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Strength
and durability of concrete with pond ash as fine aggregate
Kalgal
M.R., Pranesh, R.N. and Ravishankar S
Pond
ash is being produced at an alarming rate and efforts
are required to safely dispose it and if possible
find ways of utilising it. Fly ash collected through
hoppers has been widely accepted as a pozzolona and
is being used by the construction industry. Pond ash
being coarser and less pozzolonic, is not being used.
Some studies have been carried out earlier on partial
replacement of sand by pond ash. In view of sand becoming
scarce and pond ash accumulation posing environmental
problems, total replacement of sand by pond ash in
concrete is attempted. It is found that it is possible
to use only pond ash as fine aggregate without compromising
on strength and durability. This study opens up a
major avenue for utilisation of pond ash.
Corrosion
initiation time of reinforced concrete members in
marine environment
A.
Srividya and S.B. Allampallewar
Effective
chloride diffusion constants in partially saturated
concrete are determined for a period of thirty years
by studying the monthly temperature and relative humidity
data in Santacruz (Mumbai, India) region and assuming
diffusion decay constant as per Life-365. Temperature
and relative humidity data collected for thirty years
at Santacruz in Mumbai (India) by Indian Metrological
Department is used. Mathematical models which consider
monthly variation of chloride diffusion in concrete
for constant surface chloride concentration and step-variation
of surface chloride concentration are proposed to
predict the corrosion initiation time of reinforced
concrete members. In step-variation surface chloride
concentration (Cs) is assumed to build up in steps
to reach a maximum value (Cs,max = Co ) over a time
period to and then remaining constant i.e. for time
t ? to.The proposed mathematical models for constant
surface chloride concentration for predicting the
time and space variations of chloride content in concrete
are based on Fickian process of diffusion (Fick's
second law) and on the concept of chloride threshold
to define corrosion initiation. Corrosion is initiated
when the chloride concentration on steel reinforcement
exceeds a threshold value (Cth). The governing parameters
of the diffusion based model include surface chloride
concentration (Cs), concrete cover depth (X), chloride
diffusion coefficient in concrete (D), chloride threshold
level of steel reinforcement (Cth) and exposure age
of concrete (t - tref). Chloride diffusion coefficient
mainly depends on water-cement ratio (w/c) of concrete
and varies with age (t) depending on continued conditions
of hydration, temperature (T) and relative humidity
(h) and hence required to be modified by applying
correction. Life-365 program does calculations only
for fully saturated concrete considering user defined
monthly variation in temperature. Predicted corrosion
initiation time using effective diffusion constant
is found to be short and within five percent of corrosion
initiation time predicted by considering monthly variation
of diffusion coefficient in Santacruz region. For
fully saturated concrete condition proposed mathematical
models estimate short corrosion initiation time by
about 10% to 20% but for the same data considering
partially saturated (actual condition) condition proposed
models estimate 70% to 160% more corrosion initiation
time to that predicted using Life-365 program depending
on surface chloride concentration. With the knowledge
of proper corrosion initiation time, maintenance and
repair programmes of reinforced concrete members can
be scheduled and resources can be utilised effectively.
It also helps to fix the concrete cover depth and
water-cement ratio necessary for the design of service
life for concrete members in the region .
Mix
proportioning of plain and rice husk ash concrete
as per draft IS 10262
M.C.
Nataraja and B.M. Ramalinga Reddy
With
the revision of IS 456 in the year 2000, it is mandatory
on the part of designers and users to follow the new
specifications which are relevant for concrete mix
design. Significant modifications have been made in
many aspects of concrete mix design in the draft IS
10262 : 2004, which is currently being circulated
by Bureau of Indian Standards (BIS). The method proposed
in the draft code can be used for the design of ordinary
and standard concrete grades using concreting materials
and other supplementary materials identified for this
purpose. Here the mix proportioning is done for plain
and rice husk ash (RHA) concrete. Two percentages
of RHA namely 15% and 25 % as replacement to cement
have been tried in this work. The investigation is
only an attempt to demonstrate the use of draft code
for the design of concrete that contains cement alone
and a combination of cement and rice husk ash which
is a supplementary cementing material.
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