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Concrete Crack Concerns

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When do you start worrying about column and slab cracks? I've encountered
cracking in columns, and in a slab over a parking garage. The slab carries a
three storey wood framed apartment, and the condominium owners' group wants
to know if the cracks indicate a hazard. So far, I can't tell them.

The parking garage roof is typically an 8" slab, thickened to form a 16"x
72" beam (slab band) which spans over two, sometimes three parking spaces.
The slab spans 17' over parking and 24' over aisles, and the slab band spans
16'- 10" or 24'- 10". The slab size is 100' x 230' and there are no
expansion joints. Columns are 10"x 24", oriented with the 10" face parallel
with the span, and they have 6- #6 (fy=60ksi) verticals. Design superimposed
load is 320 psf.

The garage extends out 8' to 15' beyond the superstructure to support
landscaping (the wood framed building is smaller than the garage below).

Stresses calculate out close to allowables if it was built according to the
drawings. It was built in 1976, and the slab was probably poured in spring
or summer.

The slab is cracked right through, at several locations, over columns. The
crack extends most of  the way down the slab band, towards the bottom, over
the middle of several columns. Maximum crack width at the underside of the
slab is estimated to be about 1/16", right over the column (usually less
away from the column). The crack tapers from maximum at the top,
dissapearing near the bottom, from 2" to 4" above the soffit of the slab band.

Cracks at midspan are rare, and where visible they are hairline sized.

Many columns have spalling type cracks at the top, where the face has
separated in a wedge or pyramid shape, widest at the top. Some have fallen
away. Some of these segments are triangular and are at the corners, and
others are full width, with rectangular faces. Width of the separated pieces
is usually an inch or inch and a half (the shell width outside of the ties);
the height is 4" to 8". The biggest column cracks are not below the biggest
slab cracks, although they are in the same general area of the building.

My theory so far is that the top mat of rebar was placed, or pushed, too low
when the slab was poured. The bending over the support might have been high
enough, even under dead load, to stress the steel to yield, so the flexural
crack could develop. Shrinkage of the concrete, possibly following a pour on
a hot day, could have added high tension stresses which would increase crack
size.

There are a couple theories about the column cracks. I thought at first that
the rotation of the ends of the slab-band beams, after cracking, might have
concentrated bearing stresses on the edges of the columns, and cracked the
face shell outside the rebar cage.

Another engineer believes a better theory is that the vertical bars were
field bent to form hooked dowels into the slab, after the column pour. If
they were bent across the columns, the bars could have bowed out below the
tops of the columns and popped the face shell concrete out.

I haven't designed this type of apartment platform, so I don't know how
common these slab and column cracks are. Two other senior engineers have
reassured me that there's no problem. They are probably right about the
column cracks being caused by field bending the dowels. However, they
haven't given me a satisfactory theory to explain the flexural cracks in the
slab over columns. 

I drove around and looked at some apartments which seemed to have similar
structures, but saw no similar cracking. The cracks in the slabs of the
other apartments looked smaller, and tended to be at midspan. I had expected
that shrinkage cracks were more likely to occur at midspan than over columns.

Can someone, maybe with more experience with cracked structural slabs,
suggest the most likely explanation? 

Jim Warne, Vancouver, BC