Extending slab on grade contraction joint spacing with reinforcement

[engjg]

ACI 360-10 provides recommendations for unreinforced slabs on grade contraction joint spacing of 24 to 36 times thickness of slab

ACI 360-10 8.3 speaks to 0.5% reinforcement to eliminate the need for contraction joints

ACI 360-10 8.1 notes benefits of reinforcement allow for "use of longer joint spacings than unreinforced slabs"

ACI 332-08 8.6.2 recommended interpolating from 0.1% for 24 x slab thickness to 0.5% for 100 x slab thickness spaced joints.

While this statement appears to have been removed in later versions of ACI 332, it appears to be a rational approach.


Can anyone comment on:
Where this provision came from?
Why it might have been removed?
If your opinion is it is not a rational approach, why?
Are you aware of any other recommendations from ACI for increasing contraction joint spacing based on reinforcement ratio (for <0.5%)?

 

[Lomarandil]

What I have heard and used is that there is not any linear benefit to slab on grade reinforcement between 0.1% and 0.5 or 0.6%. Anything in that range is equally (in)effective at translating concrete shrinkage strains into steel strains and therefore controlling shrinkage cracking. So 24-36*thickness should still be used.

Once you exceed 0.5 or 0.6% steel, then longer spacings could be obtained using a procedure like the subgrade drag equation (although in some circles that has fallen out of favor).

If you really want to push the envelope with contraction joints, you'll want to get into early discussions with the mix supplier and contractor to address items like water content, aggregate size, subbase preparation, etc. Those all have a very significant impact in SOG performance.

----
just call me Lo.

 

[Settingsun]

Engjg, what did ACI 332 say the crack widths would be for the interpolation case? I haven't read it, but most slab-on-ground recommendations are loose at best when describing what cracking to expect when following their reinforcement recommendations. Most don't say at all.

ACI 360 removed the subgrade drag equation, so it sounds like they went away from being so specific as an organisational decision. I didn't trust the drag equation at all: the results were always far less than the concrete's own strength in tension indicating that reinforcement wasn't required at all, and the FOS was so high that there should be no chance of cracking - but observation says otherwise. I speculate that someone came up with the theory, which seems reasonable at first glance, and it seemed to confirm that the light meshes used in jointed slabs were correct. Then extrapolated unsuccessfully, because it was the joints doing the crack control, not the mesh.

The 0.5-0.6% number is intended to make the steel stronger than the slab, or at least not much weaker. Otherwise, a single crack can form and open wide due to yielding of the steel. 0.1% steel has about 20% of the concrete's strength in tension, so does very little to cause a second crack (and third etc) to form and distribute the shrinkage relief. Other forms of restraint (like whatever caused the first crack) are more likely to distribute the cracking if so little reo is used in a large slab.

The discussion below is from an older version of a UK design guide (Concrete Society Technical Report 34).