Rehabilitation Of Severely Damaged
Shell-Roof Structure of
DURGAPUR CHEMICAL
FACTORY
A Case Study
At Durgapur, once called Rurh of India, was
established Durgapur Chemical Limited, one of the largest chemical factories in India in
sixties. The concrete shell structure for the caustic chlorine plant was a unique
structure of its type at that time. In the sixties one could seldom find such a gigantic
shell structure with such; intricate design and aesthetic beauty. But due to various
reasons and due to absence of systematic & scientific maintenance, the reinforcements
were totally corroded. The structural stability was in question and ultimately the plant
remained non-operative. However, as a rehabilitation programme, the Government of West
Bengal decided to run the plant and sanctioned funds for the rehabilitation of both the
caustic chlorine plant and monochlorobenze plant.
Before the actual rehabilitation work was taken up,
D.C.L Authority engaged I.I.T. Kharagpur to asses the degree of deterioration by
conducting non-destructive testing of concrete such as ultrasonic pulse velocity, rebound
hammer test, chloride penetration test and corrosion of reinforcements. On the basis of
preliminary investigations by IIT, M/s. S.P. Banerjee & Associates were appointed as
consultant for finalising the basic repair scheme, selection of construction chemicals and
the tender specification.
On the basis of the repairing scheme, only few
specialised contractors having sufficient infra-structures to undertake such a huge
repairing work and who had in-house facilities for manufacturing various construction
chemicals required for rehabilitation were selected. After a detailed study, CICO was
selected for undertaking repair and rehabilitation work, which can be considered as the
largest rehabilitation work were selected of an industrial structure in eastern India,
perhaps even in India.
Basics Repairing Scheme
A) As the main-reinforcements of columns and
beams were severely corroded and the stirrup reinforcement was missing at many places,
property designed supporting structures were erected to transfer the load to the
supporting structures.
B) Appalled, cracked and any bad concrete was
removed by mechanical as well as manual methods.
C) Neutralization of acid affected concrete
by means of alkaline solution and / or high water gel was done to bring PH to more than
7.0
D) Corroded reinforcements were cleaned
thoroughly by means of sand blasting, scrubbing, wire-brushing and rubbing with Emery
paper.
E) 4 angles, one at each corner were fixed
and stirrups at a spacing as per design requirement were welded to the angles.
F) New main reinforcements were placed
wherever required to form a new reinforcement cage.
G) Polymer based anti-corrosive paints.
"Tapecrete Marine Coating" were applied on the reinforcement as a corrosion
inhibitor.
H) Epoxy based bonding agent - "CICO
BOND EPO" was applied on old concrete to have proper bond between old and new
concrete.
I) Properly designed shutters were placed for
final concreting. the design of Shuttering was done in such a way that fixing of
shuttering could be completed within 45-60 minutes after application of bonding agent
which is the open time of the Epoxy product used.
J) Concrete mix-proportion with specially
made CICO Superplasticiser and CICO Tapecrete Polymer was selected in such a way that a
flow concrete with low w/c ratio of 0.36 was achieved. Highly cohesive flow concrete was
selected, as no needle vibrator or surface vibrator could be used.
Actual Concreting Work
For long term durability i.e. to prolong the life
expectancy of the structure for 25 years, the design of polymer concrete was a Herculean
task. To withstand both acidic and highly alkaline environment from caustic and
monoshlorobenzene plant the polymer loading was selected in such a way that it could be
withstand the above mentioned aggressive environment and at the same time give sufficient
compressive strength for the progress of the work.
In the concrete testing laboratory various
mix-designs were conducted with the following variability:
a) Cement Content - 380 to 500 kg per cu.m
b) W/C ratio - 0.35 to 0.40 with
superplasticiser of varying doses so that slump in the range of 170 to 200 mm is achieved.
c) Polymer Loading - From 10% to 22% by
weight of cement.
From durability point of view, Durgapur slag cement
was supposed to be used for total rehabilitation work. However, as the early strength of
slag cement was less, the progress was slow. Since the whole job was completed in nine
months, Caustic Chlorine plant supposed to be handed over within four months for
operational purpose, ultimately L & T OPC cement were used for faster progress.
To determine the effect of both alkaline and acidic
environment, concrete cubes were cast with various polymer loading as mentioned above and
soaking cloth wrapped around the concrete in 20% of HCL solution and benzene. The cubes
were immersed alternatively in HCL and benzene and dried and wetted alternatively. As
there is no standard test to determine the effect of acidic and highly alkaline
environment on reinforcement, the reinforced concrete were subject to severe aggressive
environment as mentioned earlier. After the completion of the test the reinforcement
corrosion was tested and compared with those reinforcements which were not subjected to
alternate acidic and alkaline environment as mentioned above. From various polymer loading
was best, as at higher polymer loading the reduction in compressive strength was
appreciable.
Concluding Remarks
a) It was possible to change and select
correct materials and methodology for repair in the short time due to close monitoring of
the product.
b)A good R&D Laboratory with in house
construction chemicals proved to be beneficial for such type of rehabilitation work.
c) Practical knowledge of Structural
stability is a must for the site engineers to avoid any catastrophe.
d) A highly skilled foreman, carpenter and
mason with basic knowledge of good quality workmanship can give the best end results. |
