Page 8 - Open-Access-August-2019

TECHNICAL PAPER

2.2. Mix Proportioning with the help of four in-built thermocouples. The heating rate
of the furnace was set at 6ºC/minute. The surface temperature
In this study, concrete of M30 grade was proportioned in of specimens was monitored with the help of ‘type-K’
[35]
accordance with the IS: 10262 . A number of trial mixes were thermocouples, which was placed on the surface and connected
prepared to arrive at an appropriate proportion of different to a digital data logger. The precision of thermocouple used
ingredients that can fulfil the desired objectives in terms of was 0.5ºC. The thermocouples connections were made through
strength and workability. The mix proportion is given in Table 2. the provisions made in furnace door. The similar studies on
The mix designations with different fibre dosages are given in slow heating rate to understand the characteristics of concrete
Table 3. The sequence of adding fibres in the concrete mix was exposed to different temperatures were also carried out [41-45] .
[36]
followed as per ACI 544 . The optimized value of PP fibres, The heating rates at the surface of the specimen was around
V f = 0.25% (2.275 kg/m ) was used based on the experimental 0.5ºC/min. Temperature on the surface of concrete specimens
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investigations carried out in CSIR-CBRI [37, 38] .
took more time to reach the desired exposure temperature due
to loss of moisture on the surface of specimen and total mass
table 2: Mix proportion of M30 grade concrete
in the furnace. The specimens kept in the furnace for a period
CEmEnt FInE CoARsE WAtER sUPER of two hours at set exposure temperature. The specimens were
(kg/m ) AGGREGAtE AGGREGAtE (kg/m ) PLAstICIzER allowed to cool within the furnace at ambient temperature. A
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3
3
(kg/m ) (kg/m ) DosAGE typical heating and cooling phase of the specimens exposed
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10 mm 20 mm (kg/m ) to 800ºC is shown in Figure 2. Temperatures were recorded at
380 720 729.6 486 171 1.9 10 minutes intervals for all the specimens using data logger. In
heating phase, the surface temperature of the specimen was set
table 3: Designations of concrete mixes at 800ºC and constant heating rate was applied. It was observed
that the surface attained the set temperature of 800ºC in about
ConCREtE BAsALt FIBRE PoLYPRoPYLEnE 740 minutes. The surface temperature was maintained for two
sPECImEn %v f (kg/m ) FIBRE
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DEsIGnAtIons %v f (kg/m ) hours in the range of 800 ± 10ºC in the furnace. In the cooling
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phase, the specimens were allowed to cool down to room
Control 0.00 (0.00) 0.00 (0.00)
temperature within the furnace by opening the door of furnace.
BFRC 1 0.50 (13.0) 0.00 (0.00) Specimens were tested for their standard reference compressive
BFRC 2 0.75 (19.5) 0.00 (0.00) strength and residual compressive strength after exposure to
[46]
different temperatures as per IS 516 . The cylindrical strength
BFRC 3 1.00 (26.0) 0.00 (0.00)
is a factor of 0.8 to the cube strength.
HyFRC 1 0.50 (13.0) 0.25 (2.275)
HyFRC 2 0.75 (19.5) 0.25 (2.275)
@ 1000ºC
HyFRC 3 1.00 (26.0) 0.25 (2.275)
2.3. Mixing, casting and curing
A mixer of 0.15 m capacity was used for mixing the concrete
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ingredients. The cube of size 100 x 100 x 100 mm and cylinder
of size 100 x 200 mm and prism of size 100 x 100 x 500 mm were Figure 1: Concrete specimens exposed to elevated temperature in
cast to evaluate compressive, splitting tensile, flexural, residual muffle furnace.
compressive strength and residual elastic modulus. The fibres
were added in dry mix condition. The hardened properties of all 2.5. Fresh and Hardened Properties
concrete specimens were tested at the age of 28 days.
The workability of fresh concrete was measured by Vee-Bee
consistometer as per IS 1199 . The hardened properties of
[47]
2.4. exposure to elevated temperatures the all concrete specimens were tested at the age of 28 days
Concrete cube specimens of 100 mm were exposed to elevated as per IS 516 . The air content in the fibre concrete was
[46]
temperatures of 200, 400, 600, 800 and 1000ºC in a muffle determined as per ASTM C231 . Compressive Strength,
[48]
furnace as shown in Figure 1. The test was carried out for tensile strength, flexural strength and elastic modulus of the
‘accident conditions’ as stated in RILEM TC 129-MHT and specimens were tested in saturated surface dry conditions at
[39]
TC 200-HTC , which normally involves short-term exposure ambient temperature using universal testing machine (UTM)
[40]
to temperatures in the range 20-750ºC or above and transient of 1000 kN capacity. Elastic moduli of the concrete specimens
moisture states, i.e., the concrete is allowed to dry during were evaluated by taking slope of stress-strain curve at 40% of
[40]
heating . The temperature of furnace was monitored precisely maximum stress under compression.

18 The IndIan ConCreTe Journal | auGuST 2019