Page 8 - Open Access-June 2019

TECHNICAL PAPER

specimens of M 30 and M 90 grade heated from ambient
1400
temperature to 100°C. In the case of M 30 grade concrete,
1200 945 1006 1049 1082 1110 the residual compressive strength increased by 8.89% at first
Temperature [°C] 800 576 842 cycle and reduced gradually by 15.56% after 50 cycles with
1000
respect to control concrete. This result is in good agreement
600
with the findings of Bairagi and Dubal (1996). It can be seen
400
200 349 that strength loss of M 30 grade concrete was negligible up
to 20 thermal cycles. In the case of M 90 grade concrete, the
0 20
0 30 60 90 120 150 180 residual compressive strength increased by 6.86% at first cycle
and reduced gradually by 39.22% after 50 cycles with respect to
Time [minutes]
control concrete. Increase in strength was observed at first cycle
Figure 3.2: Standard time – temperature curve according to due to accelerated hydration of cement during heating cycle.
ISO 834 (1975).

4. RESULTS AND DISCUSSIONS 120 120 120 120

4.1 Effect of thermal cycles on compressive 100 100 100 100
strength
Residual compressive strength is the compressive strength of Percentage residual compressive strength 80 Percentage residual compressive strength 80 Percentage residual compressive strength 80 Percentage residual compressive strength 80
heated specimen is expressed as the percentage of 28 day M 30 M30 M 30
strength of the control concrete (reference concrete). The M 90 M 30 M 90
residual compressive strength of concrete reduces gradually 60 60 M 90 60 M90 60
when subjected to thermal cycles. This may be attributed due to
the following phenomenon. 40 40 40 40
0 5 10 15 20 25 30 35 40 45 50 0 5 10 15 20 25 30 35 40 45 50 0 5 10 15 20 25 30 35 40 45 50 0 5 10 15 20 25 30 35 40 45 50
As the air temperature increased in oven, temperatures on the No. of cycles No. of cycles No. of cycles No. of cycles
surface and inside the specimen also increased but at varying
rates. At this temperature, free water in the fine concrete pores Figure 4.1: Variation of percentage residual compressive strength of
began to evaporate and diffuse. A portion of the water vapour M 30 & M 90 specimens exposed to temperature of 100°C.
escaped through the heated surface of the specimen, causing a 120 120 7 7
Percentage residual compressive strength
sudden drop in temperature with respect to time on the heated 2. Figure 4.2 shows the variation of residual compressive 6
face. At the same time, the remaining gaseous mixture of water strength with the number of thermal cycles for the concrete 6
100
vapour was transported inward by pressure gradient. This mass specimens of M 30 and M 90 grade heated from ambient 100 5 5
temperature to 200°C. For M 30 grade concrete, loss in strength
transport process (vaporization and diffusion of free water) 100 C 100 C 4 4
80
continued as the temperature inside the concrete increased to was 2.22% at first cycle while it was 23.53% for M 90 grade Percentage residual compressive strength 80 200 C Percentage weight loss Percentage weight loss
concrete. M 30 grade concrete retained 80% of its original
200 C
about 100°C. Because of the vaporization and transport of water strength even after 50 thermal cycles of exposure whereas M 90 300 C 3 M 30 3 M 30
vapour in the temperature range [100 to 200°C], the transmission grade concrete has only 60% of its original strength. 300 C 60 2 M 90 2 M 90
60
of heat through the concrete is retarded as a result of which 400 C 400 C 1
the residual compressive strength of concrete got decreased. 1
120
120
Percentage residual compressive strength
As the concrete surface temperature is increased from 100 40 120 0 120 0
40
to 200°C, a more pronounced drop in temperature with time 0 5 10 15 20 25 30 35 40 45 50 0 5 10 15 20 25 30 35 40 45 50 0 5 10 15 20 25 30 35 40 45 50 0 5 10 15 20 25 30 35 40 45 50
occurred. This might have happened due to the release and 100 No. of cycles 100 No. of cycles 100 No. of cycles No. of cycles
100
diffusion of chemically bound water in the concrete matrix. The
additional volume of water vapor in the concrete pores due to
80
the release of chemically bound water caused a sharp increase Percentage residual compressive strength 80 Percentage residual compressive strength 80 Percentage residual compressive strength 80 M 30
M 30
in pore pressure. This increase in pore pressure continues until 7 M 30 8 M30 8 M 90 7
M 90
the complete diffusion of chemically bound water in concrete. 6 60 M 90 7 60 M90 7 60 6
60
After diffusion of bound water, the pore pressure decreases in 5 6 6
the concrete. The attenuation of pore pressure in the specimen 5 5 5
40
caused due to presence of micro cracks which may lead to 4 40 0 5 10 15 20 25 30 35 40 45 50 40 0 5 10 15 20 25 30 35 40 45 50 40 0 5 10 15 20 25 30 35 40 45 50 100 C 4 100 C
30
35
50
45
40
0
5
15
25
10
20
spalling of concrete. Percentage weight loss Percentage weight loss 4 Percentage weight loss 4 200 C Percentage weight loss 200 C
No. of cycles 3 No. of cycles M 30 3 No. of cycles M 30 3 No. of cycles 3
1. Figure 4.1 shows the variation of residual compressive Figure 4.2: Variation of percentage residual compressive strength of 2 M 90 2 300 C 2 300 C
2
M 90
400 C
400 C
strength with the number of thermal cycles for the concrete M 30 & M 90 specimens exposed to temperature of 200°C.
1 1 1 1
Percentage residual compressive strength 80 100 C Percentage residual compressive strength 80 100 C Percentage weight loss 4 3 2 M 30 Percentage weight loss 4 3 2 M 30
0 120
12 120 100 0 5 10 15 20 No. of cycles 40 45 50 0 7 6 5 0 5 10 15 No. of cycles 35 40 45 50 07 6 5 0 5 10 15 No. of cycles 35 40 45 50 0 0 5 10 15 No. of cycles 35 40 45 50
25
30
30
35
20
25
20
30
30
25
20
25
The IndIan ConCreTe Journal | June 2019
100
200 C
200 C
300 C
300 C
60
M 90
60
M 90
400 C
400 C
40
0 5 10 15 20 25 30 35 40 45 50 40 0 5 10 15 20 25 30 35 40 45 50 1 0 0 5 10 15 20 25 30 35 40 45 50 1 0 0 5 10 15 20 25 30 35 40 45 50
No. of cycles No. of cycles No. of cycles No. of cycles
7 8 8 7
6 5 7 6 7 6 6 5
Percentage weight loss 4 3 M 30 Percentage weight loss 5 4 3 M 30 Percentage weight loss 5 4 3 100 C Percentage weight loss 4 3 100 C
200 C
200 C
300 C
300 C
1 2 M 90 2 1 M 90 2 1 400 C 2 1 400 C
0 0 0 0
0 5 10 15 20 25 30 35 40 45 50 0 5 10 15 20 25 30 35 40 45 50 0 5 10 15 20 25 30 35 40 45 50 0 5 10 15 20 25 30 35 40 45 50
No. of cycles No. of cycles No. of cycles No. of cycles