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TECHNICAL PAPER

nUmERICAL APPRoACH

to EvALUAtE ELEvAtED

tEmPERAtURE

BEHAvIoR oF stEEL

FIBER REInFoRCED

ConCREtE CoLUmns PRATIK BhATT, VEnKATESh KODuR,

VASAnT MATSAgAR

Abstract fire exposure, cannot escape due to the high density (and low
permeability) of the HSC. When effective pore pressure exceeds
A finite element (FE)-based numerical model is developed in the tensile strength of concrete, chunks of concrete fall-off,
ABAQUS , to evaluate the response of steel fiber reinforced often in an explosive manner, from the concrete structural
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concrete (SFRC) columns under combined effects of fire and member [4, 5] . To mitigate such fire induced spalling in the HSC
structural loading. The model utilizes a sequentially coupled structural members, researchers have recommended the use of
thermo-mechanical analysis procedure to trace the fire response polypropylene and /or steel fibers in concrete [6-7] .
of concrete columns. The FE model accounts for temperature-
dependent properties of the SFRC and reinforcing steel as well The polypropylene fibers create randomly oriented micro-
as the material and geometrical nonlinearities. Predictions from and macro channels inside concrete by melting at relatively
the numerical model including temperature and axial deflection low temperature (about 160-170°C). These channels facilitate
are compared with the data measured in fire tests to establish dissipation of the vapor pressure, which in-turn reduces the
the validity of the model in predicting overall fire response of spalling. In case of steel fiber reinforced concrete (SFRC), the
the SFRC columns.
presence of steel fibers reduces the initiation and progression
of the micro-cracks which in-turn enhances the tensile and
Keywords: Elevated temperature, Fire resistance, Reinforced
concrete columns, Steel fiber reinforced concrete. flexural strengths of the HSC (both at ambient and elevated
temperatures). The increased tensile strength resists the pore
pressure generated from evaporation of water in concrete,
1. iNtRoDUctioN
thereby, minimizing the chances of explosive spalling and
Provision of fire resistance to structural members is one of enhancing the fire resistance of the HSC structural members.
the major safety requirements in design of buildings where The beneficial effects of the SFRC in enhancing fire resistance
reinforced concrete (RC) is primary choice of material for and minimizing spalling has been confirmed through high
construction. Typically, conventional normal strength concrete temperature material property tests on the SFRC cylinders as
(NSC) members exhibit satisfactory fire resistance due to the well as through limited fire tests on the SFRC columns [6, 8] .
high thermal inertia and slow degradation in strength and
stiffness properties of concrete with temperature [1, 2] . However, The addition of polypropylene fibers has minimal effect on the
high strength concrete (HSC) structural members exhibit poor strength and stiffness properties of concrete. However, addition
fire resistance due to rapid degradation in strength and stiffness of steel fibers increases the compressive, tensile, and flexural
properties of the HSC with temperature . Additionally, the strengths of concrete by 20-40 % [9, 10] . Further, it improves
[3]
HSC has low permeability as compared to the NSC, and thus resistance to material deterioration resulting from fatigue,
the HSC members are more susceptible to spalling when impact, shrinkage, and thermal stresses [11, 12] . The addition of
exposed to fire; thereby reducing the fire resistance [4, 5] . Spalling steel fibers increases the ductility of the beam-column joint in
is break-up of chunks of (or pieces) concrete from surface of reinforced concrete (RC) buildings, which is highly beneficial
fire-exposed concrete member and is primarily attributed to in earthquake-prone regions [12] . Owing to these advantages,
build-up of pore pressure during heating. The extremely high the SFRC is often used for construction of columns in the RC
water vapor pressure in concrete section, generated during buildings and built infrastructure. Since the columns form the

8 The IndIan ConCreTe Journal | auGuST 2019