what slab size relative to thickness ? ? ?

yesitsconcrete · 03-09-2009, 03:44 PM

damn'd if i can remember the rule

does anyone know the aci recommended slab sizes as the dimensions relate to thickness ? ? ? seem to recall a 20:1 rule of thumb - eg, 4" slab should be no more than 6'8" square,,, mesh or rebar wasn't a factor but also ' as i recall '

thanks, guys !

Bob Mariani · 03-09-2009, 04:52 PM

Are you talking about distance before needed crack prevention joints or size before expansion joints? Would not seem right to have a size relate to thickness. A mall floor would have to be 2 feet thick?

yesitsconcrete · 03-09-2009, 05:00 PM

thanks, bobby, yes,,, there's a rule of thumb & i can't recall it 'cause i got too used to reading engineers' plans,,, come to think of it, expansion jnts're really unnecessary in most instances - isolation & contraction, yes,,, how many times do i have to see expansion jnts in sidewalk specs but not in highways, runways, or aprons ? ? ?

Bob Mariani · 03-09-2009, 06:11 PM

They are there in highways but get filled with backer rod, butyl caulking and then whatever the road surface is. Still not clear as to what your reference is asking about

yesitsconcrete · 03-09-2009, 07:53 PM

matter of fact, i started on those very same jnts, mr bob

but NEVER butyl caulk,,, either closed cell backer rod & silicone OR heat-resistant rope & hot-applied astm 3405 or 3406 for jet-fuel resistance,,, asphalt got the 3405 or 1190 w/tape for a bond breaker.

let's try this question - IF you were placing a 4" slab, what's the largest dimension you'd wind up with & what backup information would/could you present to the client justifying the slab size ? ? ?

concretemasonry · 03-09-2009, 10:17 PM

In a typical slab, you generally do not have expansion joints unless you have street or highway!!! - You have control joints to control where the concrete cracks and can be maintained. Concrete does not expand beyond the lenth it is when it is poured. - The curing shrinkage is greater than the temperature expansion.

Joints between a slab and a structure are used to separate two different types of structures.

The problem occurs when the joints get full of "junk" that prevents the slabs to expand back to where they were. - Complicated by the movement up and down caused by loads (especially by repetitive highway loads). For a patio or a driveway, properly sawed joints that are cleaned and calked withing a month or so of pouring minimize movement, dirt and maintenance.

No matter how thick the slab is, it will shrink during curing the same amount. Rebar and wire mesh will decrease the amount of shrinkage cracking if used properly. Fiber mesh is really only good for micro surface cracking and is subject to the correct plant mixing times and placement.

Willie T · 03-09-2009, 11:39 PM

An interesting article on the subject.....

Click Here

Bob Mariani · 03-10-2009, 06:26 AM

I used 2-3 times ( in feet) the depth of the slab (in inches) for control joints. I got this from an article I read from concrete network I prefer to use more joints if customer does not complain too much.

yesitsconcrete · 03-10-2009, 07:35 AM

in the shadows of my memory

seems the thumb rule was 20 x inch thickness express'd in feet / 10 [ 4" x 20 = 80 / 10 = 8 FEET ],,, i gotta start buying those pills they shill on tv )

shoot, didn't even think about conc network & used to advertise on it,,, thanks for the jog, bobby

Willie T · 03-10-2009, 09:44 AM

Quote:

Originally Posted by yesitsconcrete

in the shadows of my memory

seems the thumb rule was 20 x inch thickness express'd in feet / 10 [ 4" x 20 = 80 / 10 = 8" ],,, i gotta start buying those pills they shill on tv )

shoot, didn't even think about conc network & used to advertise on it,,, thanks for the jog, bobby

Pretty sure you meant FEET ? It's OK... the pills start kicking in fairly quickly.

Bob Mariani · 03-10-2009, 09:50 AM

Quote:

Originally Posted by Willie T

Pretty sure you meant FEET ? It's OK... the pills start kicking in fairly quickly.

Oh.. read this too late.. now I have to many joints.. or was this a problem with someone else

Willie T · 03-10-2009, 10:20 AM

Quote:

Originally Posted by Bob Mariani

Oh.. read this too late.. now I have to many joints.. or was this a problem with someone else

Hey, I was already heading out the door to chalk off 50 or 60 more lines on my old drive.

yesitsconcrete · 03-10-2009, 01:12 PM

thanks for the laugh, guys,,, i was head'd out de door at de time

jomama45 · 03-30-2009, 02:14 PM

Quote:

Originally Posted by concretemasonry

In a typical slab, you generally do not have expansion joints unless you have street or highway!!! - You have control joints to control where the concrete cracks and can be maintained. Concrete does not expand beyond the lenth it is when it is poured. - The curing shrinkage is greater than the temperature expansion.

Joints between a slab and a structure are used to separate two different types of structures.

The problem occurs when the joints get full of "junk" that prevents the slabs to expand back to where they were. - Complicated by the movement up and down caused by loads (especially by repetitive highway loads). For a patio or a driveway, properly sawed joints that are cleaned and calked withing a month or so of pouring minimize movement, dirt and maintenance.

No matter how thick the slab is, it will shrink during curing the same amount. Rebar and wire mesh will decrease the amount of shrinkage cracking if used properly. Fiber mesh is really only good for micro surface cracking and is subject to the correct plant mixing times and placement.

I know this is an older thread, but I had to comment. This is probably the most informative & accurite post I have read on this site.

BTW, I have been taught that the ratio is 24:1, or 4" =8', 5" = 10', etc...

yesitsconcrete · 03-30-2009, 04:09 PM

that's nuttin'

you shouldda been here before i lost my memory ! ! !

dick & i usually agree, tho !