[yesitsconcrete]

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]

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]

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 ? ? ? :huh:

 

[Bob Mariani]

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 :huh:

 

[yesitsconcrete]

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

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]

An interesting article on the subject.....

Click Here

 

[Bob Mariani]

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]

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

 

[Willie T]

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

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

 

[Bob Mariani]

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

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

 

[Willie T]

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

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

 

[yesitsconcrete]

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

 

[jomama45]

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. :thumbup:

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

 

[yesitsconcrete]

that's nuttin' :yes: you shouldda been here before i lost my memory ! ! ! dick & i usually agree, tho !

 

[vikireed]

concrete slabs/pads

Hey guys, I wanted to possibly pour a concrete porch-I love the low long wide concrete porch i've seen on many craftsman style homes-I dont ever see expansion joints or saw marks for control on those. Am I right in reading that after pouring and sawing the control joints-you fill those cuts with caulk (if so what kind?) and then epoxy paint the whole slab so it looks solid?

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.

 

[yesitsconcrete]

the avg porch floor's thickness is 4" here,,, that'd mean no,,, slab larger'n 8',,, eg, if you ave a 16' deep porch, you'd need to either form OR sawcut a jnt from side to side then slice 'n' dice 8' wide,,, if you haven't seen 'em, there're crks instead,,, we use silicone install'd per mfg's recommendations over either backer rod OR closed-cell backer rod,,, imo, there's no PAINT that'd be permanent,,, howzabout acid-stain & sealer ? ? ?

 

[wildcat]

There is no rule of thumb that relates the square footage of a slab on grade to its thickness.

If a slab is a one way structural slab, spans only one direction, then the depth will be dictated by the length of the span and the loading applied.

 

[yesitsconcrete]

therefore a slab-on-grade can be unjointed & as large as possible ? ? ? 100' x 1000' x 4" ? ? ? you lost me & a bunch of other guys who're aci members,,, i never saw a slab that only spann'd 1 direction,,, if i'm placing a clear-span structural slab, i'm using steel - usually 2 mats of # 6 depending on length of span,,, knew i shouldda save those bdge plan :laughing: i'd think a porch is much different, no ? ? ?

 

[jomama45]

Boy, this thread is getting confusing! :laughing: I think the origonal ? pertained to the spacing of control joints relative to slab thickness, I think? Agan, my understandig has always been 24:1, & an aspect ratio of 1.5:1 maximum, meaning no square shall be more than 1.5 times longer than it is wide, or vice versa.