85 CHAPTER - 4 EXPERIMENTS FOR DERIVING BASIC PROPERTIES OF SIFCON 4.1 General This Chapter presents the details of experiments for evaluating the behaviour of SIFCON in compression and tension. From the detailed review of literature it is concluded that there is need to asses properties of SIFCON with locally available fibres and materials so that the actual values can be used to model non-linear response of SIFCON slabs. The various materials used in this investigation are cement, fine aggregate, fibres, super plasticizer and water and their properties are presented in the following sections. 4.2 Test Program In order to study the behaviour of SIFCON produced with locally available fibres and also to understand the stress-strain behaviour of a SIFCON a total number of 3 SIFCON mixes have been tried. Three mixes were SIFCON with 8%, 10% and 12% volume fraction of fibre. In all the mixes the same type of fine aggregate i.e. clean river sand has been used. The proportion of cement to sand has been maintained constant as 1:1. Ordinary Portland Cement 43 grade from a single batch has been used. The primary constituent materials of SIFCON specimens in this investigation are steel fibers and cement-sand based slurry. High-range water-reducing admixture (super plasticizers) has been used in order to improve the flow ability of the slurry to ensure complete infiltration without increasing the water-cement ratio (W/C).
Transcript
85
CHAPTER - 4
EXPERIMENTS FOR DERIVING BASIC PROPERTIES OF SIFCON
4.1 General
This Chapter presents the details of experiments for evaluating the behaviour
of SIFCON in compression and tension. From the detailed review of literature it is
concluded that there is need to asses properties of SIFCON with locally available
fibres and materials so that the actual values can be used to model non-linear response
of SIFCON slabs. The various materials used in this investigation are cement, fine
aggregate, fibres, super plasticizer and water and their properties are presented in the
following sections.
4.2 Test Program
In order to study the behaviour of SIFCON produced with locally available
fibres and also to understand the stress-strain behaviour of a SIFCON a total number
of 3 SIFCON mixes have been tried. Three mixes were SIFCON with 8%, 10% and
12% volume fraction of fibre. In all the mixes the same type of fine aggregate i.e.
clean river sand has been used. The proportion of cement to sand has been maintained
constant as 1:1. Ordinary Portland Cement 43 grade from a single batch has been
used.
The primary constituent materials of SIFCON specimens in this investigation
are steel fibers and cement-sand based slurry.
High-range water-reducing admixture (super plasticizers) has been used in
order to improve the flow ability of the slurry to ensure complete infiltration without
increasing the water-cement ratio (W/C).
86 4.3 Materials used in SIFCON and their properties
The materials used in SIFCON are steel fibers, fine aggregate, cement, water
and admixture. The material properties of cement, fine aggregate and water are as
follows:
4.3.1 Cement
Ordinary Portland cement of 43 grade conforming to bureau of Indian
Standards (IS 12269:1987) was used in the present investigation. The cement was
tested for various properties as per IS: 8112 -1989. The results of these tests are
presented in Table 4.1.
Table 4.1 Physical properties of 43 grade cement
S. No. Property Value I.S.Code
1 Fineness of Cement a)Blains specific surface area
320m2/kg
225 m2/kg (minimum)
2 Normal consistency 31% ----
3 Setting times
a) Initial 128 minutes >30 minutes b) Final 246 minutes <600 minutes
4 Specific gravity 3.13 ----
5
Compressive Strength at 3 days
7 days 28 days
30.00 N/mm2 39.30 N/mm2 58.30 N/mm2
23.00N/mm2 33.00 N/mm2 43.00 N/mm2
4.3.2 Fine aggregate
The locally available river sand conforming to grading zone-II of table 4 of IS
383-1970 has been used as Fine Aggregate. Following tests have been carried out as
per the procedure given in IS 383(1970) [2] and the results are presented in Tables
4.2.
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a) Specific Gravity
b) Bulk Density
c) Grading
d) Fineness Modulus of Fine Aggregate
Table 4.2 Physical properties of fine aggregate
S. No. Property Value
1 Specific Gravity 2.60
2 Fineness Modulus 2.42
3 Bulk Density
i) Loose
ii) Compacted
1482 kg/m3
1696 kg/m3
4 Grading Zone – II
4.3.3 Fibres
The type of fiber used in this study was made of steel and having unit weight
of 7850kg/m³. Black annealed steel fibers of 1mm diameter manufactured by VIZAG
STEELS were used in the present study. They were cut into required length of 50mm
using shear cutter equipment with an aspect ratio of 50. The ultimate tensile strength
was 417MPa. Black steel fibers are commercially available and are generally used for
binding the steel reinforcement in RCC works. The properties of fibre are presented
in Table 4.3 and a sample fibre is shown in Fig. 4.1.
Fig. 4.1 Fibre sample
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Table 4.3 Properties of fibre as supplied by manufacturer
Carbon 0.06
Manganese 0.39
Sulphur 0.23
Phosphorous 0.022
Silicon 0.19
Aluminum 0.022
Carbon Equivalent 0.13
Yield Strength N/mm2 331
UTS N/mm2 417
Grade SAE 1008
4.3.4 Water
Clean potable fresh water which is free from concentration of acid and organic
substances has been used for mixing the concrete. The results of various tests are
presented in Table 4.4.
Table 4.4 Physical properties of water
S. No Parameter Amount
1 pH 9.2
2 Taste Agreeable
3 Appearance Normal
4 Turbidity (NT Units) 3
5 Colour (Hazen Units) 2
6 Hardness (mg/l) 1
7 Sulphates (mg/l) 0.3
8 Chlorides (mg/l) 9
4.3.5 Super plasticizer
Requirement of right workability is the essence of good concrete. The main
action of plasticizers or water reducers is to decrease the viscosity of the mix and
89 improves the workability of concrete, mortar, or grout. In our present study
CONPLAST SP 430 A2 super plasticizer from FOSROC is used to increase
workability of slurry. At given water cement ratio, this CONPLAST SP 430 A2 of
1.5% of cement quantity was mixed to the water thoroughly before the mix.
CONPLAST SP 430 A2 is based on sulphonated naphthalene polymers and is a
brown liquid instantly dispersible in water. In the present investigation super
plasticizer is used only to improve the workability of the slurry. The properties are
given below in Table 4.5.
Table 4.5 Properties of CONPLAST SP 430 A2 (As supplied by FOSROC)
S.No. Property Value
1 Specific Gravity 1.20 to 1.21 at 300 C
2 Chloride Content Nil
3 Air entrainment 1 % (additional)
In the present investigation after making a few trials with different volume
fraction of fibres and fibre lengths, it is observed that beyond 12% fibre volume and
beyond 50 mm fibre length, placing of fibres manually in the mould is very tedious
and uniform mixing of fibres can not be achieved. So based on the observations, the
SIFCON slab specimens have been modeled with three different volume fractions of
fibres i.e., 8, 10 and 12% in the present work. A constant fibre length of 50mm has
been adopted. The aspect ratio of fibre works out to 50. The cement-mortar slurry
was prepared with 1:1 proportion using w/c ratio 0.45. Super plasticizer was added to
increase infiltration capacity. To impart additional workability, super plasticizer (1.5
per cent by weight of cement) was used. Steel fibers were procured from Vizag steels
Limited, Visakhapatnam.
90
The moulds were filled with 8, 10, and 12 per cents of fibers in layers and
slurry was poured into respective moulds. Hand tamping is done with a tamping rod.
Almost nine specimens were cast for each percent of fibers i.e. three cubes, three
beams and three cylinders. The mix proportion is briefly given below in Table 4.6
indicating the mix proportion of sand, water cement ratio, dosage of each plasticizer
for each volume of fiber
Table 4.6 Mix Proportion of SIFCON
S.NO Designation Mix proportion
Volume of fiber
W/c ratio
Dosage of plasticizer
Mode of vibration
1 Sifcon-8 Cement and sand
(1:1 by wt)
8% 0.45 1.5% Hand tamping
2 Sifcon-10 Cement and sand
(1:1 by wt)
10% 0.45 1.5% Hand tamping
3 Sifcon-12 Cement and sand
(1:1 by wt)
12% 0.45 1.5% Hand tamping
The details of experiments for deriving basic properties of SIFCON are
presented in the following sections.
4.4 Fabrication and Casting
The cubes are cast in steel moulds of inner dimensions of 150mm x 150mm x
150mm and the cylinders are cast in steel moulds of inner dimensions of 150 mm
diameter and 300 mm height Steel fibers are laid in layers and then the cement slurry
prepared as per the mix proportion is slowly poured in to the mould till the mould is
completely filled with slurry. Tamping should be done properly with tamping rod to
entrap the air between steel fibers. Six specimens were casted for every percentage of
SIFCON at the ages of 7 and 28 days to establish the average compressive strength.
The test specimens are stored in moist air for 24hours and after this period the
91 specimens are marked and removed from the molds and kept submerged in clear fresh
water until taken out prior to test. The three types of matrices explained above have
been used for producing slurry with water -binder ratio of 0.45 along with a high
range water-reducing admixture. The details of casting are presented in Figs. 4.2 to
4.4.
Fig. 4.2 Pre Placed Fibres
Fig. 4.3 Infiltration of Slurry
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Fig. 4.4 Cylinder moulds
4.5 Cube Compressive Strength
The set up for cube compressive strength is depicted in Figs. 4.5 & 4.6.
Compression test on the cubes is conducted on the 2000 kN. AIMIL - make digital
compression testing machine. The pressure gauge of the machine indicating the load
has a least count of 1 kN. The cube was placed in the compression-testing machine
and the load on the cube is applied at a constant rate up to the failure of the specimen
and the ultimate load is noted. Compressive strength test was conducted as per IS
516:1959 using compressive testing machine as shown in Fig. 4.6. Three cubes were
tested for every percentage of replacement with steel fiber at the ages of 7 and 28 days
to establish the average compressive strength. The average compressive strength are
presented in Table 4.7.
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Fig. 4.5 Cube specimen after testing
Fig. 4.6 Cube compression test set up
Table 4.7 Cube compressive strengths of SIFCON mixes
Percentage of
fiber used
Average Load P
(kN)
Average Compressive
Strength
(N/mm²)
7days 28days 7days 28days
SIFCON-8 768 1093 34.12 48.60
SIFCON-10 820 1148 36.44 51.0
SIFCON-12 853 1184 37.90 52.60
94 4.6 Split Tensile Strength
The test is carried out by placing a cylindrical specimen horizontally between
the loading surfaces of a compression testing machine and load is applied until failure
of cylinder, along the vertical diameter.
This test is conducted on 2000 kN AIMIL make digital compression testing
machine as shown in Fig. 4.7. The cylinders prepared for testing are 150 mm in
diameter and 300 mm long. After noting the weight of the cylinder, diametrical lines
are drawn on the two ends, such that they are in the same axial plane. Then the
cylinder is placed on the bottom compression plate of the testing machine and is
aligned such that the lines marked on the ends of the specimen are vertical. Then the
top compression plate is brought into contact at the top of the cylinder. The load is
applied at uniform rate, until the cylinder failed and the load is recorded. From this
load, the splitting tensile strength is calculated for each specimen. The results of split
tensile test are presented in Table 4.8.
Fig. 4.7 Split Tensile Test set up
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Table 4.8 Split tensile strengths of various SIFCON mixes
S.No. Percentage of fiber used
28 days average split tensile
strength(N/mm2)
1 SIFCON-8 5.80
2 SIFCON-10 6.12
3 SIFCON-12 6.56
4.7 Direct tensile strength
This test is conducted on 100 ton servo-controlled uni-axial tension testing
machine. The Dog bone specimens prepared for testing are of 300 x 100 x 25mm
size. The specimens were de-moulded after 28 days of water curing. The dog bone
specimen is placed between top and bottom clutches such that the specimen is
vertical. The load is applied at uniform rate, until the specimen failed and the load is
recorded. From this load, the direct tensile strength is calculated for each specimen.
The results of direct tensile test are presented in Table 4.9. In the present work, yield
strength of 1.5MPa is used to model SIFCON slab. The stress strain data needed to
model SIFCON slabs with different volume fraction of fibre obtained from tension
test are presented in Table 4.10.
Table 4.9 Direct tensile strengths of various SIFCON mixes
S.No.
Percentage of fiber used
Yield Strength
(MPa)
28 days Direct tensile
strength(MPa) 1 SIFCON-8 1.50 3.15
2 SIFCON-10 1.58 3.60
3 SIFCON-12 1.62 3.75
96 Table 4.10 Stress – Strain data of SIFCON specimens with different volume fraction of fibers
S.No
volume fraction of
fiber Data
1 SIFCON -8
Stress (N/mm2)
1.80 2.20 3.50 5.50 5.80
Strain
0.0085 0.01 0.03 0.40 0.70
2 SIFCON -10
Stress (N/mm2)
1.80 2.60 3.80 5.70 6.00
Strain
0.0085 0.01 0.025 0.35 0.60
3 SIFCON-12
Stress (N/mm2)
1.80 2.80 4.20 6.00 6.50
Strain
0.0085 0.01 0.023 0.33 0.55
An accurate model of the compressive behaviour is not necessary in case of
modelled slabs. Considering the strength of the used SIFCON and the amount of
locally available steel fibre, the specimens will never reach the yield stress in
compression. Since no crushing of compressed concrete surface is likely to occur, the
compression damage parameters are omitted in the analysis. The tensile behaviour is
described by defining the stress-strain data beyond elastic limit.
Typical tensile load–elongation response up to complete failure is recorded
from the data. For the purpose of discussion, the curve is divided into two parts: as
ascending branch up to the peak load, and a descending branch from the elongation at
the peak load to complete separation i.e. complete failure of component with wide
crack opening. But we are interested in simulating the non-linear portion in ascending
branch. The elongation up to the ultimate load can transmitted into tensile strain i.e.,
strain-elongation divided by gage length directly; the elongation beyond the peak load
represents primarily the opening of a critical crack and cannot be directly used as a
97 strain. Two observations that can be made for ascending branch are the ascending
branch of the stress-strain curve is nonlinear, and the strains to the ultimate load and
to failure are very high.
As testing begins, the stress-strain curve is linear over a small portion because
it is following hooks law, then gradually becomes nonlinear as the maximum load is
approached. It is believed that the nonlinearity of the curve is caused by multiple
cracking in the testing zone. These cracks are usually small but clearly visible during
testing and tend to be evenly distributed across the testing zone; however, they are not
through cracks as in reinforced concrete prisms. The ascending branch region up to
the peak stress represents the true stress strain response of the composite. In the final
stages of this region, failure mode and location are determined as one of the many
small tensile cracks in the testing zone begins opening to become the failure zone. A
peak point could be determined from each curve, but often a flat plateau can better
describe the behaviour near the peak. So nonlinearity of the stress-strain curves is due
to the immediate opening of cracks and possible debonding of fibres during testing.
Since the matrix involved is very strong, it is unlikely that load-induced cracking is
responsible for the initial nonlinearity of the curve. The observed nonlinearity must
exist due to the presence of cracks already in the matrix before testing. Cracks are
probably induced by drying shrinkage cracking.
Descending branch from the elongation at the peak load to complete
separation can be defined by post-cracking tension softening. Stress strain data has
been obtained based on fracture energy and stress displacement relations with known
crack band to define the tensile behaviour. (i.e. the post failure behaviour in tension
can be defined by strain, crack opening (displacement), and fracture energy (energy
absorbed during fracture).
98 4.8 Discussion of Results
4.8.1 SIFCON in Compression
From the values given in table 4.7, it can be observed that the 7 day
compressive strength of SIFCON mixes increases with increase in percentage volume
of fibres. It can further be observed that the 7 day compressive strength of SIFCON-
12 is considerably higher than that of other SIFCON mixes. Hence, it can be
concluded that addition of locally available low tensile steel fibers also contribute to
compressive strength of the mix. Similar observation was made by earlier researchers
using high tensile steel fibers. It can be observed that the 28 days cube compressive
strength also increases with increasing volume percentage of fibres. This is in
conformity with results of previous investigations. This result is expected because the
addition of fibres enhances the load carrying capacity of the mix and the increase is
also justified as per simple rule of mixtures. Thus it can be observed that addition of
fibres has significantly increased the cube compressive strength.
4.8.2 SIFCON in Tension
It can also be observed that the split tensile strength increases with increasing volume
percentages of fibre. This is also in conformity with results of previous investigations.
This result is justified because addition of steel fibres with higher tensile strength can
definitely contribute to the tensile strength of the composite. Thus, it can be observed
that addition of fibres has significantly increased the split tensile strength. The major
lacuna of low tensile strength of concrete can be thus be alleviated by using SIFCON.
4.9 Modulus of Elasticity and Poisons Ratio
The modulus of elasticity and poisons ratio of a material are fundamental
properties that are needed for modelling mechanical behaviour of structural
components and for evaluation of stiffness criteria. For safe and efficient use, the
99 stress - strain response of the composite should be known, with the peak strength and
the modulus of elasticity being two of the most important parameters of this response.
Thus, there is need to generate experimental data related to the elastic modulus of
SIFCON and poison’s ratio.
The modulus of elasticity is determined using the longitudinal Compression
meter attached to the specimen as shown in Fig. 4.8. The poisons ratio is determined
using the lateral compression meter along with the longitudinal compression meter
attached to the specimen as shown in Figs. 4.8 and 4.9. The modulus of elasticity is
determined by subjecting the cylindrical specimen to axial compression and
measuring the deformation by means of a dial gauge fixed to the longitudinal
compression meter at regular intervals. The load on the cylinder is applied at a
constant rate up to the failure of the specimen cylinder. Dial gauge reading divided by
gauge length will give the strain and load applied divided by area of cross section will
give the stress. A series of readings are taken and the stress-strain relationship is
developed. Modulus of elasticity of SIFCON is calculated as its initial tangent
modulus. Fig.4.10 shows a typical stress –strain curve to show how the modulus of
elasticity is obtained. A tangent is drawn to the stress strain curve at the origin and the
slope of the tangent is taken as initial tangent modulus of elasticity. Similarly the
poisons ratio is determined by measuring the deformation by means of a dial gauge
fixed to the lateral compression meter along with longitudinal compression meter at
regular intervals. The poison’s ratio is calculated by using expression lateral strain to
the longitudinal strain.
The modulus of elasticity of different SIFCON with different percentage
volumes of fibre and poison’s ratio obtained from the present investigation are
reported in Table 4.11.
100
Fig. 4.8 Test set up for determining Modulus of Elasticity and Poisson’s ratio
Fig. 4.9 Observations recording and test specimens after testing
