Crack Control for Deep Slab on Grade 1

[MagedPEng]

Gentlemen
We have a case of an 8 feet deep reinforced slab on grade that is going to carry train rails.
The structural drawings specify a maximum pour length of 25 ft in order to have a construction joint every 25 ft for the crack control.
Is ther's any way to run away with a pour length of 100 ft (contraction joint location) as one pour.
Please help

 

[Ron]

Post-tension the slabs.

or....

saw-cut joints every 20 feet and backfill the joints with semi-rigid epoxy.

 

[ToadJones]

8 foot deep slab?

 

[MagedPEng]

Thanks Ron Do you have a reference for that solution

 

[MagedPEng]

I am talking offcourse about the saw cut

 

[ToadJones]

so the slab in plan is 8' x 100' ?

For and 8ft width, I would think you'd have control joints at 16' intervals. They could be hand tooled at the time of the pour. The contractor I use will not wait to saw cut on a slab with that aspect ratio....he tools control joints in 2:1.

 

[MagedPEng]

Why no one wants to beleive that the slab thickness is 8'.
On the structural drawings the slab pours are 70' wide by 30'long while the depth varies between 5' to 8'. This is to carry trains running with high speed.

 

[ToadJones]

This might qualify as a mass concrete pour.

I'm not so certain you'd want to call this a slab.

The largest foundation I have ever poured was 100' ft long, 18' wide and 8' thick. I had to put thermocouples all through it and monitor the temperature to ensure that the thermal variation in the concrete was not too high.

Saw cutting an 8'ft thick slab won't do much unless you have a big a$$ saw.

 

[ron9876]

You will have to put in enough reinforcement to resist the shrinkage stresses. Also might help to have a couple of layers of vapor barrier so it is easier for the slabs to slide on the soil and relieve the stresses.

 

[hokie66]

MagedPEng,
You should be asking your questions of the design engineer, not this forum. Personally, I doubt the joints are necessary, but I won't be making the decision.

 

[ztengguy]

I want to be the concrete supplier!

 

[hawkaz]

Following up on ToadJones' point- How are you managing the thermal issues created by the chemical process for concrete this thick?

If I remember correctly, at the Hoover Dam, they limited the pours to 5' thick, and still had to add piping to pump cool water thought the concrete so they could regulate the temperature.

 

[MagedPEng]

we are pouring in cold weather and even though we have to insulate the forms and wrap the site and use heat blowers

 

[Ron]

I would agree that the joints are not necessary; however, the question at hand was how to increase the placement and, presumably, maintain cracking within the joint spacing that would have been there otherwise.

Sawcut joints would be about the only means of mitigating cracks. The depth of the sawcut would have to be relatively deep or a weakened plane for the concrete could be cast in place...in either case there's not much else you can do to stop cracking. Reinforcing will not stop the cracks.

 

[ron9876]

But reinforcing will help keep the cracks tight.

 

[ToadJones]

IMO, you will not be able to use cuts to mitigate cracking....the "slab" is too thick.
I also dont beleive that you will have any cracking problems with proper reinforcing.
The foundation I mentioned above did not crack anywhere and what minor imperceptible cracks there might have been were held tight by reinforcing.
The biggest problem is temperature variation across the cross-section, not just the heat, but a large variation from the heat in the center to the concrete on the surface.
We were able to mitigate these affects by using heaters and blankets on the outside and by designing the mix properly with fly ash etc.
You need to read up on mass concrete pours....your engineer shoudl have already designed the mix.