Reinforced SOG CCJ Spacing

[STR04]

All,

I need to increase to joint spacing for a reinforced SOG and not follow Table 9.9 from ACI 360 for plain concrete. Has anyone used the drag formula (eq. 6-3 from ACI 360) to back into a joint spacing? This procedure is noted in Section 9.9 but was wondering how others look at this situation.

TIA

 

[STR04]

Say you have a 120'x300' area and assume the slab is placed in two pours. For eq. 6-3 would you use L=120' or your CCJ spacing?

 

[miecz]

I use a drag formula published by PCA. It's probably the same formula, but I don;t have ACI 360. Can you post formula 6-3?

 

[STR04]

As = FLw / (2 x fs)

where As is the amount of reinforcing in sq. in. per lin. foot slab width

F = coefficient of friction between base and slab (typically 1.5)
L = slab length between free ends, feet, in the direction of the steel
w = weight of the concrete slab, psf (usually 12.5 psf per inch of thickness)
fs = allowable steel stress, psi 24,000 psi for 60 grade steel

 

[miecz]

Yes, this is the same formula and, I use it to back into joint spacing. In fact, I use it to eliminate joints entirely, as slabs often don't crack at the joints anyway. If your slab is placed in two 120'x150' pours, with an expansion/isolation joint in between, where the concrete and reinforcing are both interupted, I would use L=150 ft. If you are simply going to saw cut a joint and pass the reinforcing thru, I would use L=300 ft.

 

[STR04]

For my slab the thickness is 5" throughout. Per ACI 318 Section 7.12, As=.0018*bh = 0.11 si/ft. If I used ACI 360 for 150' length, As=0.29 si/ft. You state that "slabs don't crack at the joints anyway". Does this mean you accept random cracking or that in your experience with using ACI 360 that your slab does not crack at all?

 

[miecz]

When I say that slabs often don't crack at control joints, I mean that I've seen many slabs with properly cut control joints that are cracked as though the control joints didn't exist, th ecracks sometimes running near and parallel to the joints. By increasing the slab steel according to the drag formula, I intend to prevent large random cracks. I expect that the slab will have small random cracks, but that they won't open to the point where they are unsightly.

 

[bjb]

The corps of engineers has a publication that may be of help, and you can download it from their website. Check out ARMY TM 5-809-12, "Concrete floor slabs on grade subjected to heavy loads"

If I remember right, the subgrade drag equation does not result in enough steel to permit increasing joint spacing. If you do try to increase the joint spacing by using reinforcing, you won't prevent random cracking in between control joints. The steel will just hold the cracks that do develop more or less tightly together. More tightly if you have a lot of steel.

Regarding control joints not functioning as intended, this sounds like they are being cut too late. We use the Soff-Cut saw, and they can be on the slab cutting about 2 hours after finishing is completed. That way, the slab is cut while still green and before random cracks start to develop. We have had excellent results with this saw.

 

[STR04]

bib,

At your sawed crack control joints are you allowing all the reinforcing to run across the joint or do you cut every other bar or use some other procedure. I've seen it both ways and was wondering if you got onto the slab early enough then precutting bars would be eliminated. Any thought?

STR04

 

[bjb]

at our control joints in building slabs-on-grade, we don't have any steel going across the joint. In fact, we generally don't put any steel in our building slabs on grade. The steel is expensive, and in the case of wwf it usually ends up in the subbase anyway where it is not helpful in crack control.

However, sometimes we do put rebar in if there are unusual conditions or heavy loads, or whatever. In these conditions, we don't have any rebar going across the joint. That way, each slab panel has less restraint to shrinkage, and therefore, will be less likely to crack. In my opinion having all of the rebar go thru the joint renders the joint ineffective. If for structural reasons you do want some continuity between the slab panels and you can tolerate some cracking, then I would go with having every other bar go through. That way, you get at least partial benefit because you still have a plane of weakness where the shrinkage forces can be relieved.

You can also reduce the probability for the slab to have random cracks by using the largest practical coarse aggregate. We also use a w/cm of 0.45 max, and frequently use fly ash or ground granulated blast furnace slag, and have had good results.