REHABILITATION OF BEAM-COLUMN JOINT USING GEOPOLYMER ADOPTING REMOVAL AND REPLACEMENT TECHNIQUE
Date
2022
Authors
Journal Title
Journal ISSN
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Publisher
Department of Civil Engineering, NITS
Abstract
The most critical component of reinforced concrete (RC) moment resisting frame is the
beam-column joint. Beam-column joint plays an important role in load transfer mechanism
between horizontal and vertical components of a RC framed structure. Integrity of beam
column joint influences structural stability of RC frame during an earthquake. During
lateral loading, beam-column joint goes through large inelastic displacement. Substandard
beam-column joint which were designed, detailed and constructed prior to the development
of any seismic code of practice lacks in adequate confinement reinforcement and sufficient
anchorage length of the beam reinforcement. Such substandard beam-column joint when
subjected to large lateral loads during an earthquake, may lead to formation of diagonal
cracks and concrete crushing in joint core resulting in sudden brittle shear failure. With
advent of seismic code of practice, beam-column joints are being constructed with special
confining hoop reinforcement and adequate anchorage. Such seismically detailed beam
column joints exhibit better stiffness, ductility and higher energy dissipation capacity.
Recently some studies revealed that reinforced concrete (RC) structures designed as per
seismic code of practice exhibited damage in joint region when maximum force acting on
the joint exceeded the code expectation level.
If the damaged joint is not repaired and strengthened, it may cause instability in
structure. In order to mitigate structural instability, rehabilitation of damaged beam-column
joint is carried out. Rehabilitation is a simple and effective process of repairing and
strengthening the damaged structures to the service level so as to provide safety to the
occupant. Rehabilitation by removal and replacement technique is generally used for
severely damaged beam-column joint. In removal and replacement technique, the
fragmented concrete is removed from the damaged joint region and replaced with suitable
repair material. From reported literature it was observed that the repair materials that were
used to rehabilitated beam-column joint adopting removal and replacement technique were
mostly cement based (concrete, cement paste, cement mortar and commercial micro
concrete). Due to utilisation of natural resources and huge environment cost associated
with production of cement, scientist and researchers were looking for a sustainable binding
material.
Geopolymer has immerged out as an environmentally friendly cementitious material
alternative to ordinary Portland cement (OPC). Use of industrial by-products like ground
granulated blast furnace slag (GGBS), fly ash as source material contributed to lower CO2
emission and energy consumption during production stage makes geopolymer an eco
friendly green material. In last decade, geopolymer due to better physical and mechanical
properties, has shown immense potential as repair material alternative to cement-based
materials. But the repair works reported in literatures was associated with structures
subjected to monotonic loading only. Till date, no study was carried out to investigate
suitability of geopolymer based material on rehabilitation of structures subjected to cyclic
loading.
In the present investigation, an attempt has been made to rehabilitate substandard as
well as seismically detailed beam-column joints following removal and replacement
technique using geopolymer mortar (GM). Two one-third scaled substandard beam-column
joints having different percentages of beam and column longitudinal reinforcements
designed as per pre 1993s design code (substandard) of Indian Standard were used as
control specimens. Both control and rehabilitated specimens were tested under quasi-static
cyclic loading. Test results indicated that geopolymer mortar were successful in restoring
the cyclic performance of damaged substandard beam-column joint in terms of peak load,
stiffness degradation, ductility, energy dissipation and damage index up to the level of
control specimens but exhibited undesirable brittle joint failure, similar to that of
substandard control specimens. For seismically detailed specimens, two one-third scaled
beam-column joints of two different groups categorized on flexural strength ratio (1.2 and
1.4), designed as per seismic code of practice were considered in the study. To compare
performance of geopolymer mortar with that of cement mortar, another set of seismically
detailed beam-column joint (one from each group) were rehabilitated with cement mortar
following same rehabilitation technique. Seismically detailed specimens rehabilitated with
geopolymer mortar exhibited 20 – 21% higher initial stiffness, 19 – 22% higher
displacement ductility and 23 – 24% higher cumulative energy dissipation compared to
specimens rehabilitated with cement mortar. But irrespective of repair material, removal
and replacement technique was only able to partially restore the cyclic performance of
rehabilitated specimens. This was attributed to presence of yielded hoop reinforcement and
cold joints in seismically detailed specimens.
When removal and replacement technique was used to rehabilitate seismically detailed
beam-column joint, only fragmented concrete was removed and yielded hoops from the
damaged joint core are not removed and it remained inside the joint core after
rehabilitation. Therefore, investigation was carried out to study the effect of yielding of
hoops on cyclic performance of seismically detailed beam-column joint. For this purpose,
four numbers of one-third scaled beam-column subassemblies with flexural strength ratio
1.4 were prepared. All four specimens were designed and detailed following relevant
provisions of Indian Standard Code of Practice. Out of the four specimens, one was used as
control specimen and the rest three were cast with different magnitude of pre-yielded hoop
reinforcement in the joint core. It was observed that specimens with yielded hoop
reinforcement showed lower peak load, lower ductility and lower energy dissipation along
with higher stiffness degradation and damage index. On the other hand, due to removal and
replacement, there exists cold joint between old concrete and new repair material.
Moreover, yielded hoops remain inside the repaired joint core after rehabilitation adopting
removal and replacement technique. Till date, no attempt has been made to investigate
combined effect of cold joint and yielded hoop reinforcement on cyclic behaviour of
seismically detailed beam-column joint. When tested under cyclic loading, specimens with
both cold joint and yielded hoop reinforcement shown the highest reduction in cyclic
performance in terms of initial stiffness, stiffness degradation, ductility and cumulative
energy dissipation compared to individual effect of either cold joints or yielded hoops.
Between individual parameters – cold joint and yielded hoop reinforcement, cold joint
played a more dominant role. This was established by reduced cyclic performance of
specimens with cold joint in terms of initial stiffness (15 -17% lower), displacement
ductility (7 – 9% lower) and energy dissipation (18 – 19% lower) in comparison to
specimens with yielded hoops.
It was observed that though GM was able to enhance performance of rehabilitated
specimens but was not able to alter mode of failure brittle joint failure to ductile beam
failure in substandard specimens. Furthermore, when seismically detailed specimens were
rehabilitated with geopolymer mortar, rehabilitated specimens displayed beam failure
accompanied by joint failure as well as specimens exhibited lower performance compared
to control under cyclic loading. Such failure is undesirable and unacceptable as per current
seismic design philosophy. To mitigate underperformances in beam-column joints, steel
fiber reinforced geopolymer mortar (SFRGM) was used for rehabilitation of both
substandard and seismically detailed specimens adopting same repair technique. For
substandard specimens, SFRGM not only increased performance of rehabilitated
specimens substantially above the level of controls but also alter mode of failure to ductile
beam failure. In case of seismically detailed beam-column joints, specimens rehabilitated
with SFRGM exhibited beam failure along with 28 – 34% increased peak load, 20 – 26%
increased initial stiffness, 109 – 113% increased ductility, 177 – 212% higher cumulative
energy dissipation, substantial lower stiffness degradation and lower damage index.