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Re: Siporex

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Dear David:

Your observations do not surprise me.  Firstly, AAC slabs, like most 
pre-fabricated panels, are prone to sudden and catastrophic failure.
As you have already found out, the design of AAC slabs ensure that 
the shear failure precedes flexural failure -- no warning through 
cracks before failure.

The visual observation of the section, showing concentration of bubbles
near the rebars is quite surprising. I am not familiar with similar 
problems in our part of the world. It may be a quality problem or due 
the the difference in producing AAC in hot weather here and cold 
weather in Canada.  If this is typical of all AAC produced in Canada,
its use should be immediately banned unless this major technical 
defect is rectified.

Concrete structures using AAC have been generally found to be less
durable than cast-in-situ concrete structures. At our end, this may also
be due to exposure to extremely hostile atmosphere (near the sea and 
pollution) and the presence of air bubbles may be accelerating the 
carbonation process.

A much bigger problem with AAC slabs is their lower flexural stiffness
and the corresponding higher deflections. Even if the strength is not
compromised, the large deflections pose serviceability problems.
The deflections also cause minor cracks to develop at the joints, 
through which rain water invariably finds a path.  This has also 
severely affected the long-term performance of these structures.
You may be having a similar problem with rain/snow which may have 
further compromised the long-term strength of the structure.

Hope this gives some useful information based on the Mumbai experience.

ravi sinha

On Thu, 9 Sep 1999, David Handy wrote:
> I should have used AAC (aerated autoclaved concrete) instead of Siporex. I
> am pretty sure that the slabs are actually Siporex, but not 100%.
> The slabs we are concerned about are roughly 25 years old. We had one with
> a deflection of a couple of inches over 15' +/- span. This occurred during
> high snow loading condition but not higher than specified design values.
> The most common problem is unrecoverable deflection. People have been
> putting owsj at midspan in order to reduce spans in half.
> A group of engineers and maintenance staff has been assembled in order to
> address ways of determining whether the slabs are safe. With the assistance
> of a local University we have tested 5 slabs. Results vary significantly in
> terms of total load supported. The common result however was that the 6"
> thk x 18" wide slabs 12' long failed suddenly in shear and not in bending.
> Deflection was a couple of inches when it snapped. Some slabs have failed
> at design loads or just under (ie. LL * 1.5 + DL * 1.25). Cracking on the
> underside was minimal even close to failure so this cannot be used to
> determine level of distress. Results of compressive forces don't correlate
> well with load capacity so taking cores doesn't give accurate values of
> shear strength. It seems that the bubbles are quite coarse in the region of
> the reinforcing bars. We are wondering if this is typical or was a quality
> control problem and results in the wide variation. I thought that maybe as
> the mixture is expanding under the chemical reaction, the bars interupt the
> flow of the mixture and cause the resulting coarse bubbles. This may be
> typical in all AAC slabs. Does anyone know?
> Another problem is that compressive strengths of the cores are less than
> half of minimum strength values as suggested by the Siporex literature. Are
> the strengths decaying over time. If the roof leaks then one would think
> that carbonation of the "concrete" would happen very quickly as the
> moisture passes quickly through the bubbles in the slab. 
> btw AAC resembles the center of an AERO chocolate bar (if you know what
> they are)
> David Handy, P.Eng.
> dhandy(--nospam--at)
> The Thompson Rosemount Group, Cornwall, Ont. Canada
> Opinions expressed are personal only.

Ravi Sinha, Ph.D.		email: rsinha(--nospam--at)
Associate Professor		       rsinha(--nospam--at)
Department of Civil Engineering	       Phone: (91-22)-576-7336, 576-7301
IIT, Powai, Mumbai - 400 076, India    Fax: (91-22)-578-3480, 578-3557