ACI 360 slab on grade crack control reinforcement

[TS59401]

I am working on an industrial floor slab design that will also serve as liquid containment during equipment washdowns. The slab has been designed as non-reinforced for non-uniform distributed loading and wheel loads per ACI 360. I would like to extend the distance between contraction joints (beyond the 36 to 24 times slab thickness graph) to reduce the number of joints that will need to be sealed. I noticed that ACI 360 offers up using 5% steel reinforcement to eliminate contraction joints. Is there anyway to estimate the steel I would need to double the joint distance (for unreinforced slabs) without providing the full 5% to eliminate the joints?

Thanks in advance,
Tom

 

[DaveAtkins]

Yes--it is called the subgrade drag formula.

As = (F*L*w)/(2*fs)

As = area of steel required
F = 1.5
L = length between control joints, feet
w = weight of slab, psf
fs = allowable steel stress = 0.67*fy

DaveAtkins

 

[TS59401]

Thank you DaveAtkins. I have never heard of that formula.

 

[hokie66]

Are you sure about the 5%? Maybe 0.5%?

 

[DaveAtkins]

hokie66 is correct--if you use 0.5% reinforcing steel, you don't need any control joints, no matter how large the slab.

DaveAtkins

 

[hokie66]

The 0.5% Ag, or actually I prefer 0.6%, controls the width of cracks. Cracks still form, but they are very fine.

 

[dik]

What are you containing and do you have a surface coating. Can you use a vandex admixture to the concrete mix?

Dik

 

[TS59401]

We are containing wash down water from a granulation and dryer combination. The sloped floor allows the water and material to flow into floor sumps and be pumped to a storage tank. Vandex seems like a good product, but what I am really after is limiting contraction joints for my 6in floor slab.

Using ACI 318 minimum of 0.18% the subgrade drag formula tells me that I can space my free joints at 90ft intervals. That seems pretty far, but I assume I can space them at 30ft to 40 ft without large random cracks.

Thanks,
Tom