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.

 

[msquared48]

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

 

[msquared48]

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

 

[oldestguy]

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.

 

[medeek]

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.

 

[cvg]

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?

 

[CELinOttawa]

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.

 

[medeek]

Just when I though I had it all figure out.

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

 

[CELinOttawa]

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.

 

[AELLC]

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".

 

[AELLC]

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.

 

[AELLC]

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.

 

[somewhatofanerd]

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.

 

[AELLC]

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.