No control joints in concrete?

[Vintage Veloce]

We are putting a concrete patio by our garage. It will be 4" concrete about 20 feet x 18 feet.

My wife doesn't like how the control joints look. And she says she doesn't mind cracks. To some extent I understand this... a control joint is an obvious line in the concrete that you can easily see. She doesn't want the pattern of control joints. On the other hand she won't see a hairline crack from across the patio at all.

I understand her position, she is an artist, and we are doing some things with the outline of the patio that will be cool, and the control joints would disturb that look.

So, what happens if we skip the control joints? OK, the concrete will likely crack somewhere. Structurally, is there any problem? Will the cracks crumble at the edges in a way different that a control joint? (Obviously rebar, etc will prevent steps from occurring at the cracks).

Another question, is there anyway to make "invisible control joints"? Maybe some kind of divider in the bottom of the form that doesn't brake the surface? Or maybe curved control joints? She might go for that?

 

[larry_g]

You can use Zip Strips and see little or nothing. I have used them in two shops and I am happy with them. http://www.superiorprofiles.com/zip-strip.html

lg
no neat sig line

 

[Vintage Veloce]

You can use Zip Strips and see little or nothing. I have used them in two shops and I am happy with them. http://www.superiorprofiles.com/zip-strip.html

I'm assuming those come out looking just like saw cuts? She hates those too.

 

[ard]

I'd just love to read your concrete contractors' posts about this job on another forum....

 

[ItsNemo]

Multiple pours butted up to each other with rebar tieing it together. Blend the joints but your weak spots will be pre-determined if it's going to crack.

 

[ptgarcia]

Rather than foregoing control joints, I suggest you encourage your wife to incorporate them into her design. The concrete is going to crack, and without control joints it will crack irregularly and end up looking like, well.....cracks. And what starts as a hairline crack doesn't always end up that way.

 

[77Birdman]

You can certainly pour it without putting in joints. As long as you are aware that it is going to crack and youre ok with that. Being in SoCal, you don't have to worry about a frost heave. My garage slab is 30 x 36 without a single joint or saw cut and only a couple of hairline crack after 15 years. I live in a cold climate too.

 

[T_R]

Nobody can tell you what will happen. It might crack, it might not. You could put in control joints and it might not crack at them.

My garage is a on 24x24 slab with no control joints and has been through 3 winters in Maine with no cracking. We have nasty frost heaving here.

It's a crapshoot.

 

[Vintage Veloce]

You can use Zip Strips and see little or nothing. I have used them in two shops and I am happy with them. http://www.superiorprofiles.com/zip-strip.html

lg
no neat sig line

Larry, can you post some pictures of your results, maybe with a dime next to the cracks for scale? I'd love to see that. Zipstrips are available here...

 

[rlitman]

What about a post-tensioned slab? That should be crack free, and won't require joints.

Or, how about doing a stamped concrete finish? Something that hides the control joints in plain sight.

 

[texas webb]

Rebar on 18"-24" centers(subject to codes) with 6x6 steel mesh on top and add fiber to the mix.

 

[Lelandwelds]

What about a post-tensioned slab? That should be crack free, and won't require joints.

Or, how about doing a stamped concrete finish? Something that hides the control joints in plain sight.

What he said. (My post tension slab does show cracks.) I have seen herringbone stamped w/o control joints. The fine poly fiber ( not macro) limits cracks. No added water probably is the biggest difference.

 

[tlmartin84]

Multiple pours butted up to each other with rebar tieing it together. Blend the joints but your weak spots will be pre-determined if it's going to crack.

This would work well. We do it at work and the joints are often hard to find if you have a good finisher.

 

[blair683]

Evenly spaced control joints look fine. I could not imagine a contractor even agreeing to pour a slab that size with out cutting it.

 

[larry_g]

I put some pictures in an album here, https://www.garagejournal.com/forum/album.php?albumid=6037

One example close up. You can just see the plastic. This is after 6-7 years of use.
View media item 76906
The uncontrolled crack, seen in the album, is near the apron where some compacted fill meets undisturbed ground.

lg
no neat sig line

 

[Git]

We had our patio saw cut so that it would kind of look like large tiles. Still got some cracks at some of the stress points but they are not as noticeable

 

[Vintage Veloce]

I put some pictures in an album here, https://www.garagejournal.com/forum/album.php?albumid=6037

One example close up. You can just see the plastic. This is after 6-7 years of use.

Larry, I would have expected thinner lines... did they pull the top of the zip strip off while concrete was "wet" and and then float wet concrete over the top?
This does look about the size as saw cuts...

 

[Leaflessshadetree]

What about doing stamped concrete?

 

[matt_i]

You can add a lot of rebar so the concrete can't crack. Wissix will have the actual spec. I think its around 6" or 8" centers.

 

[joes169]

We routinely run into this issue with stamped patio/sidewalks/etc....

As much as I hate using "Zip Strips" this is one area where they work out okay. You're still going to see the crack though.

In the last year or two, we've been using a newer, structural macro fiber that allows us extend our joints out substantially. At 20' x 18', it would be pretty easy to do, it's not that big. It would probably cost us $25-30 per yard of concrete, or about $150 total for the correct dosage to eliminate controls joints, which would be a lot cheaper than it's steel equivalent. The downside is that the style of fiber is relatively new, and may be hard to source in many areas, and even harder to get a contractor to try.

 

[larry_g]

Larry, I would have expected thinner lines... did they pull the top of the zip strip off while concrete was "wet" and and then float wet concrete over the top?
This does look about the size as saw cuts...

In some places it is a hairline crack. What I pictured is in the main shop where it is subject to crawlers and excavators and steel wheeled tables and tool boxes. It has also been through an earthquake which produced some cracks across the shop. I'm a lot happier with this than so many that have large cuts made in the floor. Yes they did pull the stiffener off when embedding in the concrete and troweled over it.

If your doing a patio consider making your cuts as RAY's that are not parallel. You can see on the top end of the picture that the lines kinda follow the curve of the edge. We did this some what by accident and later wished we had picked a point and had all the cut lines radiate out from there. So turn you lines into a feature if you can.

lg
no neat sig line

 

[Vintage Veloce]

Thanks for all the comments guys, and the photos!
I'll definitely look into concrete fibers.

Some points:
- My wife is in charge of the cosmetic design choices. Suggestions on how to place the joints aren't really useful in this case. She PREFERS the idea of possible "thin random and probably hard to notice cracks" to the idea of clearly visible control joints.

- My big question is: Are there any structural problems with just letting the concrete crack where it wants instead of placing control joints? Control freaks may not like this, but is there an actual structural problem, or is it all just cosmetic?

 

[ItsNemo]

It depends on where it cracks, how much it cracks and what's on top of where it cracks. Not all cracks are created equal...the point of control joints is to make it happen where you want them and ensure you don't get one big crack rather than spread amongst the control joints.

I bet you'd be pretty upset if you were installing a lift and it cracked right where you planned to put a post.

 

[rburke65]

Happy wife, happy life.......don't cut the slab.

 

[Boomer343]

This is a patio ...... pour it and enjoy it. If it does crack rebar will keep it reasonably even.

I've poured lots of 24 x 24 garage pads with no control joints.

 

[ard]

is there an actual structural problem, or is it all just cosmetic?

This is a patio ...... pour it

totally cosmetic.

If it cracks at a random spot versus at a control joint it is still a crack. No structural issues for a patio laying on the ground not supporting anything....

 

[jd_1138]

Science > art. I'd have it poured correctly.

 

[cdestuck]

Would she accept a cut filled then with self leveling Silaflex? Matches the same color of concrete. I've used it on my drive and walks, looks great.

 

[wssix99]

Rebar on 18"-24" centers(subject to codes) with 6x6 steel mesh on top and add fiber to the mix.

This is not nearly enough. For a 4" slab with #4 rebar, you'd need to space it at least 9 3/4" apart. Allowing for 1/2" of variance in the thickness, best to go to 8 1/2".

I'll definitely look into concrete fibers.

Fibers don't help for this type of cracking. (They only help with the early cracking associated with moisture evaporating out of the slab too rapidly.)

You can add a lot of rebar so the concrete can't crack. Wissix will have the actual spec. I think its around 6" or 8" centers.

This is what ACI calls for:

"Reinforcement required for crack-width control is a function of joint spacing and slab thickness. To eliminate sawcut contraction joints, a minimum steel ratio of 0.5% (PCA 2001) of the slab cross-sectional area is recommended. The reinforcement should be located as close to the slab top surface as possible while maintaining minimum concrete coverage over the reinforcement."

So, by adding a high level of reinforcement, you can skip the joints entirely. The concrete still cracks like, like any slab, but what happens mechanically is the reinforcement uniformly spreads the cracks out and makes them so small that they are invisible to the eye. (It might make sense to use a good sealer on the slab after it cures if you are doing this outdoors and going this way.)

There are a few things with this method:
- The rebar should be on evenly spaced squares inside the slab
- The rebar should be at least 0.5% of the cross sectional area of the slab
- The rebar goes towards as far as it can go towards the top of the slab, while still maintaining the proper cover. ACI recommends at least 1.5"-2" of cover and you can probably find other guidelines; but for a thin slab, (4"-ish) your rebar will essentially be centered. (I understand the thinking behind this is that if you develop any cracking - best it be on the bottom of the slab, unseen vs. the functional surface, at the top.)

We are putting a concrete patio by our garage. It will be 4" concrete about 20 feet x 18 feet.

My wife doesn't like how the control joints look. And she says she doesn't mind cracks. To some extent I understand this... a control joint is an obvious line in the concrete that you can easily see. She doesn't want the pattern of control joints. On the other hand she won't see a hairline crack from across the patio at all.

I understand her position, she is an artist, and we are doing some things with the outline of the patio that will be cool, and the control joints would disturb that look.

So, what happens if we skip the control joints? OK, the concrete will likely crack somewhere. Structurally, is there any problem? Will the cracks crumble at the edges in a way different that a control joint? (Obviously rebar, etc will prevent steps from occurring at the cracks).

Another question, is there anyway to make "invisible control joints"? Maybe some kind of divider in the bottom of the form that doesn't brake the surface? Or maybe curved control joints? She might go for that?

My wife and I did this (over-reinforced) the slab in our foyer and coated it with decorative epoxy. (Being inside, we didn't want any visible joints or cuts.) The slab is around 20'X30' and (as expected) we have no cracking.

If you follow the method above, you should be fine for no visible cracking as long as your pad is rectangular. If you are doing any other shapes, then some other allowances may be needed.

 

[stm317]

There are vendors in the flooring section who sell products to fill expansion joints and saw cuts. They're hard to the touch, but more flexible than your slab, so it still controls/directs cracking. It allows guys to have seamless epoxy floors. If she's the artistic type, maybe an epoxy over the whole patio would appeal to her creative side?

 

[kj_mustang]

Mine was done with wire mesh set on chairs, 4" of I think 3500 psi rated concrete, no control joints or cuts. 32 x 28 section and another section 32 x 24 with a board separating the two sections.

 

[Falcon67]

I never saw cut anything. Go look at the 2000, 3000, 4000 sq/ft house foundations and look for saw cuts - oops, none there either.

House next door is 3300 sq/ft, 92 yards in the slab. The cracks that will occur here naturally give their stained concrete floor some nice character.

 

[ard]

Wsix99- good info.

 

[ard]

Go look at the 2000, 3000, 4000 sq/ft house foundations and look for saw cuts - oops, none there either..

I tried, but it is all under wood, tile or carpet....



If a home has a planned exposed slab, I tend to see special call outs for joints and reinforcement/ (as well as slab protection and finsihing notes during the build)

Floors in homes that were covered, and then years later exposed and stained/sealed will have that 'character'

 

[James E]

What about a thicker pour, perhaps dig the footing deeper at the center than at the perimeter so if you have settling, the slab pulls from the center, causing any cracks that form to stay close together, as opposed to falling away from each other?

What is your soil like? In San Diego, I picture sandy, soft soil. California is built on a house of cards, I imagine that most concrete slabs there over about a foot long have cracks.

Are you going to park or move anything heavy over it? If not, a 4" pour of that size might be ok as long as the substrate is solid.

 

[James E]

Cracks can be pretty and artistic but only if they develop perpendicular to the finished surface. The more acute the angle to the surface, the more likely you are to break the overlapped edge of the top piece, making the crack an ugly and deep scar that exposes the rough aggregate.

 

[Ole Slewfoot]

We have earthquakes, it is going to crack.

 

[01-7700]

I'd just love to read your concrete contractors' posts about this job on another forum....

Would make a great mashup

 

[6768rogues]

What will happen is somewhat unpredictable. Concrete shrinks, so cracks happen. With reinforcement and fiber mesh it will stay together even though it is cracked. In our climate, we use tooled joints in outdoor concrete. When I worked for the county, the specs always said that saw cut joints were not allowed in outdoor concrete. Saw cuts leave a raw edge that will deteriorate faster than a hard tooled joint. Where you are, no big deal.

 

[rsanter]

Rather than foregoing control joints, I suggest you encourage your wife to incorporate them into her design. The concrete is going to crack, and without control joints it will crack irregularly and end up looking like, well.....cracks. And what starts as a hairline crack doesn't always end up that way.

This. Put a bunch of them so it looks like large tiles.
You can even go and stain it to complete the look or add some dye to the concrete before the pour

Bob