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RE: Slab Stripped before time ...

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Thank you everyone who answered.

In this case I was told that, because the forms were "needed elsewhere". they stripped too early and yes, I'm guessing that the concrete was still "soft".  The idea that rebar bond might be lost etc is a concern I have.  I'm also surmising that, since there has been no subsequent failures or problems that the deformation occurred while the concrete was still "plastic" ...

Thor A. Tandy P.Eng, C.Eng, Struct.Eng, MIStructE
Victoria, BC
Canada 



-----Original Message-----
From: Alex Nacionales [mailto:alex1961(--nospam--at)telus.net] 
Sent: Sunday, May 08, 2011 11:38 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Slab Stripped before time ...

I thought from your statement that the trough is part of the slab section.
My statement that it would fail is based on the assumption that forms failed  after concrete has set(about 30 minutes pour is finished) but before the concrete has gained enough strength to resist horizontal shearing stresses causing it to lose the bond with the rebar causing immediate deflection of the whole slab span if not total failure. Another possibility would be creep due to high water cement ratio of concrete and/or early stripping of forms. It is also possible that the forms deflected during pouring and went unnoticed so the top of slab was not leveled by the screed.

 I would suggest a full load test of one span if you want to determine its capacity. 

It is possible that the spalling is caused by honeycombs near the surface.
I do not know of a method to detect honeycombs deep inside the core except to chip off an existing spall if it exposes  more honeycombs inside leading to a tie or rebar.

Mtcw

Alex Nacionales


-----Original Message-----
From: vicpeng(--nospam--at)telus.net [mailto:vicpeng(--nospam--at)telus.net] 
Sent: Sunday, May 08, 2011 5:13 PM
To: SEAInt
Subject: Re: Slab Stripped before time ...

There's a trough in the slab span so it happened after pour was finished ... But your comment about it would fail if it did deflect ... 
Sent on the TELUS Mobility network with BlackBerry

-----Original Message-----
From: "Alex Nacionales" <alex1961(--nospam--at)telus.net>
Date: Sun, 8 May 2011 16:35:11 
To: <seaint(--nospam--at)seaint.org>
Reply-To: <seaint(--nospam--at)seaint.org>
Subject: RE: Slab Stripped before time ...

Thor,

 It may be that the deformation occurred during pouring due to inadequate
formwork and/or shoring in which case the slab has thickened in the areas
where the form joists deflected or where shoring subsided to the ground due
to inadequate shoes. If deformation occurred after concrete set then it
would have failed completely if not after stripping, after being loaded.

MTCW

Alex Nacionales





-----Original Message-----
From: Thor Tandy [mailto:vicpeng(--nospam--at)telus.net] 
Sent: Sunday, May 08, 2011 2:41 PM
To: SEAInt
Subject: Slab Stripped before time ...

Interesting question .

 

I have been asked to do a structural review of an existing residence that
has a significant area of the footprint a suspended slab on circular
columns.

 

I understand that the slab formwork was stripped before the slab had
completely cured . yes there is a noticeable trough in the main part of the
slab.

 

It is a 6"+/- slab and the two-span is approximately 15ft and the main
deformation is probably about 2".  Built in 1990 the original owner, who is
now selling, accepted the flaw.  There has been no
perceived/recorded/admitted problem since.

 

In general the construction looks good and, except for some spalling of
columns due to bits of tie-wire at the surfaces, appears sound.  The
formwork was sonata and although I have to get the original construction
dwgs but I assume the reinforcing was tied steel cages.

 

My questions:  

 

1)      Does anyone know of literature on, or record of, impact on the
strength and/or serviceability of slabs that have deformed during the
curing/strength-gain period?

2)      Although the spalling appears localized to the tie-wire locations,
there is number of them.  Could this also be a sign of deeper issues?  I.e.
would the tie-wire spalling open up access to the stirrups etc in the
column?

3)      Has anyone scanned columns for internal content checks?

 

Thanks

 

Thor A. Tandy P.Eng, C.Eng, Struct.Eng, MIStructE
Victoria, BC
Canada 

 

 




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