Concrete slab control joints 1

[red22]

I am working on an elevated concrete tunnel, for which the floor must meet "superflat" requirements. The tunnel width is 20' and is 250' long. The floor is a cast in place concrete, one-way slab with reinforcing in two mats (each direction) top and bottom. Is it at all realistic to try to cast this slab without control joints?

 

[Ron]

If you don't care about cracks, yes. If you care about cracks, no.

 

[rowingengineer]

I'm confused, oneway slab on ground, what do you mean?

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[hokie66]

Yep, confused. He didn't mention "ground".

 

[conceng]

I picture it as being supported on each side by beams or walls the entire length, making it a 1-way slab.

Can't quite picture what "(each direction)" means, unless it just means there are two mats with reinforcing steel equally spaced in each direction. Regardless, it will crack, and the crack spacing will depend on the depth of the slab, the mix design and exposure conditions.

Greg

 

[dik]

Why superflat for a tunnel floor? The added cost will likely be $1 to $2 per square foot for this finish.

Dik

 

[rowingengineer]

I must have still been asleep.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[hokie66]

The crack spacing and width will also depend on the amount and size of the bars.

 

[dik]

250 ft is way too far to not have joints, no matter how much reinforcing you throw at it. You might want to re-assess your requirements.

What sort of environment is the tunnel in? Subject to thermal movement? You will have to break it down into a minimum of half a dozen sections and throw in a pile of rebar to minimise crack widths. If a true 'super flat' floor is required, you will have to take extraordinary care at the joints.

Dik

 

[red22]

Apparently the original post was not quite clear enough. It is an elevated concerete slab supported on CMU/CIP concrete walls. Reinforcing is top and bottom in each direction.

As Dik mentions I was hoping to use a lot of extra reinforcing to minimize/eliminate control joints, but figured that this distance would be too far to go. Hariline cracks would not be an issue. The tunnel is unheated/outdoors and will be subject to thermal movements to make matters worse.

Thanks all, particulary Ron, very insightful....

 

[dik]

So the problem is more complex... it appears to be a structural slab and you are trying to accommodate elastic and long term deflections and achieve a superflat floor... Many flatness standards require flatness measurement to be made within the first couple of days...

Good Luck!

Dik

 

[JedClampett]

Maybe you can contact someone who represents a manufacturer of Type K cement. The slight expansion of the concrete might eliminate the need for standard control joints, but for thermal you're on your own. I've never used the stuff myself, but it seems interesting.
These guys are always looking for good example projects, so I'll bet they'll be helpful.

 

[tadeng]

What about using a topping slab? You could cast the suspended slab and then put down a waterproofing membrane to prevent cracks translating through. Then pour a topping slab on top and put sawcuts in the topping slab. You would have more control with the topping slab.

If the slab was subject to heavy traffic, this may not work though.