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

Yes, the cracking will probably only be a cosmetic issue.

Reo has two functions within a concrete slab -

1. Prevent cracking from spreading, and
2. Provide tensile strength which concrete per se has relatively little of.

For factor 1 placement is irrelevant although the closer to the surface the better as one wishes to limit sub surface cracking for durability and cosmetic reasons.

For factor 2 the placement of reo is critical. It must be placed within the tensile weak plane of the concrete member. Placement within the central neutral zone (for a 100 thick slab this zone is 45 to 55 below the surface) makes the reo virtually useless.

A normal slab with vehicular traffic and non engineered substrate (less than 100% compaction) requires the reo to be placed as close as possible to the lower surface. In order to protect the steel from corrosion the reo is for that and other practical purposes placed about 20 from the bottom surface ie on 20 chairs. Any loading will cause tensile forces along the bottom plane.

If a slab is poured on an engineered substrate (100% compaction or on top of another concrete slab the reo must be placed as high as possible. Once again for corrosion purposes that is normally 20 down from the top surface ie placed on 80 chairs. This is because in this situation the tensile forces at play are along the top. This is caused by the natural curing of a concrete slab to become a saucer and lift at the edges. It is quite astounding how much lift occurs in a slab. Any loading will cause tensile forces along the top plane and the reo has to take up the load.

In my experience, considering the low price for reo in comparison with the other components of a slab it is better to place 2 layers of reo as far down and as far up as practically possible in the circumstances.

In your situation IMO the reo had to be placed as high up as physically possible.

Everyone is getting carried away again.
If you want a factory finish on anything, then the construction industry is the last place you should look.
There cannot be quality control when you are performing your job in the open environment, take a look as you mention, on our freeways.
They would have to be the most controlled environment, as big names and money are at stake,but they still get it wrong by a long margin, so give the little guy some room for error.

Lets think of it this way. A concretor can lift heavy weights, but can an engineer? So my point is, until engineers and every other arm chair critic can lift heavy weights, you will have a gap in reality as big as daylight.
And what the engineer says or asks for and what actually happens in reality are two completely different things, because what is designed in air conditioned comfort, cannot be reasonably expected to be performed in 35+ heat, or can you do it better.
I know, I started a sentence with "and".
Thats because I am a concretor.

Originally Posted by gasman

And what the engineer says or asks for and what actually happens in reality are two completely different things, because what is designed in air conditioned comfort, cannot be reasonably expected to be performed in 35+ heat

What I'm peeved with is that the engineer specified a certain procedure to be followed which the concreter ignored. The engineer then caved in to the concreter & said 'yeah, you don't really need to do such & such'.
FFS, why say it's needed on minute, then say it's not the next .










Anyway, concrete's done. Now to problems with the shed itself now.....


I have 3 1/2 walls up & they've found there wasn't enough sheeting for 4 walls.
It's now coming up to 3 weeks since they've been here. It took a week to order the extra sheets, they arrived 2 working days later but the builder must have forgotten about me.

Originally Posted by gasman

Lets think of it this way. A concretor can lift heavy weights, but can an engineer? So my point is, until engineers and every other arm chair critic can lift heavy weights, you will have a gap in reality as big as daylight.

Engineers see a problem and find a solution. Some, but not all 'Concreters' just get on with it regardless of whether it's right or wrong and typically with an outdated attitude of "that's how we do it". Sometimes old dogs learn new tricks and sometimes they just don't. Ask yourself. Do you work harder or smarter. Eventually the knees go and then it's race for the back. Concreting is a hard job but it's also as hard as you make it.

Originally Posted by gasman

And what the engineer says or asks for and what actually happens in reality are two completely different things, because what is designed in air conditioned comfort, cannot be reasonably expected to be performed in 35+ heat, or can you do it better.

Find another Engineer. Rest assured I would find another concreter. Nothing is impossible. Your statements appear to be advocating laziness and dodgy work. Intentional or otherwise.

Where I work, what is designed and specified in the office happens in the field.

How do I know, I'm an Engineer.

Regards
Andrew

P.S. By the way, reinforcement in a 100 mm thick slab is doing nothing other than minimising cracking. It's contributing almost nothing structurally. In this situation it needs to be in the centre of the slab. 20 mm from the bottom or top is technically flawed for many reasons.

Originally Posted by Scouse

....
Anyway, concrete's done. Now to problems with the shed itself now.....
I have 3 1/2 walls up & they've found there wasn't enough sheeting for 4 walls.
It's now coming up to 3 weeks since they've been here. It took a week to order the extra sheets, they arrived 2 working days later but the builder must have forgotten about me.

Aye Scott, welcome to the world of builders (including concretors). You've had the excuses from dodgy concretors now wait till the excuses for dodgy builders come in!

That's yet another reason why here in Australia we have such high housing unaffordability - the industry is rife with dodgy incompetents.

You just need to say on their case until it's all done properly and in the mean time buy some hair colouring to cover over all the grey hair you'll end up with.

Originally Posted by Daniel

in the mean time buy some hair colouring to cover over all the grey hair you'll end up with.

That's if you've got any left. Probably just another tub of wax at this stage. Get your shine on.

Well, after a phone call earlier this week, they rocked up here at lunch time.


They've put up the missing 1/2 wall & now the builder tells me that there no corner cappings. They were supposed to come with the extra wall sheets (so he says - no mention on the paperwork though).


The corner cappings don't need to go on before the roof but his concern is money. He wants to get paid now & then come back later on to fit the cappings.
FFS, as if he's going to get paid before finishing the job - I can't believe he even asked me.


I offered to pay part as I want to ensure the concreter comes back to finish what was agreed to but, no. So he's gone, left his scaffolding here & say's he'll be back in the New Year sometime...

Hello Scouse,

How goes the slab, is it all better now? We have not had the final chapter yet

Kind Regards
Lionel