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CHAPTER 6
RESULTS AND DISCUSSION
6.1 GENERAL
This chapter presents the results of the various tests conducted on
low calcium fly ash based Geopolymer concrete elements as described in
Chapter 5. This research started with finding the strength and suitability of
Geopolymer mortar cubes and based on this, the mixture proportions of
normal strength concrete and high strength concrete were fixed. This was
achieved by testing cubes made of various mix proportions. Both destructive
and non-destructive methods were employed on the cubes to find out the
compressive strength of the concrete. The durability of the cubes immersed in
various harsh solutions was observed and investigated. The results and
observations of these unreinforced elements are presented in Section 6.2.
Observations on the behaviour of reinforced beams under flexure
such as failure modes and crack patterns are presented in Section 6.3 and this
also includes a summary of the test results, including cracking load, ultimate
load, load vs deflection characteristics and the effect of the migration of
aggressive solutions in reinforced Geopolymer concrete beams with the
support of microstructural analyses.
6.2 TEST RESULTS ON PLAIN CONCRETE ELEMENTS
6.2.1 Compressive Strength Test on Geopolymer Mortar Cubes
The average compressive strength of all the four combinations of
Geopolymer mortar cubes are presented in Table 6.1.
92
Table 6.1 Compressive strength of Geopolymer mortar cubes
Sl.No. Mix identity Ultimate load
in kN
Average
ultimate load
in kN
Compressive
strength in
N/mm2
1. N1
192.20
186.32 37.38184.50
182.25
2. N2
186.25
175.25 35.16172.50
167.00
3. K1
200.75
193.20 38.76192.50
186.35
4. K2
167.50
145.60 29.21135.75
133.55
6.2.1.1 Discussions on results
A total number of 12 Geopolymer mortar specimens were cast and
tested and their strengths are shown in Table 6.1. The results indicated that
out of all the four combinations, the K1 mixture (mixture of silicates and
hydroxides of potassium) yielded a higher compressive strength than the other
three combinations. Even though the compressive strength of the K1 mixture
was 3.69% higher than the N1 mixture, N1 (mixture of silicates and
hydroxides of sodium) had been selected for the whole research work. On the
cost front, the silicates and hydroxides of sodium were much cheaper than
that of potassium, and hence the former was justified.
93
6.2.2 Compressive Strength Test on Concrete Cubes
The results of the compressive strength of both OPC concrete cubes
and Geopolymer concrete cubes are presented in Table 6.2. The compressive
strength of the cubes was evaluated by non-destructive testing methods, to get
Figure 6.1 Compressive strength test on OPC concrete cube (typical)
first hand information on the compressive strength of the specimens, and
followed by testing on a compression testing machine. The OPC concrete
cubes were tested after 28 days of curing, whereas the Geopolymer concrete
cubes were tested on the third day after curing. A total number of 24 concrete
cubes were cast, inclusive of OPC concrete cubes, and tested. The specimens
that went through the test are shown in Figure 6.1 and Figure 6.2. A plot of
the unconfined compressive strength of the concrete versus a mixture of
concretes is also presented in Figure 6.3.
94
Figure 6.2 Compressive strength test on GPC concrete cube (typical)
Figure 6.3 Compressive strength of concrete cubes
95
6.2.2.1 Discussions on results
From the observed test results on cubes for their compressive
strength, the highest strength was performed by Geopolymer concrete G30
manufactured with 12M concentration of NaOH, with 35.85 N/mm2. While
M30 OPC concrete showed the lowest strength of 31.04 N/mm2, the average
compressive strength of Geopolymer concrete G30 exceeded the compressive
Table 6.2 Compressive strength of concrete cubes
Nomenclature
of specimen
No. of
cubes
tested
Ultimate
load
in kN
Comp.
strength
in N/mm2
Average
compressive
strength in
N/mm 2
Density of
concrete
(kg/m3)
G 30
(14M-NaOH)3
939 41.73
39.96 2408.89871 38.71
887 39.42
G 30
(12M-NaOH)3
778 34.58
35.85 2390.00832 36.98
810 36.00
M 30 3
740 32.89
31.04 2418.89698 31.02
657 29.20
G 50
(14M-NaOH)3
1331 59.16
58.42 2423.811312 58.31
1300 57.78
G 50
(12M-NaOH)3
1187 52.76
55.68 2414.811270 56.45
1301 57.82
M 50 3
1219 54.18
53.50 2385.191200 53.33
1192 52.98
96
strength of its counterpart OPC concrete M30 by 15.5%, and the average
compressive strength of G50 concrete manufactured with 12M concentration
of NaOH, was slightly higher than OPC concrete M50 by 4.07 %. When the
results of G30 and G50 grade Geopolymer concrete manufactured with 14M
concentration of NaOH solution were compared with their OPC concrete
counterparts, the average compressive strength of G50 concrete exceeded that
of the OPC concrete by 9.20% and the same for G30 was 28.74%.
6.2.3 Non-Destructive Testing
6.2.3.1 Ultrasonic pulse velocity
Initially, to fix the mixture proportions for normal and high strength
concrete, cubes were cast and to minimize the labour involved in casting and
testing destructively, the Ultrasonic Pulse Velocity Method and Rebound
Hammer Method of tests were conducted to find the quality and strength of
the concrete mix.
Table 6.3 Quality of concrete of cube specimens
Mix
Identity
Time
( Sec)
Distance
travelled (m)
Pulse
Velocity
(km/sec)
Classification of
concrete quality
M30 36.5 0.15 4.12 Very good
G30(14M) 32.5 0.15 4.62 Very good
M50 21.3 0.15 7.04 Very good
G50(14M) 19.2 0.15 7.81 Very good
97
The Ultrasonic Pulse Velocity Method involved measuring the time
of travel of an ultrasonic pulse passing through the concrete to be tested. The
time travelled between initial onset and the reception pulse was measured
electronically. The path length between the transducer divided by the time of
travel gave the average velocity of wave propagation. Based on the velocity,
the quality of concrete was judged by comparing with the standard values.
They also revealed the quality of the Geopolymer concrete as given in
Table 6.3.
Figure 6.4 Quality of concrete of cube specimens
6.2.3.2 Rebound hammer test
Table 6.4 Results of the rebound hammer test
MixAverage compressive
strength in Psi
Average compressive
strength in MPa
M30 5487.5 34.288
G30 5790.69 39.84
M50 7886.63 54.26
G50 8123.55 55.89
98
Figure 6.5 Compressive strength of cube using rebound hammer
The results of the Rebound Hammer Test conducted on concrete
cubes are presented in Table 6.4. The results showed good correlation with
that of the Destructive Testing. The pictorial representation of the results of
rebound hammer test is shown in Figure 6.5.
6.3 SPLIT TENSILE STRENGTH
Three numbers of 150mmx300mm size cylinders were cast for
M30, M50, G30 and G50. Similar to the cubes, Geopolymer concrete mixes
were prepared with 14M concentration of NaOH and 12M concentration of
NaOH solution, and compared. These specimens were tested for tensile
strength at an age of three days after completion of curing. Totally 18
cylinders were tested inclusive of OPC concrete cylinders. The average test
results are presented in Table 6.5. The graphical representation of average
split tensile strength of cylinders is shown in Figure 6.7.
6.3.1 Discussions on Results
From the test result, the tensile strength of Geopolymer concrete
G30 specimens manufactured with 14 M concentration of NaOH was 13.20%
99
greater than the OPC concrete specimens. The failure of typical G30 cylinder
is shown in Figure 6.6.
Figure 6.6 Split tensile strength
Table 6.5 Average split tensile strength of cylinder specimens
Nomenclature
of specimen
No. of
cylinders
tested
Ultimate load
in kN
Split tensile
strength
in N/mm2
Average
Split tensile
strength in
N/mm 2
G 30
(14M-NaOH) 3
312.27 4.42
4.63323.57 4.58
345.78 4.89
G 30
(12M-NaOH) 3
297.44 4.21
4.22308.74 4.37
288.96 4.09
M 30 3
297.44 4.21
4.09284.72 4.03
286.13 4.05
G 50
(14M-NaOH) 3
532.70 7. 54
7.38512.21 7.25
519.28 7.35
G 50
(12M-NaOH) 3
519.28 7.35
7.29498.08 7.05
527.76 7.47
M 50 3
496.67 7.03
7.13502.32 7.11
511.51 7.24
100
Whereas G30 concrete made with12M concentration yielded 3.2% more than
that of M30 whereas the tensile strength of G50 with 14 M was 3.5% higher
than M50 and for 12M it was higher by 2.24%. Except G30, 14M all
concretes exhibited almost same tensile strength.
Figure 6.7 Split tensile strength results
6.4 RAPID CHLORIDE PENETRATION TEST (RCPT)
On passage of current, the presence of sodium hydroxide in the
Geopolymer specimen produced more heat which was measured to be
approximately about 130°C and at this temperature, the test setup started
melting and collapsed. This has proved that RCPT could not be done on
Geopolymer concrete specimens due to high alkalinity and low conductivity
of current.
6.5 DURABILITY TESTS ON CUBES
6.5.1 Sulphate Resistance Test
Visual appearance, change in mass and residual compressive
strength were evaluated and presented in 6.5.1.1, 6.5.1.2 and 6.5.1.3
respectively.
101
6.5.1.1 Visual appearance
The visual appearance of the test specimens after being exposed to
different periods is shown in Figure 6.8, Figure 6.9 and Figure 6.10. It can be
seen that the visual appearance of the test specimens after 4 weeks of
exposure showed no appreciable change in the appearance of the specimens.
There was no visible sign of surface erosion, cracking or spalling of the
specimens till 4 weeks of time, but after 8 weeks, a little erosion of surface
could be noticed on them.
Figure 6.8 OPC specimens exposed upto 8weeks in 5% sodium
sulphate solution
Figure 6.9 Geopolymer concrete (G50) specimens exposed upto 8 weeks
in 5% sodium sulphate solution
102
Figure 6.10 Geopolymer concrete (G30) specimens exposed upto 8 weeks
in 5% sodium sulphate solution
6.5.1.2 Change in mass
There was a slight increase in mass of the specimens due to the
penetration of solution. The increase in mass of the Geopolymer concrete
specimens soaked in the sodium sulphate solution was approximately 1.2%
after 4 weeks of exposure. In the case of the OPC concrete specimens, the
increase in mass was about 3.5% after 4 weeks of exposure. The change in
mass of the specimens has been illustrated in Figure 6.11. The increase in
mass of the Geopolymer concrete specimens was approximately 1.72% after 8
weeks of exposure. In the case of OPC concrete specimens, the increase in
mass was about 4.63% after 8 weeks of exposure. The test results showed that
Geopolymer concrete cubes, invariably of strength, had marginal weight gain,
which shall be attributed to the absorption of exposed liquid. But the
specimens had almost maintained their shape without any sign of severe
external deteriorations. The graphical representation of change in mass is
shown in Figure 6.11.
103
Figure 6.11 Change in mass of specimens soaked in 5% sodium sulphate
solution
6.5.1.3 Residual compressive strength
Change in compressive strength was determined by testing the
specimens after 2 weeks, 4 weeks and 8 weeks of exposure in the sulphate
solution. The test data reveals that sodium sulphate solution causes very little
reduction in compressive strength in Geopolymer concrete specimens than
Figure 6.12 Change in compressive strength of specimens soaked in 5%
sodium sulphate solution
104
OPC counterparts. The deterioration of OPC concrete due to sulphate attack
could be attributed to the formation of expansive gypsum and ettringite. The
compressive strength of the unexposed specimens was taken as the reference
strength. The residual compressive strength of OPC and Geopolymer concrete
cubes is presented in Figure 6.12. The test results show that exposure of heat-
cured fly ash-based Geopolymer concrete specimens to sodium sulphate
solution had strength loss of about 6.26% for G30 and 4.99% for G50, but in
OPC concrete, the change in compressive strength was about 13.89% for M30
grade and 8.76% for M50 grade concrete after 8 weeks of exposure.
6.5.2 Resistance to Acid Attack
6.5.2.1 Visual appearance
Figure 6.13 compares the visual appearance of OPC concrete cubes
and Geopolymer concrete cubes immersed in 5% sulphuric acid solution for a
duration of 8 weeks. It can be seen that the OPC specimens exposed to
sulphuric acid underwent a high erosion of the surface and thick white paste
formed on the surface which may be due to the high content of calcium in OP
a) OPC concrete cube b) Geopolymer concrete cubes
Figure 6.13 Concrete specimens after 8 weeks of 5% H2SO4 exposure
105
cement. There was more damage on the surface of the specimens with an
increase in the exposure period. Erosion of surface was not observed in
Geopolymer concrete specimens even after 8 weeks of exposure in sulphuric
acid solution and did not exhibit any noticeable colour change. Though the
surface had become softer with the exposure time, the specimen remained
structurally intact.
6.5.2.2 Change in mass
The test results show that Geopolymer concrete cubes exhibited
marginal weight loss of 1.70% initially and weight gain of 2.43% was noticed
after 8 weeks of observation. On the contrary, OPC specimens had weight
loss of 6.33% after 8 weeks of exposure. Figure 6.14 shows the change in
mass of specimens exposed to sulphuric acid.
Figure 6.14 Change in mass of specimens
6.5.2.3 Residual compressive strength
On observation after 8 weeks of exposure, all the Geopolymer
concrete specimens invariably had lost strength by about 4.1% in 4 weeks and
9.7% in 8 weeks, whereas the OPC specimens had a substantial weight loss of
106
about 19% in 4 weeks and 22% in 8 weeks. The residual compressive strength
of specimens is graphically illustrated in Figure 6.15.
Figure 6.15 Residual compressive strength
6.5.3 Water Absorption Test
The water absorption after 60 days of immersion of M30 grade
OPC specimens was found to be 3.1% and G30 specimens recorded water
absorption of 2.9%. For M50 grade concrete, it was 2.5% and G50 specimens
Figure 6.16 Water absorption in %
107
recorded a value of 1.9%. This shows the decrease in water absorption in
Geopolymer concrete when the grade of concrete is high. The water
absorption in percentage is shown in Figure 6.16. The reduction in
compressive strength of G30 specimen was 7.24% and 5.4% for G50
specimens. For OPC concrete M30 specimens, the reduction was 11.19% and
7.34 for M50 specimens.
6.5.4 Resistance to Chloride Attack
The test specimens were immersed in 5% sodium chloride solution.
The chloride attack was evaluated based on change in mass and change in
compressive strength after exposure up to 8 weeks.
6.5.4.1 Change in mass
Figure 6.17 shows the change in mass of G30 specimens which had
lost mass approximately by 1.87% after 4 weeks and 3.6% after 8 weeks.
Similarly, the G50 specimens showed a loss of 1.2% after 4 weeks and 2.5%
after 8 weeks of exposure. In the case of OPC specimens, the decrease in
mass was about 3.7% after 4 weeks and 8.26% after 8 weeks in M30 grade of
Figure 6.17 Change in mass of specimens soaked in 5% NaCl solution
108
concrete and 3.1% after 4 weeks and 8.40% after 8 weeks in M50 grade
concrete.
6.5.4.2 Residual compressive strength
Figure 6.18 shows the change in compressive strength obtained
after 4 weeks and 8 weeks of exposure. Test results have shown that G30
specimen exhibited a reduction in compressive strength by 2.43% after 4
weeks and 7.24% after 8 weeks of exposure. Similarly, the G50 specimen
shows 2.15% decrease after 4 weeks and 5.4% decrease after 8 weeks. For
OPC specimens the reduction in compressive strength was about 7.78% after
4 weeks and 11.22% after 8 weeks for M30 grade, and for M50 concrete it
was about 4.11% and 7.40% after 4 weeks and 8 weeks respectively.
Figure 6.18 Change in compressive strength of specimens soaked in 5%
NaCl
6.6 FLEXURE TEST RESULTS ON BEAMS WITHOUT
REINFORCEMENT
The flexural strength of concrete was calculated using the following
formula and the results are given in Table 6.6.
109
Mf y
I
• •••• •• ••• •(6.1)
where
M = WL/4, Y = d/2, I = BD3/12
M = Bending moment in N.mm
W = Load applied at centre of the beam in N
L = Span length of beam in mm
Y = Neutral axis depth in mm
I = Moment of inertia in mm4
F = Flexural strength of beam in N/mm2
Table 6.6 Flexural strength of Geopolymer and OPC concrete beams
without reinforcements
Beam ID
No.
of
specimens
Failure
Load in
kN
Average
Failure load
in kN
Flexural
Strength
in N/mm2
G30
(14M)3
9.14
9.07 5.439.09
8.98
G30
(12M)3
8.67
8.53 5.118.45
8.47
M303
6.75
6.58 3.946.62
6.37
G50
(14 M) 3
10.12
10.04 6.019.82
10.18
G50
(12M) 3
9.94
9.80 5.879.84
9.62
M503
8.2
7.80 4.677.65
7.55
110
Figure 6.19 Flexural strength of plain beams
It is obvious from test results shown in Table 6.6 and Figure 6.19
that the flexure strength of Geopolymer concrete respective to their grade and
NaOH concentration is much higher than the OPC concrete specimens.
Flexural strength of G30, 12M concrete is higher than M30 by 29.6% whereas
G30, 14M is higher than G30, 12M by 6.3% which is due to the larger
concentration of sodium hydroxide. Similarly, the flexural strength of G50,
12M Concrete is higher than M50 by 25.7% whereas G50, 14M is higher than
G50, 12M by 2.4%.
6.7 FLEXURE TEST RESULTS ON BEAMS WITH
REINFORCEMENT
6.7.1 General Behavior of Series-A Beams
This series-A beams were cast according to the specifications and
sectional details given in GC-3 research report. This test was mainly done to
ascertain the possibilities of manufacturing Geopolymer concrete in India
using Indian fly ash and other materials available to suit Indian conditions.
The beams were cast and tested to match with GBII-3 (Sumajouw et al 2006)
beams. As the percentage of steel was designed and limited to be an under
reinforced beam, all the beams showed behavior in a similar manner and
failed in compression, following high ductility of steel rods.
111
6.7.2 Crack Patterns and Failure Mode
Similar to GBII-3 beams, all the beams in this study showed the
initiation of cracks in the constant moment region of the beam, at the bottom.
As expected, the cracks propagated towards the compression zone, permitting
many such cracks to appear along the bottom span with an increase in load.
The crack patterns and failure mode are shown in Figure 6.20, Figure 6.21,
Figure 6.22 and Figure 6.23. Invariably, all the beams failed in compression.
It is interesting to note that all the Geopolymer concrete beams behaved and
failed like OPC concrete beams. The crack width at service for G30 beams
was 0.25 mm and M50 beams was 0.42 mm. The arising of a number of
cracks was also almost similar in both beams, regardless of the different
material properties.
Figure 6.20 Crack pattern and failure mode of G50 beam I
112
Figure 6.21 Crack pattern and failure mode of G50 beam II
Figure 6.22 Crack pattern and failure mode of G50 beam III
113
Figure 6.23 Crack pattern and failure mode of M50 beam (typical)
6.7.3 Load Vs Mid Span Deflection of Series-A Beams
From the load-deflection (Figures 6.24 to 6.29) curves, it can be
well understood that Geopolymer concrete beams are stiffer than its
counterpart beams and from Table 6.7, the ultimate load taken by G50 beams
is higher than M50 beams. Even though the deflection of G50 beams was
slightly lower than reference M50 beams, the load carrying capacity of G50
beams were still greater than M50 beams. Data has shown that the ductile
behavior of both the beams is well correlated. It was observed that the
maximum failure load obtained in G50 beams was243.50 kN, whereas for
M50 beam, it was197 kN only. The G50 beams showed an increase of 23.6%
in ultimate load and nearly 36% more stiffness at ultimate load values when
compared with the M50 beams. From the above results, it can be clearly seen
that all the G50 flexure beams showed enhanced performance with respect to
parameters such as initial cracking load, yield load, stiffness at yield and
ultimate load when compared with the control beams. The reduction in the
ultimate load carrying capacity of M50 beams may be due to the free water
content, resulting in a considerable increase in the workability of concrete and
thus, reducing the strength of OPC members.
114
Table 6.7 Flexure test results of series-A beams
Sl.
NoParameter
M 50-
I
M50-
II
M50-
IIIG50-I G50-II G50-III GBII-3
1.Initial crack load
in kN 32.50 30.05 33.55 40.00 28.50 36.50 -
2.Ultimate load
in kN 197.00 176.50 185.50 243.50 238.50 221.00 239.11
3.
Mid-span
deflection at
failure load
in mm
32.00 35.85 33.45 29.00 28.45 27.50 30.01
4.
Stiffness at
ultimate load in
kN/mm
6.15 4.90 5.55 8.39 8.38 8.03 7.97
5.Cracking
Moment in kNm16.25 15.02 16.77 20.00 14.25 18.25 16.65
6.
Experimental
Ultimate
Moment in kNm
98.50 88.25 92.75 121.75 119.25 110.50 119.00
115
Figure 6.24 Load Vs mid-span deflection curve of G50-I
020406080
100120140160180200220240260
0 5 10 15 20 25 30 35
Lo
ad
in
kN
Mid-span deflection in mm
G50-II
Figure 6.25 Load Vs mid-span deflection curve of G50-II
116
Figure 6.26 Load Vs mid-span deflection curve of G50-III
Figure 6.27 Load Vs mid-span deflection curve of M50-I
117
Figure 6.28 Load Vs mid-span deflection curve of M50-II
Figure 6.29 Load Vs mid-span deflection curve of M50-III
118
The comparison with experimental values of GCII-3 Beams
indicates good agreement, especially in case of ultimate moment carrying
capacity and stiffness of G50 Beams. But the cracking moments of M50
beams are found very close to GB II-3 Beams, rather than that of G50
Beams. The reinforced Geopolymer concrete flexural members manufactured
using Class F Indian fly ash behave similar to the results of
Sumajouw and Rangan.
6.8 FLEXURAL BEHAVIOR OF SERIES-B BEAMS
6.8.1 Load Carrying Capacity
The specimens were tested under monotonically increasing load
until failure. As the load increased, the beams started to deflect and flexural
cracks developed along the span of the beams. Eventually, all the beams
failed in a typical flexure mode. The progressive increase of deflection at mid-
span is shown as a function of the increasing load. The load-deflection curves
indicate distinct events that are taking place during the test. All the beams
behaved in a similar manner, as they are designed as under reinforced beams;
Table 6.8 Flexure test results of Series-B beams
Beam
ID
Companion Concrete results
in N / mm 2 Load in kN
Mid-span
Deflection
in mm Failure
Mode Comp.
strength
Split
tensile
strength
Flexural
strength
First
crack
(Pcr)
Ultimate
(Pu)
First
crack
Ultimate
!u)
G 30-I 36.95 4.72 5.05 17.80 147.00 8.60 34.50 Comp.
G30-II 36.10 4.49 5.21 17.55 142.50 8.40 33.20 Comp.
G30-III 34.50 4.53 5.07 16.25 139.50 7.90 33.40 Comp.
M 30-I 30.84 3.87 3.97 14.25 103.85 8.00 31.70 Comp.
M30-II 31.73 4.14 4.05 14.80 99.55 7.00 30.45 Comp.
M30-III 30.55 4.08 3.80 12.95 100.20 7.80 28.75 Comp.
119
therefore the tensile steel must have reached its yield strength before failure.
The flexure test results of series-B beams are tabulated in Table 6.8.
6.8.2 Crack Pattern and Failure Mode
As expected, the flexure cracks got initiated in the pure bending
zone. As the load increased, the existing cracks propagated and new cracks
developed along the span. The width and the spacing of cracks varied along
the span. In all, the crack patterns observed for reinforced Geopolymer
concrete beams are similar to the reinforced Portland cement concrete beams.
Nearer to maximum load, the beams deflected significantly, thus indicating
that the tensile steel must have yielded at failure.
Figure 6.30 Crack pattern and failure mode of typical G30 beam
Figure 6.31 Failure pattern of typical M30 concrete beams
The final failure of the beams occurred when the concrete in the
compression zone crushed, accompanied by the buckling of the compressive
120
steel bars. The failure mode is typical of that of an under-reinforced concrete
beam. The crack patterns and failure mode of typical test beams are shown in
Figure 6.30, Figure 6.31 and 6.32.
Figure 6.32 Crack pattern and failure mode of M30 beam
6.8.3 Beam Deflection
From the observed results, it can be seen that the mid-span
deflection of the G30 beams is slightly more than the deflection of the M30
beams. The load versus mid-span deflection curves of all the specimens are
shown in Figure 6.33 to Figure 6.38.
Figure 6.33 Load Vs Mid-span deflection curve of M30-I
121
Figure 6.34 Load Vs mid-span deflection curve of M30-II
Figure 6.35 Load Vs mid-span deflection curve of M30-III
122
Figure 6.36 Load Vs mid-span deflection curve of G30-I
Figure 6.37 Load Vs mid-span deflection curve of G30-II
123
Figure 6.38 Load Vs mid-span deflection curve of G30-III
The reinforced concrete beam consists of heterogeneous material-
concrete, whose property is unpredictable beyond cracking stage. However
for convenience in design, it is assumed to be homogeneous. Various
researchers assume different parameters to calculate the deformation behavior
of reinforced concrete sections. The load-deflection profile of the Geopolymer
concrete beams due to load at first crack, yield and ultimate load follows that
of RCC beams very much and obey the existing limit state design theories
given by (Ernst 1957), (Sawyer 1955), (Baker 1956) and (Cohn 1965).
6.8.4 Moments at First Crack Load and at Ultimate Load
It is well known from the experimental studies that the load
carrying capacity of G30 beams exceeds RCC beams by 41% because of the
facts that the compressive strength increases by 15% due to heat curing of
Geopolymer concrete, the bonding strength between Geopolymer paste and
deformed bars is higher by 1.6 times than that of plain bars and by getting
very dense concrete by compacting in table vibrator and as regards to
cracking moment, the percentage increase is very marginal. The cracking
moment is predicted using the formulae
124
y
xIfM
gcr
cr! (6.2)
where
Mcr = Cracking Moment,
fcr = 0.6(fck)1/2
,
Ig = Gross Moment of Inertia
y = Depth of NA
Table 6.9 Correlation of experimental and predicted cracking moment
of series-B beams
Beam ID
Moment at
First crack
Mcr in kNm
Predicted
cracking
moment in kNm
Correlation ratio
Experimental/
Predicted
G30-I 5.65 5.27 1.07
G30-II 5.20 5.04 1.03
G30-III 5.05 5.02 1.01
M30-I 4.70 3.97 1.18
M30-II 4.25 3.84 1.11
M30-III 3.95 4.01 0.99
Table 6.10 Correlation of experimental and predicted ultimate moment
of series-B beams
Beam ID
Experimental
Ultimate
moment in kNm
Predicted
Ultimate
moment in kNm
Correlation ratio
( c1)
Experimental/
Predicted
G30-I 44.63 23.77 1.87
G30-II 44.31 22.98 1.92
G30-III 43.33 22.40 1.93
M30-I 31.35 22.18 1.41
M30-II 31.23 22.13 1.41
M30-III 31.02 21.96 1.41
125
It is seen from the results in Table 6.9, the correlation ratio for both
the beams being nearly unity, the G30 beams behave similar to the M30
beams. But the ultimate moment carrying capacity of G30 beams is higher by
41.3% than M30 beams. Unlike the cracking moments, the ultimate moments
of G30 beams are not correlated with M30 beams which are given in
Table 6.10.
6.9 DISCUSSION ON RESULTS OF SERIES-C BEAMS
EXPOSED TO ACID AND CHLORIDE ATTACK
6.9.1 Surface Deterioration
Prolonged exposure of fly ash based Geopolymer concrete beams in
5% HCl + 5% H2SO4 solution (chloride attack) showed very little
deterioration of the top surface of specimens, less than 2mm, which had led to
A B R
A - HCl+H2SO4 Solution; B - Sulphuric acid solution;
R - Unexposed Specimen
Figure 6.39 Typical beam after 180 days of exposure
126
visibility of coarse aggregates whereas in beams immersed in 10% H2SO4
solution, visible yellowish green hue patches were seen on the surfaces. This
could be seen from the pictures given in Figure 6.39. The beam with
designation ‘R’ represents the specimen not subjected to aggressive exposure.
Before exposure to acid solution, the specimens possessed a fairly smooth
surface and due to exposure, the deterioration of the surfaces started which
appeared to be very marginal, less than one mm. The specimens kept in
sulfuric acid solution showed good resistance to acid. Throughout the
duration of exposure, specimens were taken from solutions periodically and
checked for any noticeable changes on the surfaces. There was no visible
rusted surface seen in all the specimens.
Figure 6.40 Uncorroded rods in broken specimen ‘A’
Figure 6.41 Uncorroded rods in broken specimen ‘B’
127
6.9.2 Change in Weight
The change in weight of Geopolymer concrete beam specimens
after exposure to acid and chloride were compared with the weight of control
element ‘R’. All the specimens, except the control elements, recorded weight
loss and it was observed to be 3.26% in the specimens subjected to acid
attack. The loss of weight in the case of specimens subjected to chloride
attack was noted and found to be 1%.Almost all the specimens retained their
shape and texture. The results are illustrated in Figure 6.42. The
reinforcement rods were taken out from the specimen at the end of 180 days
of study and weighed. There was not even a fraction of weight loss noticed in
both the beams. Figure 6.40 and Figure 6.41 show the uncorroded
reinforcement mass after exposure.
Figure 6.42 Change in weight (%) after exposure to acid and chloride
6.9.3 Scanning Electron Microscopy and EDAX
Samples for Scanning Electron Microscopy (SEM) analysis and
EDAX were taken from near the surface of the specimen. SEM micrographs
along with EDAX spectrum showing the images of Geopolymer concrete
128
beams after 180 days of exposure to chloride and acid environment,
designated as Sample A and Sample B respectively are illustrated in
Figure 6.43 and Figure 6.44. And the unexposed specimen as Sample ‘R’ is
shown in Figure 6.45. In reference Specimen R, unreacted fly ash particles
could not be noticed rendering it a high dense concrete. From EDAX
spectrum of Sample R, it shall be noted that iron oxide content was 3.7%.
Also it revealed the presence of Si, Al, K, Na and C as the main elements.
After 180 days of exposure to 10% H2SO4 solution, the specimens appeared
to have deteriorated by the acid attack and at the same time EDAX spectrum
also reported a change in the presence of elemental traces. The iron oxide
content had increased from 3.7% to 6.02% for Specimen A and the content of
iron oxide for Sample B had decreased from 3.7% to 3.11%, very marginal.
This shows that the metal was not corroded under acid attack. From SEM
images of Sample A and Sample B, the presence of light precipitates, which
might be a product of degradation, was seen.
129
2 4 6 8 10 12 14keV
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8 cps/eV
O Si C
Fe Fe
Na Al
Ca
Ca
K K
S Cl
Cl
Figure 6.43 SEM image of specimen ‘A’ and its corresponding EDAX
spectrum after 180 days of 5% HCl + 5% H2SO4 exposure
130
2 4 6 8 10 12 14keV
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4 cps/eV
O Si Al
Na Fe Fe
Ca
Ca
S K K
Figure 6.44 SEM image of specimen ‘B’ and its corresponding EDAX
spectrum after 180 days of 10% H2SO4 exposure
131
2 4 6 8 10 12 14keV
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
cps/eV
O Si Al C
Ca
Ca
Na Fe Fe K
K
Figure 6.45 SEM image of specimen ‘R’ and its corresponding EDAX
spectrum without exposure
132
6.9.4 X-Ray Diffraction Analysis
XRD patterns of the samples ‘A’, ‘B’ and ‘R’ are illustrated in
Figure 6.46, Figure 6.47 and Figure 6.48. From this spectrum of specimen A,
we could infer that the peaks for Zeolitic phases have reduced and new quartz
arising from alumino-silicate gel formed between 200 and 21
0 2" which
indicates eruption by sulphur. Also new hematite crystals appearing at 780 2"
might have reduced the peak intensity seen in the 260-29
0 2".This
characteristic of low calcium based Geopolymer concrete may very well be
adopted in structures in coastal areas, off-shore structures, tetrapods provided
on the sea shores to withstand tidal waves and erosion due to chloride attack.
Position [°2Theta] (Copper (Cu))
20 30 40 50 60 70
Counts
0
100
200
300
outside work2_807
Figure 6.46 XRD pattern of specimen ‘A’
After 180 days of exposure to acid solution, the peak intensity of
Zeolitic phases have reduced a little, without any noticeable change in the
133
Position [°2Theta] (Copper (Cu))
20 30 40 50 60 70
Counts
0
100
200
300
400
outside work2_808
Figure 6.47 XRD pattern of specimen ‘B’
Position [°2Theta] (Copper (Cu))
20 30 40 50 60 70
Counts
0
200
400
600
800 outside work2_811
Figure 6.48 XRD pattern of specimen ‘R’
134
microstructural profile. This shows the resistance to acid. Interaction of
Geopolymers with the sulfuric acid solution can also cause replacement of the
exchangeable cations(Na) in polymers by hydrogen or hydronium ions which
reduces absorption of oxygen leading to depolymerisaton, the process of
breaking polymers into manomers due to the absence of oxygen. The
observation of the mass changes of the samples exposed to acidic solutions
and results obtained from SEM, EDAX and XRD analyses gave a positive
sign of the hypothesis test of depolymerisation of alumino-silicate polymer
gel.
6.9.5 Residual Flexural Strength
The main objective of this paper was achieved by evaluating the
residual flexural strengths of the reinforced Geopolymer concrete beams after
acid and chloride attack and being compared with those of the Geopolymer
concrete beams unexposed to chemicals. All the beams cast were tested in a
Universal Testing Machine of 1000 kN capacity. A deflectometer was
positioned at the bottom middle of each beam, to find out the deflection. A
monotonically increasing single point load was applied until failure, at the
middle of the beam by a load cell of 500kN capacity. The crack propagation,
initial crack load, ultimate load and deflection were noted for further
investigation. All the beams behaved in a similar manner, as they were
designed as under reinforced section, steel to reach its yield strength before
failure. The test setup with tested beam is shown in Figure 6.51. As expected,
flexure cracks got initiated at the bottom of the beam in the tension zone. As
the load was incremented, new cracks initiated in a similar way. Invariably,
all the beams failed in compression mode, except couple of beams which
failed in shear. It was well established from the experimental studies that the
load carrying capacities of Specimen ‘A’ and specimen ‘B’ were not reduced
135
appreciably when compared to that of Specimen ‘R’. The Table 6.11 depicts
the results of this test.
Table 6.11 Flexure test results of series-C beams
Beam
ID
No.of
Days
of exp-
osure
Load in kN Mid-span
Deflec-
tion
in mm
Cracking Moment
in kNm
Ultimate Moment
in kNm
Failure
mode First
crack
(Pcr)
Ultimate
Load
(Pu)
Experi-
mental
(Mcr)exp
Calcula-
ted
(Mcr)cal
Experi-
Mental
(Mu)exp
Calcul-
ated
(Mu)cal
Specimen
R0 21.00 47.00 23 2.10 0.85 4.70 2.77 Comp.
Specimen
A
(chloride)
7 20.78 44.98 21 2.08 0.85 4.49 2.77 Comp.
15 19.47 42.76 21.5 1.95 0.85 4.28 2.77 Comp.
30 19.18 42.50 19 1.92 0.85 4.25 2.77 Comp.
60 19.15 42.18 22.2 1.92 0.85 4.22 2.77 Comp.
120 18.75 42.08 22.3 1.88 0.85 4.21 2.77 Comp.
180 18.35 42.00 22 1.84 0.85 4.20 2.77 Comp.
Specimen
B
(acid)
7 20.96 45.78 23 2.10 0.85 4.58 2.77 Comp.
15 19.98 45.32 23 2.00 0.85 4.53 2.77 Comp.
30 19.25 45.25 22.65 1.93 0.85 4.53 2.77 Comp.
60 19.15 45.05 22.65 1.92 0.85 4.51 2.77 Comp.
120 19.12 44.98 22.55 1.91 0.85 4.49 2.77 Comp.
180 19.05 44.98 22.5 1.91 0.85 4.49 2.77 Comp.
6.10 DISCUSSION ON RESULTS OF C-SERIES BEAMS
EXPOSED TO SODIUM AND MAGNESIUM SULPHATE
6.10.1 Surface Deterioration
The exposure of fly ash based reinforced Geopolymer concrete
beams in 10% sodium sulphate solution showed no considerable deterioration
of the specimens, whereas in the beams immersed in 10% magnesium
sulphate solution, white deposits were noticed on the surface of the
136
specimens. This could be seen from Figure 6.49. The amount of soft white
patches was increasing with the duration of exposure. The amount of deposits
after 180 days could be well witnessed on all sides of the beam. The beam
C D R
C - Sodium Sulphate; D - Magnesium Sulphate; R - Reference or unexposed
Figure 6.49 Typical beam after 180 days of exposure
with designation ‘R’ represents the specimen not subjected to aggressive
exposure whereas ‘C’ and ‘D’ represent specimens exposed to sodium
sulphate and magnesium sulphate solutions respectively. Throughout the
duration of the exposure, specimens were taken from solutions periodically
and checked for any noticeable changes on the surfaces. There was visible
rusted surface seen in all the specimens that underwent the magnesium
137
sulphate attack at the end of the study period. After 180 days of exposure to
sodium sulphate, specimens ‘C’ showed no traces of corrosion or colour
change.
Figure 6.50 Specimen ‘D’ after 180 days of exposure
6.10.2 Change in Weight
The change in weight of specimens after exposure to sulphates of
sodium and magnesium was compared with the weight of control element ‘R’.
All the specimens except control elements recorded weight gain and it was
observed to be 1.6% in specimens subjected to sodium sulphate and 0.45% in
the case of magnesium sulphate. Almost all the specimens retained their
shape and texture. The least amount of increase in weight observed in both C
and D might have been due to the absorption of harsh liquid. Figure 6.51 and
Figure 6.52 show dismantling of beams to take rods out. The rods taken out
from the specimen, shown in Figure 6.53, at the end of 180 days, showed no
weight loss. Figure 6.54 shows the change in weight of specimens after
exposed to sulphates of sodium and magnesium for 180 days.
138
Figure 6.51 Specimen being demolished
Figure 6.52 Uncorroded rods in Specimen ‘D’
Figure 6.53 Uncorroded reinforcement
139
Figure 6.54 Change in weight (%) after exposure to sulphates of sodium
and magnesium
6.10.3 Scanning Electron Microscopy and EDAX
Samples for scanning electron microscopy (SEM) analysis and
EDAX were taken from near the surface of the specimen. SEM micrographs,
along with EDAX spectrum, showing the images of Geopolymer concrete
beams after 180 days of submergence in 10% concentration of sodium
sulphate and magnesium sulphate environment, are illustrated in Figure 6.55
and Figure 6.56 respectively and the unexposed specimens ‘R’ is shown in
Figure 6.45. In reference specimen R, unreacted fly ash particles could not be
noticed rendering it a high denser microstructure in concrete. From EDAX
spectrum of Sample R, it shall be noted that iron oxide content was 5.3%.
Also it revealed the presence of Si, Al, K, Na and C as the main elements.
After 180 days of exposure to 10% Na2SO4 solution, specimens appeared to
have deteriorated by the sulphates of sodium and at the same time EDAX
spectrum also reported a change in the presence of elemental traces. The iron
140
oxide content had increased from 5.3% to 6.65% for Specimen C and the
content of iron oxide in Sample D had increased from 5.3% to 6.25%. This
shows that the metal was slightly corroded under sulphates, less than 1.4%.
From SEM images of sample D, the presence of shiny precipitates are seen
which might be due to the formation of ettringite and gypsum. The pores
could not be seen in the microstructure which might have got filled up by the
precipitates formed by oxides of magnesium. Thus the durability
characteristics of Geopolymer concrete activated by silicates and hydroxides
of sodium mainly depended upon the range of discontinuous pores and the
size of the pores.
141
2 4 6 8 10 12 14keV
0.0
0.5
1.0
1.5
2.0
2.5 cps/eV
O Si Al
Na Ca
Ca
Fe Fe K K Mg
Figure 6.55 SEM image of specimen ‘C’ and its corresponding EDAX
spectrum after 180 days of 10% sodium sulphate exposure
142
2 4 6 8 10 12 14keV
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
cps/eV
O Si Al
Na Ca
Ca
Fe Fe Mg K K
Figure 6.56 SEM image of specimen ‘D’ and its corresponding EDAX
spectrum after 180 days of 10% magnesium sulphate
exposure
143
6.10.4 X-Ray Diffraction Analysis
XRD patterns of the sample ‘C’ and sample ‘D’ are illustrated in
Figure 6.57 and Figure 6.58. From this spectrum of specimen C, we could
infer that peaks for Zeolitic phases have reduced and new quartz arising from
alumino-silicate gel formed between 240 and 25
0 2" which indicates eruption
by sulphur. Also new hematite crystals appeared at 760 2".
Position [°2Theta] (Copper (Cu))
20 30 40 50 60 70
Counts
0
200
400
600
outside work2_809
Figure 6.57 XRD pattern of specimen ‘C’
After 180 days of exposure to magnesium sulphate solution, the
peak intensity of Zeolitic phases have reduced a little, introducing new peaks
in 15o-16
02" without any drastic change in the microstructural profile. This
shows the formation of some ettringite and gypsum. Thus calcium and
magnesium ions have accommodated in alumino-silicate gel as network-
modifying cations. The migration of these ions into alumino-silicate gel
144
seemed to improve the flexural strength of specimen ‘D’. This is in agreement
with earlier studies (Bakharev 2005).
Position [°2Theta] (Copper (Cu))
20 30 40 50 60 70
Counts
0
100
200
300
400
outside work2_810
Figure 6.58 XRD pattern of specimen ‘D’
6.10.5 Residual Flexural Strength
The evaluation of the residual flexural strengths of reinforced
Geopolymer concrete beams exposed to sulphates of sodium and magnesium
is the main objective of the research and compared with those of reinforced
Geopolymer concrete beams unexposed to chemicals. All the beams exposed
to sulphates of sodium and magnesium were tested similar to that of the
beams exposed to acid and chloride. As envisaged, flexure cracks got initiated
at the bottom of the beam in the tension zone and with increment in load, new
cracks got initiated and propagated. Invariably, all the beams failed in
compression mode.
145
Figure 6.59 Specimen ‘C’ under testing
Figure 6.60 Specimen ‘D’ under testing
146
Figure 6.61 Specimen ‘R’ showing deflection
It is established from the experimental studies that the flexural
strength of Specimen ‘C’ decreased by 11.7%. This shall be attributed to the
formation of ettringite and gypsum to an extent due to the presence of known
percentage of calcium in the source material and unknown percentage of
calcium in aggregates. But surprisingly, Specimen ‘D’ showed an increase in
flexural strength which was 12.77%. This upward trend line might be due to
the more stable structure of alumino-silicate and the gypsum formed would
have filled up the discontinuous pores. Figure 6.59 and Figure 6.60 show
specimen C and specimen D undergoing single point load test and Figure 6.61
shows deflected shape of specimen ‘R’. The results of the test on specimen C
and specimen D are illustrated in Table 6.12.
147
Table 6.12 Flexure test results of series-C beams
Beam
ID
No.of
Days
of
expo
sure
Load in kN Mid-
span
Deflec
tion
in mm
Cracking Moment
in kNm Ultimate Moment
in kNm Failure
mode First
crack
(Pcr)
Ultimate
Load
(Pu)
Experimental
(Mcr) exp
Calculated
(Mcr)cal
Experi-
Mental
(Mu)exp
Calculated
(Mu)cal
Specimen
R0 21.00 47.00 23.00 2.10 0.85 4.70 2.77 Comp.
Specimen
C
7 20.85 45.40 22.40 2.09 0.85 4.54 2.77 Comp.
15 20.30 43.70 22.00 2.03 0.85 4.37 2.77 Comp.
30 19.95 43.20 21.70 2.00 0.85 4.32 2.77 Comp.
60 19.35 42.65 21.30 1.94 0.85 4.27 2.77 Comp.
120 18.90 41.80 20.80 1.89 0.85 4.18 2.77 Comp.
180 18.50 41.50 20.00 1.85 0.85 4.15 2.77 Comp.
Specimen
D
7 21.50 47.90 23.35 2.15 0.85 4.79 2.77 Comp.
15 21.75 48.40 23.90 2.18 0.85 4.84 2.77 Comp.
30 22.20 48.80 24.15 2.22 0.85 4.88 2.77 Comp.
60 22.50 50.20 24.60 2.25 0.85 5.02 2.77 Comp.
120 22.85 51.80 25.35 2.29 0.85 5.18 2.77 Comp.
180 23.00 53.00 27.00 2.3 0.85 5.30 2.77 Comp.
6.11 CONCLUSION
From the results of various tests conducted, it could be ascertained
that Geopolymer concrete cubes have shown to be good in strength as well as
in durability aspects. Reinforced Geopolymer concrete beams have proved to
be behaving exceedingly well and have shown more flexural strength than
OPC counterparts. Also, reinforced Geopolymer concrete beams have
withstood remarkably against very aggressive exposure to acids and
sulphates. These properties of Geopolymer concrete have led to the
conclusion that Geopolymer concretes would find certain place in structural
applications in the near future.
