Concrete slab expansion joints

PassnThru · Nov 9, 2010

I've seen several garage builds here lately where the joints are cut into the concrete after the fact with a saw. My garage was a cheapy and unfortunately does not have any joints. I was always under the impression that they were put in the concrete while still wet so the edges could be rounded off. A rounded off corner is less likely to break off than a sharp edge. So is this a problem? Is it generally accepted? Most people wouldn't do that to the edges or joints in a sidewalk - but apparently it is done to garage slabs frequently. Just curious :headscrat

DHS · Nov 9, 2010

Around here thats how they do everything. My floor with the square edge does chip off but is a work shop not a show room.

MrMark · Nov 9, 2010

I don't get it either. I have never seen it done out here in CA. Not to say that it isn't but I haven't seen it. I know my slab does not have it. I don't understand why it just wouldn't be done during the pour with an edging tool. That way there is a nice rounded corner rather than a sharp cosmetically unpleasing saw cut.

dirttracker18 · Nov 9, 2010

On another note I am note sure why people insist on cutting a line into the beautiful concrete floor in their garage.
There are few places where frost in the ground is a bigger concern then here and I have no cuts in my floor and no cracks. In fact I have seen hardly anyone around here with cuts in their floor.
The concrete guys here tell me that if the floor is conctructed properly (using rebar, mesh is just asking for cracks, proper gravel and compactment) then you should not have to worry about cracks.
Just for reference my slab is 30 by 36 with rebar and crack free.

Gary S · Nov 9, 2010

I rented a concrete saw and cut the relief cuts in my slab the day after I poured it. Before the first winter was over, the slab had cracked perfectly along each cut. If I had not cut it, the cracks would be random on my garage floor.
My cuts have the the sharp edges left by the saw, and the edges haven't broken that I've seen yet in the 6 years I've used it.

MrMark · Nov 9, 2010

Why didn't you have the relief lines put it by the edging trowel at the time of pour?

DHS · Nov 9, 2010

I like the saw cuts better than ones done at time of pour. Cleaner look but the down side is that id your drop something on the edge it will chip.

taylorguitar · Nov 9, 2010

What do you guys think about expansion joints in a 16x20 shop that is gonna sit on old concrete floor. ground doesn't come close to freezing here, so why have joints?

Jay H 237 · Nov 9, 2010

Seeing all these expansion joints has got my curiousity too. Every house I lived in and other family members never had any relief cuts done. It seems that on all new construction in the past few years these cuts have become standard practice now.

The house I'm in now, built 1989, has a minor crack in the floor. the last house I was in, built 1972, had none.

viper86 · Nov 9, 2010

the primary reason for jointing a slab is because concrete shrinks as it cures. if the slab was allowed to move freely from edge to edge (i.e. no friction between the bottom of the slab and the base course) then the shrinkage would just cause the slab to pull away from its edges. A well-reinforced slab would not be able to shrink much. There would still be cracks, but they wouldn't be very noticeable. Mesh won't do this unless you use a heavy wire size at close spacing. And even then, it only works if the monkeys placing the concrete are using bolsters to support the mesh and not pulling it up as they go.

Tooled joints take a lot more time to do, so it's going to cost more money. Sawed joints are okay if they're not done too soon, otherwise it 'tears' the concrete and you end up with jagged edges.

Another kind of joint, blind joint, can be used as well. The joint is tooled after the slab is poured, then during finishing the joint gets troweled over. The joint is essentially hidden. You'll likely have a fine crack at the surface, but it is far less noticeable than a saw joint or tooled joint. This is mostly done in an effort to reduce curling of the slab. That's probably not a big issue for most guys in the garage or shop, but it can be a big problem if you're putting any kind of flooring down that is sensitive to having a nice, flat surface.

MrMark · Nov 9, 2010

Got it. That's what I thought. Thanks Viper. It all comes down to money.

c39er · Nov 10, 2010

I had them do "Zip Strips" in my slab. It's a 2 piece plastic T. While concrete is still wet they push the Zip T into the concrete flush then pull off the top flat pice of plastic leaving the 1" vertical piece of plastic down in the cement slab-trowel/finish the concrete. The slab will crack where the remaining piece of "Zip" strip is left hidden down in the concrete. My cracks are very small and hardly noticable after 15 years. No ugly slit to fill up with dirt.

MrMark · Nov 10, 2010

That's what they do with the sidewalks here.

Does anyone have any recommendations for isolation joint material, e.g., to separate your neighbor's concrete from yours to cracks won't telegraph?

Markgyver · Nov 11, 2010

taylorguitar said:
What do you guys think about expansion joints in a 16x20 shop that is gonna sit on old concrete floor. ground doesn't come close to freezing here, so why have joints?

Shrinkage....
concrete shrinks when it cures

red69ss · Nov 11, 2010

I have a 30 x 40 shop and when they poured mine the concrete guys put what they called a keyway down the 40 ft. length, it was metal and T shaped and spiked into place. the slab cracked down the full length of the keyway a couple years after the pour, but it's not too bad. if I had it to do over I probaly wouldn't have let them put it in, even though it was probaly going to crack regardless.
they wanted to saw cut it across the 30 ft. length in three places but I didn't let them. glad I didn't, it only has one hairline crack 15 feet long in that direction six years later, and they wanted $3 a foot to saw it.

jsmith6752 · Nov 11, 2010

my 2 cents worth......from what I have read the cut needs to be 1/4 " deep for every 1 " of concrete. for a 4" slab cut should be 1" the nice part mentioned earlier was the t strips that have the proper depth

7echo · Nov 11, 2010

I have seen metal keyways used recently. Not sure why, I didn't have time to talk to the guys.
Here is a good article...
http://www.johnbridge.com/articles/concrete/concrete-slab-control-joints/

AMCguy · Nov 12, 2010

I have seen many a garage floor that has yet to crack. That doesn't mean it never will. At the same time, every floor I have ever seen that has been saw cut, has cracked on the cut. Why not predetermine where your floor will crack if it's going to.

I did all the concrete and saw cuts in my garage build. I bought a two pack of 7 1/4'' diamond turbo blades for $39.00 I mounted one in my old circular saw. I cut a total of 288 feet of concrete to a depth of 2 1/2''. I'm still on the first blade. If you cut the concrete the next day, it goes quickly with little dust. I probably did five or six feet per minute.

If having a sharp edge is a concern, just knock it off with a Rub Brick. I did this to the garage floor so the hard wheels of the floor jack or my engine lift wouldn't break the edges away.

willymakeit · Nov 12, 2010

Clean and then fill the joint with Sonneborne or comparable caulk. Go to the Ameican Concret Insitute for the suggested methods in placing of concrete and exp. joints ect.

larry_g · Nov 12, 2010

c39er said:
I had them do "Zip Strips" in my slab. It's a 2 piece plastic T. While concrete is still wet they push the Zip T into the concrete flush then pull off the top flat pice of plastic leaving the 1" vertical piece of plastic down in the cement slab-trowel/finish the concrete. The slab will crack where the remasining piece of "Zip" strip is left hidden down in the concrete. My cracks are very small and hardly noticable after 15 years.

I have also used zip strips in both my shops. One is ~25 years old and looks good. The new shop is 2 years old and looking good. The strips have a hairline crack above them but that is all.

lg
no neat sig line

knucklebusted · Nov 12, 2010

My 32 x 28 slab is 4 years old with the doors along the 32' side. I has a 32' saw cut down the middle and a 28 ft saw cut between the 16' and 9' garage doors. It has rebar in it and is 6" of 3500psi concrete. I cracked along both of those cuts... and another hairline crack down the middle of the 16' door. All of this happened about the end of the second year. Garage is insulate and heated.

jerseywild · Nov 13, 2010

willymakeit said:
Clean and then fill the joint with Sonneborne or comparable caulk. Go to the Ameican Concret Insitute for the suggested methods in placing of concrete and exp. joints ect.

I don't have a problem that you spelled concrete wrong, I don't have a problem that you spelled institute wrong but for the love of god man you spelled AMERICAN wrong. :lol_hitti

Dragster Racer · Nov 13, 2010

The zip strips are interesting. I have never seen that, but it looks cool.
The rub brick is brilliant. Good idea.
I will eventually fill mine like mentioned. The sure collect the dirt.

Justanoldguy · Nov 13, 2010

jerseywild said:
I don't have a problem that you spelled concrete wrong, I don't have a problem that you spelled institute wrong but for the love of god man you spelled AMERICAN wrong.

Ahhh .. The SPELLING POLICE have arrived.
They are typos, not spelling errors.
Happens when typing too fast. :beer:

Now, back to the joints.
In NZ, cutting the slab the day after the pour is the norm.
Caulking is available to fill the joint and if done properly, is barely visible.
The joint then does its job of containing shrinkage to that area and not at random. It is rare for a slab bigger than 20 x 20 not to crack.

StinkyBunny · Nov 13, 2010

What would be the optimal spacing of rebar? I could really care less what the code dictates in my area. I just want to hear what the experts have to say about it since I'll be pouring a 20x30 pad in the spring and I want it to last and not crack all to hell. Welding them together isn't an issue for me either.

Justanoldguy · Nov 13, 2010

I believe welding the rebar will definitely encourage cracking.
That is why it is discouraged in the construction industry.

PassnThru · Nov 13, 2010

Justanoldguy said:
I believe welding the rebar will definitely encourage cracking.
That is why it is discouraged in the construction industry.

That is a pretty bold statement. Let's get this out of the way first - I don't do concrete in any way. But I like to think I know at least a little something about it. Humor me here. I've been running a lot of things through my head - and I can't think of any reason why welding the rebar together would cause a problem. I can only guess that it would only be a problem if you expected the concrete to remain fluid and move over it's lifetime. I don't recall that being normal for concrete.
So - educate me - why would welding the rebar together be a problem????

Justanoldguy · Nov 14, 2010

PassnThru said:
That is a pretty bold statement. Let's get this out of the way first - I don't do concrete in any way. But I like to think I know at least a little something about it. Humor me here. I've been running a lot of things through my head - and I can't think of any reason why welding the rebar together would cause a problem. I can only guess that it would only be a problem if you expected the concrete to remain fluid and move over it's lifetime. I don't recall that being normal for concrete.
So - educate me - why would welding the rebar together be a problem????

OK.
Lets get the first thing out of the way.
quote: I don't do concrete in any way.
I have been doing concrete for 45 years.
That's gives me one up on you.
Next:
Welding can reduce the fatigue life of the rebar, and as a result rebar cages are normally tied together with wire.
Next:
In the US, most rebar is not suitable for welding. ASTM A 616 & ASTM A 617 reinforcing are re-rolled rail steel & re-rolled rail axle steel with uncontrolled chemistry, phosphorus & carbon content. To weld rebar you must obtain a mill statement that the reinforcing is suitable for welding.
Next:
Weld it and it puts forces on the concrete later on due to Linear expansion of steel. Nowhere to go so cracks develop.
Don't compare it to mesh which is welded as the expansion rates are not even close to comparison. Mesh can actually cause cracking as well if it is not placed exactly in the right depth from the surface.

As for "a pretty bold statement".
You be the judge. :beer:

StinkyBunny · Nov 14, 2010

Justanoldguy said:
Welding can reduce the fatigue life of the rebar, and as a result rebar cages are normally tied together with wire.

Weld it and it puts forces on the concrete later on due to Linear expansion of steel. Nowhere to go so cracks develop.

OK, but y'all still didn't answer the optimum spacing question I posed. I always wondered why they tied rebar on highway projects, now I know "The rest of the story" as it were.

PassnThru · Nov 14, 2010

Justanoldguy said:
OK.
Lets get the first thing out of the way.
quote: I don't do concrete in any way.
I have been doing concrete for 45 years.
That's gives me one up on you.
Next:
Welding can reduce the fatigue life of the rebar, and as a result rebar cages are normally tied together with wire.
Next:
In the US, most rebar is not suitable for welding. ASTM A 616 & ASTM A 617 reinforcing are re-rolled rail steel & re-rolled rail axle steel with uncontrolled chemistry, phosphorus & carbon content. To weld rebar you must obtain a mill statement that the reinforcing is suitable for welding.
Next:
Weld it and it puts forces on the concrete later on due to Linear expansion of steel. Nowhere to go so cracks develop.
Don't compare it to mesh which is welded as the expansion rates are not even close to comparison. Mesh can actually cause cracking as well if it is not placed exactly in the right depth from the surface.

As for "a pretty bold statement".
You be the judge.

That works for me. BTW - I wasn't calling you out so to speak - just looking for additional explanation.

jerseywild · Nov 14, 2010

I just got this saw and I noticed in book it said do not use to cut rebar.

motodavid2000 · Nov 14, 2010

Justanoldguy said:
OK.
I have been doing concrete for 45 years.
That's gives me one up on you.
Next:
Welding can reduce the fatigue life of the rebar, and as a result rebar cages are normally tied together with wire.
Next:
In the US, most rebar is not suitable for welding. ASTM A 616 & ASTM A 617 reinforcing are re-rolled rail steel & re-rolled rail axle steel with uncontrolled chemistry, phosphorus & carbon content. To weld rebar you must obtain a mill statement that the reinforcing is suitable for welding.

Next:
Weld it and it puts forces on the concrete later on due to Linear expansion of steel. Nowhere to go so cracks develop.
Don't compare it to mesh which is welded as the expansion rates are not even close to comparison. Mesh can actually cause cracking as well if it is not placed exactly in the right depth from the surface.

As for "a pretty bold statement".
You be the judge.

Wow, never ceases to amaze me the topics that come up on this forum. Generally, I agree with Justanoldguy. Welding of reinforcing steel is not a broadly accepted method of joining reinforcing steel. Reinforcing configurations are typically wire-tied with specific overlaps depending upon the size of the reinforcing and the specific design parameters of the concrete member.

He notes that steel's linear expansion causes cracks. As an engineer, it is actually the differential rates of expansion between concrete and steel that will cause internal stresses that MAY result in cracking depending upon embedment depths, spacing, reinforcing density and climatic or service temperature variations.

Lastly, welding (and mechanical **** connections) is accepted on certain nuclear reactor containment structures, principally for pressurized water reactor designs due to internal pressurization scenarios during a reactor blowdown event. This is only a general statement as I have seen several design variations over my career for both PWR and BWR designs.

Reinforcing steel sizing, configurations and spacing are highly dependent on the objectiuve of the concrete structure. Simply stated, concrete serves well while in compression and is vulnerable to failure when in tension. Reinforcing steel generally keeps the member together while it is being subjected to bending or tensile loads. Again, a simple answer to a complex set of questions, but I think you get the idea.

Dave

Concrete slab expansion joints

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