A
good question indeed!....there are a lot of misconceived notions about
PPC…..lets start from basics:
a) How
does action of Pozzolona help?: Significant
quantity of Calcium Hydroxide is produced during hydration of calcium
silicates. Calcium
Hydroxide is a useless material from strength/durability
considerations.
Pozzolona
converts this useless mass into cementitious produce; thereby improving
the
quality of concrete.
The
pozzolonic action can be represented as:
Calcium
Hydroxide + Pozzolona + Water---------> C-S-H (gel)
b) Advantages
of PPC
- Concrete
made with PPC is more impermeable
- Concrete
made with PPC is more durable
- PPC
produces less heat of hydration (and that too at a low rate) thereby
controlling temperature differentials, which causes thermal strain and
resultant cracking in mass concrete structures like dams.
- Its
higher resistance to cracks means that it is less prone to corrosion of
reinforcement.
- Better
resistant to sulphate attacks and to some other destructive agents.
- Contributes
to workability, reduced bleeding and controls destructive expansion
from
alkali-aggregate reaction.
- Reduces
the leaching of calcium hydroxide liberated during the setting and
hydration of
cement.
c) If
it
has all the advantages as stated above….then why are there
some concerns
over use of PPC?
Inspite
of its superior properties, there have been some concerns over its
application. The reasons
are:
¶
In
the initial
phase
of introduction of PPC, there was a question
mark (sometimes
justified) over the type & quality of pozzolona
being used. With increase
in awareness & improvements in technology coupled with better
quality
control, this concern is no more valid for products of good
manufacturers.
¶
For
manufacture of good quality PPC, uniform
blending of pozzolona material with cement
is a must. Again, with increase in awareness & improvements in
technology
coupled with better quality control, this concern is no more valid for
products
of good manufacturers.
¶
One
disadvantage of using PPC is that it
causes reduction in alkalinity which
reduces resistance to corrosion of reinforcement.
This has
been one of the
prime reasons that some restrictions on its use in RCC were imposed by
some
standards. However, it is now established that this disadvantage is more
than
offset by increased impermeability & durability of concrete.
¶
Another
disadvantage of use of PPC is its slower
rate of development of strength.
If
PPC is manufactured by using the right type of reactive pozzolonic
material,
the PPC will not in any way be inferior (in fact it will be superior to
PPC)to
OPC
except
for the
rate of development
of strength of concrete upto
7 days. Its
slower rate of development of strength does place some restriction on its
application for constructions with rapid construction methodologies
like Slip
form techniques (where 4 hr
to 6 hr strength is
important).
¶
In view of slower
rate of development of strength of concrete made with PPC,
due
care has to
be taken to ensure that formwork is
removed
only after
stipulated strength has
been achieved (which is often correlated with a time period for removal
of
formwork).
d)
Now lets come specifically to your question: Well
the
answer is:
PPC
can be used for Concreting of Heavy RCC structures…there is
no restriction on
grades of concrete upto which it can be used.
IS
1489 Part 1 (Specifications
for PPC-Fly Ash based)
& IS
1489 Part 2
(Specifications for PPC-Calcined Clay based) state that it can
“generally
be
used wherever 33 grade ordinary Portland Cement is usable under normal
conditions”.
|
|
IS
456:2000 also allows use of
PPC in CC/RCC. IS 456: 2000
is
distinctly different than its
earlier version of 1978 in that the revised code accepts and encourages the
use
of mineral admixtures
in concrete. Clause
5.2 has been specifically
added to mention the use of fly
ash,
metakaolin, rice husk ash, GGBS and silica fume.
|
It
would be worthwhile to have
a look at change in mindset on use of PPC in RCC
structures. Indian
Railways earlier did not allow use of PPC for RCC works.
However, with effect from March’2006
they have allowed
use of
PPC in RCC
structures while stipulating that proper damp curing of concrete for at
least
14 days is to be ensured & that supporting formwork should not
be removed
till concrete attains at least 75% of the design strength.
PPC
can be used & is being used for high grade concretes. In fact,
presently
most of the ongoing projects with high grade concretes use blended
cement.
A
paper by Tahir
Kibriya titled
“Performance
of Blended Cement in High Strength Self Compacting
Concrete”(presented
in
2006
Structures
Congress, St.
Louis, Missouri; Sponsored
by the Structural Engineering Institute (SEI) of the American Society
of Civil
Engineers) gives details of
a
experimental study aimed at evaluating
the properties of high strength Self
Compacting Concrete made from blended cements using rice husk ash,
portland
cement, natural aggregates and sand.
In
this study, wide
ranging investigations
covering most aspects of mechanical behaviour and permeability were
carried out
for various mixes for compressive strengths of 60N/mm2,
80N/mm2
and 100N/mm2.
Compressive strengths of
high strength SCC
specimen with blended cements for
60N/mm2, 80N/mm2
and 100N/mm2
were observed
to be higher
by about 4 to 9% than
the control specimen, for
concrete with 50% Portland
cement blended with 50%
rice husk ash. Higher
elastic moduli and reduced permeabilities were observed
along with better sulphate and acid resistance.
The
paper concluded
that
“Better
strengths and improved durability of such high strength SCC make it a
more
acceptable material for major construction projects”.
A
reading of Use
of Fly Ash in Concrete would
give a good appreciation of this
topic.
