IF....the settling problem has been understood and mitigated, then the new to old connection can be successfully accomplished by ensuring a very clean surface on the "old" side.  Preferably roughened a bit (some prescribe etching with muratic acid/water mix).  Free from any loose, crumbly material.

If the slab is thin (i.e. 4" or so), drilled dowels probably wouldn't be a good idea.  

We have, in some cases, detailed a thickened edge along the joint, where the new slab increases in thickness by an additional 4 inches over a 12" width.  About 3 inches of the existing grade below the edge of the old slab can also be excavated such that the new slab thickens, and extends a bit under the old slab.  

There are products out there called bonding agents (elmer's glue) that can be applied to the joint prior to concrete placement but I have always had doubts about the benefits of doing so.

If you haven't determined what caused the original settlement, then your new concrete may just continue settling as well, and the joint will most likely re-open.

Since this is a load bearing slab (traffic!)the old concrete should be drilled and doweled, or "shelved".  This "shelf" is created by excavating slightly under the existing slab portion such that when you place the new concrete it will flow under the old slab, creating a support "shelf".  An epoxy or acrylic bonding agent should be applied to the old concrete to provide bond to the new concrete.  Several products are specially formulated for this.  Check www.sika.com for their products.

Ideally you will dowel into the old slab and extend the dowels into your new concrete.  Make sure the dowels placed into the old concrete are greased and unbonded.  You do not want the dowel bonded on both sides of the joint or additional cracks will develop.

Also, if your repair section is long and narrow, expect to get additional transverse cracks unless you reinforce the section.

Good luck.

Ron  

Lets not forget to found the slab on appropriate subbase and subgrade.  Make sure that what material is taken out of the subgrade is replace and compacted not just filled in.  Definitely clean the concrete like JAE says.

With traffic, I would definitely dowel the existing and perhaps grease the ends for the new to accomondate shrinkage and temperature.

Hey guys, thanks for the great info.  As for the settlement, it has been static for 5 yrs.  The contractor has assured me that adequate compression well be done.

To sumarize, I should dowell and shelved the old joint?  The joint by the way goes the whole length of the garage.
And we believe it's a 4" thick slab.

Steve:)

Steve,
The doweling/shelving is an either-or proposition.  No need to do both.  If 4-inch slab, JAE is right...risky to dowel unless you use small dowels.  I have successfully doweled 4-inch slabs, but it requires care.  Use 3/8 or 1/2 inch smooth dowels, greasing the inserted side.  Space dowels at about 9 to 12 inches apart.  No, this doesn't meet pavement design specifications, but it should be fine for your light loadings.  Also, add a small piece of foam at the end of the inserted dowel (about 1/2-inch long) to allow for dowel expansion.

Putting shelf in is easy and doesn't require a lot of brain damage on the contractor.

Also, if you didn't have faulting at the crack (one side lower than other) then you didn't have a settlement crack, though if 2 inches wide....a bit much for shrinkage only!

For bonding, either decide to fully bond the concrete or don't bond it at all!  Partial bonding only creates an opportunity to crack in other places.

Good luck.

Ron

Ron,
Thanks for the advice.  The crack is definitely fault generated, one side is about 2" lower.

I am interested in your bonding paragraph, could you please elaborate. Is it something you apply after the new slab has been poured and dried or do you apply it when you pour?

Steve:)

Steve,
It is applied just prior to pouring.  There are several on the market.  One I'm comfortable with is SikaDur 32, an epoxy resin material (see www.sika.com).  There are numerous latex bonding agents on the market.  Some work well, others don't.  If you prefer latex, I have had reasonably good luck with Thorobond (www.chemrex.com).

The intent of the bonding is to do away with a joint in the concrete. Be sure you dowel with bonded dowels if you select this route.  Don't use unbonded dowels for this case...they should only be used if you have a "working" joint.  Bonded dowels are rebar epoxied into the existing slab and new concrete poured around the rebar.  For narrow trenches, run a continuous piece of rebar from one side of existing slab to the other, epoxied into both, then pour the concrete into the "trench".

Ron

If you have any sawed contraction joints in the old concrete that you will be extending into the new concrete, make sure that your dowels are at least 12 inches away from that contraction joint.  Otherwise, when the new concrete shrinks, there won't be enough concrete to resist cracking caused by the shrinkage against the dowell.  For wheel loads of 10,000 lbs or less, TXDOT uses #5 deformed rebar expoxyed into the old concrete a depth of 6 inches on 12 inch centers.