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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


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.



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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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.



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.



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.



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.

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