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Original
Compressive strength of the concrete: It
can be said that generally concrete strengths higher than 55 MPa are
more susceptible to spalling and may result in lower fire resistance.
Ways for enhancing Fire Performance:Take
appropriate precautions to prevent spalling when concrete strength
exceeds 55 MPa.
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Moisture content of the concrete:
The
moisture content, expressed in terms of relative humidity, influences
the extent of spalling. Higher RH levels lead to greater spalling.
Ways for
enhancing Fire Performance:
Fire resistance tests on full scale HSC columns have shown that
significant spalling occurs when the RH is higher than 80%. The time
required to attain an acceptable RH level (below 75%) in HSC structural
members is longer than that required for NSC structural members because
of the low permeability of HSC. In some cases, such as in offshore
structures, RH levels can remain high throughout the life of the
structure and should therefore be accounted for in the design.
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Concrete
Density: The
extent of spalling has been found to be much greater when lightweight
aggregate is used. This is mainly because the lightweight aggregate
contains more free moisture, which creates higher vapour pressure under
fire exposures.
Ways for
enhancing Fire Performance:Use
normal-weight aggregate (instead of lightweight aggregate) to minimize
spalling.
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Fire Intensity: The
spalling of HSC is much more severe in fires characterized by fast
heating rates or high fire intensities. Hydrocarbon fires pose a severe
threat in this regard.
Ways for
enhancing Fire Performance:When
HSC is to be used in facilities where hydrocarbon fuels are present,
such as offshore drilling structures and highway tunnels, the probable
occurrence of spalling should be considered in the design.
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Specimen Dimensions:The
risk of explosive thermal spalling increases with specimen size. This
is due to the fact that specimen size is directly related to heat and
moisture transport through the structure, as well as the capacity of
larger structures to store more energy.
Ways for
enhancing Fire Performance:
Careful consideration must be given to the size of specimens when
evaluating the spalling problem; fire tests are often conducted on
small-scale specimens, which can give misleading results.
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Lateral Reinforcement: The
spacing and configuration of ties both have a significant effect on the
performance of HSC columns. Both closer tie spacing (at 0.75 times that
required for NSC columns) and the bending of ties at 135° back into
the core of the column, as illustrated in Figure below, enhance fire
performance. The provision of cross ties also improves fire resistance.
Ways for
enhancing Fire Performance:
- Employ
both closer tie spacing and cross ties to improve fire resistance.
- Install
bent ties (at 135° back into the concrete core) instead of straight
ties.
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Tie
configuration for reinforced concrete column
Conventional tie configuration (shown on
left); Modified tie configuration (shown on right)
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Load Intensity: A
loaded HSC structural member will spall to a greater degree than an
unloaded member. The load adds to the stresses created by the pore
pressure generated by steam. Also, a higher load intensity leads to
lower fire resistance, since the loss of strength with a rise in
temperature is greater for HSC than for NSC.
Ways for enhancing Fire Performance: Consider
the probable occurrence of spalling appropriately in the design |
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Type
of Aggregate: Of
the two commonly used aggregates, carbonate aggregate (predominantly
limestone) provides higher fire resistance and better spalling
resistance in concrete than does siliceous aggregate (predominantly
quartz). This is mainly because carbonate aggregate has a substantially
higher heat capacity ( specific heat), which is beneficial in
preventing spalling. This increase in specific heat is likely caused by
the dissociation of the dolomite in the carbonate concrete.
Ways for enhancing Fire Performance: Use
carbonate aggregate (instead of siliceous aggregate) to reduce spalling.
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Fibre
Reinforcement: The
addition of polypropylene fibres minimizes spalling in HSC members
under fire conditions. This approach works on the basis that, as the
concrete is heated by fire, the fibres melt, creating passageways
along which water vapour can dissipate, so avoiding a build-up of
pressure.
Studies have shown that the amount of polypropylene fibres needed
to minimize spalling is about 0.1 to 0.25% (by volume).
Steel fibres also reduce spalling in HSC and improve fire resistance.
The fibres enhance the tensile strength of concrete, even at high
temperatures, and help to withstand the pore pressure generated due to
vapourization of water under fire exposure. With these fibres the
tensile strength increases to between 5 and 7 MPa, which in many cases
may be sufficient to achieve two to three hours of fire resistance
without significant spalling.
Ways for enhancing Fire Performance:
- Add
polypropylene fibres to the mix to reduce spalling.
- Add
steel fibres to enhance tensile strength and reduce spalling.
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