Joints in reinforced slab on grade 1

[Despy]

I have looked all over, but have been unable to find a definitive answer regarding the placement of shrinkage control joints in reinforced slabs on grade.

The PCA publication "Design and Control of Concrete Mixtures" lists a table showing slab thickness vs. spacing. The table lists a maximum spacing of 10-ft for 4" slabs. This agrees with the old rule of about joints being spaced 30 times the slab thickness, but this is all based on unreinforced slabs. This is also less than we have traditionally used for reinforced slabs (max. around 16-ft o.c.).

What would be proper slab joint spacing for a 4" S.O.G. with #4 bars at 18"o.c. each way? Is there a calc or publication I can point to as backup?

Thanks

 

[JAE]

Check out ACI 360 perhaps?

 

[Lion06]

Yes, ACI 360 gives formulas (one is the subgrade drag theory) for spacing of control joints based on area of steel.

 

[Despy]

Thank you for the immediate feedback. I do not have that reference at my fingertips, but a coworker in our branch office does and is forwarding me the information.

 

[asixth]

When you say 'shrinkage control joints' are you referring to soffcut joints. We typically provide soffcut joints at 20' (6000mm) centres and try not to exceed panel ratios of 1:1.35. I have seen this detailed for 5" (125mm) pavement slabs reinforced with WWF top reinforcement subject to light traffic loads and the slab has performed quite well to date. I will try and find some reference material tomorrow at work.

I think I remember seeing something posted in the Q&A section of a recent Concrete International magazine on this topic but I am going to have to dig around for it.

 

[BarryEng]

When I started work using concrete slabs, your question was one I also tried to find an answer to. Perhaps if I answer a question that you did not ask - it may open up discussion from other readers. What level of reinforcement is required in a concrete slab so that joints are not required?

It has been suggested (years ago), that crp (continuously reinforced pavements) concrete in the USA, should have a minimum reinforcement percentage of about 0.7% (I don't know the currect practices). This assumed that 0.7% was the minimum quantity of reinforcement required in a pavement so that any stresses applied (by concrete shrinkage restrained by the soil) would not create one large crack - rather it would distribute any cracking to a large number of very fine cracks. This means that NO joints are required - except for construction joints due to a pour conclusion at the end of the day. Reinforcement would be continuous thru' all joints & the joint would be prepared thoroughly (keys for shear transfer, surface preparation etc etc). Remember the environmental conditions (in the weather all the time) & the stress reversals due to traffic (in the life of the crp) require a solution that may be excessive for a 'normal' SOG.

I have designed reservoir slabs for many years (up to 200 m square with 10 m depth of water) & have used about 0.5% reinforcement. For my 150 slab, the reinforcement is N12-150 EW top (#4 @ 6"). Pours were continuous for up to 150 m X 10 m, with a movement joint only located at junction of the sloping wall/floor concrete. For your slab of 4" (100 mm), 0.5% would require 500 mm2/m for N12-220 (#4 @ 9"). This is the basis of the minimum requirements of the Australian Standard (AS 3735) - 'concrete structures for retaining liquids'. Fully restrained concrete, & minimum reinforcement with no joints.

Back to your problem - a 4" slab with #4 @ 18".

It appears to me that you are some way between a completely restrained slab (using #4 @ 9") & an unreinforced slab. I suppose that it depends on the degree of restraint offered by the soil. This will vary depending on the degree of surface preparation. I suppose that you could use two layers of plastic on the subgrade but the effect would be dependent on the 'roughness' of the soil & plastic layers. Any local imperfections would ensure that there would be full restraint between the slab & the soil surface profile (the plastic sheets would hence have no effect). I have been told (by a local CACA Engineer) of a roller skating rink was constructed using a reinforced concrete slab, & topped with two layers of plastic sheeting with grease in between the two sheets, & topped with a non reinforced slab. This system ensured that there was no cracking (no restraint due to a smooth interface & the grease layer). I assume that it was very expensive.

Our local airport (for 747's) has used a 535 mm thick slab, unreinforced, but supported by an extensive sub grade construction. The joint spacing does not appear to be > than 8 or 10 m.

For info on local experience, have you tried your local CACA, concrete paving contractors, road designers, etc?

 

[Despy]

In response to BarryEng:

In the USA, the ACI 360R code section 7.3 says that slabs can be constructed without sawcut contraction joints if you use 0.5% reinforcing steel in the upper half of the slab depth. I do not have this level of reinforcement, and for a pedestrian slab I do not feel it is warranted.

As mentioned, what I am using is a design somewhere between unreinforced and the 0.5% reinforcement for no joints. According to multiple sources this should allow me to extend the spacing between control joints from the default value used for unreinforced slabs. What I am searching for is some way to quantify that conclusion. I have found some information from the Wire Reinforcement Institute (WRI) at the following site:

http://ijmweldmesh.com/AdditionalResources/Innovative ways on Reinforce slab on Ground.pdf

None of these methods appear to be referenced in ACI, but using the five different methods found there (including subgrade drag theory) it looks like my original design is fine.

Any thoughts?

 

[BarryEng]

Thanks for the reference.
It contains some information that I have seen in a CACA paper on concrete slabs. I'll see if I can find the reference & post it.
I only work with completely restrained slabs & it appears to me that you are on the right track.

 

[B16A2]

How about trying an internally cured concrete? No joints or reinforcement required because it doesn't shrink. Works wonders.

 

[BarryEng]

I don't know if I can post the full publications on this site, but here is a brief list. If you want to know some excerps, reply to this post.

Concrete Society, TR34, 2003, 148p

Concrete Institute of Australia, Design of joints in concrete structures, CP24, 2005, 20p

CACA Australia, permanent industrial pavements, 1996, 200+p
suggests reinf of 0.1 to 0.3% (I think for sheltered pavements - eg under a factory roof)

The last reference gives (for a 100 slab)
slab length reinf area
0 to 13m 90 mm2/m (#4 @ 48")
13 to 30m 90 to 220 mm2/m (linear with length)

Seems a bit light to me but I suppose it depends on your exposure, loading, expected pavement life, importance, curing practices, soil surface preparation etc etc

 

[OHIOMatt]

I have used the .5% reinforcing on slabs and had relatively good succes with them. The slabs were constructed with no joints.

I have also used a minimal amount of reinfocing as well. Such as #3 bars at 18" in a 6" slab (about .1%. The slab was saw cut at about 15' c.c. The minimal amount of reinforcing would yield at the joints and allow them to activate. The bars would still have shear capacity. (Please note that this does not meet the minimum criteria for T and S steel as required by ACI., but since you are installing joints, I have taken the approach that this is acceptable, as an unreinforced slab is permitted by ACI).

Your idea of #4 at 18" in a 4" slab will likely create cracks that leave the joint and follow the bar to some degree. In slabs you do not want to be in the middle in terms of reinforcing.

 

[cancmm]

Many of your questions are answered in the Army's pavement design manual (TM 5-822-5). They address contraction joint spacing, reinforcement requirements, slab thickness, joint details etc. for both unreinforced and reinforced concrete pavement. They also address flexible pavement too to some extent. If you are dealing with very heavy loads, they have a separate manual which addresses this. The theory is rather lacking and I don't recall any references to ACI, but it is a solid reference in my opinion.