Control Joints in Concrete

[Dustin Crawford]

Hopefully this is the correct section.

My slab has been poured inside my pole barn. Its about a week old. It measures 24'x28' with the two garage doors on the long side.

I planned on cutting a control joint between the garage doors which would make two 14x24 panels.

My contractor said I should do one the other way as well to make 4 14x12' panels.

Sounds good to me but it would run directly under where my 2 post lift will bolt down to. That's a little disconcerting to me.

This is a 6" slab with lots of rebar and 4'x4'x12" pads where the lift goes.

What do you guys think?

 

[dcs Inc]

The crack control cuts should haver been placed right after finishing the concrete. Early entry cuts help protect against shrinkage cracks. If the thickened slabs for your lifts are poured separately, I would be concerned about cracks coming off the corners of this slab as it will expand and contract differently than the rest of the floor. If you can line up the saw cuts to the corners of your thickened slab, that could be the way to go.

If the thickened slab was poured at the time of the whole area placement, this too can cause cracking so cuts would help control random cracks at these points also. You can't have too many crack control cuts in a slab. I like seeing them at 8 foot intervals each direction.

Also, remember the slab cuts should be 1/4 of the depth.... at least. Anything less than that will only be cosmetic. gene ec-Indy

 

[Dustin Crawford]

Thanks Gene. The thickened lift areas and the area in front of the garage door threshold are poured as one. However they are tapered at 45* to full depth in an effort to help with cracking.

So cut ASAP and who cares if they go under a lift right?

 

[Dustin Crawford]

Oh and what can I expect from waiting to cut the concrete? How bad did I screw up?

 

[dcs Inc]

Well the shrinkage stage of the concrete is probably over with by now. Concrete cracks from the bottom up so you may a a visit from a guy named random down the road. You have this slab filled with rebar, hopefully it was poured on some kind of supports so it in the middle of the slab. Laying on the bottom does nothing.

I wouldn't run the cuts through the thickened concrete if thats what your asking. If you can configure them to run parallel with it, that would be better.

If you can draw something so I can see it, I may be able to give you more of an informed answer. It's not hard to figure and mostly it's just common since. Just gotta think like a crack. It will take the path of least resistance. gene ec-Indy

 

[ConCretin]

Dustin,
The purpose of saw cutting is to control where the cracks occur during the curing process occur by creating weak spots.

Shrinkage cracks are most likely to occur as mix water is consumed by hydration and evaporation. A 24x28 slab that didn't crack in the first 24 hours may not crack at all.

Saw cuts at this point will just weaken an otherwise intact slab. You may get a random crack in the future but personally I'd take the chance.

 

[pauls340]

I love this forum. DCS and LL have good points. My next slab of concrete will have Vapor Lock 20/20 Admixture in it and no control joint saw cuts. Not needed with VL. But, you're dealing with post-placement concrete. Track down a distributor for Vapor Lock 5/5. Pre-wet the slab and then spray one or two coats of VL5/5. When or if you ever want to epoxy the floor, mechanically blast it and do your application. You will have zero moisture drive issues with your slab...mine is going on three years and zero moisture. Good luck.

 

[Dustin Crawford]

Great point. That falls inline with what I was just going to post. And for the record I'll post a diagram for cutting tomorrow.

So I might be way off base here but I got to ask- do I really need control joints on a 24x28 garage?

Here me out here, everyone I ask about this around here looks at me like I am retarded. It goes like this "so you want to not have cracking be a problem in your shop floor? so your going to cut grooves in the floor?" thus implying that the cuts would be just as inconvenient as cracks albeit a little more appealing.

Not saying it's right but I can't think of one garage around here that is even remotely my size that has any expansion or control joints. Not to mention I have a WAY better subgrade, am 2" thicker and have a ton of rebar. I see slabs with control joints cut in often, but it is on outside exposed slabs and usually much larger in size. Lots of people say to do it, but I don't see it done in applications like mine so what the heck am I missing here?

If the subgrade is prepared well, you have rebar to add strength in tension, and you have a covered enclosed and insulated application I dont really see how you could do any better.

And one other thing. What exactly is the difference between a crack, and a sawcut control joint? They both sound like a pain in the **** to me.

Just thinking out loud. The way I see it is I have 2 choices. No cuts at all, or 3 cuts to avoid the thickened lift pads. This would leave me with 6 squares which seems like a lot of cuts for a garage my size.

DC

 

[pattenp]

My garage is 26 x 30 with no control joints. My slab was to be 4" but actually ended up being about 5". The concrete has fiber in it and the concrete guy said that there was no need for control joints. If you do cut control joints beware that 2 post lift install specs usually say not to place lift within a certain distance of a control joint.

 

[ConCretin]

Dustin,
You answered your own questions well and outlined the issue pretty well to boot.

Saw cut control joints are just pre-planned, straight cracks. If everything is done correctly, you have an excellent chance of avoiding random cracks on a 24 x 28 slab with no control joints.

A proper sub grade and reinforcing are part of it but the concrete and curing are far more important. We are trying to minimize early age plastic shrinkage cracks.

The way to do this is by using a good mix design with at least 3/4 aggregate, keeping the slump as low as possible (use a mid range water reducer) and curing the slab properly to keep it from 'drying out' too quickly.

If you pay attention to the details, it is very easy to have a modest sized slab free of cracks and control joints.

 

[ConCretin]

I'll just add this. saw cut joints are good insurance if a random crack will ruin your day.

 

[Dustin Crawford]

Thanks a lot for the help and great explanations! I'll post up a pic of the floor layout so you can see where the thickened portions are. Then you guys can tell me if I should do it or not. I just want you to see where everything is and how it may be not ideal to cut. Or at least without at least 3 I'm thinking.

Oh and one last thing. Is there any way me cutting this could actually hurt not help?

 

[Dustin Crawford]

Here is my floor plans

The shaded areas are the deeper areas. The rest of the slab is 6"

Here is my rebar schedule

some pics

Square recievers

Subgrade before 12mil plastic

Hopefully this shows you why I am concerned about cutting in 4 squares.

1. close to the lift
2. right through the thickened pads.

 

[dcs Inc]

here's how I would lay out the cuts.

 

[slickgt1]

I would take the chance, and not cut it at this point.

 

[Dustin Crawford]

Sorry, I am not sure. Do you mean you would cut the floor or not cut?

 

[cderalow]

here's how I would lay out the cuts.

This seems like overkill given the size of the slab.

I'd go with the central, and then the two Y shaped cuts along the door thickened slabs.

 

[dcs Inc]

You asked, I answered. Do what you will. I gave you the best set up for cuts. Good luck. By the way slick, what the hell are you thinking? gene ec-Indy

Oh, a 12x24 slab is way too large to protect against random cracks. Cd, what are you thinking?

 

[Dustin Crawford]

You asked, I answered. Do what you will. I gave you the best set up for cuts. Good luck. By the way slick, what the hell are you thinking? gene ec-Indy

Oh, a 12x24 slab is way too large to protect against random cracks. Cd, what are you thinking?

Hey Gene, I'm not sure what the deal is. I wasn't arguing with you. I appreciate your input.

In my last post I was trying to get clarification on what Slick was trying to tell me.

DC

 

[dcs Inc]

I wasn't pointing that reply towards you, sorry. I should have been more specific.

gene
Elite Crete of Indiana

 

[slickgt1]

****, type to fast sometimes and not check what I wrote. I was trying to say, that I would take the chance and not cut the slab.

Reason I say this, my mason poured cement at a project 7 years ago. Huge sidewalk. 40' long, 14' wide. No control joints, just decorative slits. 4.5" thick, compacted, and thoroughly prepped ground. Not one crack yet. He gave a 10 year warranty in writing as well.

Needless to say, I have been doing business with him since that time.

Recently he poured a garage, 23' x 19', 5" thick, rebar and mesh. No control joints either. He did come to hose it fairly often for about a month.

Your slab is even thicker. I'll ask him what he thinks next time I see him.

So if it was me, I would just leave it without cuts, and deal with it if it cracks. Plus you get a smooth floor without crevices for **** to hide or get caught in.