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STUDY ON STRENGTH OF BEAMS RETROFITTED USING FERROCEMENT JACKETS
ABSTRACT
l Our main
attention in this dissertation is the use of ferrocement as retrofitting
material.
l This disseration
is carried out by casting cubes, cylinders, prisms to evaluate compressive
strength, tensile strength and flexural strength of concrete.
l The dissertation
carried out the study on the strength of stressed beams retrofitted with
ferrocement with different orientation.
l In that two
control beams are tested to failure to find safe load carrying capacity to
allowable deflection as per IS 456-2000 and six beams are stressed upto 75% of
the safe load and these beams are retrofitted with different orientation wire
mesh as 00, 450 and 600.
INTRODUCTION
l In this
developed world RCC structures are suffer from damage and distress before their
service period is over, such structure required immediate attention to bring
the structures into its functional use again.
l There are
various types of retrofitting techniques used to bring such structure into its
functional use again, apart from which plate bonding technique is considered as
the best.
l The plate
bonding technique consists of plate of different material such as CFRP, GFRP,
ferrocement etc.
l Ferrocement
sheets are most commonly used or retrofitting material these days due to their
easy availability, economy, durability, and their property of being cast
without needing significant formwork.
l The National
Disaster Mitigation Agency (NMDA), Government of India also accepted the use of
ferrocement as retrofitting material.
l The behaviour of
ferrocement in flexure depends upon various parameters such as mortar, type of
wire mesh, orientation of wire mesh etc.
ØFERRORCEMENT
Ferrocement
is a composite material consisting of rich cement mortar matrix uniformly
reinforced with one or more layers of very thin wire mesh with or without
supporting skeletal steel.
v
Compressive Strength:
Compressive
strength of ferrocement is more in use of welded wire mesh due to its lateral
restraint provided by welded transverse wires.
Kameshwara
Rao and Kamasundra (1986) (13) studied the behaviour of ferrocement in
compression it indicates that compressive strength depends upon specific surface
area of the composite.
v
Flexural Strength:
Mansur
and Paramasivam (1986) (14) studied the behaviour and strength of ferrocement
in flexure. It was found that the ultimate moment increase with increase matrix
grade and increasing volume fraction of reinforcement.
v
Shear Strength:
Venkata
Krishna and Basa Gouda (15) (1988) found that shear strength depends upon
mortar, strength of wire mesh, volume fraction and shear span
v
Impact Resistance:
Impact
strength is defined as the energy absorbed by the specimen when struct by a
swinging pendulum dropped from a constant height.
Shah
and Key (1972) (16) tested ferrocement slabs using impact tester. It indicated
that the higher the specific surface of the meshes and the higher the strength
of the mesh, the lower the damage due to impact loading.
EXPERIMENTAL
PROGRAMME
v
Test Programme
To carryout the
investigation, Eight Prototype Underreinforced Beams of size
2300X130X230mm(lxbxd) reinforced with 2-12mmΦ and 2-10mmΦ in compression are cast using M20 grade concrete.
Out
of these Eight beams two are used as control beams (Type – A) and tested to failure to find out the safe load
carrying capacity corresponding to the allowable deflection as per IS: 456-2000
i.e. Span/250. The other six beams are then stressed to 75 percent of
the safe load obtained from the testing
of the control beams and are then retrofitted with 20mm thick ferrocement
jackets made with 1:2 cement sand mortar and w/c ratio 0.40. The jackets are
reinforced with single layer of 40 mm x 40 mm square welded wire mesh. The
three wire mesh orientation viz., 00, 450, 600 degree are used in the
ferrocement jackets.
v
Test for compressive strength of concrete (IS:
516-1959):
Compressive strength is calculated by
dividing load by area of specimen.
Fc =P/A
Where
Fc
= Cube compressive strength in N/mm2.
P
= Cube compressive load causing failure in N
A
= Cross sectional area of cube
No.
of cubes tested for plain cement concrete are shown in table below
v
Test for Split Tensile Strength of concrete (IS:
5816-1970)
Tensile Strength = fcs =
2F/πdl,
Where:
F=
fracture compression force acting along the cylinder generatrix,
d= cylinder diameter;
l = cylinder length.
Numbers of cylinder tested for plain concrete are shown in table below
v
Test for Flexural Strength of concrete
(IS:516-1959)
The maximum tensile stress is called
modulus of rupture and is computed from the standard formulae.
F = M/Z
Where
M =
Bending moment at the section where rupture occurs
Z =
Section Modulus = I/Y
I =
Moment of inertia of the section
Y =
Distance from neutral axis = d/2
Number
of prisms for plain concrete are shown in table below
v
Testing Methodology
First,
the two control beams are tested to failure. The load corresponding to an
allowable central deflection of 8.6mm (span/250) is obtained from the load
deflection curve as 59.25kN.
The
remaining six beams are stressed to 75 percent of this average safe load i.e.,
44.44kN.
Subsequently,
the retrofitting of beams using different orientaitons of wire mesh in the
ferrocement jackets are carried out with cement mortar of thickness 20mm along
with wire mesh bonded on three sides for all six beams. After one week of
curing the beams are tested again with the same method as the control beams are
tested initially and the corresponding results are recorded in the form of load
v/s deflection.
DISCUSSIONS
During
the experimental work, close observations have been done to study the
following.
l The curves shows
that with an increase in the load carrying capacity there is a considerable
increase in the deflection for all the beams except two control beams.
l The percentage
increase in the ultimate load w.r.t the control beams is highest for the beams
retrofitted with 450 orientation (i.e 39.75%)
l The percentage
increase in the ultimate deflection w.r.t the control beams in highest for the
beams retrofitted with 450 orientation (i.e 24%)
l
The beams retrofitted with 450
orientation is the best among all the three orientation because of its enhanced
maximum load carrying capacity.
l
The deflection at the centre at ultimate load is
maximum in the case of beams retrofitted with wire mesh at 450 (15.55mm) as compared to those with wire mesh at zero degree,
(13.68 mm) and 600 (14.16mm).
l
The ductility ratio is highest in case of beams
retrofitted with wire mesh at 00orientation followed by 450
and 600 orientation.
l
Retrofitted beams with wire mesh oriented at zero
degree are the most efficient of the three orientations as its cost to strength
ratio is the lowest at 1.25 as compared to the other two orientation i.e. 00
(1.28) and 600 (1.41).
CONCLUSION
l
The first crack appeared for control beams at early
stage and for retrofitted beams the first crack appeared lately.
l
The load carrying capacity of retrofitted beams is
more compared to control beams.
l
The beams retrofitted with wire mesh at different
orientation do not de-bond when loaded to failure.
l
Wire mesh oriented at 450 has the
highest load carrying capacity as compared to the control beams as well as the
other orientation of the beams.
l
The failure of the composite is characterized by
the development of flexural cracks on the tension side.
l
In case of retrofitted beams it is observed that
the spacing of cracks is increased, indicating better distribution of stress when
compared to the control beams.
l
Beams retrofitted with wire mesh oriented at zero
degree are the most efficient as their cost to strength ratio is lowest.
SCOPE
FOR FURTHER STUDY
l
In this present investigation the retrofitting of
ferrocement laminates is done on three sides of the specimens. Further work can
be carried out by the retrofitting of ferrocement laminate is done only bottom
face of the specimens.
l
Further work
can be carried out by using higher grades of concrete i.e., M 25, M 30
etc.,
l
For the present investigation the specimens are
stressed to 75% of safe load. Further work can be carried out the specimens are
stressed to varying percentage of safe load.
l
Presently the study of RCC rectangular beam is
carried out on a limited span of 2.3 m. further work can be extended for
greater span.
l
In this investigation only flexural behaviour is
studied. Further work can be carried out by combined flexure and shear
behaviour.
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