Assessment of the residual strength of concrete structures

Magnel Laboratory for Concrete Research – Department of Structural Engineering

Assessment of the residual strength of concrete structures after fire exposure

Luc R. Taerwe, Emmanuel Annerel

PhD E. Annerel – 26/03/2010

Magnel Laboratory for Concrete Research 2

Magnel Laboratory for Concrete Research

Temperature θ?

fy(θ)

fc(θ)

M = As.fy(θ).z

Assessment techniques

Physico-chemical alterations

Residual & post-cooling strength

3


fy(θ)

fc(θ)



M = As.fy(θ).z

Temperature θ?



Heating of the cement matrix

Wei

ght l

oss

[%]

Tem

pera

ture

diff

eren

ce [°

C]

T e m p e r a tu r e [ ° C ]2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0

- 3 0

- 2 0

- 1 0

0

- 7

- 6

- 5

- 4

- 3

- 2

- 1

0

Evaporation free water Dissociation of ettringite

Start breakdown of CSH

Decompositon of portlandite :

Ca(OH)2 CaO + H2O

Decarbonation:

CaCO3 CaO + CO2

SCCDecompositon of CSH

Differential thermal + thermogravimetric analysis

5


Heating of the cement matrix

A

A A

CH

CSHettrettr

CH

CSH*

CSH*

20°C 350°C 550°C

20°C

>450°C>100°C

20°C 20°C

UVFLUVFL

more pores more pores and cracksand cracks

PPTLPPTLCPLCPL

CH↓

6


Post-cooling behaviour

•During heating:

700°C : CaCO3 CaO + CO2

450°C : Ca(OH)2 CaO + H2O

CaO + H2O Ca(OH)2

CaO + CO2 CaCO3

•Post-cooling storage:

(re)hydration CSH, CH

~> strength loss

~> corrosion steel

~> strength recovery

7


Post-cooling behaviour

0

5

10

15

20

25

30

35

40

0 100 200 300 400 500 600 700

Temperatuur [°C]

Car

bona

tion

dept

h [m

m]

TC1-8_27 months air storage

TC2k-1_27 months air storage

If not removed coating with CO2 inhibitor

Strength loss Strength recovery

8


Aggregates

9

Refractory aggregate

Lightweight aggregate

Dolomitic limestone

Siliceous limestone

Calcareous limestone

Quartz

200 400 600 800 1000 1200 1400 1600

Stable

Phase change

Decarbonation

Large expansion

Contraction

Temperature [°C]Aggregate type

Calcareous CaCO3

Siliceous SiO2

α-β phase transformation: 5.7% volume increase

20°C 800°C

β-quartz cristobalite

oxidation of Fe hydroxidesdisconnection disconnection

of sand particlesof sand particles


Interfacial debonding

20°C 300°C 700°C 1160°C

overview (35x)overview (35x)transition zone (210x)transition zone (210x)matrix (210x)matrix (210x)

10


Interfacial bonding

Interfacial transition zone = more porous = weaker cement paste

aggregate

Thermal incompatibility

Interfacial cracking

Strength lossStrength lossGraph: Kordina

11


fy(θ)

fc(θ)


M = As.fy(θ).z

Temperature θ?




fy(θ)

fc(θ)




M = As.fy(θ).z

Temperature θ?



14


Schmidt Rebound Hammer

15

0

20

40

60

80

100

120

0 100 200 300 400 500 600Temperature [°C]

Rel

ativ

e R

ebou

nd In

dex

[%]

TC1-12_fccub150_0d TC1-8_0d

TC1-8_28d_water TC1-8_28d_air

TC1-8_824d_water TC1-8_824d_air

Storage in air Storage in water


Ultrasonic Pulse Velocity

y = 8,8382e0,0009x

R2 = 0,7846

y = 7,9494e0,0008x

R2 = 0,9542

y = 11,421e0,0007x

R2 = 0,9098

0

10

20

30

40

50

60

70

0 500 1000 1500 2000 2500 3000Pulse velocity [m/s]

Com

pres

sive

str

engt

h fc

cil [

N/m

m²]

TC -- 20°C TC -- 175°CTC -- 300°CTC -- 520°CTCk -- 20°CTCk -- 520°CSCC -- 20°CSCC -- 205°CSCC -- 315°CSCC -- 530°C

Vbc eaf ⋅⋅=

16


Image analysis

20°C 300°C 700°C 1160°C

Surface blackened; pores and cracks filled with white BaSO4 powder

Flatbed scanner: pore size ≥ 50 µm

17


Image analysis

Interfacial cracks

18

0

5

10

15

20

25

30

35

40

45

0 200 400 600 800Temperature [°C]

Tota

l por

osity

[%]

SCCTCGuise

0

0,2

0,4

0,6

0,8

1

1,2

1,4

1,6

10 100 1000 10000

Pore diameter [µm]

Poro

sity

[%]

20°C

150°C

300°C

500°C

Cement matrix cracks

Onset of strength lossOnset of strength loss

voidsvoids

550°C

crackscracks

SCC: better interfacial bond and less coarse aggregates

increase in porosity: spread over all measuring classes

Interfacial and cement matrix cracks increase at same rate


-14

-12

-10

-8

-6

-4

-2

00 100 200 300 400 500 600 700 800 900

[M(T

)-M(1

20°C

)]/M

(120

°C) [

%]

TC1-8 -- 0dTC1-10 -- 0dTC1-12 -- 0dTC2k-1 -- 0dTC2k-4 -- 0dTC2k-5 -- 0dSCC1-5 -- 0dHPC1-5 -- 0d

Decarbonation

4,5

5

5,5

6

6,5

7

7,5

8

8,5

0 100 200 300 400 500 600 700Temperature [°C]

100*

[M(T

,xd)

-M(R

ef)]/

M(R

ef)

TC1-10 -- waterTC1-10 -- L, ovenTC1-10 -- L, 20°CTC2k-1 -- L, ovenTC1-8 -- L, oven

Water immersion

Weight increase due to water absorption

R² = 0,93

Concrete specimen is the reference!

Weight loss during heating


Colorimetry

950°C1150°C

250°C

20°C

250°C 450°C

700°C20°C

20°C

700°C

1150°C

>700>700°°CC

SiO2 + Al, K

SiO2 + CaCO3

CaCO3

SiO2 + Mg

SiO2

Red

YellowYellow

20


Colorimetry

21

-0,4

-0,2

0

0,2

0,4

0,6

0,8

1

-6 -4 -2 0 2 4 6

[a(T)-a(120°C)]/a(120°C)

[b(T

)-b(1

20°C

)]/b(

120°

C)

TC1-8

TC1-12

TC2k-4

TC2k-5

SCC1-5

HPC1-5

200°C

400°C

300°C

500°C

600°C

700°C

800°C

Red

Yellow

elimination of moisture influence → transformation: shift + scaling


Colorimetry

20°C 300°C 700°C 1160°C

400°C

350°C

300°C

600°C700°C

800°C

1000°C

500°C

150°C

200°C250°C

150°C

200°C

250°C

300°C

350°C400°C

500°C

600°C

700°C

800°C

4

5

6

7

8

9

10

0 0,5 1 1,5 2 2,5 3

a* [-]

b* [-

]

HPCSCC

300-400°C

500-600°C

700-800°C

Red

Yellow

22


Colorimetry Influence of moisture content

600°C500°C

400°C

300°C200°C

120°C

20°C

600°C 500°C

400°C

300°C

200°C

120°C20°C

5

5,5

6

6,5

7

7,5

8

8,5

9

9,5

10

0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8

a* [-]

b* [-

]

0d_Surface 7d_Surface14d_Surface 28d_Surface56d_Surface 90d_Surface

Red

Yellow

23


Visual inspection + Schmidt Rebound

Index

RIT/RI20°C> 85(90)%

40% < RIT/RI20°C < 85(90)%

Core drilling

Colour measurement

Stereomicroscope

Reinforcementdetection

Water immersion

RIT/RI20°C< 40%


fy(θ)

fc(θ)


M = As.fy(θ).z

Temperature θ?






fy(θ)

fc(θ)Residual & post-cooling strength

M = As.fy(θ).z

Temperature θ?

26


Influencing parameters

DEVELOPMENT OF PORE STRUCTURE AND CONCRETE STRENGTHconcrete (aggregates, W/C, cement, porosity),H2O (rain, household moisture production) and CO2

FIRE:Heating rateTarget temperatureDuration of fireExternal load

EXTINGUISHING: water! additional strength loss

VISUAL INSPECTION & SAMPLING

HYDRATION REHYDRATION

Sealed storage

FIRST WEEKS OF ADDITIONAL STRENGTH LOSSTHEN POSSIBLE STRENGTH RECOVERYH2O (extinguishing water, sprinkler, rain) and CO2

LAB RESEARCH

Time

27








Sealed storage


LAB RESEARCH

Time

28


Test setup - strain evolution during heating- post-cooling strength

29


Test setup

30


Test setup

31


Constitutive model εtot = εth(T) + εσ(σ, T) + εcr(σ, T, t) + εtr(σ, T)

Strains during heating

εth(T)εtot

0

0,5

1

1,5

2

2,5

3

3,5

4

4,5

5

0 50 100 150 200 250 300 350 400Temperature [°C]

Stra

in [1

0¯³]

EN 1992-1-2

TC_α=0.0: 2.17E-11.T³ + 8E-06.T - 2.21E-04

TC_α=0.2: -9.65E-13.T³ + 8E-06.T - 2.38E-04

TC_α=0.3: -5.66E-12.T³ + 7E-06.T - 1.72E-04

SCC_α=0.0: 1.23E-11.T³ + 1E-05.T - 3.62E-04

SCC_α=0.2: 1.89E-13.T³ + 8E-06.T - 2.54E-04

SCC_α=0.3: -1.44E-11.T³ + 9E-06.T - 3.71E-04

Load ratio 0.0

LITSLoad ratio 0.2

Load ratio 0.3

32




0

10

20

30

40

50

60

70

80

0 0,001 0,002 0,003 0,004 0,005 0,006Strain [-]

Stre

ss [N

/mm

²]

SCC1-7_Reference 20°C_1SCC1-7_Reference 20°C_2SCC1-7_Strength0d 205°C_load0.0SCC1-7_Strength0d 315°C_load0.0SCC1-7_Strength0d 530°C_load0.0model 20°C_0d_load0.0model 205°C_0d_load0.0model 315°C_0d_load0.0model 530°C_0d_load0.0

205°C

20°C315°C

530°C

( ) ηηησ⋅−+

−⋅=

21

2

, kk

f Tcm

T

Tc ,1εεη =

εc1,T = f(T,α). εc1,20°C

fcm,T = f(T,α). fcm,20°C

k(T,α)

33




-4

-3,5

-3

-2,5

-2

-1,5

-1

-0,5

00 50 100 150 200 250 300 350 400

Time [min]

Stra

in [1

0¯3]

205°C -- Load ratio 0,3 (model) 205°C -- Load ratio 0,2 (model)315°C -- Load ratio 0,3 (model) 315°C -- Load ratio 0,2 (model)530°C -- Load ratio 0,3 (model) 530°C -- Load ratio 0,2 (model)SCC1-7_Creep205°C_load03_1 SCC1-7_Creep205°C_load02_1SCC1-7_Creep205°C_load03_2 SCC1-7_Creep205°C_load02_2SCC1-7_Creep315°C_load03_2 SCC1-7_Creep315°C_load02_1SCC1-7_Creep315°C_load03_1 SCC1-7_Creep315°C_load02_2SCC1-7_Creep530°C_load03_1 SCC1-7_Creep530°C_load02_1SCC1-7_Creep530°C_load03_2 SCC1-7_Creep530°C_load02_2

Load ratio 0.2

Load ratio 0.3

( )2000413.03.0

20, 180000615.0 −⋅

°⋅⎟

⎠⎞

⎜⎝⎛⋅⋅−= T

Cccr et

fσε

34


Strains during heating=- - -

-5

-4,5

-4

-3,5

-3

-2,5

-2

-1,5

-1

-0,5

00 50 100 150 200 250 300 350 400

Temperature [°C]

Tran

sien

t str

ain

[10¯

³]

TC_α = 0.2_testTC_α = 0.3_testSCC_α = 0.2_testSCC_α = 0.3_testTC_α = 0.2_modelTC_α = 0.3_modelSCC_α = 0.2_modelSCC_α = 0.3_model

Load ratio 0.3

Load ratio 0.2


TCc

tr Af

c ⋅⋅=°20,

σε

⎟⎟⎠

⎞⎜⎜⎝

⎛

+⋅⋅−

⋅⋅+⋅⋅−⋅⋅−=

−

−−−

202.0103.1

1045.31082.11011.12

2537410

T

TTTAT

TC c = 1.06

SCC c = 0.94

35








Sealed storage


LAB RESEARCH

Time

36







REHYDRATION

Sealed storage


LAB RESEARCH

TimeHYDRATION

37


Compressive strength (α = 0.0)

0,50

0,60

0,70

0,80

0,90

1,00

0 10 20 30 40 50 60

Time [days]

fccu

b150

(T)/f

ccub

150(

20°C

) [-]

SCC - 350°C - water storage

SCC - 350°C - air storage

TC - 350°C - water storage

TC - 350°C - air storage

0,20,30,40,50,60,70,80,9

11,11,2

300 350 400 450 500 550 600

Temperature [°C]

fc(θ

)/fc(θ0

) [-]

SCC ref SCC 20°C/minSCC 3600min SCC waterTC ref TC 10°C/minTC 3600min TC water

Small strength recoverySmall strength recovery

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

0 100 200 300 400 500 600 700 800 900

Temperature [°C]

fccu

b150

(T)/f

ccub

150(

20°C

) [-]

EC Siliceous -- 0d (hot)EC Calcareous -- 0d (hot)TC1-12 -- 0dTC2k-4 -- 0dSCC1-5 -- 0dHPC1-5 -- 0d

Storage after cooling: Storage after cooling: + 20+ 20--30% strength loss30% strength loss

Strength immediately Strength immediately after coolingafter cooling

Watercooling (fire brigade): + 30Watercooling (fire brigade): + 30--35% loss35% loss

38


Compressive strength (α ≥ 0.0)

-100

-80

-60

-40

-20

0

20

40

60

Stre

ngth

loss

[%]

TemperatureLoadWatercoolingStorage in airStorage in water

Load : 0% 0%20% 40% 20%

350°C 550°C

Strength recovery!!

39


Residual & post-cooling strength models

0

10

20

30

40

50

60

70

80

0 0,001 0,002 0,003 0,004 0,005 0,006Strain [-]

Stre

ss [N

/mm

²]

SCC1-6_Reference 20°C_1SCC1-6_Reference 20°C_2SCC1-7_Reference 20°C_1SCC1-7_Reference 20°C_2SCC1-6_ResStrength 205°C_load0.0SCC1-7_ResStrength 205°C_load0.0SCC1-6_ResStrength 315°C_load0.0SCC1-7_ResStrength 315°C_load0.0SCC1-6_ResStrength 530°C_load0.0SCC1-7_ResStrength 530°C_load0.0model 20°C_8we_load0.0model 205°C_8we_load0.0model 315°C_8we_load0.0model 530°C_8we_load0.0

( ) ηηησ⋅−+

−⋅=

21

2

, kk

f Tcm

T

Tc ,1εεη =

fcm,T = f(T,α). fcm,20°C

k(T,α)

εc1,T = f(T,α). εc1,20°C

40



41


fy(θ)

fc(θ)

M = As.fy(θ).z

Temperature θ?




42


fy(θ)

fc(θ)

M = As.fy(θ).z

Temperature θ?



Load bearing capacity

Σ[xibfifcd,fi(θm)]

fcd,fi(θm)

43


CONCLUSION

Assessment techniques- Techniques are available that are easy to use in situ and in the lab- Schmidt Rebound Hammer/ UPV give values that can help the visual inspection- The shape of the colour path results in different temperature zones- Water immersion is an economic method

Residual strength- Additional strength losses due to fast cooling and post-cooling storage- These losses are important for calculating the remainig bearing capacity- Superposition of the influencing parameters is possible- Strength recovery will occur during storage after water cooling- Stress-strain models as function of both temperature and load level

44

Assessment of the residual strength of concrete structures

Documents

Transcript of Assessment of the residual strength of concrete structures