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GalileoG (Structural)
5 Jul 10 11:56
I am curious as to what a typical detail is when it comes it doweling rebar into an existing slab-on-grade. Do they drill a hole on the edge of the slab and epoxy rebar in the holes or do they saw cut the slab from the top and place the rebar that way? How is it usually done? Does anyone have a standard detail they would like to share or communicate to me?

Thanks!

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

Ron (Structural)
5 Jul 10 12:05
Drill holes from the side, epoxy the rebar in the hole, place new concrete against old.  I usually specify a small blockout for a sealant joint at the top, unless there is no need for a joint.

Make sure the hole diameter is in accordance with the epoxy manufacturer's recommendations.  It makes a difference in the load capacity of the dowel, particularly in tension.
Helpful Member!  GalileoG (Structural)
5 Jul 10 12:30
For an exterior slab on grade, I have heard sometimes that they do not even epoxy. They would just place the rebar through the hole since we only require shear action. Have you seen this done before or do you always epoxy?

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

Ron (Structural)
5 Jul 10 13:06
If the slab is a rigid pavement, we do not epoxy the dowels...in fact we grease the inserted side and provide a small expansion space at the end to allow for movement.  In short, pavement dowels only transfer shear and do not need to be epoxied.
Ron (Structural)
5 Jul 10 13:27
ps...dowels used for pavement are smooth dowels...dowels used for bonding concrete are deformed rebar.
msquared48 (Structural)
5 Jul 10 14:05
If your are talking about a 3.5" to 5.5" sog, forget it from the side as you will more than likely spall the concrete during the drilling process.  Better in those slabs to sawcut into the top of the slab, place the rebar in the slot and epoxy fill back.  8" and thicker from the side, far better chance of no spalling.

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

GalileoG (Structural)
5 Jul 10 18:13
Thanks all! Very informative.

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

hokie66 (Structural)
5 Jul 10 18:32
Another thing to consider is that the new slab will shrink, so the new dowels will restrain this shrinkage parallel to the joint and tend to cause cracks.  Thus, it is wise to keep dowels well clear of the ends or corners.
Ron (Structural)
5 Jul 10 20:03
I guess this posting didn't get through when the site went down for a while today...

You can use plate dowels instead of rod dowels, particularly for thin slabs.  A saw-cut is made into the side of the exposed slab, and a plate dowel is placed, then the new concrete placed adjacent.  Works well for those slabs in the thickness range Mike noted.
hokie66 (Structural)
6 Jul 10 2:38
To expand on Ron's suggestion, using a diamond shaped plate dowel would also alleviate the problem of cracking in the new slab which I mentioned.  It would also limit the length of sawcut required.
asixth (Structural)
6 Jul 10 6:38
How would a diamond plate dowel behave when it is post-installed into an existing slab?

http://www.danley.com.au/Product_Joints_Diamond_Dowels.htm

Whenever doweling into an existing slab on ground, I have always provided round bar dowels, 16mm diameter (5/8"), hot-dipped galvanized, 450mm long (18"), 200mm (8") into the existing slab and spaced at 1000crs (3').

I have never give any consideration to the shrinkage of the new slab or spalling of the existing slab due to inadequate edge distances during drilling.  
hokie66 (Structural)
6 Jul 10 8:27
As Ron said, you could sawcut a groove in the edge of the slab for the plate to fit in.  Not sure what type saw, but I imagine there is a suitable one.  Then set the plate into the groove with sand filled epoxy.

The old slab has already done its shrinking, or most of it, and the new slab will want to shrink, both normal and parallel to the joint.  The idea of diamond dowels is that shrinkage in the perpendicular direction also provides clearance to allow the parallel shrinkage to occur without stress.  This can also be accomplished with square dowels with compressible material on the sides, and these are also furnished by Danley.

I haven't used plate dowels in thin slabs, and would be concerned about consolidation under the plates and also that the concrete on top of the plate would break out.

The size and spacing of dowels, regardless of the type, needs to be related to the slab thickness and loading.
civeng80 (Structural)
6 Jul 10 21:03
hokie66

Ive always used R20 dowels at 600mm centres in 150mm RC concrete pavements used for industrial applications.  Ive just drilled holes and inserted the dowels, always thinking of shear transfer, never of shrinkage parallel to joint, but I guess for long joints it would be significant. Also Ive attended quite a few workshops on pavement design and the point was never discussed.

Maybe I'll bring that up in my next workshop. (It's a very good point !)

Cheers
hokie66 (Structural)
6 Jul 10 21:45
civeng80,

What you have been doing is fairly typical, but the shrinkage parallel to the joint can be a problem, especially if there is some time between pours.  The corners are the most vulnerable for cracking if dowels are placed too close.  The Danley site which asixth referenced has some good information, and the Danley folks will probably come and give you a lunchtime presentation if you wish, and may even bring sandwiches.
Helpful Member!  csd72 (Structural)
13 Jul 10 11:40
Civeng80,

Shrinkage along the joints can be a significant issue especially at the corners where a diagonal crack often forms.

Best to avoid placing too long a section at once and also other items of good practice to minimise this..

I would recommend against dowelling into a 150mm slab as it very often breaks out the concrete. Undercut the slab and pour a lip of the new slab under the existing.

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