Connecting Driveway to Foundation with Rebar

[medeek]

I've seen this done a lot while working for a general contractor, however recently I'm told it is a bad idea and frost could destroy the foundation stem wall so its better not to connect the driveway slab to the foundation wall. Detail shown below. I've also seen a lot of concrete "experts" promote connecting in this manner or with dowels. Thoughts and comments appreciated.

 

[CELinOttawa]

Bad idea... The reliable function of this detail is heavily dependant on the quality of the compaction of fill and little settlement of the foundations. Many residential structures settle, at least in the majority of soils, and this detail is wholly unforgiving of settlement.

 

[JAE]

or frost heave.

 

[Splitrings]

Really a bad idea. I'll take a picture tonight of my driveway just outside my garage door. It has heaved 8-10".

 

[medeek]

Take a look at these two links:

[URL unfurl="true"]http://www.standard.net/stories/2014/02/26/proper-installation-key-driveway-lasts-longer[/url]

[URL unfurl="true"]http://www.douglasfoundationrepair.com/the-anatomy-of-a-driveway-pour-austin-concrete-raising-or-mudjacking/[/url]

 

[Archie264]

Add me to the list saying it's a very bad idea. It's going to extra effort and expense to make something worse. In addition to the issues of settlement and frost-heave as noted above, there is also the issue of the connecting rebar rusting and that rust traveling into both slabs and the foundation wall.

Be wary of any recommendations coming from someone who thinks this is a good idea.

 

[TXStructural]

What purpose is the connection intended to serve? It will not restrain a heaving driveway. You detail an expansion joint, but with a rebar connection, there will be no movement across the joint. If you want to allow horizontal movement at the joint, use de-bonded smooth dowels. Or better yet, form a ledge in the stem wall so the driveway cannot drop but can lift.

 

[oldestguy]

One way to avoid or at least minimize the heave is to replace the subgrade soil with non-heaving soil. I'd remove the original soil next to the garage down about 3 feet or so. Extend this excavation out at a taper depth to perhaps 8 or 10 feet or to the first joint. Replace with clean sand or clean sand and gravel. At least some compaction is needed, but not necessarily 95 percent. If you have the ability to drain it, fine, but water being present may not be bad. The reason for this is that when it freezes it gives off heat. That helps to reduce the depth of freezing. However, heave is caused by the build up of ice lenses in fine grained soil. Clean sand may freeze, but heave, if any is very slight. An interesting fact: Depth of freezing in clean sand is deeper than in silty soil for same surface situations. Why? Silty soil has more water per cubic unit. The more water, the more heat given off when freezing occurs. It is not unusual in cold areas to have deeply buried frozen water pipes in sand country. Now a problem in my sandy area of Wisconsin due to this cold winter. Cities are recommending "let the water run".

 

[Splitrings]

If you were concerned about the concrete driveway settling at the garage, cast a 10" thick wall, leaving a 4" shelf for the slab to rest on. Use an expansion joint material on the vertical portion of the shelf and a bond breaking material on the horizontal shelf. The risk will be the settled fill under the driveway concrete.

 

[medeek]

I'm liking Oldestguy's idea of removing the soil and replacing with sand or gravel. I'm thinking some heavy clean gravel replacing the soil all the way down to the footing would prevent settling and allow drainage so there would be no significant frost heave. What size of aggregate would be optimal in this situation. I would rather detail this out than leave it to chance or the contractor since that usually ends up in something like what I have shown being done as a countermeasure which then creates more problems than it solves.

 

[cvg]

of course removing the frost susceptible soil is a good idea, but also very expensive to excavate 3 feet down and replace with 3 feet of granular fill.

setting the slab on a ledge will only crack it when it settles, it is not a structural slab. same with dowels, it is a thin, un-reinforced or only lightly reinforced slab, it will settle or heave and it will crack.

just use care in the earthwork and good compaction, provide good drainage and slopes so so water does not run towards the foundation wall, install good expansion joint material in the gap to fill it and seal it properly and then maintain the seal to keep water out. nothing else is needed.

 

[CELinOttawa]

Take a look at these two links:

http://www.standard.net/stories/2014/02/26/proper-installation-key-driveway-lasts-longer

http://www.douglasfoundationrepair....y-pour-austin-concrete-raising-or-mudjacking/

First link is for new repairs to existing homes; Here I think there could be some merit in the detail, but I would still think a better result could be obtained with less problem-prone replacement details.

Second link is specific to southern climates, and frankly a great deal of details can work through tremendous abuse thanks to the additional load paths, reserve capacity, etc inherent in residential construction. That said, we are expected to do better as Professionals.

 

[medeek]

To pour the footer you already have to excavate down and the soil is disturbed. So do you just backfill with the regular soil or wouldn't it be better to replace some of this soil with granular fill.

 

[CELinOttawa]

Yes, you have to have disturbed the soil, but not for any meaningful distance. Oldestguy's suggestion is to underlay the full driveway with new soils; Just the few feet up at the foundation lip isn't going to do that trick.

 

[medeek]

Ok, remove the offending rebar, leave the expansion joint and the other backfill notes as they are. Would that be good enough?

 

[CELinOttawa]

I see no issue with that, so long as this is in an area with near zero frost heave suceptibility.... 1'6" is not alot of soil to insulate the base of that footing.

 

[medeek]

The 1'6" is the minimum, probably for zero frost regions. Now assuming I was to build this in Utah where we do get a lot of frost heave and the depth of the footing is 36", what then?

Do we just go with what cvg commented?

"just use care in the earthwork and good compaction, provide good drainage and slopes so so water does not run towards the foundation wall, install good expansion joint material in the gap to fill it and seal it properly and then maintain the seal to keep water out. nothing else is needed."

I may be overthinking this.

 

[cvg]

I think you are mixing the driveway and stemwall designs a bit. the stemwall should be set below the frost depth and recommend granular backfill top to bottom.

the driveway slab is separate, not connected and has its own design. dependant on site soil, traffic and climate conditions you may need 4 inches gravel base or 12 inches gravel base. you might need 4 inch thick un-reinforced, in other areas perhaps thicker or reinforced is recommended. your subgrade may be expansive or frost susceptible in which case either you plan for the heave and settlement movement or try to prevent it

 

[CELinOttawa]

I agree with cvg.

 

[medeek]

I agree we are mixing the stemwall and driveway design into one detail, but I don't see why one can't do that to some extent. If the climate, soil and traffic require a more involved driveway slab design then the homeowner/contractor can detail that out further. Where I call out the driveway slab note on the top of that put in brackets (Driveway by Others).

Then call out a 4" min. gravel base as required. I think we can all agree I should have at least 4" of gravel, possibly more.

Change the backfill note to "95% compacted granular backfill",

and then get rid of that bent out rebar.