Concrete cracking

[Ben537]

Hello,
I recently saw this forum and that I might get some good information. I hope someone can help. My question is. What is the best method of joining old concrete and new concrete to prevent cracking? I recently purchased an old house and I'm in the process of adding a full bath and perimeter drainage system in the basement. I had a good friend {who is a plumber} help me saw cut the floor, and remove the sections of concrete. The sections that were removed go from the foundation block walls out 18 inches and run around the perimeter of the house. I am now left with the job of pouring the new concrete. I have seen many jobs where contractors will drill holes into the old concrete and install rebar into the pre drilled holes. What is the best overall method for pouring new concrete and preventing it from cracking where the new and old concrete join? Will the rebar restrict the concrete from contracting as it cures and cause it to crack? Thank You

 

[haynewp]

I use smooth rods to connect new to existing slabs. Something standard is smooth 1/2" diameter A36 rods about 2'-0" long at 18" o.c. Drill into the existing slab, grease the rod side going into the existing and place the rod into the existing slab leaving some of it sticking out into the new portion to be poured. This will allow shear to transfer across the 2 slabs while reducing restriction during shrinking.

 

[JAE]

Ben537
If you are simply replacing an 18" wide strip of concrete all around the perimeter of your basement, then a doweled connection to the existing floor slab may not be really necessary. In most cases, dowels in slabs are used to do one of two things, first - to prohibit vertical differential movement between the old and new concrete, which would create a small trip hazard across the joint.

Second - as the concrete cures, it shrinks to varying degrees and fully bonded dowels may help to keep the crack/joint closed.

Neither of these purposes are needed in your case as the old slab portion has already cured out and most of its shrinkage has occurred and the new 18" wide strip is so short (18&quot that the accummulation of shrinkage will be quite small. Thus your joint gap will remain small. Also, for a residential basement you really don't have significant vertical loads imposed on the slab so differential movement across the joint shouldn't be a big problem. At the very least, you could put back a slightly thicker slab in the 18" and undermine the edge of the existing slab such that the new concrete provides a small shelf underneath to limit any differential movement.

 

[shgizmo]

Ben537
Probably the remaining perimeter gap (depending on how much depth was removed from the gap)and assuming a depth of 0.25m could be filled with two layers of fill or soil each evenly compacted to a level up to 0.05m lower than the existing concrete slab then either place fresh sand and tile the gap with the rest of the basement or just pour lean concrete over the compacted soil. In my point of view if you dont have any loaded work benches or other heavy equipment placed around the perimeter of the basement that's the way I would go.

 

[haynewp]

Is your basement wall supporting soil? The slab section you removed may have been helping to support the base reaction and shorten the wall span.