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Article based on contribution by: Mr Saransh Kataria & Mr Satyajeet Harne , Civil engineering Students, S.G.S.I.T.S, Indore
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Introduction
Starting
in the 1980s concretes with increasing compressive strengths started to become commercially available and
primarily utilized in the construction of high-rise buildings. High-strength concrete
(HSC) provides a high level of structural performance, especially in strength
and durability, compared to traditional, normal-strength concrete (NSC). Use of HSC offered economic advantages
because concrete column size could be reduced, thus permitting more usable space. It also had
application in the construction of prestressed girders for bridge construction
and other specialized applications like offshore structures and infrastructure
projects in
which high performance (e.g., low permeability) is required. Today concretes having compressive
strengths up to 140 MPa and above can be produced, with strengths of 172 MPa and above attainable
through use of special fabrication procedures.
Generally,
concrete structural members perform well under fire situations. Studies show,
however, that the performance of HSC differs generally from that of NSC and may
not exhibit good fire performance. Spalling under fire conditions is one of the
major concerns with HSC.
A fire in the English
Channel tunnel in 1996, for example, caused severe damage to tunnel rings owing
to the spalling of concrete and resulted in injuries to eight people and a
property loss of £50 million. The
spalling was attributed to the high strength of the concrete.
What is "Spalling"?
Spalling can be
described as the breaking of layers or pieces of concrete from the surface of a
structural element when it is exposed to the high and rapidly rising
temperatures experienced in fires. Spalling is an
umbrella term, covering different damage phenomena that may occur to a concrete
structure during fire. These phenomena include:
• Violent
Spalling,
• Progressive Gradual Spalling,
• Corner Spalling
• Explosive Spalling,
• Post-Cooling Spalling.
Mechanisms of Spalling
Phenomena of Spalling is caused through different mechanisms of Spalling as given below:
1. Pore
pressure rises due to evaporating water when the temperature rises;
2. Compression of the heated surface due to a thermal gradient in the cross section;
3. Internal cracking due to difference in thermal expansion between aggregate and
cement paste;
4. Cracking due to difference in thermal expansion/deformation between concrete
and reinforcement bars;
5. Strength loss due to chemical transitions during heating.
Different mechanisms (as above) give rise to different types of spalling as tabulated below:-
| . |
Pore Pressure due to evaporation of moisture |
Compression of the heated surface due to a thermal gradient in the cross section |
Internal
cracking due to difference in thermal expansion between aggregate and cement
paste
|
Cracking due to difference in thermal expansion/deformation between concrete
and reinforcement bars |
Strength loss due to chemical transitions during heating |
| Violent Spalling |
x |
x |
x |
. |
. |
| Sloughing off |
. |
. |
x |
. |
x |
| Corner Spalling |
. |
. |
. |
x |
kk |
| Explosive Spalling |
x |
x |
. |
. |
..jggjgj |
| Post-Cooling Spalling |
. |
. |
x |
. |
x |
Why Spalling is a problem in HSC & not in Normal Strength Concrete (NSC)
The conventional theory of explosive
spalling is that it is chiefly caused by the build-up of water vapour pressure
in concrete during fire. If the concrete is not very permeable, water
vapour formed within it during heating will not be able to dissipate and
pressure is formed. When that pressure exceeds the tensile strength of
the concrete, explosive spalling will result.
HSC
is produced primarily through use of a
relatively low water/cementitious ratio and incorporates silica fume.
This leads to a reduced permeability relative to normal weight concretes. Because of the
relatively high permeability of normal strength concrete, when subject to fire,
water vapour is able to readily dissipate through it. Vapour pressure
within the concrete therefore remains within its tensile strength and spalling
is generally avoided. Whereeas HSC is more susceptible to
explosive spalling under fire conditions due to the buildup of pore pressure in the
cement paste (due to its reduced permeability).
The extremely high water
vapour pressure, generated during exposure to fire, cannot escape because of
the high density (and low permeability) of HSC. This pressure often reaches the
saturation vapour pressure, which at 300°C is about 8 MPa. Such internal
pressures are often too high to be resisted by the HSC, which has a tensile
strength of about 5 MPa.
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Factors affecting fire performance & ways to enhance fire performance of High Strength Concrete Structural members
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Factors
affecting Fire performance |
.. |
Ways for enhancing Fire Performance |
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1
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Original
Compressive strength of the concrete
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It can be said that generally concrete strengths higher than 55 MPa are more susceptible to
spalling and may result in lower fire resistance.
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Take appropriate precautions
to prevent spalling when concrete strength exceeds 55 MPa.
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2
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Moisture content of the concrete
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The
moisture content, expressed in terms of relative humidity, influences the
extent of spalling. Higher RH levels lead to greater spalling.
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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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3
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Concrete
Density
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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.
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Use normal-weight aggregate (instead of lightweight aggregate)
to minimize spalling.
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4
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Fire Intensity
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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.
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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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5
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Specimen
Dimensions
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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.
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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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6
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Lateral
Reinforcement
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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.
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(i) Employ both closer tie
spacing and cross ties to improve fire resistance.
(ii) 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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7
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Load Intensity
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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.
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Consider the
probable occurrence of spalling appropriately in the design
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8
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Type of
Aggregate
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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
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Use carbonate aggregate
(instead of siliceous aggregate) to reduce spalling.
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9
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Fibre
Reinforcement
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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.
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(i) Add polypropylene fibres to
the mix to reduce spalling.
(ii) Add steel fibres to enhance
tensile strength and reduce spalling.
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Some other ways to enhance fire performance are:-
Spray
coating of finished concrete with a substance that slows down the rate of heat
transfer from fire. It is the rate of temperature change in the concrete
that has been proven to be at least as important a cause of spalling as the
ongoing exposure to high temperature itself.
Placement of a preformed thermal barrier over the concrete surface- a method sometimes
used in tunnel construction.
A relatively new concept to
counter the spalling threat is to provide vents in the concrete to alleviate
pore pressure.
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References:
1. "Fire Performance of
High-Strength Concrete Structural Members" by V.K.R.Kodur
2. "The effect of fire-Concrete Spalling" from http://www.promat-tunnel.com
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