7314356 Latex Modified SFRC Beamcolumn Joints Subjected to Cyclic l
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Transcript of 7314356 Latex Modified SFRC Beamcolumn Joints Subjected to Cyclic l
Latex modified SFRC beam-column joints subjected to cyclic loading
ABSTRACT
This topic describes the experimental results of ten
exterior beam-column joints that employ steel fibre reinforced concrete
(SFRC) and natural rubber latex under cyclic loading. Test results have
indicated that latex modified SFRC increases joint strength and
enhances the ductility and energy absorption capacity of the joint. Also
the congestion of steel reinforcement in the joint and construction
difficulties could be reduced by using latex modified SFRC in the
conventional joint.
C.O.E. & T., Akola
Seminar 2000-2001
Dept. of Prod. Engg.
Latex modified SFRC beam-column joints subjected to cyclic loading
INTRODUCTION
In the design and construction of seismic-resistant
structures, one of the most critical areas is the beam-column joint. In
these area, a high percentage of transverse hoops in the core of joint is
needed in order to meet the requirement of strength, stiffness and
ductility under cyclic inelastic flexural loading. Provision of high
percentage of hoops leads to congestion of steel leading to construction
difficulties.
Several researchers have reported their test results
using SFRC in framed beam-column joints. All these tests have shown
the effectiveness of using steel fibres to increase the joint strength,
ductility and the energy absorption capacity. Studies on polymer
modified concrete also reveal that many of the engineering properties
like ductility, energy absorption capacity etc could be improved by the
addition of polymers to plain concrete. However, no attempts have
been made so far to study the combined effect of steel fibres and
polymer on the strength and behaviour of beam-column joints. Hence
C.O.E. & T., Akola 1
Seminar 2000-2001
an attempt is made in this paper to study the combined effect of steel
fibres and polymer on the strength and behaviour of beam-column joint.
EXPERIMENTAL PROGRAMME
In the present investigation, 10 numbers of exterior
beam-column joints were cast and tested under flexural cyclic loading.
The overall dimensions and details of reinforcement of beam-column
joint are given in Fig 1. The column was reinforced with six 12-mm
diameter high yield strength deformed (HYSD) bars and the beam was
provided with an equal reinforcement of two 12-mm diameter HYSD
bars at top and bottom. HYSD bars of 6-mm diameter were used for
transverse ties in columns and stirrups in beams. Two different values
of volumetric ratio of transverse reinforcement in the core of the joint
(p.) namely, 0.89 and 1.22, four different values of dry rubber content
(DRC) of natural rubber latex(L) namely, 0.5 percent and 1.0 percent
were used. As the value of volume fraction of steel fibres exceeds two
percent, the normal concrete mix becomes less workable. Hence the
maximum value of V1 considered is two percent. From the earlier
studies, it has been noted that as the percentage of DRC exceeds above
Dept. of Civil. Engg. 2
Latex modified SFRC beam-column joints subjected to cyclic loading
one percent, the load carrying capacity of latex modified concrete
decreases drastically. Hence it is limited to one percent. The details of
specimens tested in this study are given in Table 1.
C.O.E. & T., Akola 3Fig. 1 :- Detail of a typical beam-column joint which was cast for testing
Seminar 2000-2001
CASTING OF SPECIMENS
Materials used:-
Cement : Ordinary portland cement (43 grade) conforming to IS:8112-
1989.
Fine aggregate : River sand passing through 4.75 mm IS sieve and
having a fineness modulus of 1.96%.
Coarse aggregate : Crushed granite stones passing through 20 mm and
retained on 4.65 mm. IS sieve and having a fineness modulus of 6.8.
Fibres : Galvanised straight round steel fibres of diameter 0.88 mm
having an equal aspect ratio, A, of 50.
Polymer : Natural rubber latex having DRC of 41.1 percent.
Dept. of Civil. Engg. 4
Latex modified SFRC beam-column joints subjected to cyclic loading
CASTING:-
For casting the specimens, wooden moulds were used
Reinforcing cage was fabricated and placed inside the mould. The ratio
of cement, sand and coarse aggregate used in the nominal mix was 1:2:4
by weight, with a water-cement ratio of 0.5 by weight. Required
quantities of cement, sand and coarse aggregate were mixed thoroughly.
To start with, 50 percent of the water was added and the remaining 50
percent of the water, mixed with latex and a superplasticiser was added
later and mixing was done till a uniform mix was obtained. The
superplasticiser Complast P211 was added to concrete to prevent the
premature coagulation of the mix, on addition of latex. The quantity of
superplasticiser for different values of percentage of DRC was obtained
from workability test for a flow of 40 cm. The concrete mix was
poured into the mould in layers and the mould was vibrated for
thorough campaction. After 24 hours, the specimen were demoulded
and air dried for 24 hours and then cured under wet gunny bags for 28
days.
C.O.E. & T., Akola 5
Seminar 2000-2001
TESTING OF SPECIMENS:-
All the specimens were tested in a universal testing
machine (UTM) of 2943 kN (300 t) capacity. The specimen was
mounted in a vertical position. A constant axial load of 75 kN which
consists of 20 percent of the axial load capacity of the column was
applied to the column to keep the column in vertical position and to
stimulate column axial load. A hydraulic jack was used to apply the
load at the free end of the beam. To record the load precisely, a load
cell with a least count of 0.098 kN(10 kg) was used. The increment of
loading selected was 4 kN. The beam was loaded gradually upto 4 kN,
then unloaded and reloaded to the next increment of load and this
pattern of loading was continued for each increment until failure. The
deflection of the beam at the point of loading during the test was
measured using a dial gauge with a least count of 0.01 mm. Other
instrumentation used during the tests consisted of three linear variable
differential transducers (LVDTs) to record the curvature of the beam
near the joint. These LVDTs measured deformation over a guage
length of 200 mm. The locations of LVDTs are shown in Fig 2. From
these readings, the strains and the curvature were computed.
Dept. of Civil. Engg. 6
Latex modified SFRC beam-column joints subjected to cyclic loading
C.O.E. & T., Akola 7
Deflection dial guage
Fig.2 :- Locations of LVDT’s on the beam-column joint
Seminar 2000-2001
BEHAVIOUR OF THE SPECIMENS
In all the specimens cracks appeared near the joint
after the first crack load. As the loading is increased additional cracks
formed. With further increase in loading, the cracks propagated up the
beam and initial cracks started widening. Specimens without fibres
developed wide cracks at the joint and the cracking was more or less
concentrated at the joint. The SFRC joint developed large number of
closely spaced finer cracks and width of such cracks was smaller than
those developed in conventional reinforced concrete (RC) joint. It was
observed that the use of SFRC in the joint core could increase the joint
stiffness and minimise damage to the concrete. The core and cover
concrete were found to be intact.
Dept. of Civil. Engg. 8
Latex modified SFRC beam-column joints subjected to cyclic loading
RESULTS AND DISCUSSIONS
Table 1 gives the peak load, P u and the deflection at
peak load δu for all the specimens.
Referring to these the following points may be noted.
The addition of 0.5 percent of V f of steel fibres,
improves the ultimate load by 30 percent. When the fibre content
increases, that is, at one percent of V f of fibres, the ultimate load
increases by about 50 percent. The gain in strength could be attributed
to the following effects of ‘fibre bridging’. As and when microcracks
develop in the matrix, the fibres in the vicinity of such microcracks
intercept these cracks and arrest them which prevents further
propogation of cracks in the same direction. Hence the crack which
appear inside the matrix have to take a meandering path, resulting in
the demand for more energy for further propagation, which in turn
increases the ultimate load. Addition of fibres above one percent of V f
did not enhance the ultimate strength. This may be due to ‘balling
effect’ of fibres and this cn be explained as follows. At higher
C.O.E. & T., Akola 9
Seminar 2000-2001
percentage of fibre content, the mix becomes less workable and the
fibres tend to knit themselves in the form of balls with little or no
Table 1: Details of specimen and test results
Desciption ρ s V f Lp PukN δu mm φu x 103/m φu
φy
J1E0L0 0.89 0.0 0.0 21.582 9.35 18.11 1.22
J1E1L0 0.89 0.5 0.0 29.430 18.15 23.88 1.62
J1E2L0 0.89 1.0 0.0 32.373 30.64 35.59 2.44
J1E1L0 0.89 1.5 0.0 28.253 25.22 25.34 1.71
J1E4L0 0.89 2.0 0.0 23.544 19.18 19.19 1.30
J1E0L1 0.89 0.0 0.5 22.563 13.85 20.38 1.36
J1E0L2 0.89 0.0 1.0 18.639 16.85 22.85 1.50
J1E2L1 0.89 1.0 0.5 33.648 32.02 38.09 2.56
J1E2L2 0.89 1.0 1.0 29.430 35.58 45.32 3.04
J2E0L0 1.22 0.0 0.0 28.449 29.09 30.29 2.06
concrete in between which causes difficulty in compacting the
specimens. This in turn may have led to lot of voids in the concrete
and hence might have caused reduction in the value of ultimate load.
With the addition of 0.5 percent of latex, there is a slight increase in
Dept. of Civil. Engg. 10
Latex modified SFRC beam-column joints subjected to cyclic loading
the strength. However, at higher values of latex content, that is, at one
percent a reduction in strength is observed and the specimen failed
relatively in a ductile manner. This can be explained as follows. When
latex is added to the fresh concrete, the polymer particles are uniformly
dispersed in the concrete phase. As the hydration of cement proceeds,
and the water in the pore drains out, the adhesive and autohesive
polymer particles fill the resulting micropores and the closely packed
polymer and cement particles coalesce into continuous films which
improves the density of concrete and hence the ultimate strength.
However, as the latex content increases, the excess latex will lead to
the formation of weak spots in the specimen, due to air entrainment
which causes reduction in strength. When fibres and latex were added
to concrete the load carrying capacity improved considerably at the
combination of 0.5 percent of latex and one percent of V f of fibres.
C.O.E. & T., Akola 11
Seminar 2000-2001
Dept. of Civil. Engg. 12
Fig. 3. :- Energy absorption capacity versus F
Latex modified SFRC beam-column joints subjected to cyclic loading
ANALYSIS OF TEST RESULTS
Energy absorption capacity
The area under the load-deflection curve represents
the energy absorption capacity of the specimen. Due to the inherent
limitations of the test setup, the load-deflection could be traced only
upto 80 percent of the post-peak loading in the descending portion of
the curve. Hence the area under the load deflection curve considered in
this study consists of the area under the ascending portion up to the
peak load and under descending portion up to 80 percent of peak load.
As several variables like volumetric ratio of
confinement, ρy volume fraction, V f and aspect ratio, A f of fibres and
the percentage of DRC of latex, L p affect the behaviour of concrete, an
attempt was made to obtain a parameter which gives the combined
effects of all these variables. One such parameter known as
confinement-latex-fibre index was introduced after trying several
combinations of these variables and is given by,
C.O.E. & T., Akola 13
Seminar 2000-2001
F = ρs (1+ V f Af +Lp) .................(1)
An attempt is made to relate the energy absorption
capacity with this parameter F. Fig 3 shows the plot relating the energy
absorption capacity and F.
It can be seen from the plot that as the value of F
increases, the energy absorption capacity increases upto a value of F =
1.35 and beyond that value, the energy absorption capacity decreases.
The reason for this decrease in energy absorption capacity at higher
values of F are due to
(i) the balling effect at higher values of fibre
content and
(ii) formation of weakspots at higher values of
latex content and these have been already explained.
Moment curvature behaviour
An attempt is made to study the moment-curvature
relationship for all the specimens using the test results. The strains
measured at 15 mm below the extreme compression fibre and 15 mm
above the extreme tension fibre have been used to calculate the
curvature, φ of the beam for every loading stage using the relation.
å1+ å1
φ = ............................(2)
Dept. of Civil. Engg. 14
Latex modified SFRC beam-column joints subjected to cyclic loading
d1
where,
å1 = strain at the level of top LVDT
åb = strain at the level of bottom LVDT
d1 = distance between top and bottom LVDT
The values of moment M were computed using the
experimental values of load and lever arm. These values of M and φ
were used to obtain moment-curvature plots for the joint.
CURVATURE DUCTILITY FACTOR :-
The capacity of the member to deform beyond its
initial yield deformations with minimum loss of strength and stiffness
depends upon the ductility factor which is defined as the ratio of the
ultimate deformation to its deformation at first yield.
Ductility may be defined easily in the case of
elastoplastic behaviour. However, reinforced concrete members which
are lacking such characteristic, there is no universal definition for
ductility. Thus in evaluating the performance of beam-column joint and
studying the effect of different variables, curvature ductility is defined
C.O.E. & T., Akola 15
Seminar 2000-2001
as follows. Ductility factors have been defined in terms of curvature at
critical section and is,
Φu
Curvature ductility factor = .........................(3)Φy
Where
Φu = curvature of peak load
fy
Φy = curvature at yield = ............(4) Es ( d – x )
Where
d = the effective depth
fy = the yield strength of reinforcement,
x = neutral axis depth
Es = Modulus of elasticity of steel.
The curvature at peak load and curvature ductility
factor thus calculated for all specimens are given in Table 1. From the
table it may be noted that the latex modified SFRC specimens have
given better values of ductility factor than other specimens. This
Dept. of Civil. Engg. 16
Latex modified SFRC beam-column joints subjected to cyclic loading
indicate that addition of steel fibres and latex improve the ductility of
conventionally reinforced concrete beam-column joints appreciably.
Referring to Table 1, it may be seen that specimen
with ρs= 0.89 (6 mm diameter ties spaced at 120 mm on centres) with
V f = 1.0 percent and Lp = 0.5 percent has given higher values of
ultimate strength (33.648 kN) and curvature ductility (2.56) than the
specimen having ρs = 1.22 (6 mm diameter ties spaced at 80 mm on
centres). This indicate that the ties/stirrups at the core of the beam-
column joint can be replaced by the addition of steel fibres and latex
without losing strength and ductility.
C.O.E. & T., Akola 17
Seminar 2000-2001
CONCLUSION
Based on the experimental study, the following
conclusions have been drawn
(i) Addition of steel fibres and latex, to the core of the conventional
RC beam-column joint region improve the strength and ductility
of the joint.
(ii) By using latex modified SFRC, the spacing of hoops provided in
the core of the beam-column joint can be increased while
maintaining ductile behaviour of the frame. This reduces
congestion of reinforcement in the joint and hence ease
construction difficulties.
(iii) Deflection, curvature at peak load and energy absorption capacity
were significantly increased with the increase in fibre content and
addition of latex. It is true only up to a certain percentage of
fibre content and latex. The combination of V 1 = 1 percent and
Lp = 0.5 percent appears to be a better combination than others.
Dept. of Civil. Engg. 18
Latex modified SFRC beam-column joints subjected to cyclic loading
REFERENCES
1. The Indian concrete Journal
No. 7 ( July 2000)
2. The Indian concrete Journal
No. 11 ( Nov. 1999)
3. Journal of cement and concrete
Vol. 18 ( 1988)
4. Journal of structural Engineering
No. 1, Vol. 24 ( April 1997)
5. The text book on Concrete Technology
By M.S. Shetty
C.O.E. & T., Akola 19