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RE: seaint Digest for 17 Aug 2011

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My two cents: The first question that comes to mind is---what level of
crack control was required? Topping on an "existing" slab will shrink
with restraint to that shrinkage occurring due to bond with or roughness
of the substrate, and the potential for restraint is much greater than a
slab-on-grade placed over a vapor sheet or even stone. See ACI 222,1R
Chapter 6, 224,3R, and 302.1R.2.3.2---some of this is contradictory but
intuitively, added joints may be necessary.

As far as crack width is concerned, 0.009 in is pretty small. You
describe it as adjacent to a "seawall" adjacent to a brackish (to us
that means non-seawater) water body. A "seawater" or "coastal" exposure
is probably not warranted; but if it is a pedestrian use, and it gets
icy, and they treat it with salt, and being that salt and moisture
together are pretty aggressive, and even if not in the "splash" zone,
the smaller width may be appropriate. However, having a contractor
remove and replace slabs with that level of cracking (especially if
guidance in joint requirements was unconservative) is onerous. IMHO, I
suggest that you consider using a crack-healer or sealer that will
bridge the cracks instead of removing and replacing the topping; there
is a good chance that cracks would occur even if you gave the pour a
second chance.....

Subject: Reasonable Crack Width in a Concrete Slab
From: Larry Reynolds <larryr31(--nospam--at)>
To: seaint(--nospam--at)

Our office is involved in a project in which shrinkage cracks, varying
width from 0.003=94 to 0.009,=94 have developed in a new 3=94
topping slab.  The topping slab was placed over an existing, exterior
structural slab.

This project is located in a mid-Atlantic state and the slab is adjacent
a seawall.  This seawall forms the edge of plaza area that is adjacent
to a
pool of brackish water.   The plaza is not subject to overtopping from
brackish water.

We are trying to determine which exposure condition from ACI 224, Table
is applicable to our situation, so we can determine which cracked
need repair/replacement.  We have narrowed it down to the following:

1)      Humidity, moist air, soil - reasonable crack width 0.012=94

2)      Seawater and seawater spray, wetting and drying =96 reasonable
width 0.006=94

I was not able to find any information in ACI 224 or from other sources
would provide additional insight on how the exposure conditions are
For example, does wetting and drying mean wetting and drying in the
zone, or wetting and drying due to weather?  Or both?

We having a robust debate here in the office as to which exposure
applies.  My personal opinion is that our project falls somewhere
the two exposure conditions described above.

Does anyone have any insight to offer?


Larry Reynolds, P.E.

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