dowel spacing question
dowel spacing question
(OP)
Hello,
I have a dowel question for everyone. I am saw cutting an existing slab(150'x240') with a 9' wide x 220' long cut and putting back 18" of reinforced concrete to support some new loads. The existing slab is inside of a building and is also 18" thick; however, the existing slab does not have any reinforcing, which is why I am removing it. I need to epoxy a piece of rebar into both sides of the existing slab at the saw cut to reduce the bearing pressure of the new equipment and reduce the amount of concrete that has to be removed and repoured. I have roughly 1500-2000 lbs/ft of shear across each side of the joint. I planned on putting one dowel at mid-depth. I used SAFE to analyze the new slab with soil springs. Anybody have any suggestions on how to calculate spacing, size and embedment for my dowels? I don't believe shear-friction is an option as the sides of the cut will be nice and smooth.
Thanks in advance.
Schoolie
I have a dowel question for everyone. I am saw cutting an existing slab(150'x240') with a 9' wide x 220' long cut and putting back 18" of reinforced concrete to support some new loads. The existing slab is inside of a building and is also 18" thick; however, the existing slab does not have any reinforcing, which is why I am removing it. I need to epoxy a piece of rebar into both sides of the existing slab at the saw cut to reduce the bearing pressure of the new equipment and reduce the amount of concrete that has to be removed and repoured. I have roughly 1500-2000 lbs/ft of shear across each side of the joint. I planned on putting one dowel at mid-depth. I used SAFE to analyze the new slab with soil springs. Anybody have any suggestions on how to calculate spacing, size and embedment for my dowels? I don't believe shear-friction is an option as the sides of the cut will be nice and smooth.
Thanks in advance.
Schoolie






RE: dowel spacing question
In Russia building design you!
RE: dowel spacing question
RE: dowel spacing question
RE: dowel spacing question
I would think shear friction would still be applicable - there is a factor in the ACI equation for smoothness-roughness with a smooth surface.
Alternatively you could shot blast the surfaces or hand chip them - perhaps below the top surface - such as leaving a 1 1/2" wide smooth strip at the top and roughening the lower 16.5 inches.
RE: dowel spacing question
Dik
RE: dowel spacing question
Yes, I read it that way also. Like dik, I wonder why the existing slab is not good enough. I wouldn't necessarily take "SAFE's" word for it.
RE: dowel spacing question
Per ACI
Concrete placed against hardened
concrete not intentionally roughened ................ 0.6λ
Do ya'll agree that this covers a saw cut edge in order to use shear friction? I need to transfer some of the load into the adjacent slab for soil bearing purposes.
Thanks,
RE: dowel spacing question
RE: dowel spacing question
RE: dowel spacing question
I am trying to transfer some shear across the joint from the new pour to the existing, unreinforced slab. Minimal shear, but some. On the order of 1k-2k/dowel at 24" O.C. I am using the dowels to reduce the soil bearing pressure on the new strip footing and to take care of any differential settlement between the footings. I think its prudent to make sure there is no tripping hazards. No moment transfer at the joint, just some shear. I have to keep the bearings below 1000PSF. Soils as not so good in the swamp lands of South Louisiana and they may settle at different rates if no dowels are used. Using 11.6 of ACI with the friction factor of 0.6λ, the #4 bar works fine if I can assume that this applies to saw cut edges. I could bush the edges down, but that is a lot of work. I appreciate the thoughts guys.
Schoolie
RE: dowel spacing question
You stated that you only need shear transfer, and shear transfer is achieved in slabs on ground by dowel action, not shear friction. I would definitely use larger bars than 1/2", but they would be smooth dowels.
RE: dowel spacing question
I would bite the bullet and tell your client you have to cut out for a pad-and-grade-beam foundation sized for the
soils you have, maybe reinforced with 2" pin piles, then pour the foundation against asphalt fiber board on the old,
so there is zero load transfer. Not telling you want to do, just some advice from 30 years doing heavy industrial SE.
If you get differential settling from relying on that unknown slab, the annual trackage inspection on the crane will
tag it out, and then your client is gonna be robo-calling you to pay for the crane trackage re-adjustment every year.
You will be amazed at how much trackage inspectors and crane adjusters charge by the hour, ...and at their own pace.
How much money do you have in your wallet?
RE: dowel spacing question
RE: dowel spacing question
On the shear friction issue, I'm not sure I agree with hokie66's comment that, ..."and in plain concrete, there is nothing to resist the development." Hopefully, ya'll can enlighten me. You can resist tensile forces from an anchor or a piece of rebar in plain concrete. The shear friction method uses the two concrete contact surfaces to transfer the shear across the joint.. ACI says, "For concrete cast against hardened concrete not roughened in accordance with 11.6.9, shear resistance is primarily due to dowel action of the reinforcement and test indicate that reduced value of 0.6λ specified for this case is appropriate."
Vn = Avf*phi*fy*u, u= 0.6λ(friction coefficient)(ACI 11-25)
2kips*1.5 = Avf*0.9*60ksi*0.6*1.0
Avf = .083in2 or a #4 bar
I can see the dynamic effect petertor is referencing.
So, using Hilit HY 150 max, #4 with 14" embedded into existing, plain concrete develops the full bar, at which point, the shear is transferred through the face of the concrete shear planes in contact. I would roughen the saw cut side and provide additional dowels at the column locations. I know I can develop the bar on the new 9' wide foundation side.
ACI 11.6.8 says "Shear friction reinforcement shall be appropriately placed along the shear plane and shall be anchored to develop fy on both sides of the embedment, hooks, or welding to special devices."
Sorry to rehash, and I'm sure you guys may be right, but I don't understand where this logic fails?
In any case, if ron and hookie66 are correct, could you guys point me to any reference to help me design these smooth dowels for shear, greased up on one side?
Many thanks,
schoolie
RE: dowel spacing question
My 2 cents..
RE: dowel spacing question