Dowel or expansion joint

[KellyM77]

I know there are applications where using dowels is the right thing when adding concrete and other times when expansion joint should be used. I’ve had a patio extended and the contractor used dowels the new patio moved and busted the old existing concrete in the area of the dowels. Now I have just poured a new mono slab for a new shop. I will soon be adding a driveway to the shop. What is the right method to use where driveway and shop floor come together. Do I use an expansion joint or do we use dowels into the shop footing?

 

[The Cobbler]

no to doweling the driveway to the shop pad .
that could vary with your location possibly...

 

[tinmanwpk]

Driveway in the hot sun may have freezing cold in winter and will expand and contract differently from an enclosed garage.

 

[jack stand]

I'd think that you'd do both in an interior situation and only the expansion for the interior to exterior condition.
Our resident concrete experts will hopefully show up soon.

 

[CraigStu]

Expansion joint. As tinmanwpk says at times there will be a wide variation in conditions driveway vs inside garage. Also the shop has whatever footers/foundation is appropriate for your area. The driveway will have less than that so it may move differently. I would want the garage to be somewhat of a monument and the driveway to be somewhat sacrificial. Not sure those are the best words but hopefully you understand what I mean.

 

[lolaetype]

Would this be the proper application for a sleeved dowel?

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[ConCretin]

Expansion joint is a bit of a misnomer because concrete usually doesn't get any bigger than the day it's placed. Concrete shrinks as it cures and usually doesn't expand any more than it shrunk. There are exceptions of course such as when concrete is exposed to sun and high temps. The foam we all call expansion joint might better be described as isolation joint because it's most common purpose is to prevent binding or restraint

So when do you dowel vs isolating concrete placements? It's obviously project specific but I would offer the following as a general guide.

Use 'expansion' material between elements that will undergo differential movement to prevent binding. An example would be between a concrete wall and sidewalk or around the perimeter of a slab.​

​

Use rebar dowels when you are placing or re-placing part of an monolithic structure where movement isn't expected or desired. An example might be replacing part of a slab or a construction joint in a wall or footing.​

​

Use smooth dowels between elements to prevent vertical displacement but allow for horizontal movement. The base under the two elements must behave similarly because the concrete can't resist much vertical loading without cracking. An example would be construction joints in large slabs where you want each section to expand and contract freely but maintain a level surface.​

In the OP's case, he has a joint between his shop floor and driveway. If the shop floor is contained within a foundation, I'd lean towards expansion material because each element will want to move differently and you don't want the joint to bind or break especially if frost is a concern. If you don't get frost, you could use smooth dowels to allow each slab some movement while keeping the top of the slabs flush.

Keep in mind that smooth dowels need to be kept square and level or they will bind just as completely as rebar does. A plastic sleeve or grease will facilitate free movement.

​

 

[Leaflessshadetree]

In some cases I've doweled to restrain the slab from moving vertically. (keep the outdoor slab from "heaving up" ).
Then places expansion joint to allow horizontal expansion, due to heat of summer.

 

[chinboys]

Dowels are for "cold" seams between two concrete "plates".
If both sides of the plates or the groundwork have been prepared correctly then you have less of a chance of the two plates moving up, down, left, or right.
IMHO, get the two plates doweled to each other.

 

[KellyM77]

Expansion joint is a bit of a misnomer because concrete usually doesn't get any bigger than the day it's placed. Concrete shrinks as it cures and usually doesn't expand any more than it shrunk. There are exceptions of course such as when concrete is exposed to sun and high temps. The foam we all call expansion joint might better be described as isolation joint because it's most common purpose is to prevent binding or restraint

So when do you dowel vs isolating concrete placements? It's obviously project specific but I would offer the following as a general guide.

Use 'expansion' material between elements that will undergo differential movement to prevent binding. An example would be between a concrete wall and sidewalk or around the perimeter of a slab.​

​

Use rebar dowels when you are placing or re-placing part of an monolithic structure where movement isn't expected or desired. An example might be replacing part of a slab or a construction joint in a wall or footing.​

​

Use smooth dowels between elements to prevent vertical movement but allow for horizontal movement. An example would be construction joints in large slabs where you want each section to expand and contract freely but maintain a level surface.​

​

A caveat - I thought this through as I typed so it may be less than comprehensive.

In the OP's case, he has a joint between his shop floor and driveway. If the shop floor is contained within a foundation, I'd lean towards expansion material because each element will want to move differently and you don't want the joint to bind or break especially if frost is a concern. If you don't get frost, you could use smooth dowels to allow each slab some movement while keeping the top of the slabs flush.

Keep in mind that smooth dowels need to be kept square and level or they will bind just as completely as rebar does. A plastic sleeve or grease will facilitate free movement.

​

Great info. I have never thought of dowels as smooth or rebar. So this brings another question. My concrete contractor really screwed up this slab and we have to add 6” to one end. I never questioned weather or not to use dowels for this. So the footing is currently 12” wide but will soon have 6” added to it. And make the total slab length 6in longer. I normally wouldn’t question a contractor, but this guy has already made too many mistakes and in fixing the problem I want to make sure it’s done correctly. Clearly my trust in him has depleted. So what Dows and how far apart would you suggest for the 6 inch wide 18 inch deep addition? Smooth dolls and grease are something I’ve never heard of but then I’m not a concrete guy.

 

[ConCretin]

My concrete contractor really screwed up this slab and we have to add 6” to one end. I never questioned weather or not to use dowels for this. So the footing is currently 12” wide but will soon have 6” added to it. And make the total slab length 6in longer. I normally wouldn’t question a contractor, but this guy has already made too many mistakes and in fixing the problem I want to make sure it’s done correctly. Clearly my trust in him has depleted. So what Dows and how far apart would you suggest for the 6 inch wide 18 inch deep addition?

Depending on the nature of your structure and your local building department, this situation may require the involvement of an engineer since the slab extension will bear the weight of the structure above. The extension will have a very small 'footprint' and depend entirely on the mechanical connection to the existing slab for support. It's basically a cantilevered ledge.

If it's a typical single story residential garage, the loads probably aren't that high and it may be doable but a better approach would be to remove and recast the entire footing section of the slab, which would be self supporting. Doweling would still be required but it wouldn't be holding the building up, just holding the slab together. I'd be directing my contractor to take this route whether he likes it or not. A 6" extension is sketchy and he'll be long gone when problems occur.

If you go this route and opt not to consult an engineer, I'd think 4 hooked dowels, drilled and epoxied into the existing footing on each side, two top and two bottom would work. I'd also drill and epoxy a #3 bar into the slab every 12 or 18". Good luck resolving this.

 

[PopcornSutton]

Over the years, design engineers have spec'd different types of "doweling". In addition to what has been said, CC mentioned smooth and greased. There has been plastic caps with foam inserts in the bottom so there is room at the end of the bar for movement. There is also a "system" where a rebar end was swaged with a flange and a hole tapped, and a mating rebar that is threaded. The flanged end is nailed to a form, so when the form is stripped, the threaded end simply screws into it.

Drilling and epoxying a dowel is fine, but most times the hole is not drilled perfectly perpendicular and the dust is hardly ever cleaned/blown out for the epoxy to adhere.

All sounds to be nit-picking, but when dealing with the likes of the Corps of Engineers you have to jump through their hoops.