[Ferro List] microcracking, tom's crud testing, swimming pool on clay

[tp sing]

There was some discussion on microcracking and comments saying that it
does'nt matter if the cracks remain micro. Perhaps, somewhere I read a
figure of 200 microns too !
Friends, after having been associated with Dr. P.K.Mehta of Berkeley and Dr.
V.M.Malhotra of CANMET on the subject of Sustainable, Durable High Volume
fly ash Concrete what I have learnt is as follows:
If the aim is to get a long life, durable concrete structure, we need to
understand what causes deterioration of reinforced cementitious composites (
including FC) . It is the ingress of the atmospheric deleterious substances
through the thousands of microcracks which may be invisible to the eye or
the microscope but are invariably there in ordinary concrete due to thermal
stresses at the time of hydration. In due time during the life of the
structure, this network of cracks expands as well as the cracks grow wider
due to stresses from temperature and service loads . The ingress leads to
carbonation and subsequently reactions with the steel reinforcement corrode
it and deterioration sets in.
This deterioration can be postponed greatly by incorporating SCMs(
Supplementary cementitious materials like fly ash and GGBS) in the mix,
which prevent microcracks in the first place due to a lower heat of
hydration. Secondly  the slow pozzolanic reaction products keep filling the
microcracks and the pores making the concrete more and more impermeable.
This growing impermeability with time  has been verified in numerous RCPT
tests on HVFAC mixes all over the world.

I would therefore never like to have ANY microcracks in My FC if I can.
Fibres in the mix in my opinion, will help in retaining the impermeability
longer. On loading beyond a certain point the cementitios matrix may fail in
tension but Catastrophic sudden failure  will not happen since there is a
well distributed reinforcement in the form of Mesh. Just my opinion as I
understand things !

Back to TOM !
I had sent a message last week which seems to have been eaten up by a hungry
server on its way in some developing nation. I had asked for a min. 1 kilo
cement for the repeat test. Conclusions should wait for the second test. If
you are in your tenth month, I deeply sympathize with you. We can discuss
the remedial for the tank if you want .

Regarding the *swimming pool*, I agree with peter in most of the content of
his last post. I would also add that a single cementitious layer with all
the meshes is preferable over a sandwhich. One should not consider it UNLESS
insulation is important for heating etc. In case it is, one should be
prepared to spend extra ( making up in energy savings ) with enough care in
design and execution .
PETER  your server continues to hate me . I 'm keen to have Handling and
transportation details of your precast Tanks. Regds and good wishes.

-- 
T.P. Singh

Director (R&D),
Construction Research Centre,

Ph.: 91-11-26802388
http://www.geocities.com/tps_har
E-mail: sing.tp at gmail.com