Garage slab rebar question

[anthonyjorgensen]

I'm building a 22 x 32 garage and I'm almost at the stage where I'm going to be pouring the slab. I've had a few people tell me that I should be putting dowels from the foundation wall to tie into the slab and some say that I shouldn't. Curious what other people have done. The slab is going to be 5" thick with 10m (#4) rebar 18" on center each way.

 

[Firebird 1]

I am not a structural engineer and I am sure someone on here will contradict my advice, but there is no need to dowell the slab to the sidewall. I have been a contractor for over 35 yrs, and have seen smaller slabs than you are talking about that have settled and either snapped the bars or sliced thru the foundation. If you have a solid concrete foundation that will probably not happen though. We have pinned one slab to another with keyways and dowels but one side of the dowel will be sleeved and greased to allow movement. If your sub soil is compacted properly I see no reason why you would need to do it.

 

[BlackTalon]

Not normal to dowel a slab-on-grade to foundation walls.

 

[Cyberbear]

We live in an area that is seismic active and on decomposed granite soil. Taking no chances that a later adjoining slab would shift over time, we doweled into the building footing at the door openings. That was ten years ago and all is still good.

 

[hnsy]

I'm building a 22 x 32 garage and I'm almost at the stage where I'm going to be pouring the slab. I've had a few people tell me that I should be putting dowels from the foundation wall to tie into the slab and some say that I shouldn't. Curious what other people have done. The slab is going to be 5" thick with 10m (#4) rebar 18" on center each way.

Thanks for asking this. Good question.

 

[wssix99]

As noted above, doweling the slab would defeat the purpose of a floating slab. If anyone tells you to do it, ask for the technical engineering reference that instructs why. When they don't provide one, slap them with a wet leather glove for putting your slab on jeopardy. You can be so confident in this, that you can have the glove at the ready when you ask the question.

Floating slabs are continuously supported by the base/ground below. As long as they have that support, they are incredibly strong. If the ground pulls away from the slab and they loose contact, then the slab can fall apart. (The rebar you have in your slab is there to control shrinkage stresses as the cement in the mix cures and will help you control cracking. It isn't anywhere near what you would need for actual structural strength. The slab is also too thin to act as a good reinforced floor.)

We live in an area that is seismic active and on decomposed granite soil. Taking no chances that a later adjoining slab would shift over time, we doweled into the building footing at the door openings. That was ten years ago and all is still good.

If I had this problem, I'd either have an engineer design reinforcing to deal with a potential void at the door opening under the slab or pour a sacrificial sill through the opening instead of pinning. I have separate sills in my garage now and they work great. One was off and all I had to do was break it out and re-pour. Doing this would be a lot easier than dealing with cracks or damage to the main slab.

 

[dallasjeannie]

Depending on your location, the walls are often a frost wall so they do not move with frost heaving. The floor is inside and unless the garage is heated well the soil below can freeze and the floor can move, thus the term floating floor. If the floor moves and the walls do not you will get cracks if they have been joined by rebar. In fact it is a good idea to put a thin expansion joint during the pour between the floor and wall.
By the way, using the rebar in your slab is a good idea as compared with mesh.

 

[JoeFin]

My wife is an architect specializing in "Concrete Post and Beam Construction" and she insisted I dowel the Slab to the Stem Walls in my Non-Monolithic poured foundation.

So much for "I'm a contractor", "I poured a slab one time and" ... Nontechnical without references from those that don't have a Masters Degree in Architecture advice

Besides - rebar is relatively cheap insurance against cracks rather then dealing with cracks, separation

 

[DougWil]

Your foundation holds your walls in place, not the slab.

As posted above a floating slab should be free to move and settle without creating stresses to the stemwall or the slab itself.

 

[The FIB]

My wife is an architect specializing in "Concrete Post and Beam Construction" and she insisted I dowel the Slab to the Stem Walls in my Non-Monolithic poured foundation.

So much for "I'm a contractor", "I poured a slab one time and" ... Nontechnical without references from those that don't have a Masters Degree in Architecture advice

Besides - rebar is relatively cheap insurance against cracks rather then dealing with cracks, separation

I would think in your location you would not be dealing with frost heave conditions, what works well in your part of the country, would not work so well in others.

When you pin a slab to a foundation wall that is below frost depth in an unheated garage, one will move and the other will not. The results wont be good.

 

[matt_i]

While my slab is filled with rebar, I did not do any pinning to the walls because I didn't want the slab breaking the walls or vice versa. I did dowel the new slab to the existing slab to maintain alignment.

 

[anthonyjorgensen]

Thanks for all your replies. I wasn't planning on tieing into the walls and based on the responses here I'm going to stick with that plan!

 

[like2wheel]

My wife is an architect specializing in "Concrete Post and Beam Construction" and she insisted I dowel the Slab to the Stem Walls in my Non-Monolithic poured foundation.

So much for "I'm a contractor", "I poured a slab one time and" ... Nontechnical without references from those that don't have a Masters Degree in Architecture advice

Besides - rebar is relatively cheap insurance against cracks rather then dealing with cracks, separation

Could you have her please explain the logic in doing this? because to most of us commoners, it simply makes no sense.

 

[DougWil]

Could you have her please explain the logic in doing this? because to most of us commoners, it simply makes no sense.

I am guessing that his slab went over the stem walls, in which case it makes sense, since it isn't a floating slab and the walls are anchored to the slab and slab to the stem wall.
Otherwise in a seismic event the slab and building could slide right off the stem walls.

 

[wssix99]

^ This particular detail would not work in a place that has freezing weather and would also require meticulous attention to the compaction to the material under the slab.

 

[wssix99]

My wife is an architect specializing in "Concrete Post and Beam Construction" and she insisted I dowel the Slab to the Stem Walls in my Non-Monolithic poured foundation.

Architects don't typically study pavement design. The continuously supported (by a base underneath) condition wouldn't be covered in a typical concrete structures class that an architect would take. They'd have to go to a higher level Civil class for that. (And a licensed structural engineer should be involved wherever something exceptional like this may be appropriate.)

You can always take a concrete saw to where the slab meets the wall and slice those bars to relieve any stress that might be built up. Your wife can clean up the mess. (If you don't have freeze/thaw cycles, then it may not be a big deal.)