Page 7 - March-2022

TECHNICAL PAPER

Electron Microscopy (SEM) is used to study the micro 24.91 %
characteristics of the optimum FRM. The broken FRM samples 17.33 %
were polished with different sized sand polishing papers for
a polished surface. The samples were then subjected to final 0 % 1.60 %
polishing with cloth to obtain a mirror like polished surface, Compressive strength reduction in % -9.31 %
and the samples were further coated with gold to avoid surface -18.11 % -19.05 %
charging. A Scanning Electron Microscope having a field
emission gun for EDX facility (Zeiss SUPRA 55-VP FEG-SEM) was
used for this analysis.
-59.52 %
3. RESULTS AND DISCUSSION Room Temperature 300°C 600°C
B-S-P 0 % -18.11%
0 % -59.52% -19.05 %
B-S-P S
3.1 Compressive strength studies B-S-P P 0 % -9.31 % 17.33 %
B-S-P H 0 % 1.60% 24.91%
Temperature
From Figure 1, it can be seen that specimens without or B-S-P B-S-P S B-S-P P B-S-P H
without fibers at 300°C temperature had improved compressive Figure 2: Percentage reduction of compressive strength after elevated
strengths in comparison to specimens that were cured at temperature exposures
room temperature (unexposed samples). The specimens (N.B.-The negative results indicate improvement in residual compressive
B-S-P (without any fibers) show a gain of 18.11 % at 300°C in strength of the specimens)
comparison to the compressive strength of the mortars at room to the fact that the PP fibers would have melted at 160°C, which
temperature. However, when the temperature increased further
to 600°C, the cubes could not sustain the high temperature and could provide a network for escaping channels for the vapour
busted inside the furnace. The possible reason for this behavior nullifying the effect of vapour pressure.
could be due to the accumulation of internal vapour pressure In case of the specimen with steel fiber (i,e., B-S-P S ) shows a
owing to a compact matrix (without any fibers), and thus, it strength gain of 59.52 % and 19.05 % at a temperature of 300°C
contributed to an explosive spalling. From Figure 1, it also can and 600°C, respectively. The mix with steel fibers exhibited a
be observed that at 300°C, there is an increase in compressive compressive strength of 33.70 MPa without any temperature
strength for most of the mixes taken in this investigation. exposure, whereas, at 600°C and 300°C temperature, the same
The increment in compressive strength could be considered mix gave a strength of 53.76 MPa and 40.12 MPa, respectively.
primarily due to the rapid drying of the mortar mixtures [15] .
The mortar mixtures with PP fibers show a comparatively The specimens containing Hybrid fibers performed poorly in
better fire resistance in terms of both spalling and reduction of both the high-temperature conditions. The mortar mixture with
compressing strength than that of the mix without any fibers. hybrid fibers had the lowest thermal resistance in comparison
The mortar cubes showed a good increment in compressive to all the taken mixes. The mix B-S-P H shows 1.6 % and
strength at a temperature of 300°C, but the strength was 24.91 % reduction in compressive strength at 300°C and 600°C
reduced by 17.33 % (Figure 2) when the samples were exposed temperature exposures, respectively. Though the mixture
to a temperature of 600°C. The specimens with PP fibers were composition with PP fibers shows enhanced compressive
not subjected to explosive spalling at high temperatures, owing
strength at room temperature, their melting at 165°C reduces
60 the strength at elevated temperature conditions, which could
have contributed to the reduction of the compressive strength
50
for the mix B-S-P H . However, the specimens with steel fibers
strength 40 performed excellently in comparison to the other mixes at both
the elevated temperature conditions. This high-temperature
Compressive 30 resistance performance of the steel-reinforced mortars could
be associated with the high melting pointing of the fibers that
20
prevented the steel from melting, and thus, the fibers restricted
the spalling of the specimens.
10
3.2 Microstructural studies
0
B-S-P B-S-P S B-S-P P B-S-P H
Room Temperature 37.71 33.7 37.78 38.65
300°C 44.54 53.76 41.3 38.03 The scanning electron microscopy (SEM) images were taken for
600°C 0 40.12 31.23 29.41
TEMPERATURE the mortar mixture with optimum fire resistance performance,
Room Temperature 300°C 600°C .i.e., steel fiber reinforced mortars. The specimens that are
Figure 1: Comparative evaluation of specimen at different temperatures exposed to 300°C and 600°C temperatures were examined

40 THE INDIAN CONCRETE JOURNAL | MARCH 2022