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Transcript of FSMS Concrete1 Material
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Reinforced concrete at high temperature
Materials' behavior and structuralimplications
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Fire Safety of Materials and StructuresConcrete
Outline
Concrete microstructure
Thermal properties of steel and concrete as a function of thetemperature
Thermal analysis of reinforced-concrete and prestressed-concrete
sections
Mechanical properties of concrete and steel as a function of the
temperature
(Simplified calculation methods)
(Evaluation of the bearing capacity)
Concise overview on structural effects
Concluding remarks
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Fire Safety of Materials and StructuresConcrete
Concrete microstructure 3
fresh concrete
hardenedconcrete
water/cement 0.65 0.45 0.25
unhydrated cement
grain
ettringite
(sulfoalluminated calcium)pore calcium hydroxide
Ca(OH)2
hydrated cement
grain
(from Atcin e Neville, 1993)
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Fire Safety of Materials and StructuresConcrete
Concrete microstructure 4
Componenti del clinker
Tricalcium silicate (alite, C3S) 3CaO-SiO2 30-70%
Bicalcium silicate (belite, C2S) 2CaO-SiO2 10-50%
Tricalcium alluminate (C3A) 3CaO-Al2O3 7-15%
Tetracalcium ferroalluminate (C3AF) 4CaO-Al2O3-Fe2O3 6-20%
Transition zone in concrete
(a) Fresh concrete without silica fume
(b) Mature concrete without silica fume
(c) Fresh concrete with silica fume
(d) Mature concrete with silica fume
(pc) Portland cement grain
(sf) Silica fume particle
(CH) Calcium hydroxide = Ca(OH)2(CSH) Calcium silica hydrate gel
(ett) Ettringite (calcium sulphoaluminate)
(agg) Aggregate particle
a b c d
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Fire Safety of Materials and StructuresConcrete
Concrete microstructure
Up to 20% of the cement paste volume is occupied by pores:
nanopores(pores in the gel) contain adsorbed water, i.e. water thatis chemically bound (d 50 nmm)
microporescontain free water (d 500 nm)
Above 100C, the free water tends to become vapour, thus
producing vapour pressure in the micropores Above 150-200C, the adsorbed water tends to dissociate, thus
enhancing pore pressure
Above 450C, the portlandite Ca(OH)2dissociates into calcium oxide
CaO and water H2
O
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Fire Safety of Materials and StructuresConcrete
Above 700C, calcium carbonate CaCO3 dissociates into calcium
oxide CaO and carbon dioxide CO2, thus starting concretebreakdown (calcination)
Above 500C, the cement paste is almost completely dehydrated;
aggregates (particularly siliceous aggregates) have their
crystallization water expelled, and start breaking down
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Concrete thermometer 7
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Thermal properties - Concrete
Good insulation properties, incombustibleand stable (T 400-450C)
but sensitive to: high temperature(T 500C)stress state (preloading)
thermal gradients(T/t ; T/x)
spalling (explosive/gradual/local/extended)
strength increase by using optimized mixes
Mix-design optimization:
aggregate (siliceous < mixed < calcareous < light < basalt)
binder (cement, microsilica, fly ash)
water content and water/binder ratio (w/c ; w/b)
added materials (calcareous powders)
fibers (metallic, polymeric, inorganic, hybrid)
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9Comparison between concrete and other
building materials
R/C and P/C structural members behave well at high
temperature, because of concrete very good insulation properties:at 20C/800C
thermal conductivity concrete 1.0-2.0/0.5-0.85 W/mK
steel 54/27 W/mK
timber 0.12/0.18 W/mK
specific heat c concrete 900/1250 J/kgK
steel 425-650 J/kgK
timber 1500/750 J/kgK
thermal diffusivity D concrete 0.3-0.8/0.3-0.4 mm2/s
steel 17.0/5.5 mm2/s
timber 0.05-0.25/0.15-0.30 mm2/s
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NSC, HPC, HSC fc20= 50, 80, 90 MPa
Example of concrete highly-variable thermal
properties - (SCC)
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11Example of concrete highly-variable thermal
propertiesLWC/HPLWC
NSC, LWC, HPLWC fc20= 30, 40, 60 MPa
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Example of concrete highly-variable thermal
propertiesShotcrete
12
fc20= 15*, 50**, 45** MPa (*,** with/without alkali)
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Thermal properties given in EC2 13
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Thermal analysis of R/C and P/C sections
Inside a concrete member, heat transfer is by pure conduction,
controlled by Fouriersequation The reinforcement (ordinary or pre-/post-tensioned) is IGNORED,
because (a) of the low steel ratio, and (b) of steel very high diffusivity
(a reinforcing bar very quickly reaches thermal equilibrium with the
surrounding concrete) Ts,i= Tc(xi,yi,zi)
Boundary conditions are generally based on heat convection and
radiation, should a temperature-time curve be assumed, but they can
also be expressed in terms of heat flux
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Temperature profiles for slabs (thickness
h = 200 mm), fire duration 30-240 minutes
16
x is the distance from
the exposed surface
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Temperature profiles (
C) for a column
h x b = 300 x 300
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R30
R90 R120
R60
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Position of the 500C isotherm in a square
column
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Temperature profiles (
C) in a beam
provided with a top slab
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hxb=150x80- R30 hxb=300x160-R30
f 00C
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Position of the 500C isotherm in a beam
provided with a top slab
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Fire Safety of Materials and StructuresConcrete
21Temperature profiles (
C) in a circular column,
diameter = 300 mm
R30
R90 R120
R60
P iti f th 500C i th i i l
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Position of the 500C isotherm in a circular
column
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T t fil i t b
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Fire Safety of Materials and StructuresConcrete
Temperature profiles in concrete beams
provided with a top slab (standard fire)
23
St t l l f th th l ti
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Fire Safety of Materials and StructuresConcrete
24Structural role of the thermal properties
M h i l ti f t t hi h
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Fire Safety of Materials and StructuresConcrete
Mechanical properties of concrete at high
temperature (400-600C)
The following data are typical of ordinary concrete, whose specimens
were heated without pre-load(unstressed specimens):
Compressive strength: fc600/fc
20 = 70-30%
Tensile strength: fct600/fct
20 = 20%
Modulus of elasticity: Ec600/Ec
20= 15%
Poissons coefficient: c400/c
20 = 200%
Fracture energy: Gf400/Gf
20= 125-133%
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T i l t t d liti
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Fire Safety of Materials and StructuresConcrete
Typical test modalities 26
The heating rate (dT/dt) is usually very low (0.5-2.0C/min).
27Mechanical decay
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Fire Safety of Materials and StructuresConcrete
27Mechanical decay
fc20= 40 MPa (Takeuchi et al.)
28Role of the stress state
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Fire Safety of Materials and StructuresConcrete
28Role of the stress state
in uniaxial compression
29Comparison between hot and residual
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Fire Safety of Materials and StructuresConcrete
29Comparison between hot and residual
behaviour (SCC)
30Mechanical decay and thermal properties
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Fire Safety of Materials and StructuresConcrete
30Mechanical decay and thermal properties
fc= 45 MPa, Basaltico, di miscela
fc= 65-70 MPa
Basaltico, Portland
fc= 45 MPaSiliceo, Portland
fc= 65-70 MPa
Calcareo, Portland
0.10
0.20
0.30
0.40
0.50
0.60
0.00 0.20 0.40 0.60 0.80 1.00
Dc[mm2/s] (mean values 200-600C)
fc
600/
fc
20
Siliceous-blended Siliceous-blast
Siliceous-portland Calcareous-blended
Calcareous-blast Calcareous-portland
Basalt-blended Basalt-blast
Basalt-portland
94
91
92
93
106
95
96
97
98
110
105
108
116
109
120
increasing insulation efficacy
increasing
heat sensitivity
Mean diffusivity (200-600C)
Residual fracture energy 31
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Fire Safety of Materials and StructuresConcrete
Residual fracture energy 31
32Mechanical decay (from EC2)
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Concrete temp. (
) 20 100 200 300 400 500 600 700 800 900100
01100 1200
Siliceous aggregates 1.00 1.00 0.95 0.85 0.75 0.60 0.45 0.30 0.15 0.08 0.04 0.01 0.00
Calcareous aggregates 1.00 1.00 0.97 0.91 0.85 0.74 0.60 0.43 0.27 0.15 0.06 0.02 0.00
c ( )k
c ( )k
Mechanical decay (from EC2)
Compressive strength
33Typical design values for the compressive
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c,T
c,T
1.0
910 / 560
k
k T
c,T
c,T
1.0
1000 / 500
k
k T
350 C
350 C
T
T
500 C
500 C
T
T
For normal weight concrete: For lightweight concrete:
Typical design values for the compressive
strength
34Mechanical decay (from EC2)
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for 20100
for 100600
c,t 1.0k
c,t 1.0 1.0 100 / 500k
ck,t c,t ck,t( ) ( )f k f
Mechanical decay (from EC2)
Tensile strength
35Typical design values for the modulus of
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Fire Safety of Materials and StructuresConcrete
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E,T
E,T
1.0
700 / 550
k
k T
150 C
150 C
T
T
Typical design values for the modulus of
elasticity
Stress-strain relationships for concrete at 36
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Fire Safety of Materials and StructuresConcrete
Stress-strain relationships for concrete at
elevated temperatures
36
Example of stress-strain curves 37
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Fire Safety of Materials and StructuresConcrete
Example of stress strain curves
Alkali-free shotcrete (fc= 45-50 Mpa)
37
Creep in concrete one day after loading at 38
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Fire Safety of Materials and StructuresConcrete
Creep in concrete one day after loading at
10% of the initial strength
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Mechanical properties of reinforcing steel 39
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Fire Safety of Materials and StructuresConcrete
Mechanical properties of reinforcing steel
Hot-rolled steel
Elastic-plastic-hardening behaviour up to 350C
Elastic-hardening behaviour beyond 400C
(Almost) Elastic-plastic behaviour beyond 600C
Below 400-500Cthe ultimate strength is ft20fy
20
At 600Cthe ultimate strength is ft20
39
Mechanical properties of reinforcing steel 40
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Fire Safety of Materials and StructuresConcrete
Mechanical properties of reinforcing steel
Cold-worked steel
Elastic-hardening-softening behaviour up to 200C
(Almost) Elastic-plastic behaviour beyond 200C
At 500Cthe ultimate strength ft500 is1/3ft
20
At 600Cthe ultimate strength ft600 is1/6ft
20
40
41Steel High-temperature behavior
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Fire Safety of Materials and StructuresConcrete
41Steel High temperature behavior
Carbon steel (R/C) High-strength steel (P/C)
42High-temperature behavior
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Fire Safety of Materials and StructuresConcrete
42High temperature behavior
Stainless steel vs. carbon steel
Fire duration / Temperature (Standard Fire)
43Residual behavior
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Fire Safety of Materials and StructuresConcrete
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Stainless steel
Tempcore/Termex and high-strength steel
0
0.2
0.4
0.6
0.8
1
1.2
0 100 200 300 400 500 600 700 800 900
Temperature (C)
fy
T/f
y20
12 Inox Cold-Drawn fy = 666 MPa
24 Inox Hot-Rolled fy = 494 MPa
24 Tempcore fy = 496 MPa
12 Tempcore fy = 519 MPa
0.5'' Strand fy = 1730 MPa High-Bond Bars
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Fire Safety of Materials and StructuresConcrete
Coefficient ks() to be applied to the characteristic strength (fyk)
of tension and compression reinforcement (Class N)
45C ffi i t k () t b li d t th h t i ti
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Fire Safety of Materials and StructuresConcrete
Coefficient kp() to be applied to the characteristic
strength (fpk) of prestressing steel
Steel temp. () 20 100 200 300 400 500 600 700 800 900100
01100 1200
Cold worked Class A 1.00 1.00 0.87 0.70 0.50 0.30 0.14 0.06 0.04 0.02 0.00 0.00 0.00
Cold worked Class B 1.00 0.99 0.87 0.72 0.46 0.22 0.10 0.08 0.05 0.03 0.00 0.00 0.00
Quenched & tempered 1.00 0.98 0.92 0.86 0.69 0.26 0.21 0.15 0.09 0.04 0.00 0.00 0.00
p ( )k
p ( )k
p( )k
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Fire Safety of Materials and StructuresConcrete
Mathematical model for the stress-strain relationship of
reinforcing and prestressing steel at elevated temperature
General overview on structural effects 47
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Fire Safety of Materials and StructuresConcrete
In R/C and P/C structures : (1) restrained thermal elongation
(2) materials mechanical decay
(3) geometry reductions (spalling)
In steel structures : (1) steel mechanical decay
(2) increasing deformability and buckling
phenomena (because of decreasing
elastic modulus with the temperature)
In timber structures : (1) geometry reductions because of timber
charring (charring rate 0.5-0.9
mm/min; temperature-damaged sub-
layer 35-40 mm)
Main structural effects in concrete members 48
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Fire Safety of Materials and StructuresConcrete
Some structural effects in tunnels 49
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Tunnel sotto la Manica (1996) Frejus (2005)
San Gottardo (2001) concio prefabbricato
The St. Gotthard Tunnel 50
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Fire Safety of Materials and StructuresConcrete
Stress state in tunnel linings 51
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Buckling 52
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Pentagon Building, 11.9.2001
Local effects on geometry 53
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Spalling 54
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Moisture content and spalling sensitivity 56
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Fire Safety of Materials and StructuresConcrete
(Khoury, 2000)
3%
Role of polipropylene fibers 57
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Fire Safety of Materials and StructuresConcrete
with fibres
(0.15-0.50%)
without fibres
Role of polypropylene fibers 58
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Fire Safety of Materials and StructuresConcrete
An explanation on why they reduce spalling
Concrete spalling with load- and
heat-induced stresses, and pore
pressure
Effect of pp fibers according
to Jansson and Bostrm
(2008)
Concluding Comments - Concrete 59
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Fire Safety of Materials and StructuresConcrete
Concrete is a rather heat-tolerant material (500C), with very good
insulation properties, to the advantage of bar protection
Cracking and buckling are favored at high temperature, because of the loss
affecting (less) the tensile strength and (more) the elastic modulus
Because of its low thermal conductivity and high stiffness, concrete is rather
sensitive to thermal self-stresses, that may contribute to cover spalling
Because of concrete composite nature, spalling is favored by both moisturevaporization (with pressure peaks in the pores) and kinematic incompatibility
between the coarse aggregates and the cementitious mortar
Polypropylene fibers markedly reduce concrete spalling, even for rather low
fibers contents
Basalt aggregates give concrete superior resistance to high temperature andlower thermal conductivity
Light-weight aggregates improve concrete properties at HT, because they
reduce the hollow-aggregate/mortar kinematic incompatribility
ConcludingcommentsR/C structures 60
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Concrete structures are rather stable at high temperature (thanks to
their intrinsic stiffness), but they are sensitive to axial restraints, that
may improve the behavior of heated members (because of extra
compression), while worsening the behavior of the contiguous
members (because of extra shear and bending)
Generally, a properly-designed reinforcement against seismic actions
is also effective in fire, and vice-versa Evaluating the maximum temperature reached by a fire-affected R/C
structure is badly needed (and not easily done!), should the structure
survive the fire, as often occurs in R/C constructions, contrary to what
occurs in either timber or steel constructions
The static redundancy of R/C structures improves structuralrobustness (thanks to load redistribution in fire) but allows the static
effect of fire to propagate up to the members further from the
compartment in fire (thermal elongation of the heated members)