concrete / shrinkage vs strength gain

[boffintech]

The particulars:

1. parking deck elevated slab 9” thick with 5” drop panels
2. columns F’c = 10,000psi
3. slab F’c = 5,000psi
4. actually placing concrete F’c = 7,000psi to eliminate puddling 10ksi column mix at column heads during slab placements
5. five levels of parking deck are below street level
6. a permanent shotcrete foundation wall borders 3 sides of the below street level slabs
7. the slabs will be tied to the fdn wall with #4s @ 12” crossing a 3’ wide pour strip and having a 2’ splice
8. the specs read to not place concrete for the pour strips until 28 days after the PT is stressed.


The question:

The 7ksi slab mix is high-early type III which rockets to 4500psi in 24 hours. By 7 days the breaks are >6,500psi. The 24 hour breaks are super accurate; I’m using Command System and 6x12 cylinders kept at similar temps.

Considering that the PT is stressed the day following the placement and considering the rate at which the concrete gains strength is there any reason to wait beyond 7 days to place concrete for the pour strips?

More shrinkage? Equilibrium?

 

[hokie66]

Restrain shrinkage is the problem, and I consider that the specification is appropriate, although I would wait as long as possible to cast the pour strips. Six months would be good, but I realize that is not practical in most cases.

The walls restrain the slab, inevitably leading to cracking of the slab. Strength of the concrete has very little to do with it. The earlier the pour strip is cast, the more cracking will occur. I hope the pour strip has enough reinforcement in the longitudinal direction to control the cracks to an acceptable width.

I assume you are using the ACI Code, and I don't know its requirements for column force transmission through the slab. If you were in Australia, the code here allows for transmission without "puddling" as long as the column/slab strength ratio does not exceed 2.0, and provided the column is fully restrained by the slab.

 

[JAE]

I could be wrong, but doesn't the higher strength concrete equal more cement in the mix, which in turn means higher levels of shrinkage? I know that the aggregate gradation, curing methods, etc. all affect shrinkage.

Check out this article re: a similar project.

http://www.cement.org/buildings/buildings_hotel_denver.asp

It appears they fiddled a bit with the mix to minimize shrinkage in an undergroung parking garage similar to yours.

Here's the relevent paragraph:

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With specified concrete strengths from 4,000 to 10,000 psi, this unique and complex project afforded many opportunities for innovation and for the use of specialized concrete admixtures. The concrete specifications were performance based to ensure the durability of the below-grade, post-tensioned parking garage floors. The concrete supplier developed a mix design to limit the expected shrinkage to 0.05%. Through a collaborative effort with the contractor, slab support details were modified and improved to allow the slab to shorten with minimal restraint. As a result, the garage floors (which measure 400 feet in length by 200 feet in width without an expansion joint) are crack free and expected to perform without maintenance for many years.
[/blue]

 

[hokie66]

I think the 7000 psi concrete would definitely shrink more than the 5000 psi concrete, and if your code will allow it, I would stick with 5000 psi as I suggested above. JAE is correct, concrete technology and curing can assist with limiting shrinkage, but not eliminate it.

 

[boffintech]

The code will not allow the use of 5ksi concrete in the slab without puddling at the columns. The ACI Code allows for transmission without "puddling" as long as the column/slab strength ratio does not exceed 1.4. The EOR allowed the 7ksi in lieu of puddling. 7000x1.4=9800=close enough

BTW, the strip is 3' wide with 3 #4 longitudinally.

 

[VTPE]

I did (2) similar garages. The first, we tied the slab into the foundation wall and spent the next year chasing cracks. The wall will be extremely rigid, even after the shrinkage is out, the slab will tend to move. If it is restrained, you will get cracking.

The second garage we detailed a joint at the wall to allow the slab to slip. This was very tricky and led to some other design opprotunities for the lateral system, but the result was far less restraint cracking.

Anpother thing to think of in the below grade slabs is the stability of the slab in compression. The soil loads will add to the compression forces and could potentially buckle your slab. Combined with the prestress force, you have to be careful the slabs don't become unstable when not loaded. We had to thicken our slabs to make them work.

 

[hokie66]

VIPE has some excellent points, but they are for the designer, and I gather from your posts that you are not the designer.

Again, for the designer, I think the delayed pour strip will crack noticeably in the transverse direction, because the 3-#4 are much too light to control the cracking which will occur when the strip shrinks. It will be restrained on both sides. The good news is that the cracking should mostly be confined to the pour strip.

 

[wadakis]

I have used a 56 day delay for the pour strips in similar cases. About half of the total slab shrinkage should have occured in this time. The concern is not only cracking, but P/A washing out into the walls. I would require a mix design with as little water as possible (30-34 gallons) and require shrinkage testing (ASTM or SEAOC). Then run the numbers and see what kind of cracks you'll get. We have had good results with shrinking reduction admixtrures (Tetra Guard?). No matter how much steel you throw at it, it will crack. Depending on your lateral system, you can wrap the dowels with foam to allow for additional shrinkage after the pour strip has been placed.