|
1
|
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.
|
|
| 2 |
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.
|
|
|
3
|
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.
|
|
|
4
|
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.
|
|
|
5
|
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.
|
|
|
6
|
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.
|
|
|
|
|
|
|
.
|
Tie configuration for reinforced concrete column
Conventional tie configuration (shown on left); Modified tie
configuration (shown on right)
|
|
| 7 |
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 |
|
|
8
|
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.
|
|
|
9
|
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.
|
|
