Control joints in 12" SOG 1

[cec17]

I have an equipment slab that is fairly large 130' x 20' and 12" thick for traffic reasons. I would like to break up the "long" direction with control joints. However, usually the depth of our control joint sawcut is D/4 (and I have seen D/3).

That would put the saw cut in a 12" slab at 3" or 4" deep which is deeper than our top mat of steel. What have you done about control joints in a 12" slab? Just reinforce to 0.5% and don't install control joints? 12" seems like the cutoff zone, where detailing at control joints comes into play due to top mat depth.

 

[dik]

I'd put them at 21'8" centres and use 3" deep sawcuts with reinforcing 4" cover to the top. 0.5% is a lot of rebar...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Tomfh]

We would tend to use cradled dowel joint systems for a slab of that depth.

Continually reinforced pavements (jointless) often have in the order of 1% steel. There is something to be said for omitting joints altogether.

 

[Greenalleycat]

I suspect the depth of the cut doesn't really matter - just set it up to avoid the steel
Alternatively, use crack inducers and that way you know you won't hit the steel

Generally though, the first professional conference I ever went to was for slab-on-grade design and it talked about industrial applications
Sawcuts in industrial slabs (i.e. slabs with forklifts and other traffic over them) being the limiting factor for the slab life was a large focus of the talk
If that is your situation here then you could reach out to a specialist supplier and ask for their control joint input
I know there are guys I follow on Linkedin here that are always touting their control detail products that are designed to help with these situations

 

[Tomfh]

It’s common to just saw cut the steel too. Some people don’t it because the bar ends are exposed, but in practice it seems to work. Of course, sometimes you want the reinforcement intact, for dowel action.

 

[Greenalleycat]

Depends how confident you are in the subgrade I suppose
If you have traffic loads then this seems like a recipe for differential movement leading to the edge fettering away
Maybe a greased dowel bar detail is a better play?

 

[JAE]

Well in highway design, where the pavement is sometimes very thick, the detail is to use smooth dowels in chairs at mid-height and cut as deep as needed.

But then again, most pavements don't have any reinforcement other than the dowels.

 

[Greenalleycat]

The thought of ginormo unreinforced slabs with big loads on them scares my little structural engineering brain
I'll leave that to the expert civil folk

 

[cec17]

Thanks all for response.

@dik I considered increasing cover but something about having 4" top cover in a slab makes me feel antsy about excessive cracking. I don't see anything in ACI224 that provides a MAXIMUM cover in a SOG, though.

We do have construction joints that utilize a greased dowel at mid height. They will also function to allow contraction in the long 'span'.

If that is your situation here then you could reach out to a specialist supplier and ask for their control joint input

Can you elaborate? A specialist supplier for a crack inducer or cradle product?

 

[Greenalleycat]

Well this is the dude I was thinking of - he loves posting about their armouring details for slab pours

https://www.linkedin.com/feed/update/urn:li:activity:7236477736326733824/

Maybe this will give you a steer
I'd be more helpful but this is an area that I only know just enough to be dangerous in...

 

[cec17]

Well now I am in a bit of a rabbit hole on continuously reinforced pavements. CRSI recommends 0.6% longitudinal steel ratio, placed with a minimum cover of 3-1/2". I assume this means we can use 0.6% and split it between both faces.

 

[STrctPono]

Based on the information you are giving, it sounds like you need to follow civil highway PCC pavement design practice. This is actually pretty specialized design work. FHWA has a lot of good design reference guides on the matter. There are even software programs that will help you do the design. A 12" thick pavement is in keeping with the thicknesses you would find in a Jointed Plain Concrete Pavement (JPCP). This is sort of the standard nowadays. Some people still use reinforced panels but that is falling out of favor in most states.

In order to do this kind of design, you need to work closely with a geotechnical engineer that is familiar with this kind of work. They dictate the slab thickness based on the design of the subbase material and thickness given the maximum wheel load.

Jointing is the most important feature of designing PCC pavement (other than proper thickness and subbase design). Figuring out joint locations based on the shape and knowing when to put deformed tie bars or smooth dowel bars across joints is important. Many states use epoxy coated for dowel bars. Smooth GFRP bars are nice in lighter load situations but typically inappropriate for highway type loading. Stay as close to 1:1 with your panel aspect ratio as you can go. I would split the slab right in the middle in the transverse direction and go with 2 panels. In the longitudinal direction, I would go with 13 panels. PCC pavement concrete mixes are also normally specified by their flexural capacity which is different than other structural concrete.

 

[STrctPono]

You should be looking at ACPA, AASHTO, and FHWA for information related to concrete pavement.... I would not use CRSI.

 

[dik]

OP...
Most thick slabs like that, I have done, have been for runways and taxiways for heavy aircraft (747s, etc.). They are often non reinforced. The dowel baskets you reference are common, even for highway work.

As soon as you sawcut the slab, you move out of the 0.5% (25M@12/#8@12) range.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[EngDM]

It’s common to just saw cut the steel too. Some people don’t it because the bar ends are exposed, but in practice it seems to work. Of course, sometimes you want the reinforcement intact, for dowel action.

Saw cut the slab, if it hits rebar whatever. Add a urethane sealant if its exposed to corrosives.

 

[jayrod12]

At 12" thick if you need dowel action, just provide additional dowels at mid-ht. I had a job recently where we did that and the contractor actually left a gap in the top steel and each joint so the saw didn't need to cut anything but concrete

 

[LittleWheels]

Most saw cut contractors don't like the additional blade wear from cutting reinforcement.

 

[cliff234]

We have used these at the control joints in thick heavily loaded SOG's:

https://www.pna-inc.com/products-designs/diamond-dowel-system

When we have 12" slabs on grade we reinforce the slab with continuous EW,T&B reinforcing for crack control.

Just as important, pay attention to the mix design and subbase and subgrade preparation. Use well-graded aggregate to minimize the amount of cement required and specify f'c=4 ksi. (Higher strength concrete requires more cement which leads to more shrinkage, which leads to more cracks.) Use epoxy-coated reinforcing if the slab is exposed to the weather. Interrupt the rebar at the joints.

 

[cec17]

Thanks everyone I really appreciate your input. I am planning to interrupt the top mat and use dowels, basically replicating our construction joint detail.

 

[dik]

You may want to look into epoxy bars... many places have stopped using them.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik