Polished slab for extension

[Shedhand]

G'day all.
Looking for some expert advice on the merits of a polished concrete slab versus polished T&G Tas Oak versus a Structaflor floor for our extension.

We have the following quotes for a 40m2 open plan extension - essentially a living entertaining area.

Polished T&G Tas Oak floor = approx $8,500
Polished Concrete slab = Approx $7,000
Structaflor Green Tongue sheet floor = Approx $5,500

Our architect has recommended the polished slab - which we first suggested and still prefer for energy savings reasons - but we've been told by a builder we know that the slabs are prone to cracking. It will be suspended on 3 block foundation walls spanning 2 x 4.0 metres and tied into a Concrete filled block wall. The site is ancient sand dune on massive sandstone bedrock. If this builder is correct what questions should I be asking the architect or engineer to ensure that the job can be done without the slab eventually cracking. Apparently, according to the builder, engineers are prone to demand overspec for slabs which results in excess steel being used where the slab ties into the block wall. The cracks are claimed to appear where there is no steel reo (ie, between where the reo HAS been placed). Clear as mud I s'pose but hopefully someone will get my drift. Any advice will be appreciated.
Cheers:confused:

[Master Splinter]

Concrete, no matter how well specified and poured, will crack, just as sure as wood will change dimensions with the seasons.

Most of the time, big(ish) cracks (more than a few mm wide) show themselves within a few months of the pour as the slab dries...5mm per ten meters is roughly the amount of shrinkage movement in concrete.

The best way to cope with this is to put some control joints into the finished slab. The neatest way is to have a guy with a diamond saw put cuts in as soon as the concrete is hard enough to take being cut cleanly, rather than have the concretor do it with a grooving trowel.

Some months later when you are doing the final tidy-up prior to move in fill the control joints with some epoxy concrete crack filling goo, and bring the boys with the concrete grinders in to do their stuff.

It'd be good to place some MDF into the groove and cover it with a strip of gaff tape to stop siteworkers using it as a convenient fulcrum for straightening bent screwdrivers and so on (edge chipping on concrete looks yuk). Pull the tape and the MDF prior to filling, of course!

[Master Splinter]

Just noticed you are from Tassie - it would be worth looking into putting polystyrene insulation under the slab to keep heat loss down if you want to use it as thermal mass.

[Shedhand]

Just noticed you are from Tassie - it would be worth looking into putting polystyrene insulation under the slab to keep heat loss down if you want to use it as thermal mass.

Thanks for the advice mate I'll put it to the architect to sort it out with the concreter.
Cheers

[Shedhand]

Just noticed you are from Tassie - it would be worth looking into putting polystyrene insulation under the slab to keep heat loss down if you want to use it as thermal mass.

Crikey mate!
It's NOT cold here. It is in the Central Highlands and on the west coast. The climate here is officially classed as Mediterranean. Melbourne is cooler than Hobart (though they would never admit it). I don't know where people get the idea Tassie is cold. Every state has cold places. I lived on the nullabor once - smack in the middle of the desert - and the water pipes used to freeze :eek: . That's never happened here. And don't get me started on Canberra . It's 11.34PM here and its 26C still after 33C this afternoon.
Off the box now. Going down the beach ( 2min walk) for a midnight swim.
Cheers

[ian]

Our architect has recommended the polished slab - which we first suggested and still prefer for energy savings reasons - but we've been told by a builder we know that the slabs are prone to cracking. It will be suspended on 3 block foundation walls spanning 2 x 4.0 metres and tied into a Concrete filled block wall. The site is ancient sand dune on massive sandstone bedrock. If this builder is correct what questions should I be asking the architect or engineer to ensure that the job can be done without the slab eventually cracking. Apparently, according to the builder, engineers are prone to demand overspec for slabs which results in excess steel being used where the slab ties into the block wall. The cracks are claimed to appear where there is no steel reo (ie, between where the reo HAS been placed). Clear as mud I s'pose but hopefully someone will get my drift. Any advice will be appreciated.
Cheers:confused:

You can pour a 4m slab AND not have it crack, BUT and it's a big BUT, your concreter and his supplier both need to know what they're doing, which means it will probably COST extra.

For starters the concrete supplier must use low shrinkage cement, which might mean a special order AND the supplier can't be too free with additatives because some will increase the drying shrinkage.
then the water cement ratio needs to be below about 0.5 (it's a while since I've done this but my memory is that it should be below 0.45) this results in a very stiff mix — you're looking for a slump less than 50mm, possibly as low as 40mm, normal structural concrete has a slump of about 80mm and pumped concrete a lot more. This makes placing the concrete a real challenge for a concrete crew who hasn't done it before.
Once in the form the concrete needs to be well vibrated, probably for five times as long as your concreter is used to AND he needs to use a vibrating screed to get the surface properly compacted AND it shouldn't be done on a day with high evaporation — low humidity and/or a wind.
Lastly, the slab needs to be moist cured for about 15 days, preferably with a membrane to prevent evaporation, though wet hessian can be a substitute.

The only times I've seen concrete crack between the reo bars is when there is something wrong with the mix or its palcement.


ian

[Shedhand]

Excellent info there Ian. Thanks. My architect has told me the guy who quoted for the polished slab is the best in the business and has been doing it for years. I went and looked at a polished slab in a commercial premises in Hobart that he did earlier this year ( a large area on the ground admittedly) and it looked brilliant. I'm going back tuesday the look for cracks.

I'm printing out the words of wisdom you guys have given and will pass them to my architect.
Cheers

[TARLOX]

the Cement and Concrete Association of Australia has a useful document on Polished Concrete Floors. Go to www.concrete.net.au and do a search for Polished Concrete. Hope this Helps

[bitingmidge]

If this builder is correct what questions should I be asking the architect or engineer to ensure that the job can be done without the slab eventually cracking.

Notwithstanding that it is possible (but very expensive and challenging) to prevent cracks in concrete, if you can't live with cracks, don't use concrete.

Small cracks are just part of the character of the material, so no need to get your knickers in a twist over them.

I'd still prefer timber over the concrete as a surface that I can live on though, but then I like to go barefoot.....

Cheers,

P

[Shedhand]

the Cement and Concrete Association of Australia has a useful document on Polished Concrete Floors. Go to www.concrete.net.au and do a search for Polished Concrete. Hope this Helps

Thanks mate. I've printed it out so I can iclude the recommendations in the contract.
Cheers

[Shedhand]

Notwithstanding that it is possible (but very expensive and challenging) to prevent cracks in concrete, if you can't live with cracks, don't use concrete.

Small cracks are just part of the character of the material, so no need to get your knickers in a twist over them.

I'd still prefer timber over the concrete as a surface that I can live on though, but then I like to go barefoot.....

Cheers,

P

I prefer barefoot too. But the SWMBOATT doesn't like bare boards for some odd reason. Small cracks don't worry me its the big ones that I've been told can happen if the job isn't done to proper specs.
Cheers

[PeterR10]

Hi Shedhand,
sorry to hijack thread but we're desperate to find good concrete contractor for a coloured slab in Hobart area. Any chance of a name. I have just registered on this forum and I think I can be emailed if need be.

Regards,
Peter

[Exador]

You can pour a 4m slab AND not have it crack, BUT and it's a big BUT, your concreter and his supplier both need to know what they're doing, which means it will probably COST extra.

For starters the concrete supplier must use low shrinkage cement, which might mean a special order AND the supplier can't be too free with additatives because some will increase the drying shrinkage.
then the water cement ratio needs to be below about 0.5 (it's a while since I've done this but my memory is that it should be below 0.45) this results in a very stiff mix — you're looking for a slump less than 50mm, possibly as low as 40mm

Not with admixtures these days. I've tested special purpose mixes with WC ratios as low as 0.42 and still had the slump spec at 80. Silica fume is your friend, as is a good air entraining agent - look for air content of 4% or so.. What is really critical is that the truck driver doesn't add water on site and that the batch plant is not too far away if you're specifying low-shrinkage stuff.

normal structural concrete has a slump of about 80mm and pumped concrete a lot more. This makes placing the concrete a real challenge for a concrete crew who hasn't done it before.
Once in the form the concrete needs to be well vibrated, probably for five times as long as your concreter is used to AND he needs to use a vibrating screed to get the surface properly compacted AND it shouldn't be done on a day with high evaporation — low humidity and/or a wind.

And (a really important bit) they must place the concrete in its location with shovels/wheelbarrows or the pump - not chuck it in a heap and use the vibrator to make it flow into place. What happens then is that the fines, including the pozzolans (cement, fly ash, silica fume) flow well and the aggregate just fall down where it is, leading to all sorts of problems. Most commercial N25 mixes these days have a higher proportion of fine aggregate (sand) to carry the coarser stuff along, as it's well known that concretors love to use the vibe as a spreader.

Lastly, the slab needs to be moist cured for about 15 days, preferably with a membrane to prevent evaporation, though wet hessian can be a substitute.

The only times I've seen concrete crack between the reo bars is when there is something wrong with the mix or its palcement.
ian

Absolutely spot on. 9 times out of 10, the problem is the driver "pissing it up" on site, with the slump often going as high as 160 - 200, which is virtually water. Apart from the shrinkage and segregation issues this causes, the compressive strength may be as much as halved compared to the spec.

The only other factor is thermal effects and I wouldn't recommend any more than about 3 metres between control joints if you don't want the slab to crack over time, depending on the size of mesh used.

[Shedhand]

Not with admixtures these days. I've tested special purpose mixes with WC ratios as low as 0.42 and still had the slump spec at 80. Silica fume is your friend, as is a good air entraining agent - look for air content of 4% or so.. What is really critical is that the truck driver doesn't add water on site and that the batch plant is not too far away if you're specifying low-shrinkage stuff.




And (a really important bit) they must place the concrete in its location with shovels/wheelbarrows or the pump - not chuck it in a heap and use the vibrator to make it flow into place. What happens then is that the fines, including the pozzolans (cement, fly ash, silica fume) flow well and the aggregate just fall down where it is, leading to all sorts of problems. Most commercial N25 mixes these days have a higher proportion of fine aggregate (sand) to carry the coarser stuff along, as it's well known that concretors love to use the vibe as a spreader.



Absolutely spot on. 9 times out of 10, the problem is the driver "pissing it up" on site, with the slump often going as high as 160 - 200, which is virtually water. Apart from the shrinkage and segregation issues this causes, the compressive strength may be as much as halved compared to the spec.

The only other factor is thermal effects and I wouldn't recommend any more than about 3 metres between control joints if you don't want the slab to crack over time, depending on the size of mesh used.

this is excellent stuff. i'm gonna drive the architect nuts.
cheers and thanks heaps for the sage advice.

[TassieKiwi]

I have more for you - always good to annoy an architect, as they love to annoy engineers!

I have recently completed a topping slab in a 100yr old round reservior, about 1200m2 of 100-170mm thick concrete. Everyone told us it would crack, esp. as the old 300mm thick conc. floor underneath had very bad radial cracks.

poured in late November - no cracks. Not a one. Not even sub 0.2mm ones.

Things you will need to specify:
Design:
Avoid having the new concrete tied to existing, eg pouring on top of a footing. The fresh concrete tries to shrink, the old has already shrunk and this can cause constrained shrinkage cracks.
2 layers of SL62 mesh (used to control cracks)
Concrete:
32mPa
low water content (0.36)
Superplasticiser/water reducer

Pouring - all surfaces to be damp prior to pour. Use concrete pump/hand placement to minimize distance concrete moves afler placing
Use concrete vibrators
We used an evaporation retardant (Masterkure 111CF) sprayed immediately before bullfloating. Helps in final finishing too
CURING
Most everyone doesn't do this correctly. We were able to IMMEDIATELY fill the reservior with 300mm of water - ideal curing! As soon as they can the guys will need to place hessian and wet it down, and cover this with poly sheeting. THe concrete must be kkept wet for at least 7 days, longer the better. Most important. Can use a soaker hose under the poly.

Discuss all of this with the concretor - if you've already seen his work, you're halfway there son!