Rebar spacer for watertight construction (slabs and walls)

[Engr1888]

Hi my fellow Engineers,

I am working on a project involving a floating concrete box with bottom slabs, wall and top slab. To bring the best in water tightness, what are the recommended spacers for rebar? Are concrete spacers better than plastic spacers?

Appreciate if you can be specific if possible on the spacers (shape, etc). The concrete mix will use fine aggregates (3/8") with 10" slump.

Thanks in advance.

 

[ATSE]

The basics:
Use ACI 350 as basis.
Use larger aggregate if possible / available.
Use lots of smaller bars spaced 5" to 8" apart, with As/Agross = 0.005 to 0.006.
Stagger laps horizontally.
Use w/cm = 0.40 to 0.42, up to to 0.45 max.
Use plastic spacers.
Consider post-tensioning (fully grouted).
That's a start.

 

[Engr1888]

@ATSE, thanks!

My concerns about using plastic spacers is whether the spacers will create small voids at the underside of slabs due to potential inability for concrete mix to completely cover and fill the spacers.

 

[TehMightyEngineer]

Working with precast concrete plastic spacers I've found the following:

Chair type plastic spacers with lots of nooks and such may cause slight voids around the spot where the chair contacts the form but likely wont penetrate to the bar (I would hope).
Large plastic wheel spacers will cause voids at the edges of the wheel where it contacts the form but likely not beyond that.
Long, multi-leg bolsters will likely not cause voids.
Anything made of wires (wire ties and any wire chairs) will be the least likely to cause voids (darn things grab concrete better than rebar).

I also second all of ATSE's suggestions and include a few of my own:

-Go precast if possible, better curing conditions resulting in less shrinkage cracking, better concrete and quality control, and lower clear cover minimums.
-Try lightweight concrete, we've poured a floating concrete slab for a wave tank (using foam anchored to the bottom to float the slab) and it worked well enough.
-Use minimum air content acceptable.
-Your joints will need compression to properly seal whatever joint material you have, casting in bolt pockets to prestress the joints would be a good idea. A better idea would be post-tensioned ducts to pull all your pieces together. This has other issues though and the joints will need a lot of quality control to ensure this works as designed.

Finally, why? There are many better materials for things that float. I highly suspect you can find another alternative that will be far superior.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com