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CHAPTER - VII
DISCUSSION OF TEST RESULTS
The results of the experimental investigations on conventional and
bacterial concrete are discussed in the following sections.
The discussions and interpretations are grouped in three phases
of study. They are
7.1 PHASE
I: CULTURE OF BACTERIA
7.2 PHASE - II: STRENGTH STUDIES
1] To study the strength of cement mortar.
2] To study the compressive and split tensile strength of concrete.
3) To study the stress-stain behavior of concrete.
4) To study the flexural behaviorof concrete.
7.3 PHASE - III: DURABILITY STUDIES
1) To study the strength loss, weight loss, Acid Durability Factor and
Acid Attack Factor of concrete.
2) To study the accelerated corrosion of cracking in concrete.
7.1 CULTURE OF BACTERIA
Bacillus subtilis JC3 is a laboratory soil bacterium, which can be
cultured in the laboratory. Dendrogram depicting the phylogenetic
relationships of strain JC3, within the family Bacillaceae determined
using 16S rRNA gene sequence analysis. The dendrogram is
constructed using the maximum-likelihood program in the PHYLIP
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package. Numbers at nodes are bootstrap values. Bootstrap values
below 50 are removed from the dendrogram. Strain JC3 has shown
100% sequence similarity and is clustered with Bacillus subtilis as
shown in Table 5.2.1.
7.2 STRENGTH STUDIES
7.2.1 TO STUDY THE STRENGTH OF CEMENT MORTAR
The compressive strength at 3 days, 7 days and 28 days for
different cell concentrations are given in Table 5.3.1.1. It is observed
that the compressive strength of cement mortar showed significant
increase by 16.15%for cell concentration of 105 cells per ml of mixing
water. So, for the further investigation bacteria with a cell
concentration of 105 cells per ml of mixing water is used. Plate.1.1.1
shows the Scanning Electron Microscopy analysis. It is noted that
pores are partially filled up by material growth with the addition of the
bacteria. Reduction in pore due to such material growth will obviously
increase the material strength.
7.2.2 TO STUDY THE COMPRESSIVE STRENGTH AND SPLIT
TENSILE STRENGTH OF CONCRETE
In ordinary grade concrete the compressive strength of concrete at
7 days, 14 days, 28 days, 60 days, 90 days, 180 days, 270 days and
365 days are given in Table 5.3.2.1. It is observed that with the
addition of bacteria the compressive strength of concrete showed
significant increase by 13.93% at 28 days. The percentage of
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compressive strength improvement is in the order of 13.93% to
18.65% as the age of concrete varies.
In standard grade concrete the compressive strength of concrete at
7 days, 14 days, 28 days, 60 days, 90 days, 180 days, 270 days and
365 days are given in Table 5.3.2.2. It is observed that with the
addition of bacteria the compressive strength of concrete showed
significant increase by 14.92% at 28 days. The percentage of
improvement in compressive strength varies at different ages.
In ordinary grade concrete the Split Tensile Strength on standard
cylindrical specimens at 28 days, 60 days, 90 days and 180 days are
given in Table 5.3.2.3. It is observed that with the addition of bacteria
there is a significant increase in the tensile strength by 12.60% at 28
days. The percentage of split tensile strength improvement is in the
order of 12.6% to 14.62% at different ages.
In standard grade concrete the Split Tensile Strength on standard
cylindrical specimens at 28 days, 60 days, 90 days and 180 days are
given in Table 5.3.2.4. It is observed that with the addition of bacteria
there is a significant increase in the tensile strength by 12.09% at 28
days. The percentage of split tensile strength improvement varies at
different ages.
7.2.3 STRESS-STRAIN BEHAVIOUR OF CONVENTIONAL AND
BACTERIAL CONCRETE MIXES
The relationship between stress and strain is important in
understanding the basic elastic behaviour of concrete in hardened
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state. The modulus of elasticity of concrete is one of the important
mechanical properties, which is useful in design purpose.
From the values of stresses and strains, average stress-strain
curve for each mix is plotted, taking the average values of the results
of the three cylinders. The stress-strain curves for conventional and
bacterial concrete mixes at 28 days, 60 days, 90 days and 180 days
for ordinary grade concrete and standard grade concrete are shown in
Figures 5.3.3.1. to 5.3.3.56. and the corresponding stress-strain
values are given in Tables 5.3.3.1. to 5.3.3.56.
From the stress-strain values of conventional and bacterial
concrete mixes and the corresponding stress-strain plots, normalized
stress-strain values are calculated by dividing each stress value by the
peak stress and dividing each strain value by strain at peak strain.
From the normalized stress-strain values of conventional and bacterial
concrete mixes, the average normalized stress-strain curves are
plotted for conventional and bacterial concrete separately and
empirical equations are proposed in the form of Y= Ax/(1+Bx2) for
ascending and descending portions of conventional and bacterial
concrete mixes for an average strength of ordinary grade concrete and
standard grade concrete. From the stress-strain curves of
conventional and bacterial concrete mixes and average normalized
stress-strain curves, different parameters like modulus of elasticity,
energy absorption capacity, peak stresses, strains and stress-strain
behavior are discussed in the following sections.
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7.2.3.1 Stress-Strain response of conventional and bacterial
concrete mixes
From the observations made from stress-strain curves of all the
conventional and bacterial concrete mixes, the stress-strain behaviour
is observed to be almost similar. The only difference is that bacterial
concrete mixes have shown improved stress values for the same strain
levels compared to that of conventional concrete mixes.
7.2.4.2 Mathematical Model for Stress-Strain Curves of
Conventional and Bacterial Concretemixes
Empirical equations for the stress-strain response of conventional
and bacterial concrete mixes have been proposed in the form of
Y = Ax/(1+Bx2). The same empirical formula is valid for both
ascending and descending portions with different values of constants.
A set of constants A,B and A1, B1 have been determined to get
empirical equations for ascending and descending portions of
normalized stress-strain curves for conventional concrete and another
set of constants for bacterial concrete mixes. The constants for
conventional concrete and bacterial concrete at 28 days are
A=2.11, B = 1.11 for ascending portion
A1=3.4, B1=2.94 for descending portion of conventional concrete
and
A=2.07, B = 1.07 for ascending portion
A1=2.33, B1= 1.55 for descending portion of bacterial concrete.
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Thus the equations for ascending and descending portions of
conventional and bacterial concrete are
Y = 2.11x/(1+1.11x2) and Y = 3.4x/(1+ 2.94x2 ) for conventional
concrete,
Y = 2.07x/(1+1.07x2 )and Y = 2.33x/(1+1.55 x2)for bacterial concrete.
Where Y = f / f0and x = / 0
The proposed empirical equations can be used as stress block in
analyzing the flexural behaviour of sections of conventional and
bacterial concrete. For both the ordinary grade concrete and standard
grade concrete the proposed equations have shown good correlation
with experimental values at all the ages.
7.2.3.3 Toughness of conventional and bacterial concrete mixes
Energy absorption capacity expressed in terms of area under
stress-strain diagram of conventional and bacterial concrete mixes are
shown in Table 5.3.3.57. The average value of area under stress-strain
diagram for ordinary grade concrete and standard grade concrete for
conventional and bacterial concrete mixes at are observed to be 0.61,
0.73 units and 0.64, 0.77 units respectively.
7.2.5FLEXURAL BEHAVIOUR OF CONVENTIONAL AND
BACTERIAL CONCRETEBEAMS
Beams of size 100mm x 150mm x 1200mm of ordinary grade
concrete and standard grade concrete with selected conventional and
bacterial concrete mixes are cast and tested at 28 days, 60 days, 90
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days and 180 days under two point flexural bending tests under
strain rate control. Beams are tested until the load drops to 15-20% of
peak load in the descending portion of load deflection curves. While
testing, the ultimate load, curvature, crack width and crack pattern
are observed for all the beams. Test set up is shown in Plate No.1.4.2.
The following discussions are based on the experimental results
shown in Tables 5.3.4.1.to5.3.4.8. and the moment-curvature Plots
are shown in Figures 5.3.4.1to5.3.4.8.
7.2.4.1 Moment Curvature Relationship
Moment curvature plots for conventional and bacterial concrete
mixes for ordinary grade concrete and standard grade concrete
reinforced beams are drawn and shown in Figure 5.3.4.1 to 5.3.4.8.
In beams, curvature increases gradually with increase in moment
up to the multiple cracking stages and beyond, the curvature
increased drastically at constant moment or with small variation in
the moment. The M-curve becomes more or less flat till the ultimate
moment is reached. Finally all the beams failed by compression of
concrete and the moment curvature plots are drawn up to failure
stage.
In conventional concrete beams the visible flexural cracks
developed at 70% to 80% of the ultimate load of each beam and in
bacterial concrete beams they developed at 75% to 85% of the
ultimate load of each beam. The crack started to widen considerably
indicating higher strains in steel than the yield strain in steel. All the
beams exhibited a tension failure which is a ductile failure. The cracks
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are accompanied by pronounced bulging. When the load is further
increased, cracks propagated towards the top of the beam. As the
beams are forced to deform further, the cracks became more
pronounced and the concrete crushed at one or both the ends. With
further increase in beam deflection, the load decreased, accompanied
by concrete spalling. This crack pattern is observed to be same for all
the conventional and bacterial concrete beams. Crack pattern for the
beams are shown in Plates 1.4.4. to 1.4.8.
7.2.4.2 Theoretical Moment-Curvature Values
The values of experimental moments and curvatures are
calculated, from the loads and curvature meter readings obtained
from the beams tested under two point flexural bending tests under
strain rate control.
An analytical procedure was developed for the calculation of the
theoretical moments and curvatures for the beam sections. The
procedure developed for obtaining the theoretical moment-curvatures
are explained in the section 6.4.The obtained values of the theoretical
moments and curvatures are compared with the observed
experimental values in conventional concrete and bacterial concrete
beams. These diagrams indicate that the procedure developed based
on the strain compatibility is satisfactory. The estimated moment
values are within 10 percentage compared to the experimental
values. However the variation between the theoretical and
experimental curvatures at ultimate is more than 10 percent. This
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may be due to the difference in calculation of theoretical curvatures.
Theoretically obtained curvature represents the curvature at a section,
where as the experimental curvatures represent the curvature over a
region (Gauge length).
7.2.4.3 Comparison of experimental and theoretical Moment-
Curvature values and validation
The values of experimental moments and curvatures are calculated
from the loads and curvature meter readings obtained from the beams
tested under two point flexural bending tests under strain rate
control. Theoretical moments and curvatures are calculated using the
analytical equations developed for stress-strain behaviour of
conventional and bacterial concrete. These equations are developed by
conducting axial compression tests on cylindrical specimens made
with conventional and bacterial concrete without stirrup confinement.
Using these equations theoretical moments are developed for
conventional and bacterial concrete beams. Curvatures are calculated
from the strain distribution over the cross section. With these values
M- curves are plotted.
The experimental and theoretical M- relationships are compared
and discussed.
In ordinary grade concrete and standard grade concrete at all the
ages of the conventional and bacterial concrete beams, the
experimental moment values are higher than their theoretical values.
There is a variation of 6.52% between the experimental values and the
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theoretical values in ordinary grade conventional concrete at 28 days,
and for bacterial concrete beams, there is a variation of 7.14%
between the experimental values and the theoretical values.
There is a variation of 7.14% between the experimental values and
the theoretical values in standard grade conventional concrete at 28
days and for bacterial concrete beams, there is a variation of 10.56%
between the experimental values and the theoretical values.
This difference in experimental and theoretical curvature values is
mainly due to the measurement of curvature over a short specified
gauge length during the experiment. Theoretically obtained curvature
represents the curvature at a section, where as the experimental
curvatures represent the curvature over a gauge length. Hence the
experimental curvature values are much higher than the theoretical
values.
7.3.1 STRENGTH LOSS, WEIGHT LOSS, ACID DURABILITY
FACTOR AND ACID ATTACK FACTOR OF CONCRETE
From the durability studies in ordinary grade concrete the
percentage weight loss and percentage strength loss with 5% H2SO4at
30 days for conventional concrete are 0.99 and 2.80 and for bacterial
concrete they are 0.79 and 0.67 respectively are shown in the Table
5.4.1.1. Similarly in standard grade concrete the percentage weight
loss and percentage strength loss with 5% H2SO4 at 30 days for
conventional concrete are 1.14 and 0.94 and for bacterial concrete
they are 0.91 and 0.68 respectively are shown in the Table 5.4.1.2.
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From the durability studies in ordinary grade concrete the
percentage weight loss and percentage strength loss with 5% HCL at
30 days for conventional concrete are 0.67 and 0.53 and for bacterial
concrete they are 0.51 and 0.40 respectively are shown in the Table
5.4.1.5 and in standard grade concrete the percentage weight loss and
percentage strength loss with 5% HCL at 30 days for conventional
concrete are 0.55 and 0.43 and for bacterial concrete they are 0.48
and 0.32 respectively are shown in the Table 5.4.1.6.
Durability studies carried out in the investigation through acid
attack test for ordinary grade and standard grade concrete with 5%
H2SO4and 5% HCL revealed that bacterial concrete is more durable in
terms of Acid Durability Factors than the conventional concrete,
similarly in standard grade concrete with 5% H2SO4 and 5% HCL
revealed that bacterial concrete is more durable in terms of Acid
Durability Factors than the conventional concrete as shown in the
Tables 5.4.1.3., 5.4.1.4., 5.4.1.7. and 5.4.1.8.
Similar durability studies carried out in ordinary grade concrete
with 5% H2SO4 revealed that Acid Attack Factors for bacterial
concrete and conventional concrete are 1.19 and 1.41 respectively and
in standard grade concrete with 5% H2SO4revealed that Acid Attack
Factors for bacterial concrete and conventional concrete are 1.09 and
1.53 respectively as shown in the Tables 5.4.1.3. and 5.4.1.4.
Durability studies carried out in standard grade concrete with 5%
HCL revealed that Acid Attack Factors for bacterial concrete and
conventional concrete are 0.63 and 0.72 respectively and in standard
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grade concrete with 5% HCL revealed that Acid Attack Factors for
bacterial concrete and conventional concrete are 1.03 and 1.22
respectively as shown in the Tables 5.4.1.7 and 5.4.1.8.
7.3.2 RELATIVE CHARGE, TOTAL CHARGE AND DETERIORATION
FACTORS OBTAINED FROM ACCELERATED CORROSION TEST
7.3.2.1For 10 mm effective cover, the time of passing total charge at
failure of bacterial concrete and conventional concrete beams for
ordinary grade concrete are 22 and 19 hours respectively and relative
charges are 100 and 100 respectively, and for standard grade concrete
are 29 and 24 hours respectively, and relative charges are 82.76 and
100 respectively, as given in the Tables 5.4.2.1. and 5.4.2.2.
7.3.2.2For 20 mm effective cover, the times of passing total charge at
failure of bacterial concrete and conventional concrete beams for
ordinary grade concrete are 60 and 51 hours respectively and relative
charges are 100 and 100 respectively, and for standard grade concrete
are 86 and 74 hours respectively, and relative charges are 97.67 and
100 respectively, as given in the Tables 5.4.2.1. and 5.4.2.3.
7.3.2.3For 30 mm effective cover, the times of passing total charge at
failure of bacterial concrete and conventional concrete beams for
ordinary grade concrete are 131 and 109 hours respectively and
relative charges are 91.60 and 100 respectively, and for standard
grade concrete are 178 and 146 hours respectively, and relative
charges are 87.64 and 100 respectively, as given in the Tables 5.4.2.1.
and 5.4.2.4.
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7.3.2.4For 40 mm effective cover, the times of passing total charge at
failure of bacterial concrete and conventional concrete beams for
ordinary grade concrete are 292 and 248 hours respectively and
relative charges are 86.30 and 100 respectively, and for standard
grade concrete are 379 and 327 hours respectively, and relative
charges are 88.65 and 100 respectively, as given in the Tables 5.4.2.1.
and 5.4.2.5.
7.3.2.5The Charge Deterioration Factor for 10 mm effective cover,
at a given time for bacterial concrete and conventional concrete beams
for ordinary grade concrete are 7.53 and 6.51 and for standard grade
concrete are 7.65 and 6.33, as given in the Table 5.4.2.2.
7.3.2.6The Charge Deterioration Factor for 20 mm effective cover,
at a given time for bacterial concrete and conventional concrete beams
for ordinary grade concrete are 20.55 and 17.50 and for standard
grade concrete are 22.69 and 19.53, as given in the Table 5.4.2.3.
7.3.2.7The Charge Deterioration Factor for 30 mm effective cover,
at a given time for bacterial concrete and conventional concrete beams
for ordinary grade concrete are 44.86 and 37.33 and for standard
grade concrete are 46.97 and 38.52, as given in the Table 5.4.2.4.
7.3.2.8The Charge Deterioration Factor for 40 mm effective cover,
at a given time for bacterial concrete and conventional concrete beams
for ordinary grade concrete is 100 and 84.93 and for standard grade
concrete are 100 and 86.28, as given in the Table 5.4.2.5.