FSMS Concrete1 Material

8/10/2019 FSMS Concrete1 Material

1/60

Reinforced concrete at high temperature

Materials' behavior and structuralimplications


2/60

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

2


3/60


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)


4/60


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


5/60


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

5


6/60


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

6


7/60Fire Safety of Materials and StructuresConcrete

Concrete thermometer 7



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)

8



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



10

NSC, HPC, HSC fc20= 50, 80, 90 MPa

Example of concrete highly-variable thermal

properties - (SCC)



11Example of concrete highly-variable thermal

propertiesLWC/HPLWC

NSC, LWC, HPLWC fc20= 30, 40, 60 MPa



Example of concrete highly-variable thermal

propertiesShotcrete

12

fc20= 15*, 50**, 45** MPa (*,** with/without alkali)



Thermal properties given in EC2 13



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

14


15/60



Temperature profiles for slabs (thickness

h = 200 mm), fire duration 30-240 minutes

16

x is the distance from

the exposed surface



Temperature profiles (

C) for a column

h x b = 300 x 300

17

R30

R90 R120

R60



Position of the 500C isotherm in a square

column

18



Temperature profiles (

C) in a beam

provided with a top slab

19

hxb=150x80- R30 hxb=300x160-R30

f 00C



Position of the 500C isotherm in a beam

provided with a top slab

20


21/60


21Temperature profiles (

C) in a circular column,

diameter = 300 mm

R30

R90 R120

R60

P iti f th 500C i th i i l


22/60


Position of the 500C isotherm in a circular

column

22

T t fil i t b


23/60


Temperature profiles in concrete beams

provided with a top slab (standard fire)

23

St t l l f th th l ti


24/60


24Structural role of the thermal properties

M h i l ti f t t hi h


25/60


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%

25

T i l t t d liti


26/60


Typical test modalities 26

The heating rate (dT/dt) is usually very low (0.5-2.0C/min).

27Mechanical decay


27/60


27Mechanical decay

fc20= 40 MPa (Takeuchi et al.)

28Role of the stress state


28/60


28Role of the stress state

in uniaxial compression

29Comparison between hot and residual


29/60


29Comparison between hot and residual

behaviour (SCC)

30Mechanical decay and thermal properties


30/60


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


31/60


Residual fracture energy 31

32Mechanical decay (from EC2)


32/60


32

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


33/60


33

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)


34/60


34

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


35/60


35

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


36/60


Stress-strain relationships for concrete at

elevated temperatures

36

Example of stress-strain curves 37


37/60


Example of stress strain curves

Alkali-free shotcrete (fc= 45-50 Mpa)

37

Creep in concrete one day after loading at 38


38/60


Creep in concrete one day after loading at

10% of the initial strength

38

Mechanical properties of reinforcing steel 39


39/60


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


40/60


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


41/60


41Steel High temperature behavior

Carbon steel (R/C) High-strength steel (P/C)

42High-temperature behavior


42/60


42High temperature behavior

Stainless steel vs. carbon steel

Fire duration / Temperature (Standard Fire)

43Residual behavior


43/60


43

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

44


44/60


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


45/60


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

46


46/60


Mathematical model for the stress-strain relationship of

reinforcing and prestressing steel at elevated temperature

General overview on structural effects 47


47/60


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


48/60


Some structural effects in tunnels 49


49/60


Tunnel sotto la Manica (1996) Frejus (2005)

San Gottardo (2001) concio prefabbricato

The St. Gotthard Tunnel 50


50/60


Stress state in tunnel linings 51


51/60


Buckling 52


52/60


Pentagon Building, 11.9.2001

Local effects on geometry 53


53/60


Spalling 54


54/60



55/60

Moisture content and spalling sensitivity 56


56/60


(Khoury, 2000)

3%

Role of polipropylene fibers 57


57/60


with fibres

(0.15-0.50%)

without fibres

Role of polypropylene fibers 58


58/60


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


59/60


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


60/60

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)

Documents

FSMS Concrete1 Material