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RE: Slab Cracks

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-----Original Message-----
From:	ASLCSE(--nospam--at)aol.com [SMTP:ASLCSE(--nospam--at)aol.com]
Sent:	Wednesday, September 17, 1997 11:13 AM
To:	seaoc(--nospam--at)seaoc.org
Subject:	Slab Cracks

Dear fellow engineers:

I am looking for a book or guidelines regarding cracks in 
slabs-on-grade
(mainly in residental construction).
How else do you explain to a lay person, after she or he removed 
the carpet
and finds cracks (hairline to 1/16") in the slabs (no vertical 
off-sets, no
signs of of differential settlement or settlement) that these 
cracks are
shrinkage cracks (high water content ehen paced and/or little or 
no curing)
and that some may have gotten bigger due to the earthquake and 
aftershocks.
A (real estate) inspector friend of mine is being taken to court 
because he
should have seen these cracks without removing the 
carperting!!!!!!

Thanks for your upcoming answers and comments.

Antonio S. "Tony" Luisoni
Consulting SE
Granada Hills CA
phone/fax (818) 363-6531


Tony,
You raise an interesting issue. You might check with ACI, 
however, I think that the answer is to issue a report from an 
engineer to explain the problems associated with differential 
shrinkage and drying techniques of slabs on grade.
I received a call the other day from a couple moving into a new 
home for seniors in our city. He noticed a crack running the 
full length of the living room, yet it did not proceed down the 
edge of the slab. As in your case, there was not elevation 
changes one ether side of the crack and the width did not exceed 
about 1/16" in width.
I explained how concrete dries from the inside out and how 
construction practices vary as to how to maintain a moist 
surface as the curing process continues. I also explain how 
tensile mesh is often left at the bottom of the slab where it 
may be in contact with earth which may exaggerate the cracking 
condition.
Most of all, I try to insure the owner that the crack is not 
structural and offer suggestions as to whether it is necessary 
to repair the crack (to prevent moisture from seeping through) 
and what methods are available.
In most cases I find that the concerns are more with the 
installation of tile over a cracked surface. I explain that 
Builders papers are not enough to cover the crack since they do 
not have the elasticity needed to allow the crack to move but 
keep the tile rigid. The tile setter must use a membrane which 
is commercially available and consists of two layers of 
elastomeric or rubber separated by a cloth fabric. Although this 
is not cheap, it provides the best protection when ceramics, 
terracotta and stones are to be set.  Some of the newer thinset 
contain a non-shrink and additive to maintain a degree of 
elasticity to add additional protection.
I don't know of any way to detect a crack below carpet when 
there is no elevation change over the width of the crack. Except 
for moisture migration, there is no way to tell other than 
removing the carpet.
Your report is probably the best support he has at this time. 
You also might check the web by searching for concrete or 
"concrete slabs" and uncover additional information.
Good Luck,
Dennis Wish PE