new patio concrete too high

[Terrian]

An old engineer that I hired once told me that he likes the mesh to be no more than 25mm from the top of the slab, because when the slab cracks, as all slabs do, the cracks will be in the bottom of the slab and not the top. This bloke knew concrete - he was the engineer at the Sydney Show Ground and was in charge of making all the suspended concrete ramps and so on that the horses walk on.

He'd been an engineer for 50 years, so who was I to challenge his opinion

to get open cracks on the top in the middle, the ends of the slab have to drop, or the middle has to raise

[Terrian]

Who said I was a builder at all? I'm just a 15 year old kid playing on his parent's computer.

[autogenous]

journeyman Mick: Bad move in my opinion. I was taught a long time ago to build and detail any penetration/possible water entry point as if sealant didn't exist. Sealants can and do fail, gravity never does. If the water can't get under the threshold (sills are under windows, thresholds under doors) then it won't matter if the sealant breaks down. If it can get to the threshold there's a good chance you'll get water into the house even with sealant.

Sure Id have at least a 20mm step down from the brick sill. In this case the concrete is level with the sill.

There is fall in his concrete. Silicons are pretty good, especially under cover.
Theres more than one builder who do a level straight through to FFL.
I wouldn't say its great practice but under cover you have two chances of water ingress.
Fat chance and none.

If theres that much water in that area with overhead cover then theres bigger issues of concern.

In this case grinding 15 mm off the top will be a hell mission. It may also reduce the concrete thickness below its structural need.

I agree journeyman Mick, your definitely right. Build with failure in mind.

[silentC]

to get open cracks on the top in the middle, the ends of the slab have to drop, or the middle has to raise

Yup I'll admit right now I'm no expert on concrete. Only just mastered the broom finish on small areas

I think the type of crack he was talking about are the hairline ones you get as the concrete contracts, not open gaping cracks. The idea of the mesh being near the top is that it holds the top of the slab relatively intact and allows the bottom to expand and contract. I should have paid more attention, I was still trying to deal with the concept of the stuff moving once it has set.

[autogenous]

To deal with surface shrinkage cracks is to run a sprinkler on the concrete 10 hours after the initial pour for 24 hours.

Mesh prevents cracking or crazing placed in the place of tension which is 25-50mm below the surface.

There is modern fibres etc which can be added to the concrete to prevent cure crazing cracking.

Too much water can cause shrinkage cracks.

Too much heat such as 38 degree days and over accelerates the curing leading to cracks.

Modern concrete building structure floors are surface sprayed with a resin to slow the evaporation from the surface of the concrete to prevent cracking.

In the past some concrete pads especially multistory were ponded with a mix of 3:1sand/lime around the edge then filled with water to an approx half an inch. Once the slab had been ponded for a few days the lime sand edging was removed with a scraper.

[sports fan]

Who said I was a builder at all? I'm just a 15 year old kid playing on his parent's computer.

i didnt say you were a builder your obviously far far from it, i said you sound like a textbook builder

[silentC]

Yep you're right, I'm not a builder, never said I was, but I do hate dodgey work. I've seen plenty of she'll be right behaviour from builders and tradies too, so don't think that just because you're one of the most respected builders in the business or once worked for one that you're against cutting the odd corner, right?

You just keep hacking at those slabs, mate, she'll be right. Just keep away from my place and we'll all be happy, OK?

[Terrian]

Yup I'll admit right now I'm no expert on concrete. Only just mastered the broom finish on small areas

broom finish is good, provides some grip when walking, and getting the brushed look is an art in itself

I think the type of crack he was talking about are the hairline ones you get as the concrete contracts, not open gaping cracks. The idea of the mesh being near the top is that it holds the top of the slab relatively intact and allows the bottom to expand and contract.

OK, I think I see where he is coming from, I personally would be adding 2 layers of reo, top & bottom if I was that concerned.Or I would use just the 1 layer of reo, and have fibremesh added to the mix

I should have paid more attention, I was still trying to deal with the concept of the stuff moving once it has set.

[Terrian]

Mesh prevents cracking or crazing placed in the place of tension which is 25-50mm below the surface.
There is modern fibres etc which can be added to the concrete to prevent cure crazing cracking.

there were times when we would add fiber glass fibers (about 6" long) or steel 'pins' (about 1mm thick, 5" long) to the mix in the barrel, mainly when to pond pours, no need for traditional reo to be used