91 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.
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91
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.
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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.
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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.
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Figure 6.2 Compressive strength test on GPC concrete cube (typical)
Figure 6.3 Compressive strength of concrete cubes
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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
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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
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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
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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%
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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
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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.
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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