Zone 2 Slab Question, Residential - Garage Entry at Driveway 1

[trevorpan]

Hi,

I have a simple residential project. Using IRC 2012 tables and recommendations.

Concerned the driveway slab will heave in rain storms. Do you think it better to have smooth dowels and bond break as shown (duct tape), or provide a key?

I've also thought of adding a bond break on top of the footing, so the slab can move. And removing the dowels altogether. Just have a turndown butt up against the stem wall, and garage opening turndown.

http://files.engineering.com/getfil...3d-91cf17adf481&file=Slab_at_Garage_Entry.png

Thoughts?

Trevor
Trevor Pan-Architect, PLLC

 

[KootK]

Is frost an issue where you are? Expansive clays?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[trevorpan]

Hi,

Thank you for answering.

The project is in the Phoenix area. Certainly no frost issues. Tried to convince client to get a soils report, but they don't want to. (happen to have worked for a structural engineer for two years).

Anyhow, the soils are clayey. This particular slab will replace one that is cut out to accommodate the new garage extension, shown in the detail. The existing slabs don't exhibit any cracking beyond normal.

My concern is to keep the top edges of garage and drive equal.

Maybe coating the stem, and footing with form oil would be enough to keep the bond broken, so it can move?

Trevor

 

[BAretired]

I would dowel the garage slab and driveway together with dowels but omit dowels to the footing. If the driveway wants to heave, let it heave. Just ensure the slab heaves by the same amount.

BA

 

[BAretired]

Actually, the footing is redundant through the door opening.

BA

 

[trevorpan]

You are correct. It is redundant through the door opening when taken in isolation.

It is a continuous footing and connects to the jambs of the garage opening. There it supports the stemwalls, isn't that a desirable thing?

I have never seen a footing cut short for an opening, but if you have a reason to do so please share ~

 

[BAretired]

I don't believe it is necessary or even desirable to remove the footing in the garage opening and yes, it is a necessary item under the stem walls. Structurally, it is redundant within the garage door opening but rather than remove it, I would simply avoid tying it to the thickened slab above.

In central Alberta where I used to practice, the predominant soil was an expansive clay which had been compressed by thousands of feet of snow and ice during glacial periods. When covered with polythene, as is usual under a grade slab, the evaporation of capillary water was suddenly terminated and the moisture content of the soil would increase, not from rainfall but from the capillary rise of water from the water table below. An equilibrium moisture content would eventually be reached after five or six years at which time the soil would stop swelling and the heaving would stop. In most cases, the swelling would be one or two inches at most which was not deemed to be a problem.

I cannot comment on the clay soil at your site but from what you have stated, it doesn't seem that there will be significant heaving.


BA

 

[oldestguy]

Your detail allows the dowels to be exposed to water. With rust probably forming and locking in the dowel and possibly breaking out that 2 inches cover, why any dowels at all? If the soil there is high swell type clay, dowels likely won't do much to keep the two slabs even.

 

[trevorpan]

Ok, I think I have a better picture of what is going on. Dowels are great for interior, not for weather exposed conditions.

In this revision, I've removed the dowels at garage entry. Is a key necessary between the two turndowns, or is it better to put an expansion material in between the slab turndown and stemwall? Interior slabs would not require expansion material?

The desert does actually pop up sidewalks now and again, not having expansion material, but is fairly rare. I saw about (3) heaves last summer in Phoenix. Amazing how it can pop up hundreds of pounds of concrete.

The only thing I don't like about expansion material, at least asphalt based fiber ones, is they eventually fade out and leave a gap.

 

[OzStruct]

Hi

I think your original detail is pretty good. Dowel bars can be used externally and commonly are. Round dowels will allow the slab to shrink away at the joint which is good. Also in external applications make sure the dowel bars are hot dip galvanised which protects the steel from corrosion. Pathway slabs will 'pop' if they are very long without expansion and contraction joints. So if you are worried about that you could included a thin compressible void former (Ableflex is what is used here) up in between the 2 slab edges to allow for expansion as well.

 

[hokie66]

There is probably no ideal solution. I would leave the ties to the footing in. If the slab ever wants to "pop up", it might not want to go back down without anything to hold it. Dirt/debris can get into a joint very quickly to prevent movement. May not be the case where you are, but my experience is most heaving slabs are caused by tree roots, so best to keep trees well clear.