Thread: Laitance on a new slab - how to approach removal/repair?

Curing oil or visqueen stops the water evaporating so you don't have to keep it wet

AM

Somehow, I don't think the concreter is going to have 225sqm of hessian handy.


He's out today to finish off the formwork so I'll just casually mention tomorrow's forecast & ask him if the heat will be a problem.


He'll be going over the court order allotted time to have this finalised anyway so I'll see if he wants to postpone it.

Originally Posted by p38arover

In the past few weeks, a footpath has been laid down our street - often on very hot days.

I walked along it today. I was surprised at how much cracking there was on the panels. I wondered whether the concrete quality was poor or whether it was due to the temperature.

I think Council specify a concrete that cracks & lifts within a year or so .

Temperature per se is not a problem - the problem is the difference between the under slab temp (ground temp) which is usually about 20°C and the surface temp. It should never be any more than 10°C maximum.

Therefore it is not good practice to pour and lay concrete at anything above 30°C air temp. Any competent concretor/engineer would know that.
Concrete curing is a chemical reaction which is temperature dependent Otherwise the surface cures far too fast compared to the underside and problems will result.

Also the higher the air temp and the lower the air humidity then the more curing problems will result. Once again as soon as the surface of a new slab is floated it must be irrigated and get 100% damp for at least 7 days. In practice concretors do not do that and less than optimum concrete surface strength is the result.

Originally Posted by Ancient Mariner

Curing oil or visqueen stops the water evaporating so you don't have to keep it wet

AM

evaporation is not the main issue - concrete cure is a chemical reaction that requires water input constantly throughout the process - the hotter the concrete is then the faster the reaction and more water is utilised in the process - that's
yet another reason to not pour concrete on a hot day.

Originally Posted by Ancient Mariner

Curing oil or visqueen stops the water evaporating so you don't have to keep it wet

AM

Personally I would go both and have done so many times. Belts and braces method. Two thumbs up.

Originally Posted by Daniel

Temperature per se is not a problem - the problem is the difference between the under slab temp (ground temp) which is usually about 20°C and the surface temp. It should never be any more than 10°C maximum.

Temperature difference is important.

Further info. Two situations. Mass concrete pours and bog standard stuff such as house slabs.

Mass concrete pours require you to maintain the temperature to ensure a good result. Mass concrete pours can be defined as a pour in dimensions greater than 1 metre in all directions. Mass pour temperatures will get up to 80 degrees (depending on the project specification) and this temperature then must be managed for >7days, ideally ~14 days. The centre of the slab will be 80 degrees (like the core of the earth is the hottest) and outer surface has the potential to be the coolest if not managed. There is a skill to manage the core and external temperatures and the variance is what needs to be managed. The longer it takes to control the temperature decline the more you prolong the follow on trades.

Slabs are kept warm by retaining the formwork and insulating the top and potentially the sides. The ground surface acts as the insulation on the underside.

With regards to ground slabs (not mass pours) I would dampen the substrate. Ensure the concrete is delivered at a temperature <28 degrees. If you can get it at 20 degrees then all the better. Don't accept any concrete over 35 degrees (this is in the Australian Standards).

Originally Posted by Daniel

Therefore it is not good practice to pour and lay concrete at anything above 30°C air temp. Any competent concretor/engineer would know that.

Can be done though and quite successfully. Try Darwin. High temperature and humidity and no problems. Not everyone can pour at night.

Originally Posted by Daniel

Also the higher the air temp and the lower the air humidity then the more curing problems will result.

And also wind and most importantly the temperature of the concrete. Temperature of the concrete is the most important of all followed by humidity, wind and the air temperature. It can be 40+ degrees if you have looked after the other three. Concrete has been placed successfully in Equatorial Guinea and Alaska. Two extremes.

Originally Posted by Daniel

Once again as soon as the surface of a new slab is floated it must be irrigated and get 100% damp for at least 7 days.

Don't agree, mainly because it subject to the performance of humans (humans are lazy and unreliable). Basically get rid of the human and get rid of variability in the work and end product. Visqueen does this job for you and so do curing compounds.

Regards
Andrew

Read the following.

Holcim Australia - Curing of concrete techniques

[ame]http://www.cementandconcrete.com/wp-content/uploads/2011/05/UNO-PJ-Plastic-Shrinkage-Cracking-ACI98-vol95-M342.pdf[/ame]

Was driving down the Pacific Hwy a couple of years ago on a very hot day... a major concrete pour in the northbound lanes was underway...to reduce the temperature they had a simple idea... two BOC tankers carrying liquid nitrogen were in attendance... was worth stopping at a discreet distance and watching the technique... passenger's unthinking response "Cool"!

Well, the concrete slab has been done.


When I asked the concreter on Thursday if the heat would be a problem, he said "should be OK, better than rain".
I also reminded him about cutting every 2nd rebar along the proposed cut lines (as per the engineer report). He was amazed that this would be done & rang the engineer to confirm.
He didn't say anymore to me & left an hour or so later. I had a look at the reo & saw that it had been cut as per the report.


Come Friday morning, he's finishing off the reo & blowing off the leaves so I thought I'd ask him which bonding agent he's using.
"Bonding agent? What bonding agent?"
I said "as per the report".
Aah, he said. I spoke to the engineer about that yesterday (Thursday) - because we're laying a 100mm slab, he said I don't need a bonding agent.


I call the engineer - yes, he spoke to the concreter but only an hour before my call.
So the concreter lied to me about having it cleared the day before.


I can't say I'm impressed with the engineer. Most of what he recommended has been ignored & he seems OK with it .
Laitance still present on 99% of slab & no bonding agent used.




Anyway, back to the pour.


One half of the slab was done by mid morning. By this time the temps were well into the 30s.
Then they had a break of what seemed like 30-45 min between trucks before doing the other half.


The concrete trowelling was finished by around 1pm & the formwork came down at the same time.


They had nothing to cover the slab with, nor any sprinkler set up so they used my soaker hose along the centre line of the first half poured. This didn't cover much of the area.


They left by 1.30 & told me the leave the second half of the slab another 30min before using the sprinkler.


2.00 comes around & the soaker hose is doing stuff all so I use the hand held hose instead, gently spraying the surface.


2.30 comes & I have to go to see my son be presented with an award at school (see, there is a smart one in the family!).


Back home by 4.30 & I water the slab again.




The next day, I walk on the slab to carry out the hose down & find the surface cracking up (so it appears). 90% of the slab is covered in what looks like cracks but you can't feel them.


Googling this seems to indicate it's a common occurrence but normally doesn't present a strength issue.


Since then, I've been hosing the slab 3-4 times a day & the cracking doesn't appear to be getting any worse so I'm hoping it will be OK.