrane of house slabs to get the mix to dry at a faster rate on house slabs. I have often wondered how many people discovered mouldy patches of carpet and wondered how it happened. Just thought I would throw that in. 
I recently put in a single garage slab of 6m x 3m for a mate and the plastic cost $18 off the hardware store rack and that is the most expensive way to buy it. If you are going to go to the expense of 25MPa there is no reason to skimp on either reo or if you think it suitable the plastic. 
John

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## Exador

> Craig and Untamed, 
> The cost of black plastic is nothing, probably less than the cost of a counter meal even at Bunnings rates. You would not leave it out on the basis of cost. I would never have considered using it on a driveway in the past but after reading the arguements for it's use would consider that the use of plastic has merit. The speed of cure has a lot to do with final strength and minimisation of cracking. Cracking will also occur with sand with to high a clay content or organic matter present in the aggregate.

  The most important reasons I can see are:
It maintains the w/c ratio of the mix as designed when placing it, simply because it prevents water draining away or being picked up.
It prevents the sand from being picked up and creating pockets of weakness when it gets picked up in lumps. The same goes for clay if no sand was used, of course.
It creates a long-lasting moisture barrier that protects the slab from a wet subgrade and the subgrade from a wet slab. On driveways it's the latter.
It creates a shear plane that helps to prevent cracking due to differential drying shrinkage, thermal stresses and the movement of the slab under load. 
Your comments regarding aggregate are also spot on, although if buying batched concrete the agg will have been tested for each of those properties and a whole lot more (AS 1141 covers concrete aggregates)   

> The old concretors would slow down the cure by covering with hessian and hosing down, a practice you no longer see around here.

  It's still used on large slabs although it's largely been replaced by the use of wax curing compounds that are sprayed on after the pour and have a life of perhaps a week. They will UV degrade and can be easily washed off with a gerni. Some means of water retention is very desirable if you want to achieve design strength in a reasonable time-frame. Many engineers now specify higher-strength concrete to provide a FOS in the knowledge that both construction times and curing are being cut shorter and knowing that the stronger mix design will achieve the desired strength more quickly.  
 My experience is garage slabs, driveways and like Boban learnt the skills from my father which was a mix of hand mixed small pours and solid plastering but small areas. You can't beat a truck mix for consistancy and if chasing a set slump and MPa rate only a truck mix will achieve it.   

> You have nothing to loose using plastic, coming into winter the concretor may want to add some dryers to the mix to get it to set before dark, which does involve a bit of a strength trade off I believe.

  Depends, really. Not much of a problem here in Qld, funnily enough. :Biggrin:

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## XT4V

> I recently put in a single garage slab of 6m x 3m for a mate and the plastic cost $18 off the hardware store rack and that is the most expensive way to buy it. If you are going to go to the expense of 25MPa there is no reason to skimp on either reo or if you think it suitable the plastic.John

  hey bud, I am not trying to skimp on anything, I have gone F72 reo, I think the recommended was 52? Just wondering where to buy it as I work full time and we are pouring this Saturday.  So bunnings would be my best option I guess.  The thing is, i dont need a 100m roll of course  :Smilie:

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## TEEJAY

> hey bud, I am not trying to skimp on anything, I have gone F72 reo, I think the recommended was 52? Just wondering where to buy it as I work full time and we are pouring this Saturday. So bunnings would be my best option I guess. The thing is, i dont need a 100m roll of course

  Look up steel reinforcing in your local telephone book. A supplier of this may be able to sell you a smaller amount of the polythene sheeting you are after. Recommended to tape the joints too. 
For sure SL52 mesh is light on - it is about half the amount of steel as in SL72 mesh. 
I don't remember from this thread (and it is too long to reread) but you have edge beams or thickenings?

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## XT4V

> Look up steel reinforcing in your local telephone book. A supplier of this may be able to sell you a smaller amount of the polythene sheeting you are after. Recommended to tape the joints too. 
> For sure SL52 mesh is light on - it is about half the amount of steel as in SL72 mesh. 
> I don't remember from this thread (and it is too long to reread) but you have edge beams or thickenings?

  no there are no edge beams, but the front of the slab has been depened to 300mm for strength as advised by the architecht.

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## TEEJAY

Raft slabs are called a "raft" cuz they have a stiff perimeter beam and a thin slab between the beams. The stiff element is to all of the perimeter. 
It is normal to have a thin slab between walls with the walls continuing to the strip footing but if the slab is not between walls on strip footings then it is normal to have integral edgebeams.  
See attachment for code requirements for raft slabs

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## ausdesign

The 100mm concrete can be laid directly onto the existing base. There is no need for sand unless the grounds uneven & you want to save on concrete. a plastic membrane is not needed but if you want to convert the garage into a 'habitable' room in the future as many do then put it in.
Sounds like the garage walls are constructed. If so you don't need any beams. (The 100 thick should sit on the strip footing across the garage opening.) 
Good practice to place expansion material between concrete & bricks.
SL72 mesh.

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## XT4V

Thanks for everyones input.  I have approximately 50mm of crushed rock as the sub base and have put down the polyethene plastic which only costed $40 from trusty old bunnies. 
We are putting the concrete down on Saturday, so I will let everyone know how it went  :Biggrin:

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## Exador

> We are putting the concrete down on Saturday, so I will let everyone know how it went

  You shouldn't have said that! Don't you realise it's now going to start raining on Friday night, with the result that your excavation will look like a swimming pool on Saturday morning? Never tell the rain gods anything!

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## XT4V

> You shouldn't have said that! Don't you realise it's now going to start raining on Friday night, with the result that your excavation will look like a swimming pool on Saturday morning? Never tell the rain gods anything!

  I will take photos of the swimming pool hahaha  :Biggrin:

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## ausdesign

I don't think he has any problems Exador. The rain gods are too busy watching my place. Every time I order in a load of water it rains & we're now getting low. But I'm going to fool them this time with a quater of a load !

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## XT4V

if I go to my block tonight and do a rain dance, I think it will definately NOT rain  :Cool:

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## Exador

> But I'm going to fool them this time with a quater of a load !

  You do realise you've just condemned us all to drought for the next umpteen years, don't you?

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## shep

> You do realise you've just condemned us all to drought for the next umpteen years, don't you?

  gday exador
a drought would be welcome in the top end i just checked the rain gauge 253mm last 24hr. hope it stops soon.
shep

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## Exador

> gday exador
> a drought would be welcome in the top end i just checked the rain gauge 253mm last 24hr. hope it stops soon.
> shep

  Good for shed time, I would have thought. Too much of a good thing is still too much, I guess.

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## XT4V

we came, we poured, we kicked its ass.. it did rain in the evening, but not heavy enough to come into the garage and damage it. 
all went well, will post some photos next week  :Biggrin:   
next to be done, exposed aggregate driveway & path  :Cool:

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## Exador

> we came, we poured, we kicked its ass.. it did rain in the evening, but not heavy enough to come into the garage and damage it. 
> all went well, will post some photos next week   
> next to be done, exposed aggregate driveway & path

  Good to hear all went well. Keep that concrete moist for a few days and you'll have a magnificent slab for years.

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