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Connecting Driveway to Foundation with Rebar

Connecting Driveway to Foundation with Rebar

Connecting Driveway to Foundation with Rebar

(OP)
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.






RE: Connecting Driveway to Foundation with Rebar

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.

RE: Connecting Driveway to Foundation with Rebar

or frost heave.

RE: Connecting Driveway to Foundation with Rebar

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

RE: Connecting Driveway to Foundation with Rebar

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.

RE: Connecting Driveway to Foundation with Rebar

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.

RE: Connecting Driveway to Foundation with Rebar

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".

RE: Connecting Driveway to Foundation with Rebar

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.

RE: Connecting Driveway to Foundation with Rebar

(OP)
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.

RE: Connecting Driveway to Foundation with Rebar

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.

RE: Connecting Driveway to Foundation with Rebar

Quote:


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.

RE: Connecting Driveway to Foundation with Rebar

(OP)
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.

RE: Connecting Driveway to Foundation with Rebar

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.

RE: Connecting Driveway to Foundation with Rebar

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

RE: Connecting Driveway to Foundation with Rebar

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.

RE: Connecting Driveway to Foundation with Rebar

(OP)
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.

RE: Connecting Driveway to Foundation with Rebar

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

RE: Connecting Driveway to Foundation with Rebar

I agree with cvg.

RE: Connecting Driveway to Foundation with Rebar

(OP)
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.

RE: Connecting Driveway to Foundation with Rebar

To make this work, I would have to detail it totally different, with a stepped, thicker wall and ledge to bear on below frost, and a thickened slab edge exte3nding to the ledge. Kinda depends on where you are (how deep the frost depth is) as to whether it is worth the trouble.

Mike McCann
MMC Engineering

RE: Connecting Driveway to Foundation with Rebar

Another thought. I would make the slab structural for a distance of about 6 to 8 feet from the stem wall to alleviate any vertical settlement next to the fill area by the stem wall.

Mike McCann
MMC Engineering

RE: Connecting Driveway to Foundation with Rebar

On the replacement of frost susceptible soil with non-frost susceptible soil, the gravel size is not critical. I am assuming you might be buying road gravel or similar. The main criteria of the type to be non-frost susceptible is that of the fraction passing the Number 4 sieve, not more than 3% passing the Number 200 is ideal 5% is near OK. Thus, a base course with a mix of sand and gravel should have even less P-200 since a larger sample is tested. Many states allow a base course to have significant silt content, which translates to a base that is frost susceptible (Wisconsin is one of those). The most suitable then would be the mix coming from a ready mix concrete plant, but with no cement. However, a bank-run sand with low silt content still probably is OK for most areas. A material made from crushing of rock probably is OK, assuming the fines are not finer than the 200 sieve to any significant amount. Thus any size stone is OK. In Wisconsin even crushed sedimentary rock can be less suitable, since good sources are now scarce. Crushed concrete also probably is OK unless a lot of dirt has been included.

RE: Connecting Driveway to Foundation with Rebar

(OP)
In summary the four best options are:


1. Dowel with expansion joint (for zero frost environments)
2. Concrete ledge with expansion joint and breakaway joint for vertical uplift due to frost heave.
3. Replace all of the soil down to the footer/frost line with gravel, crushed rock etc... and slope it out gradually to about 10 ft in front of garage (mostly costly option)
4. Leave as is with an expansion joint and remove bent out rebar and detail it out so driveway slab is shown as "by others". The theory is that driveways eventually need replacing anyhow and if special considerations are needed then the owner and the contractor can address them in the field.

RE: Connecting Driveway to Foundation with Rebar

really? that's your conclusion?

dowels or rebar are unnecesary
a ledge will promote cracking
option 3 is ok but expensive
and why do you insist that driveway is by others?

RE: Connecting Driveway to Foundation with Rebar

NO Medeek,the dowling is NOT a good idea, just particularly bad in a frost suceptible zone. Read through the replies; It is *still* a bad idea anywhere.

RE: Connecting Driveway to Foundation with Rebar

(OP)
Just when I though I had it all figure out. :(

Option 4, but do not say "by others"?

RE: Connecting Driveway to Foundation with Rebar

By others is not a problem, as long as your scope also clearly defines that you are not responsible.

Your two, three and four are not bad; They are simply options. Where and when to apply them will come with experience, but each can be adapted. The fun part is that most of the time the RIGHT solution is the safe, effective and economical choice.

RE: Connecting Driveway to Foundation with Rebar

Usually there is an expansion joint, there is no rebar nor dowels, and the driveway is cast as part of the contract for the home so there are no "by others".

RE: Connecting Driveway to Foundation with Rebar

You also should thicken the edge of slab to 8" and provide a 40# felt as bond-breaker between the slab and top of stem wall.

RE: Connecting Driveway to Foundation with Rebar

ooops when said no dowels I meant the one from stem wall to driveway slab. You still need dowel ftg to stem, but 48" in lieu of 24" is enough.

RE: Connecting Driveway to Foundation with Rebar

If the plans are generic house plans being sold to anyone in the US under the IRC 2012 code you *definitely* do not spec out the driveway. And you should have a disclaimer that the foundation has been designed according to an allowable soil bearing pressure of 2000 psf without consideration given to clay and local geotechnical site specific report may be required and may require additional modifications/changes to the foundation design.

AND DON'T DOWEL GARAGE TO DRIVEWAY.... EVER! :)

And yes, 8" turndown is a good idea, not sure why the need for a 40# felt bond breaker though.

RE: Connecting Driveway to Foundation with Rebar

The 40# felt is an IMO thing, no big deal. Has to do with conc slab shrinking, don't want to restrain it by virtue of slab-stem interlock.

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