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Concrete Mat Expansion Joint

Concrete Mat Expansion Joint

Concrete Mat Expansion Joint

For 50ft x 25ft x 18in thick concrete mat with 5ft height x 10in thick wall all around (tank spill containment area) is expansion joint required?

RE: Concrete Mat Expansion Joint

It seems unlikely to need an expansion joint. What do you think might cause your concrete to expand?

RE: Concrete Mat Expansion Joint


RE: Concrete Mat Expansion Joint

Agree that no joint is needed, but consider adding reinforcing in the long direction to prevent shrinkage cracking.

RE: Concrete Mat Expansion Joint

The problem does not lie with expansion; as mentioned, shrinkage (or contraction) should be addressed. Assuming that the slab will sub-divide itself into two equal sections of 25' x 25', you might want to induce a crack at the mid point.

I submitted the following suggestion to the engineers designing a new USDA facility in Iowa and it was implimented.

Centered in the 18" slab, place a 12 inch tall piece galvanized, expanded metal lath (like Stay-Form) from side to side at the middle of the long axis; tying it to supplimental pieces of reinforcing steel to hold it in place. Horizontally attach an expanding, bentonite waterstop (like CETCO RX101) to the expanded metal lath with a minimum of 4" of concrete cover to the top of the slab. Provide a mix design that includes a crystalline concrete waterproofing (like Xypex Admix C-1000 or C-500).

Here is what occurs:
  The galvanized, expanded metal lath prevents aggregate
  interlock and a crack is induced at a specific location.

  The bentonite waterstop is precisely where it needs to be
  in order to prevent fluid from passing through the joint.

  The crystalline admixture will reduce the heat of
  hydration as well as binding up a lot of the mix water.
  There will be less shrinkage and the entire concrete
  slab will be internally waterproofed.

The entire volume of concrete can be placed at one time; right on through the control joint(s). During the placement, care must be taken to equalize the level of concrete each side of the metal lath; as to not dislodge it. Normal curing practies are to be followed; membrane or water.

This approach is very predictable; you'll need to decide how many locations to induce the crack(s).

RE: Concrete Mat Expansion Joint

Depends on how much reinforcing you are willing to use.

A top mat of bars 2" below the top of the slab with an area of steel of 0.5% (.005) of the gross cross sectional area of the slab will keep any cracking water tight.

For an 18" thick slab, that is a #9 @ 11" O.C. each way. If this seems like a bit much, you may want to think about a contraction joint like scj53 describes above.

RE: Concrete Mat Expansion Joint

Better #7 bars at 6" o.c. If you space the bars too far apart you can get shrinkage cracking between the bars.

RE: Concrete Mat Expansion Joint

And to limit shrinkage cracking, pay a lot of attention to the curing of the concrete.  Very good "bang for buck"  

RE: Concrete Mat Expansion Joint

OK, I have a question regarding the 5' high containment walls - any expansion joints there?  I ask because we had a project (which included 6' high walls, no mat) where the client insisted the design follow ACI 350 (?) Concrete for Environmental Structures.  Including the recommended expansion joints.  We disagreed with the need for EJ's, to no avail.

RE: Concrete Mat Expansion Joint

I don't know where ACI 350 requires expansion joints.  It gives some very loose and vague recommendations. It also recommends minimum reinforcing based on movement joints.  If you go with .005 you're good to up to 150 ft. or so.
For your size tank, you don't need any expansion joints in the slabs or walls. Expansion joints are not cheap and they're the most likely spot to leak.  Avoid them unless they're absolutely necessary.
My background includes many many tanks of this size and larger. I can remember the number of expansion joints on one hand.

RE: Concrete Mat Expansion Joint

Just completed a monolithic pour 5000psi 18" thick 116' diameter #7 bars 12" both ways top and bottom.  We experienced no cracking and had no joints

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