Control joint questions for a new slab.

[Simplytodd]

Hello,

Just like a lot of other first time poster I have been lurking around in the background trying to absorb all the great info this site has to offer. Anyway I am getting ready to break ground on my shop and had a few questions about control joints in a new slab. The following is the specs that the contractor provided after our initial consultation.

Slab is 30'x62'
Shop is 30'x50' feet enclosed with the remaining 12' being covered parking on one of the end walls.
There will be 244' of interior and exterior beams.
Beams will be 24"x12" with 4 5/8" rebar with 3/8" stirrups 24"OC
Pour will be continuous 5" thick of 4000 psi concrete.
Slab mat will be 3/8" rebar 12" OC supported by chairs.

I will be meeting the contractor this coming Tuesday to review site prep and to nail down a more exact figure on pricing. The initial quote and specs didn't include saw cuts. I have come to the conclusion that I would like to include them in my build. Now on to the questions.

First. From the searching I have done with the thickness that the slab will be poured too a 10'x10' saw cut pattern seems to be the consensus. That would leave me with a grid of 18 10'x10' squares. That seems like a lot. I don't mind the cuts so much because they can be filled with caulk or some other material to keep dirt and general nasties out but I believe they charge by the foot and that's 274' of cuts. Would a rectangular pattern of 20'x10' be okay considering the amount of rebar being placed in the slab?

Second. I will be placing a 9k lift in one of the bay's and the columns for the lift would bridge between two separate squares using the 10'x10' pattern. Is this a no go or perfectly acceptable? Also I couldn't seem to find a definitive answer on pouring a thicker pad depth for the area under the lift columns. Something like a 4'x4' area poured to a 10"-12" depth? There were about as many people saying that this can cause cracking in the slab as there were saying that they want the added security. The lift manufacturers mostly call for 4" @ 3500 psi so I would seem to be okay as is without the extra concrete.

Any help or insights will be greatly appreciated.

 

[Dhagan887]

2.5x the thickness (in feet) as a minimum for the control joints. Always want to keep them within a 1.5 to 1 ratio of square (i.e if doing ~10' joints none should be more than 15'w the other direction.

 

[tlmartin84]

See Red....

First. From the searching I have done with the thickness that the slab will be poured too a 10'x10' saw cut pattern seems to be the consensus. That would leave me with a grid of 18 10'x10' squares. That seems like a lot. I don't mind the cuts so much because they can be filled with caulk or some other material to keep dirt and general nasties out but I believe they charge by the foot and that's 274' of cuts. Would a rectangular pattern of 20'x10' be okay considering the amount of rebar being placed in the slab?

15x15 would be my max, I would never recommend anything over 15'. 10' x 10' is better. In the end you would rather have a nice straight line than an odd crack.

Second. I will be placing a 9k lift in one of the bay's and the columns for the lift would bridge between two separate squares using the 10'x10' pattern. Is this a no go or perfectly acceptable?

With the rebar yes this is fine. HOWEVER consult your lift source. Mine recommends 6" off of the control joint

Also I couldn't seem to find a definitive answer on pouring a thicker pad depth for the area under the lift columns. Something like a 4'x4' area poured to a 10"-12" depth? There were about as many people saying that this can cause cracking in the slab as there were saying that they want the added security. The lift manufacturers mostly call for 4" @ 3500 psi so I would seem to be okay as is without the extra concrete.

Follow the manufacturers recommendations, slabs tend to crack when a "beam" is placed, on your beams request they angle them up to the slab at a 45 and not a 90.

Any help or insights will be greatly appreciated.

 

[Cyberbear]

I'm a little confused, out here in Kalifornia I've never heard of "beams" being put into a slab, just what is meant by beam? When I had my 2400 sq/ft slab poured we used a 6 bag concrete mix 4" thick with 3/8" rebar at 12" o/c. Also, I didn't have any saw cuts made and there are still no cracks after 21 years. When I attended college back in the mid 1960s and took the class on concrete, saw cuts were thought of as only weakening the slab and actually creating a place to create an unwanted crack in the slab, plus also creating a place to trap dirt and chip when steel wheels were rolled over them. I guess times have changed since then and several slabs later over the years.

 

[matt_i]

If you did 15x15 squares roughly, 1 cut down the center @ 62' and 4 cuts @ 30' = 182'. Roughly half the linear distance. What's the "per foot" charge? If its $5 per foot I might rethink the spacing to larger. If its $1 per foot I would just roll with the 10x10 personally. The biggest danger is trying to attach an anchor bolt next to a saw cut. Because of the propensity for the slab to crack all the way thru, the potential is there to have a bolt hole/stress riser where it can chip off a small "rock" that can separate from the main mass. Rule of thumb is attempt to stay 6" back from the saw cuts for light duty anchoring and 12" back would be my choice for anchoring a more serious machine like a lift.

I would not worry about bridging a saw cut personally. Its theoretically inadvisable to set something like a machine tool across a saw cut, but a vehicle lift is no big problem. The thickened slab in my opinion is to give extra pullout strength to the anchor bolts that you choose to use. I'd say a full 6-7" is probably sufficient, especially if you use epoxy anchors and follow the prep. So that's an inch or two more than nominal, should not become a serious stress riser there.

Something to think about if you are erecting a clear span metal building (because of the grade-beam foundation perimeter you mentioned). In my opinion, you really don't want the saw cut to go directly thru an anchor bolt location for the building columns. So there is that to consider in your layout. Stick built or floating slab would not be a concern at all.

 

[nehog]

... 10'x10' saw cut pattern seems to be the consensus. That would leave me with a grid of 18 10'x10' squares. That seems like a lot. I don't mind the cuts so much because they can be filled with caulk or some other material to keep dirt and general nasties out but I believe they charge by the foot and that's 274' of cuts. Would a rectangular pattern of 20'x10' be okay considering the amount of rebar being placed in the slab?...

My 30x60 is cut on 10x10 pattern. The contractor didn't charge for the cuts, they took about two hours to do, IIRC. Cuts are made only about 3/4" deep, when the concrete is very soft (just after the pour is firmed, for me it was the next morning.)

 

[wssix99]

Slab mat will be 3/8" rebar 12" OC supported by chairs.

This is the work of an engineer... You may ask your contractor about the cost of 3/8" vs. 1/2" rebar. 1/2" is probably less expensive. Kick it back to the engineer to re-calculate the spacing based on using 1/2" rebar and it should drop the cost of your slab. It will use less expensive bar, decrease the lineal footage, lessen the labor cost, and increase the spacing - making it easier for your contractor to pour and get you a quality finish!

It's so easy to design when they are using your money...

Slab is 30'x62'
Shop is 30'x50' feet enclosed with the remaining 12' being covered parking on one of the end walls.
There will be 244' of interior and exterior beams.
Beams will be 24"x12" with 4 5/8" rebar with 3/8" stirrups 24"OC
Pour will be continuous 5" thick of 4000 psi concrete.
Slab mat will be 3/8" rebar 12" OC supported by chairs.

I will be meeting the contractor this coming Tuesday to review site prep and to nail down a more exact figure on pricing. The initial quote and specs didn't include saw cuts. I have come to the conclusion that I would like to include them in my build.

This sounds/looks like an engineered slab. Thus, you will need to consult the engineer for your questions. You will also need to confirm with the engineer the placement of your lift and how that will interact with your slab.

In some situations (many in Texas - I'm not sure if Houston has the issue) the soils require engineered slabs due to expansive clays, etc. It sounds like your slab is structural and integral to your grade foundations so the whole thing will move as a unit as the ground swells/moves. So, practices typical to a common "floating slab" may not apply to your building.

I expect an engineer is already engaged to sign off on your structural plans, so getting the answers to your questions will hopefully not be an incremental expense.

With a structural slab, you may not need control joints at all! Again, your engineer can confirm this. You may have enough structure and reinforcing already there to control cracking, so control joints could be redundant, an unnecessary expense, and counter-productive.

First. From the searching I have done with the thickness that the slab will be poured too a 10'x10' saw cut pattern seems to be the consensus. That would leave me with a grid of 18 10'x10' squares. That seems like a lot. I don't mind the cuts so much because they can be filled with caulk or some other material to keep dirt and general nasties out but I believe they charge by the foot and that's 274' of cuts. Would a rectangular pattern of 20'x10' be okay considering the amount of rebar being placed in the slab?

Common practice for a typical unreinforced, "floating slab" is to have the joints spaced 10-12' in squares. The further out you go, the higher the chance you'll get cracking outside of your cuts. You can make rectangles, but you don't want any of the sides to go longer than 10-12'.

If you add reinforcement to the slab, that 10'-12' distance can be expanded. The American Concrete Institute has a design guide for slabs-on-grade that has a chart where your engineer can look this up. If he/she says that you do need saw cut joints, they can look up this distance for you based on your final reinforcing design. (above) However, I suspect with cris-crossing grade beams in your slab, you won't need any saw cut joints - but it all depends on how far they are spaced from each other.

Second. I will be placing a 9k lift in one of the bay's and the columns for the lift would bridge between two separate squares using the 10'x10' pattern. Is this a no go or perfectly acceptable?

It could be a no-go because your saw cut joints could violate the guidelines the lift manufacturer gives for the posts being in proximity to "cracks." Thus, it's important to plan these cuts out (if you need them) along with your lift placement ahead of construction.

Your lift manufacturer also gives specs for a floating slab. With your structural slab and beams underneath, you'll need your engineer's help/blessing with this, anyway.

Also I couldn't seem to find a definitive answer on pouring a thicker pad depth for the area under the lift columns. Something like a 4'x4' area poured to a 10"-12" depth? There were about as many people saying that this can cause cracking in the slab as there were saying that they want the added security. The lift manufacturers mostly call for 4" @ 3500 psi so I would seem to be okay as is without the extra concrete.

The lift manufacturers call for a consistent depth, because that is best practice. A thickened slab builds up greater shrinkage stresses in the concrete. (because it has a wider cross section) With the same cross section of reinforcing steel as the rest of the slab, you build up differential stresses in that thickened area. That will lead to an increased chance of cracking.

there were saying that they want the added security.

People who think that a thicker slab provides increased strength or security misunderstand the mechanics of the lift/slab interaction and the behavior of concrete, which is a heterogeneous material. (It doesn't behave like a homogeneous material - ie: a plate of steel.) A thicker slab does give you some anchoring attachment advantages but at 5", this shouldn't be a concern for you.

 

[Simplytodd]

Second. I will be placing a 9k lift in one of the bay's and the columns for the lift would bridge between two separate squares using the 10'x10' pattern. Is this a no go or perfectly acceptable?

With the rebar yes this is fine. HOWEVER consult your lift source. Mine recommends 6" off of the control joint

If I go with the 10'x10' pattern the bay with the lift would be split right down the middle with the lift being several feet from the anchor points for the columns.

Follow the manufacturers recommendations, slabs tend to crack when a "beam" is placed, on your beams request they angle them up to the slab at a 45 and not a 90.

I'm not sure I follow. Are you saying ask the contractor to bevel both the interior and exterior footers and beams or only if I were to add the extra pad depth below the lift columns.

I'm a little confused, out here in Kalifornia I've never heard of "beams" being put into a slab, just what is meant by beam?

I'm in Texas and my building site has clay after you dig down a couple of feet. Slabs with footers and beams are quite normal here. As far as the saw cuts, it is more likely to make the slab crack and that's exactly why I want them. They will crack at the saw cuts and not some random place in my foundation. Hopefully.

Something to think about if you are erecting a clear span metal building (because of the grade-beam foundation perimeter you mentioned). In my opinion, you really don't want the saw cut to go directly thru an anchor bolt location for the building columns. So there is that to consider in your layout. Stick built or floating slab would not be a concern at all.

It is a clear span metal building and I hadn't even thought of the building columns as far as saw cut placement. It looks like I might need to wait to get the actual building plans in my hands before trying to decide on saw cut placement. The quote I got from the two building suppliers only includes rough sketches on placement for columns with no measurements shown.

 

[machsnell]

We place concrete everyday.

I would get no 4 bar (1/2") space 18" on center.

5 inches of 4000 is good.

Slump no higher than 3 inch ( you just have to hope for the best on this unless you know what 3 slump looks like)

Add microfiber. Cheap insurance trust me. If you aren't getting epoxy don't care about finish go reg fiber. Micro you can't see.

Do vapor barrier at a minimum and 2 inch insulation if you can. Insulate the edges vertically at a minimum if not under slab. It is like a big expansion joint and you should go down as deep as your frost line.

Dont pour in freezing temps or when ground is frozen. You want the ground dry and tight and not frozen.

Install stone. Well compacted 21a or stone with fines like read base. 3/4 inch washed is fine too but more for a capillary break which is what your vapor barrier is for.

Move your control joints away from where your lift will go. It's OK if they aren't evenly spaced.

Typically it's a max of 15 foot for cuts. But it really comes down to soil type and prep and concrete type, metal etc. and the confidence you have in what's under the concrete.

When I did my slab which has an odd shape I did all my prep with my guys it was spring and I made sure all my backfill was tight and stone was compacted very well. Jumping jack on footing backfill after block was cured. Real 3 to 6 ton roller on stone on slab not a plate tamper. All of this matters.

I have 5 inches 9f 4500 psi with no 4 18" centers and microfiber 1600 sf and one saw cut joint on the odd angle of my slab. 2 adn a half years ago and not a Crack yet.

I am not saying to do this and I have fully expected getting one but after the first winter where I didn't get one I kind of let it ride.

Remember basements never get control joints and are usually never poured on tight or dry ground and always with a high slump and thin section. Even tho they are temp controlled they Crack much less than most would imagine.

Where are you anyway?

Subgrade amd Prep prep prep.

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[rburke65]

He said he was in Houston

 

[Simplytodd]

 

[Simplytodd]

View attachment 512328

So the long story is I was originally going to to do the slab myself with my father in law and a friend. From the high point to the low point of the slab is a difference of just over two feet. I undercut and graded the ground and had some select fill brought in to level the site.

After I did this I joined the site and started researching some questions that I had. Right off the bat I realized that the site prep was done wrong. The select fill was just brought in all in one day and not in lifts. There was no compaction done between loads. So I started to question the info I was getting from the friend who at one time was in the construction industry. To top that off there was one section in the pad that my friend said had fractured and was pumping. I ended up discing the entire pad and bringing in another 12 yards of bank sand to amend the select fill with. At this point I decided it best to have a professional do the whole thing from start to finish.

My neighbor and I had a falling out about this time and I needed to get a new power easement from him to makes some changes to the existing power coming into the property. After a year the power has finally been resolved and I am ready to move forward with the shop

So the pad has been sitting for over a year untouched. This is why I will be meeting the builder this week to discuss the site prep. The slab design is not a engineered slab it is what the builder designed. The builder is from my area and comes highly recommended from the two building supply companies I am considering.

 

[joes169]

This is the work of an engineer... You may ask your contractor about the cost of 3/8" vs. 1/2" rebar. 1/2" is probably less expensive. Kick it back to the engineer to re-calculate the spacing based on using 1/2" rebar and it should drop the cost of your slab. It will use less expensive bar, decrease the lineal footage, lessen the labor cost, and increase the spacing - making it easier for your contractor to pour and get you a quality finish!

Just an FYI, most concrete contractors buy rebar by the ton, and the price is the same, or nearly the same, for 3/8" through 1", by the ton. Obviously, there's more length in a ton of 3/8" rod (5320') vs 1/2" (3000'), but the difference is negligable. Personally, I'd tie 3/8" on 24" centers at the very closest................

 

[wssix99]

The slab design is not a engineered slab it is what the builder designed. The builder is from my area and comes highly recommended from the two building supply companies I am considering.

If you have grade beams crossing the entire slab, you can't just make that up. Maybe your builder has adapted it from someone else's engineered design? If you really do have beams running across the field of the slab, you should consult an engineer regarding the control joints and your lift to figure out how all that will interact. Doing saw cuts "by the book" and your lift instructions both apply to floating slabs, which doesn't sound like what you'll be getting.

Just an FYI, most concrete contractors buy rebar by the ton, and the price is the same, or nearly the same, for 3/8" through 1", by the ton. Obviously, there's more length in a ton of 3/8" rod (5320') vs 1/2" (3000'), but the difference is negligable.

Not for #4 and not if you buy a lot of it. In my area, there is a good price break for #4. Every other size is a wash based on weight. When you take in to account the reduced labor of handling the #4 vs. the #3, the savings are larger.

Personally, I'd tie 3/8" on 24" centers at the very closest................

If you are eliminating saw cut joints, you need to go closer.

 

[tlmartin84]

I'm sorry but I am in agreement with Joe, here rebar is always based on poundage UNLESS you are buying it from a lowes, hd, etc. but from a REAL steel supplier it will be based off of the poundage, and if there are any bends, epoxy coat, etc.

If his is on 12" centers, the savings for labor will be negligible. You have to tie No.4 bar at every intersection if it is on 2' centers, with the No. 3 at 12" he can get away with every other intersection. So the only savings would be placing the extra bars, not tying.

On a side note, I don't think he will need any insulation since he is in TEXAS..... AN ENTIRE DIFFERENT SITUATION THAN US NORTHERN GUYS OFFERING ADVICE.

 

[Pwrgeek]

One thing to account for is that in Houston expansive clay is very common. I just had my shop poured and I had about twice the rebar the book said I needed put in because I didn't want to have problems later.


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[joes169]

Not for #4 and not if you buy a lot of it. In my area, there is a good price break for #4. Every other size is a wash based on weight. When you take in to account the reduced labor of handling the #4 vs. the #3, the savings are larger.

I buy 20+ tons of rebar a year, I'm all ears if you can give me a heads up of who's selling #4 rebar at a reduced price in the Chicago area. I'm not that far away, and it's winter, I'll make the drive.

BTW, from someone who handles rebar almost everyday (not on paper, but in reality) #3 is much, much easier to work with, and the what little time savings there may be from spacing with #4's can be made up in cutting, bending, and carrying #3's typically.............

 

[wssix99]

I buy 20+ tons of rebar a year, I'm all ears if you can give me a heads up of who's selling #4 rebar at a reduced price in the Chicago area.

White Cap was, two years ago, when we ordered the steel for our building.

It's not that much, but at least one to two cases of beer of savings per ton. If one invests that beer (after work) for the crew, you may end up with a better quality product.

Most of the flatwork companies around my area have #4 on their trucks. That also tips the equation, a bit. Since the OP's design calls for a bunch of different sizes, stirrups, etc. it may not be as big of a deal on the material cost side of things.

If his is on 12" centers, the savings for labor will be negligible. You have to tie No.4 bar at every intersection if it is on 2' centers, with the No. 3 at 12" he can get away with every other intersection. So the only savings would be placing the extra bars, not tying.

That's a few hours of work. For a crew that comes to the site with machines and operators, the savings are significant - for the owner/OP. (not necessarily for the contractor)