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Re: Siporex

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>What exactly has been the problem with Siporex slabs that you mention?
>We have several structures in Mumbai using Siporex slabs, and the 
>performance has been *very* interesting.  Please also note that 
>Siporex is a brand-name and other companies may also be making similar
>products under a different name.
>Ravi Sinha


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.
The Thompson Rosemount Group, Cornwall, Ont. Canada
Opinions expressed are personal only.