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3.0 EXPERIMENTAL INVESTIGATIONS
3.1 INTRODUCTION
In the present experimental investigation the following
properties of steel fibre reinforced concrete are studied using OPC
concrete and Metakaolin concrete. The experimental investigations of
these properties are given in sections 3.3 to 3.12
Compressive strength
Splitting tensile strength
Modulus of rupture
Modulus of elasticity
Impact resistance
Residual compressive strength and weight loss at elevated
temperatures
Residual compressive, split tensile strength and modulus of
rupture at different thermal cycle
Durability studies
Flexural behavior of beams
Flexural behavior of slabs
In the present experimental investigation, M20 and M50 grade of
concrete designed as per IS: 10262 - 1982 was used. The details of
the crimped steel fibres and mix proportions of M20 and M50 grade
OPC concrete used in this investigation are given in table 4.1.8, 4.2.3
and 4.2.4 respectively.
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The following mixes were used to cast the specimens for finding the
above properties given in section 3.1
a) Ordinary Portland Cement Concrete specimens using 53 grade
OPC (ultra tech cement).
b) SFRC specimens using OPC and crimped steel fibres of aspect
ratio 60 and 80 each with volume fraction as 0.5, 1 and 1.50%.
c) Metakaolin concrete specimens (MK 10) with 10% cement
replacement with Metakaolin (binary blending)
d) SFRC-MK specimens with 10% Metakaolin and crimped steel
fibres of aspect ratio 60 and 80 each with volume fractions as 0.5, 1
and 1.50%.
3.2 MATERIALS
3.2.1 Cement:
Ordinary Portland cement available in local market of standard brand
was used in the investigation. Care has been taken to see that the
procurement made from a single batch and is stored in airtight
containers to prevent it is being affected by atmospheric, monsoon
moisture and humidity. The Cement is tested for its various
proportions as per IS 4031-1988. The specific gravity was 3.10 and
fineness was 3200 m2/Kg. The details are given in Table 4.1.1. The
cement confirms to 53 Grade.
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3.2.2 Coarse Aggregate:
Machine Crushed angular granite metal of maximum size of 20mm
retained on 4.75mm I.S. sieve confirming to I.S. 383-1970 was used in
the present investigation. It is free from impurities such as dust, clay
particles and organic matter etc. The coarse aggregate is also tested
for its various properties. The specific gravity and fineness modulus
are found to be 2.56 and 7.15. The details are tabulated in 4.1.3 and
4.1.4.
3.2.3 Fine Aggregate:
The locally available river sand was used as fine aggregate in the
present investigation. The sand is free from clayey matter, salt and
organic impurities. The sand is tested for its various properties like
Specific Gravity, Fineness modulus, Bulk Density etc in accordance
with IS 2386-1963. Fine aggregate passing through 4.75mm I.S. sieve
and retained on 0.075mm I.S. sieve was used. It confirms to grading
zone – II of I.S. 383-1970. The specific gravity and fineness modulus
are found to be 2.50 and 2.79. These test results are tabulated in
Table 4.1.4. Sieve analysis is carried out and results are shown in
Table 4.1.2.
3.2.4 Super plasticizer:
Conplast SP 430 obtained from Fosroc chemicals (I) Ltd. was used in
this experimental research. It confirm to I.S. 9103-1999, Table 3.7
shows the properties of this super plasticizer. Plate 3.1 shows the
view of super plasticizer SP 430 (FOSROC) container of 5 liters.
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3.2.5 Crimped steel fibres:
In the present investigation, round crimped steel fibres of 27mm and
36mm length and diameter of 0.45 mm with aspect ratio as 60 and 80
are procured from M/s. Stewols India (P) LTD, Nagpur, Maharashtra
State. The properties of these fibres are given in table 4.1.8. Plate 3.2
shows the view of crimped steel fibres.
3.2.6 Metakaolin
Metakaolin is obtained by calcination of pure or refined kaolin clay at
a temperature between 6500C and 8500C, followed by grinding to
achieve a fineness of 15000 m2/kg (B.E.T).The specific gravity is found
as 2.50. The resulting material has high pozzolanic property. Plate
3.3 shows the view of Metakaolin.
Metakaolin is manufactured from pure raw material to strict
quality standards and not a by-product. Other pozzolanic materials
are currently available, but many are by products, which are available
in chemical composition. They may also contain active components
(such as sulphur compound, alkalis, carbon, reactive silica) which can
undergo delayed reactions within the concrete and cause problems
over long time periods.
Metakaolin is a high quality pozzolanic material, which is
blended with Portland cement in order to improve the durability of
concrete and mortars. Metakaolin removes chemically reactive
calcium hydroxide from the hardened cement paste. Metakaolin
reduces the porosity of hardened concrete. Metakaolin densifies and
reduces the thickness of the interfacial zone thus improving the
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adhesion between the hardened cement paste and particles of sand or
aggregate.
Metakaolin manufactured from pure raw material to strict
quality standards. Metakaolin is a high quality pozzolanic material,
which when blended with Portland cement improves the strength and
durability of concrete and mortars. Metakaolin removes chemically
reactive calcium hydroxide from the hardened cement paste. It
reduces the porosity of hardened concrete. Metakaolin densifies and
reduces the thickness of the interfacial zone, thus improving the
adhesion between the hardened cement paste and particulars of sand
or aggregate.
Properties of Metakaolin
Metakaolin grades of calcined clays are reactive allumino silicate
pozzolan formed by calcining very pure hydrous China clay.
Chemically Metakaolin combines with calcium silicate and calcium
processed to remove uncreative impurities producing almost 100
percent reactive material. The particles size of Metakaolin is
significantly smaller than cement particles. IS:456-2000 recommends
use of Metakaolin as mineral admixture.
Metakaolin is a ultra fine pozzolanic which replaces industrial
by-products such as silica fume/micro silica. Commercial use of
Metakaolin has already in several countries worldwide. Metakaolin
removes chemically reactive calcium hydroxide from the hardened
paste. Metakaolin reduces the porosity of hardened concrete.
Metakaolin densifies, reduces the thickness of the interfacial zone,
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thus improving the adhesion between the hardened cement paste and
particles of sand or aggregate. Blending Metakaolin with Portland
cement improves the properties of concrete and cement products
considerably by:
Increasing compressive and flexural strength
Providing resistance to chemical attack
Reducing permeability substantially preventing Alkali-Silica
Reaction
Reducing efflorescence & Shrinkage
Protecting corrosion
Pozzolanic Reactivity
Metakaolin is a lime-hungry pozzolan that reacts with free
calcium hydroxide to form stable, insoluble, strength-adding,
cementitious compounds. When Metakaolin – HRM(AS2) reacts with
calcium hydroxide (CH), a cement hydration byproducts, a pozzolanic
reaction takes place whereby new cementitous compounds, (C2ASH8)
and (CSH), are formed. These newly formed compounds will
contribute cementitious strength and enhanced durability properties
to the system in place of the otherwise weak and soluble calcium
hydroxide.
Cement Hydration Process
OPC + H2O - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -> CSH + CH
Pozzolanic Reaction Process
AS2 + CH - - - - - - - - - - - - - - - - - - - - - - - - - - - - -> C2ASH8 + CSH
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Unlike other commercially available pozzolanic materials,
Metakaolin is a quality controlled manufactured material. It is not a
by-product of unrelated industrial process. Metakaolin has been
engineered and optimized to contain a minimum of impurities and to
react efficiently with cement‟s hydration byproduct-calcium hydroxide.
Table summarizes the relative reactivities of six different pozzolans,
including high reactive Metakaolin-HRM.
Reactivity of Pozzolanic Materials:
Material Pozzolanic Reactivity mg Ca(OH)2 per g
Blast furnace slag 40
Calcined paper waste 300
Micro silica, silica fume 427
Calcined bauxite 534
Pulverized fuel ash 875
High reactivity Metakaolin 1050
3.2.7 Water
Water is the least expensive but most important ingredient of the
concrete. The water, which is used for making concrete should be
clean and free from harmful impurities like oil, alkalis, acids etc. in
general, the water which is fit for drinking should be used for making
concrete.
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3.2.8 Mix design of M20 grade OPC concrete:
M20 grade of concrete designed as per IS: 10262-1982 is given
in Appendix-A. Trial mixes were cast as given in table 4.2.1 and the
quantity of cement is optimized to a value of 320 kg. The compressive
strength obtained was 30.69 MPa at 28 days and workability obtained
was 0.976 in terms of compacting factor. The mix proportions of this
OPC concrete are given in table 4.2.3. The ratio of the quantities
obtained were cement: Fine aggregate: Coarse aggregate = 1:1.92:2.64
with w/c = 0.55.
3.2.9 Mix design of M50 grade OPC concrete:
M50 grade of concrete designed as per IS: 10262-1982 is given
in Appendix-A. Trial mixes were cast as given in table 4.2.2, and the
quantity of cement is optimized to a value of 470 kg. The compressive
strength obtained was 61.40 MPa at 28 days and workability obtained
was 0.892 in terms of compacting factor. The mix proportions of this
OPC concrete are given in table 4.2.4. The ratio of the quantities
obtained were cement: Fine aggregate: Coarse aggregate = 1:0.96:3.64
with w/c = 0.33.
3.2.10 Factors affecting the workability and strength of fibre
concrete
Previous investigations13, 15 shows that for a given mix of specific
proportions and water-cement ratio, there is a maximum quantity of
fibre which can be introduced into it without causing balling and
interlocking of the fibres. Increasing the sand content of the mix
makes it possible to increase the fibre content. Increasing the fibre
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content or the aspect ratio of the fibres will cause a reduction in
workability and / or increased balling of fibres during mixing.
Obviously, improvements in strengths due to fibre addition are
only likely to occur if the mix workable and free from balling. In a
workable, homogeneous mix, improvements in strength can be
ascribed to two major factors: the fibre volume fraction in the mix and
the resistance offered by the fibres to the crack formation and
propagation. For similar materials, the latter is affected by two
variables: fibres having larger aspect ratio lf/df exhibit greater pull-out
strengths and can be considered to be more effective than fibres with
smaller values, crimped fibres will possess higher bond strengths than
similar un crimped ones.
In this study, the influence of these factors is incorporated into
a single parameter called the fibre factor F, given as15:
F = (lf/df) Vf .β
Where
lf/df = The ratio of length of the fibre to its diameter (also called
as aspect ratio of the fibre)
Vf = Volume of fibres per unit volume of concrete (also called
as volume fraction)
β = Bond factor which accounts for different bond
characteristics of the fibres.
Based on large series of tests15 on different types of fibres, the
following relative values are assigned for β.
β = 0.50 for un crimped fibres of circular cross section
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β = 0.75 for crimped and hooked fibres of circular cross
section
β = 1.0 for indented fibres
For crimped steel fibres of circular cross section used in this
study, β has been assigned a value of 0.75.
3.2.11 Mixing, casting and workability tests
The mixing of SFRC was done in a laboratory PAN mixer
available in the concrete laboratory. The following procedure is
followed for mixing of ingredients. First aggregates were added to the
mixer. Then during mixing 75% water-super plasticizer mixture was
added and mixed for 3 to 5 minutes continuously. Then 75% binder
was added. When Metakaolin is to be used it was first mixed with
cement until a uniform mix is obtained and then fed into the pan
mixer. Fibres were gradually added by sprinkling them into the pan
mixer. Addition of fibres usually takes 5 to 10 minutes. Lastly the
remaining water-super plasticizer mixture and remaining binder was
added one after the other and mixed for 5 minutes. After 5 minutes
rest, concrete was mixed for another 2 minutes to get a homogenous
mix. Plate 3.4 shows the view of the pan mixer used for mixing of
concrete. Plate 3.5 shows the view of mixing of fibres in the pan
mixer.. Plate 3.6 shows the view of dry mixing of fibres in the pan
mixer. Plate 3.7 shows the view of SFRC in fresh state. Plate 3.8 to
3.10 show the view of casting of cubes, cylinders and prisms.
Workability tests such as slump test, compacting factor test and
Vee-Bee test were conducted on all mixes in order to check uniformity
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in concrete workability. For SFRC and SFRC-MK mixes the super
plasticizer content was suitably adjusted so that workability in terms
of compacting factor is within 0.80 to 0.95. The quantity of super
plasticizer used was 1% of the weight of binder for various mixes to
get workable mixes.
The compaction of concrete was made using a table vibrator. To
eliminate the effect of possible fibre orientation, the cube moulds were
initially half filled, the mix was vibrated and then the remaining half
was filled and the vibration was continued for one minute. No signs of
segregation or air bubbles were observed during mixing or
compaction. After casting, the moulds containing compacted concrete
were covered with a thin plastic sheet in order to prevent the
evaporation of water from the surface of concrete. The specimens were
de-moulded after 24 hours and cured by immersing them in water for
28 days as shown in plate 3.13.
3.3 COMPRESSIVE STRENGTH
3.3.1 Introduction
Concrete is strong in compression but it is weak in tension and
has low strain at fracture. The low tensile strength of concrete is due
to the presence of numerous micro cracks. These micro cracks
further propagate under load and result in poor strength of concrete.
In this study, the effect of fibres on compressive strength is studied by
casting cubes and testing them in compression.
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3.3.2 Casting of cubes
For each mix 9 cube specimens of size 100 x 100 x 100mm were
cast in C.I. moulds as shown in plate 3.14a. 3 cubes was tested at 7
days, 3 cubes was tested at 14 days and the remaining 3 cubes were
tested at 28 days of curing. Each compressive strength result is the
average of 3 test results.
3.3.3 Testing of cubes for compressive strength
After 7,14 and 28 days of curing the cubes were removed from
the curing tank, weighed and tested for compressive strength in a
3000 KN digital compression testing machine with the cast face
parallel to the axis of loading at the rate of 140Kg/cm2/minute as per
IS: 516-1959114 as shown in plate 3.14b. The load at which the
specimen fails is recorded. The experimental compressive strength
was obtained by dividing the maximum load applied on the specimen
during the test by its cross sectional area.
3.4 SPLITTING TENSILE STRENGTH
3.4.1 Introduction
Tensile strength of concrete greatly affects the extent and size of
cracking in concrete. It is of great importance while designing liquid
retaining structures, prestressed concrete structures and concrete
pavements. Tensile strength of concrete is very less when compared to
its compressive strength. The determination of tensile strength of
concrete can be classified as direct and indirect methods. The direct
methods suffer from a number of difficulties related to holding the
specimens properly in the testing machine without introducing stress
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concentration and to the application of uniaxial tensile load which is
free from eccentricity to the specimens. Even a very small eccentricity
of load will induce bending and axial force conditions and the concrete
fails at apparent tensile stress other than the true tensile strength.
Because of the difficulties involved in conduction the direct
tensile test, a number of indirect methods are available to determine
the tensile strength. In these tests, in general a compressive force is
applied to a concrete specimen in such a way that the specimen fails
due to tensile stresses induced in the specimen. The tensile stress at
which failure occurs is the tensile strength of concrete.
The splitting tension test and flexure test (modulus of rupture
test) are some of the indirect tests for finding tensile strength of
concrete.
3.4.2 Splitting Tension Test
The test consists of applying compressive line loads along the
opposite generators of a concrete cylinder placed with its axis
horizontal between the platens of a compression testing machine as
shown in plate 3.15. Due to the applied line loading a fairly uniform
tensile stress is induced over nearly two-third of the loaded diameter.
The magnitude of this splitting tensile stress (acting in a direction
perpendicular to the line of action of applied compression) is given by
σsp = 2P/πdl
Where P = The applied compressive load at failure
d = Diameter of the cylinder
l = Length of the cylinder
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Due to this tensile stress, the specimen fails finally, by splitting along
the loaded diameter. Immediately under the load, a high compressive
stress is induced. Therefore the load is applied through a packing of
plywood strip, 25 x 4 mm in cross section. As the cylinder splits into
two halves, the test is known as split test.
This test can also performed on cubes by splitting either-
(i). Along a centre line parallel to the edges of a cube by applying
two compressive forces through 15 cm square bars of sufficient
length.
(ii). Along one of the diagonal planes by applying compressive force
along two opposite edges.
In this investigation splitting tension test was performed on cylindrical
specimens of size 150 mm diameter and 300 mm length. This test
was conducted as per IS: 5816 – 1999.
The determination of flexural tensile strength is essential to estimate
the load at which the concrete member may crack. The flexure tensile
strength at failure is called modulus of rupture. This test is described
in the next section.
3.4.3 Casting of cylinders
For each mix three cylinders of size 150 mm in diameter and
300mm in length were cast and cured for 28 days. Each splitting
tensile strength results is the average of 3 test results.
3.4.4 Testing of cylinders for splitting tensile strength
After 28 days of curing the cylinders were removed from the
curing tank, weighed and tested for splitting tensile strength in a
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3000KN digital compression testing machine as per IS: 5816 – 1999 at
a rate of loading, (1.2 to 2.4) (π/2) l*d, N/min. The maximum load
applied on the specimen was recorded Here l = 300mm and d =
150mm. The experimental splitting tensile stress was calculated
according to the above equation.
3.5 MODULUS OF RUPTURE
3.5.1. Introduction
The determination of flexural tensile strength is essential to
estimate the load at which the concrete members may crack. As it is
difficult to determine the tensile strength of concrete by conducting a
direct tension test, it is computed by flexure testing. The flexural
tensile strength at failure is called modulus of rupture. The knowledge
of modulus of rupture is useful in the design of pavement slabs,
airfield runways, finding deflections and crack widths as flexural
tension is critical in these cases.
3.5.2 Modulus of rupture test
The modulus of rupture is determined by testing standard test
specimens (Prisms) of size 100 mm x 100 mm x 500 mm over a span
of 400 mm, under symmetrical two-point loading according to IS: 516-
1959. The modulus of rupture is determined from the moment at
failure as:
σr = (M * y) / I
Where σr = Modulus of rupture
M = bending moment at failure
Y = distance of extreme fibre from neutral axis
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I = Moment of inertia of the section
Thus the computation of σr assumes a linear behaviour of the
material up to failure.
3.5.3 Casting of prisms
For each mix three beams (prisms) of size 100 x 100 x 500 mm
were cast, de-moulded after 24 hours and then cured for 28 days.
Plate 3.16 shows the view of casting of prisms.
3.5.4 Testing of prisms for modulus of rupture
After 28 days of curing the prisms were taken out from the
curing tank, weighed and tested for modulus of rupture under two
point loading in a flexure testing machine according to IS: 516 –
1959114. The maximum load „P‟ and the distance of the crack from the
nearer support „a‟ measured on the centre line of the tensile face of the
specimen are recorded. The modulus of rupture was calculated
according to the clause 8.4 of IS: 516 – 1959114 as given below.
Case (i) If „a‟ is less than 133mm but greater than 110mm for 100mm
specimens,
σr = P*l/bd2
Case (ii) If „a‟ is less than 133mm but greater than 110mm for 100mm
specimens,
σr = 3P*a/bd2
Where σr = Modulus of rupture
A = Distance of the crack from the near support, measured along
the centre line on the tensile face of the specimen
L = length of the span = 400mm for 100mm specimens
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b = width of the cross section = 100mm
d = height of the cross section = 100mm
Case (iii) If „a‟ is less than 110mm for 100mm specimens the results of
the test are discarded.
3.6 MODULUS OF ELASTICITY
3.6.1 Introduction
The modulus of elasticity of concrete is largely controlled by the
volume and modulus of the aggregate. The modulus of elasticity of
concrete is a constant, defined as the ratio of axial stress to axial
strain within the elastic limit (i.e. when the load does not exceed 1/3
of the ultimate load) under uniaxial loading. The modulus of elasticity
of concrete is designated in various ways as:
(a) Initial tangent modulus, defined as the slope of the straight line
drawn at the origin of the stress-strain curve.
(b) Tangent modulus, defined as the slope of the tangent drawn at
any point on the stress-strain curve.
(c) Secant modulus, defined as the slope of the line joining any
point on the stress-strain curve to the origin.
(d) Modulus is the slope of the line joining any two points on
stress-strain curve.
The modulus of elasticity most commonly used is the static modulus.
The modulus of elasticity in this investigation was determined under
load control mechanism by subjecting a concrete cylinder of 150mm
diameter and 300mm height to stress in uniaxial compression in a
digital compression testing machine of 3000 KN capacity and
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measuring the deformations on a dial gauge using a longitudinal
compressometer. The modulus of elasticity determined in this
investigation is the initial tangent modulus.
3.6.1.1 Factors affecting the modulus of elasticity of concrete
Strength of concrete – higher strength shows higher modulus of
elasticity.
Volume and modulus of aggregate.
Wetness of concrete – wet concrete shows higher modulus of
elasticity than dry concrete.
Age of concrete – modulus of elasticity increases with age of
concrete.
Richness of the mix- richer mixes shows higher modulus of
elasticity.
3.6.2 Procedure for finding Modulus of Elasticity of concrete
After 28 days of curing, the specimens were taken out from the
curing tank and for modulus of elasticity while the specimens were
still in the wet condition using a longitudinal compressometer in a
digital CTM of 3000 KN capacity. The specimens were tested as per
clause 9 of IS: 516 – 1959114.
The compressometer consists of two circular frames for
clamping to the concrete specimen by means of five tightening screws.
The two circular frames are held in position by means of two spacers.
Spacer screws are provided to fix the spacers to the frame. The centre
distance of the tightening screws of the bottom and top frame is
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200mm. A dial gauge of 0.002 x 10mm gives the deformation over a
gauge length of 200mm.
The top and bottom frames are assembled by keeping the
spacers in position. The pivot rod is kept on the screws and adjusted.
The specimen (cylinder) after removing from water is placed on a level
surface. The compressometer is placed centrally on the specimen so
that the tightening screws of the bottom and top frame are at an equal
distance from the two ends of the concrete cylinder. The tightening
screws are tightened so that the compressometer is held on the
specimen. The two spacers are removed by unscrewing the spacer
screws before applying the load on the specimen. Plate 3.17 shows
the view of the compressometer fitted on to the concrete cylinder.
The specimen with the compressometer is placed centrally on
the platen of the Digital compression testing machine. The load was
applied continuously, at a rate of 140Kg/cm2/min (4.12 KN/sec) until
an average stress of (C + 0.5) N/mm2 is reached, where C is one-third
of the average compressive strength of the cubes calculated to the
nearest 0.5 N/mm2 . The load was then reduced gradually to an
average stress of 0.15 N/mm2 and dial gauge readings were taken.
The load was then applied a second time at the same rate until an
average stress of (C + 0.15) N/mm2 was reached and dial gauge
readings were taken. The load was gradually reduced and dial gauge
readings were taken. The load gradually reduced to 0.15 N/mm2 and
dial gauge readings were taken.
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The load was applied a third time and dial gauge readings taken
at ten approximate equal increments of stress up to an average stress
of (C + 0.15) N/mm2. If the overall strains observed on the second
and third readings differ by more than 5%, the loading cycle has to be
repeated until the strain between consecutive readings at (C + 0.15)
N/mm2 does not exceed 5%.
The strains are calculated and plotted against the stress and a
curve is plotted through these points. The slope of this curve gives the
modulus of elasticity expressed to the nearest 100 N/mm2.
3.6.3 Casting of cylinders
For each mix three cylinders of size 150mm diameter and
300mm long were cast in cast iron moulds, de-moulded after 24 hours
and cured for 28 days.
3.6.4 Testing of cylinders for modulus of elasticity
After 28 days of curing the cylinders were taken out from the
curing tank, weighed and tested for modulus of elasticity using a
compressometer in a digital compression testing machine of 3000KN
capacity according to IS: 516 – 1959114. The load applied and the
deformations up to 1/3 of the compressive strength are recorded. The
modulus of elasticity was calculated according to the clause 9 of IS:
516 – 1959114.
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3.7 IMPACT RESISTANCE
3.7.1 Introduction
Concrete structures are often subjected to short duration
(dynamic) loads. Loads originate from sources such as impact from
missiles and projectiles, earthquakes, wind gusts and machine
vibrations. Due to a relatively low tensile strength and fracture
energy, impact resistance of concrete that exhibits improved impact
resistance than conventional concrete. Fibre reinforced concrete
(FRC) has emerged as a viable structural material for use in such
applications.
One of the significant aspects of randomly distributed fibres in
cement composites is their ability to slow down the propagation of
tensile cracks, thereby improving the post cracking behavior, flexural
toughness and ductility of the cement composite. These
characteristics have rendered the fibre reinforced concrete, especially
steel fibre reinforced concrete (SFRC), as one of the suitable materials
for the construction of structures which are subjected to impact and
suddenly applied loads.
The improvements in the impact resistance of SFRC come
primarily from the large amount of energy absorbed in de-bonding,
stretching and pulling out the fibres which occur after the concrete
has cracked. This improvement in the impact resistance is measured
generally by using different types of impact tests. They are:
Weighted pendulum charpy-type impact test
Drop-weight Impact test
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Constant-strain rate test
Projectile impact test
Split-Hopkinson bar test
Explosive test
Instrumented pendulum impact test
Blast and projectile impact tests are generally used to evaluate the
impact resistance of structural members. Charpy impact test is
employed for metals, even though some researchers have tried for
concrete as well. Modified charpy impact test set up simplest among
all the above tests and results in quantitative estimate of the impact
resistance of SFRC has been employed in this investigation.
3.7.2 Casting of prism specimens
For each mix three prism specimens of size 100x100x500mm
were cast and cured for 28 days and then tested for impact resistance.
Plate 3.16 shows the view of casting of prisms for impact test.
3.7.3 Testing for impact resistance
After 28 days of curing the prisms were removed from curing
tank, and when the surface was dry they are kept ready for testing.
Plate 3.19 shows the view of testing method for impact
resistance. The pendulum is made to hit repeatedly and the number
of blows required to cause the first visible crack on the top surface of
the specimen are recorded as the first crack strength. The loading
with the drop pendulum is continued till the prisms fails by opening of
the cracks in the specimen sufficiently so that the specimen
completely fails. The number of blows that causes this condition is
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recorded as the failure strength. With fibre reinforced concrete
specimens, the prism was held together by fibres as shown in plate
3.20 where as in plain concrete prisms, there was a brittle failure as
shown in plate 3.21.
3.8. DURABILITY STUDIES
3.8.1 Introduction
A durability concrete is one that performs satisfactorily in the
working environment during its anticipated exposure conditions
during service. Inadequate durability manifests itself by deterioration
which can be due to external factors or to internal causes within the
concrete itself. The various actions can be physical, chemical or
mechanical. Mechanical damage is caused by impact, abrasion,
erosion or cavitation. The chemical causes of deterioration include
the alkali-silica and alkali-carbonate reactions. External chemical
attack occurs mainly through the action of aggressive ions, such as
chlorides, sulphates, or of carbon dioxide, as well as many natural or
industrial liquids and gases. The damaging actions can be of various
kinds and can be direct or indirect.
Physical causes of deterioration include the effects of high
temperature or of the difference in thermal expansion of aggregate and
of the hardened cement paste. An important cause of damage is
alternating freezing and thawing of concrete and the associated action
of de-icing salts. It is observed that the physical and chemical
processes of deterioration can act in a synergetic manner.
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3.8.2 Experimental investigation
In this experimental investigation the external chemical attack
due to exposure in 5% HCL and 5% H2SO4 was studied by immersing
100 x 100 x 100mm size concrete cubes in the above three liquids for
a period of 120 days, beyond 28 days of water curing. Plastic tubs
were used to immerse the cubes in the acid solutions. Care is taken
to maintain a minimum of 40mm distance between the cubes placed
in tubs containing acid solutions.
The weight and compressive strength of the cubes were found at
various ages of 30, 60, 90 and 120 days of exposure in the above
solutions. After every 30 days the cubes were removed from the tubs,
brushed with a soft nylon brush and rinsed in tap water to remove
loose surface material and placed in fresh acid solutions to study the
properties up to 120 days of exposure.
Plate 3.22 shows the view of specimens, after normal curing.
Plate 3.23 shows the view of specimens, after exposure to acid attack
in 5% HCI.
Plate 3.24 shows the view of specimens, after exposure to acid attack
in 5% H2SO4.
The loss in weight and loss in compressive strength are
calculated as follows
Loss in weight % = (W1 – W2) * 100
W1 Where
W1 = Weight of concrete cube specimen before immersion in
acid.
93
W2 = Weight of concrete cube specimen after immersion in acid.
Loss in compressive strength % = (σ1 – σ2) * 100
σ1
Where
σ1 = Compressive strength of concrete cube before immersion
in acid
σ2 = Compressive strength of concrete cube after immersion in
acid
The mixes studied for finding loss in compressive strength and
loss in weight is given in Table 3.8.0. At the end of each period of 30,
60, 90 and 120 days, the specimens were removed from the chemical
solutions cleaned with water and weighed. Similarly the specimens
were tested for compressive strength. The loss in weight and loss in
strength were calculated using the above equations.
For determining the resistance of concrete specimens to
aggressive environment like acid attack, the durability factors are
used based on relative compressive strength. The relative strength are
found at the end period of water curing i.e., at 28 days.
The acid durability factor (ADF) is given by
ADF = Sr * N
M
Where Sr = Relative strength at N days in %
N = Number of days at which the durability factor is
calculated
M = Number of days at which the exposure to acids is to be
terminated
94
Acid attack test was terminated at 120 days, so M is 120 in this case.
Table 3.8.0
S.No.
Type of
concrete
mix
Mix ID Percentage
of fibres
Aspect
ratio
Cement
%
MK
%
1 Plain
concrete
OPC
Concrete 0 0 100 0
2 MK
concrete 0 0 90 10
3
SFRC
AR - 60 1.50 60 100 0
4 AR - 80 1.50 80 100 0
5
SFRC-MK
AR - 60 1.50 60 90 10
6 AR - 80 1.50 80 90 10
95
3.9 EFFECT OF THERMAL CYCLES ON VARIOUS MIXES
OF M20 AND M50 GRADES
The present investigation is to study the effect of thermal cycles
on the compressive strength, split tensile strength and modulus of
rupture of various mixes of M20 and M50 grade specimens subjected
to various thermal cycles at a temperature of 500C and 1000C. This
was planned to be carried out through an experimental programme on
concrete cubes for compressive strength, cylinders for split tensile
strength, and prisms for modulus of rupture. The test specimens
were demoulded after 24 hours of air cooling and kept in water curing
for 28 days.
One thermal cycle constitute a heating period of 8 hours and
subsequent cooling (in air room temperature) period of 16 hours. The
standard specimens after curing period were placed in electric ovens
at 500C and 1000C for 0, 28, 90 and 180 thermal cycles. The
specimens were removed from oven and then allowed to cool in air for
2 hours after specified time. Then the specimens were tested for
compressive strength, split tensile strength and modulus of rupture.
Plate 3.25 show the view of specimens placed in electric oven.
The details are tabulated in 4.5.1 to 4.5.6.
96
3.10 TEMPERATURE EFFECTS ON VARIOUS MIXES OF
M20 AND M50 GRADES
The present investigation is to study the temperature effects on
compressive strength, pulse velocity and percentage weight loss of
various mixes of M20 and M50 grade specimens at room temperature,
when subjected to elevated temperatures of 2000C, 4000C and 6000C
at different time intervals of 4 hours, 8 hours and 12 hours and
allowed to cool for a duration of 24 hours. The test specimens were
demoulded after 24 hours of air cooling and kept for water curing for
28 days. The standard specimens after curing period were placed in
electric furnace at requisite temperatures of 2000C, 4000C and 6000C
at different time intervals of 4 hours, 8 hours and 12 hours. After the
specimens were removed from the furnace the specimens were allowed
to cool in air for 2 hours. Then the specimens were tested for
compressive strength, percentage loss of weight and pulse velocity.
Later the specimens were cooled to room temperature for duration of
24 hours and then tested in compression.
The details are tabulated in tables 4.6.1.1 to 4.6.1.14 and 4.6.2.1 to
4.6.3.2.
Plate 3.26 show the view of specimens placed in electric furnace for
exposure to elevated temperatures.
Plates 3.27 and 3.28 show the view of specimens after exposure to
elevated temperatures.
97
3.11 FLEXURAL BEHAVIOUR OF SFRC AND SFRC-MK
BEAMS
To study the suitability of the SFRC and SFRC-MK beams
investigations were carried out for ultimate load and load deflection
characteristics.
Steel fibre reinforced concrete and steel fibre reinforced
Metakaolin concrete beams by varying the percentage of steel fibres
from 0 % to 1.5% and of size 1200 x 150 x 100 mm were cast with
reinforcement of 2 nos of 12mm diameter HYSD bars as tension
reinforcement and 6 mm diameter stirrups spacing at 150 mm c/c as
shear reinforcement. To hold stirrups in position two hanger rods of 2
nos of 10mm diameter HYSD bars at top were used. These specimens
were cured in water for 28 days and tested for ultimate load,
deflections and failure characteristics under one third point loading.
The test setup is shown in plate 3.29. Plate 3.30 shows the view of
beam under testing. Plates 3.31 to 3.33 show the view of crack pattern
in the beams after failure. The results are tabulated in Table 4.9.1 to
4.9.4.
3.12 FLEXURAL BEHAVIOUR OF SFRC AND SFRC-MK
SLABS
To study the suitability of the SFRC and SFRC-MK slabs
investigations were carried out on ultimate load and load deflection
characteristics.
98
Steel fibre reinforced concrete and steel fibre reinforced
Metakaolin concrete by varying the percentage of steel fibres from 0 %
to 1.5% of size 1400 x 1200 x 100 mm were cast with reinforcement of
8mm diameter HYSD bars with a spacing of 200mm c/c on either side
of the slabs by varying the percentage of crimped steel fibres from 0%
to 1.5%. These specimens were cured in water for 28 days and tested
for ultimate load, deflections and failure characteristics under one
third point loading. The test setup is shown in plate 3.34. Plate 3.35
shows the view of noting of central deflection for slab under loading.
Plate 3.36 shows the view of highlighting the crack pattern of slabs
after testing. Plates 3.37 to 3.40 show the view of crack pattern of the
slabs after testing. The results are tabulated in Tables 4.10.1 and
4.10.2.