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RE: Slabs on grade at garage and people doors

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Option 1 with some caveates.
I would be most concerned about differential vertical movement at the joint either due to settlement or soil heaving.  To control this, I would suggest backfilling with 3/8" crushed rock (not gravel).  The crushed rock compacts well, the angularity of the crushed rock holds the aggregate in place pretty well while providing a sufficient void to mitigate heave.  Compact the crushed rock with a vibrating compactor.
Thicken the slab, tie the interior slab side to the foundation wall with hooked rebar, provide joints to minimize shrinkage cracking, add rebar to control the shrinkage cracking.  The outside slab can be tied to the interior slab with square dowels and PNA square dowel clips which will allow for differential in-plane shrinkage. 

Harold Sprague


From: Jim Wilson [mailto:wilsonengineers(--nospam--at)]
Sent: Tuesday, January 08, 2008 9:04 AM
To: seaint(--nospam--at)
Subject: Slabs on grade at garage and people doors
I'm looking for some opinion about slabs on ground at door openings - Which detail has more pros or cons?  Are there other details that perform better than these?
The typical slab detail in this case is to have an expansion joint at the perimeter to allow it to float inside the foundation wall.
1. Drop foundation wall at door 8"-12", extend slab over and pin to wall with reinforcement bent out of the wall and into the slab.
2. Drop foundation wall at door 8"-12", extend slab over with a horizontal slip joint between the wall and slab to allow the slab to shrink unrestrained.  Additional reinforcement to be provided in this section of slab to mitigate cracks.
This is in non-seismic areas, northeast U.S., light to heavy commercial use.
Jim Wilson

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