Thread: garage slab concrete - subbase

Originally Posted by Exador

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?

Originally Posted by untamed

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:

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

Originally Posted by 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 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)

Originally Posted by johnc

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.

Originally Posted by johnc

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.

Originally Posted by johnc

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

Originally Posted by untamed

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?

Originally Posted by TEEJAY

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.

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

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.

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

Originally Posted by untamed

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!

Originally Posted by Exador

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

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 !

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