slab on grade without control joints?

[AaronMcD]

Slab on grade, only residential foot traffic. The architect HATES the suburbia look of squares cut into the slab and far prefers cracking. We will have long narrow walkway slabs with no control joints. Only a few expansion joints. But the concern is BIG cracks and semi-regular cracking. Is rebar good enough to keep cracks small and irregular? Or would polypropylene fibers + rebar be better?

http://www.hellodwell.design

 

[dik]

Outside, you need rebar... don't rely on polypropylene... I understand it works by regulating bleed water for a better cure... nothng more. I prefer well defined cracks (aka sawcuts)but have to be done at the right time. You can add as much as 0.5% instead of 0.2% to minimise the crack widths and make them more distributed.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[DaveAtkins]

I believe the theory is if you use 0.5% reinforcing you don't need any control joints, no matter how large or long the concrete slab area.

Of course, concrete has a mind of its own

DaveAtkins

 

[OHIOMatt]

We have done this quite a bit. We like to use small bars, #3 or #4 chaired as close to the top of the concrete as possible. Obviously this will change with location and exposures. Use no less than .5% reinforcing. I also don't use less than a 5" slab, specify a well compacted base, and have a sprinkling of sand added to the surface to help reduce the coefficient of friction between the bottom of slab and the base. Then, weather permitting like to use a moist cure. The cracks that develop, are tight and usually fairly uniform in spacing. Basically, they end up creating a length to width ratio of close to 1.

 

[AaronMcD]

Thanks guys. Looks like the 0.5% near the top is the common rule of thumb. Read that on a couple other sites also.
The ability to tie the slab into other elements seems a good bonus. I don't see base friction as an issue when anticipating numerous cracks, as the slab as a whole shouldn't need to move much.

http://www.hellodwell.design

 

[XR250]

I would think that chairing as close as possible to the top would be asking for rusty rebar and spalling

 

[Settingsun]

Agree the crack expectations in writing. 0.4mm cracks can look bigger than people think they will, and you might get that with 0.5% reo depending on the concrete.

What do you mean by semi-regular? The cracks will tend towards the same spacing.

 

[dik]

If salt de-icing, I'd use 2" cover, else 1-1/2"... to the top...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[MIStructE_IRE]

Can that 0.5% be split between the bottom and top?

Where would I find the 0.5% recommendation written?

 

[dik]

For a slab greater than 8" thick... and then I'd likely use 60-40 for top... that's where the cracks show up...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

from ACI 360R-27... to a point, concrete shrinks and cracks, and the more reinforcing steel you have, the finer and more distributed the cracking will be... I don't know where the .5% comes from... it 'always has been'.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Settingsun]

See here for an Australian document which gives indication of crack width vs reinforcement quantity:

https://www.eng-tips.com/viewthread.cfm?qid=479073

 

[ATSE]

Rebar for crack control can be found in ACI 350.
Better, 0.6% vs 0.5% for lengths > 50ft.
I would not run any exterior exposed concrete more than 100ft, even if 0.6%. As the saying goes, concrete doesn't always follow the law of large numbers, and the concrete doesn't know which code you used.
You need to inform the architect about consequences of hating joints. Shrinkage and joints will defined happen, either planned or unplanned.
The intent of planned joints is that you get a "straight crack." Otherwise, you will get an irregular (unsightly to most) crack.

 

[AaronMcD]

@ATSE, The architect knows there will be cracks and prefers natural cracking to straight cracks in this setting. He said cracks are fine, just not the big nasty ones where slabs move.

@steveh49, I said semi-regular, but I meant I don't think they'd like big natural cracks forming obvious sidewalk squares.

http://www.hellodwell.design

 

[dik]

In these environs, it could cycle through 50C to -30C...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik