Slab on Grade - Control Joints Revisited

[pelotoner]

I have seen control joints detailed with just a saw cut (usu. 1/5 to 1/3 of slab depth) and steel continuous, a saw cut with every other bar cut, and a saw cut with the steel discontinuous.

I'm relating to reinforced slabs on grade in Florida; we have primarily 2 types: 4" with WWM (standard) and 6" with rebar (heavy loads or differential settlement).

It seems exceedingly rare that discontinuous steel, or every other bar/wire cut occurs at the actual saw cut location. I also cannot see how not cutting the steel would restrain the crack. When the slab shrinks it puts the slab and the steel in tension. Even with #4 bars @ 16" in a 6" slab, the transform area of steel to concrete is only 1.35 sq in./ft, which is much less than the 1/5 of the slab depth (14.4 sq in./ft) and should effectively weaken the slab to illicit a crack there.

Are these details stopping/cutting the steel just archaic, or is there a concept I have missed? What depth do you use (1/5 is our standard)?

 

[SrVaro]

The idea behind a contraction joint (control joint) is for the concrete to have a place to crack, as all concrete will crack over time. This allows the crack locations to be controlled and if detailed properly the cracks will be sealed.

I have added steel within unreinforced slab at contraction joints via load plates and dowel baskeets. The idea of stop every-other bar is to limit the amount of steel at the location you want the concrete to crack so it will crack "there". If the steel is the full amount and continuous, then only the reduced section of the concrete at the saw cut would attract the crack. By removing steel you are allowing the concrete to crack "there" instead of someplace else.

The depth of the saw cut I usually use is 1/4 the thickess else a min of 1".

Remembering of course that this is only one element in crack control of concrete. Mix, placement, curing, etc also need to be considered.

 

[DaveAtkins]

With a lightly reinforced slab-on-grade, the wire mesh should yield at a control joint, allowing the crack to form.

DaveAtkins

 

[pelotoner]

DaveAtkins,
What depth of saw cut do you use?

 

[ron9876]

The steel isn't in tension until the concrete cracks. The purpose of the steel is to keep the cracks relatively tight.

I typically stop part or all of the horizontal steel in a retaining wall at the joint because if the wall decides to crack at a location other than the joint the crack will be tight and it will then force movement back to the joint.

I am in Florida also and almost all WWF that I see is on the bottom so there is no way to cut it.

 

[pelotoner]

Thanks Ron,
What depth of saw cut do you use on a SOG?

I assume you're talking about a vertical control joint in a retaining wall? In those cases, we use an isolation joint with rubber joint material and sealant.

 

[ron9876]

t/5 or 1". Yes I am talking about vertical joints.

 

[Den32]

I believe ACI recommends 1/4 of slab thickness for control joints (1/3 of slab for fiber renforced).

You should read ACI 302 & ACI 360 regarding Design and Construction of Slabs-on-ground.

There is alot of information how all this works and what ACI recommends. ACI will cost money if your firm doesn't have, but there is great information in there on slabs. The questions you are asking are 1/10 of the 'slab performance pie' (i.e. curing, finishing, joint spacing, joint locations, mix design, etc.). Also, the Wire Reinforcing Institute has free technical pubs available on their website.

You should look into Fiber Reinforcing as well. WWF usually ends up on the vapor barrier and doesn't help with crack control. I like to spec slab fibers unless the Owner objects. Contact your local Propex Systems Rep, or read further on their website.

 

[a2mfk]

Den- respectively disagree with your comment on fibers, they do nothing but possibly reduce the formation of plastic-shrinkage cracks at the surface. They will not prevent a full-depth shrinkage crack or keep it tight and are therefore not even close to an equal substitute for WWF (that is properly placed).

REF ACI 302.1R-26, Section 5.9.3

Also it has been my experience inspecting residential slabs that they do very little in terms of shrinkage cracks. But if used in conjunction with WWF, reinforcement, CJs, mix design, etc. they may help a little with the micro plastic shrink cracks.

 

[epitome1170]

I actually agree with Den.

I understand the reservations a2mfk and you are probably correct with some fibers; however, if you get a good micro fiber specified and added to the mix at the correct dosage, you can eliminate WWF entirely on a project (as long as it is not needed structurally). The problem is that there some fibers better than others to achieve these results.

Additionally, the fibers are not a cure all. If your w/c ratio or curing preps are not adequate, the reinforcement or fibers wont matter at all. The slab will crack.

 

[DaveAtkins]

I generally specify a 1/8" wide sawcut joint that is 1/4 of the slab depth. The wire mesh "should" be 1/3 of the slab depth from the top of the slab.

DaveAtkins

 

[hokie66]

Plastic fibres have been discussed a lot on this site. The salesmen have evidently done a good job of convincing some that their useless product can replace steel reinforcement.

 

[Den32]

Everyone has their opinions. I am not a Fiber rep, nor have I ever been one.

I have specified and seen both fibers and WWF installed. In my opinion, most of the time WWF ends up on the vapor barrier (or through it after they step on it) and is useless. I have even used steel fiber blends in heavy industrial slabs in lieu of rebar to great success.

I used fibers on my own 20 foot by 30 foot patio slab with no cracks.

We are all grown ups here, use what you want.

 

[Ron]

Not a fan of WWF and certainly not a fan of fibers other than steel. As I've stated in some of those numerous threads that hokie66 mentioned, fiber is a property enhancement...not a reinforcement.

I am a fan of proper mix designs, proper grade and thickness control, proper placement techniques and proper finishing/curing. Sawcuts should be 1/4 of slab thickness and they should be cut early...as early as possible on the same day as placement.

Joints should be spaced such that no joint is more than 15 feet from adjacent and ratio of slab size bordered by joints should be no more than 1.2:1.

Den32...you got lucky. Polypropylene fibers will spread the cracks a bit further apart...but won't stop them completely.

 

[a2mfk]

Den- I agree with Ron, I think you got lucky my friend... Did you place CJs or cure the slab?

Read this section for their opinion on POLYPROPOLENE FIBERS:
ACI 302.1R-26, Section 5.9.3

I posted a pic we have all seen, but this is a lot of CJs and I found no cracks in this driveway. That is rare to find no cracks.

I do several residential inspections a week, and document cracks in driveways, garages, and pool deck slabs-on-grade. Since I don't know the mix or reinforcement, and I was not there for placement and curing, the only conclusions I can draw are based on crack and joint locations and quantities. And based on 100+ and counting such investigations, CONTROL JOINTS appear to be the common factor in crack control. Those with no joints usually crack in relatively predictable patterns, and those with joint spacings in the 10'-15' o.c. range have much less cracking, and the few times I see spacing below 8' there are rarely cracks. And like Ron said, keep the panels as square as possible and avoid odd angles with your CJs.

A few years ago fibers were all the rage, and I was sucked in for a while. Like others have said, good marketing I think. I still use fibers but just for micro crack control. A few years back we put a large section in our general notes on slabs-on-grade leaving the decision up to the contractor and owner with our opinions clearly stated. We also clearly state that all slabs-on-grade will crack, and we make no guarantees to their performance. We let them select their slab:
Level one- fibers only
Level two- fibers and WWF (we say use sheets, but...)
Level three- fibers and reinforcement

Well, you can probably guess what level they usually go with but at least we were up front with everyone. After all, a SOG is non-structural anyway (not talking about mats)..

Steel fibers are a completely different beast.

I agree that WWF is rarely placed correctly.

 

[a2mfk]

This is an example of exceeding the 1.5:1 ratio on the CJ locations. The slab panels cracked down the center predictably where another joint should have been placed. See this all of the time too.

Sometimes I so site visits in a neighborhood where almost every driveway has CJs at like 5-6' o.c., amazing that a contractor if Florida would voluntarily do that (or tired of call backs over slab cracks)... Rarely find a crack in this neighborhood...

I know this is all a bit anecdotal and not a scientific study. But at 100+ investigations and counting, a clear pattern has emerged for me. And now because of my current line of work, I find myself sadly looking and slabs and joints everywhere I go. Yes, I have other hobbies...

 

[Den32]

Yes, I had them place control joints. So, panels were 15 feet by 10 feet. No curing.

Unfortunately, don't currently have access to ACI 302.

Yes Ron, agree with you in that there are many other ways to better the slab-on-grade final product.

There are also slabs poured without reinforcement (fibers, WWF, etc.) that perform well (with proper mix, finishing, curing, joints,etc)

 

[Ron]

There are also slabs poured without reinforcement (fibers, WWF, etc.) that perform well (with proper mix, finishing, curing, joints,etc) [\quote]

...exactly, Den32