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KYT 2014 Loppuseminaari 18.3.2015
Betonisten vapautumisesteiden säilyvyys
voimalaitosjätteen loppusijoituksessa
Osaprojektit 1&2
Olli-Pekka Kari
Aalto-yliopisto/Rakennustekniikan laitos
INTRODUCTION
MODEL
EXPERIMENTAL WORK
SIMULATIONS
CONCLUDING REMARKS
Background
•
The safety of the repository is ensured
by multiple concrete barriers for
several hundreds of years
•
Experiences of existing structures
based on modern type of concrete
covers tens rather than hundreds of
years
•
Simulations
and
experimental
analyses are needed to justify the
long-term behaviour of these concrete
structures
Layout of the low- and intermediate-level
nuclear waste disposal concept in Finland
INTRODUCTION
MODEL
EXPERIMENTAL WORK
SIMULATIONS
CONCLUDING REMARKS
Project Overview
Subproject 1 Aalto
Durability of
engineered concrete
barriers for nuclear
waste repositories
Developing the
model for ageing
of concrete
Subproject 2 VTT
Experimental
analyses of
concrete
Simulations of the long-term
ageing of concrete
Experimental validation of the
model introduced
Laboratory analyses of the
test concrete specimens
Investigations of current
condition of the test conctetes
INTRODUCTION
MODEL
EXPERIMENTAL WORK
SIMULATIONS
CONCLUDING REMARKS
Ageing of concrete structures in Finnish rock caverns
AIR
•
CONCRETE
carbonation changes the composition
of pore solution reducing alkalinity,
the compounds of cement paste, and
the pore structure
ClMg2+
SO42-
STEEL
Post-closure period
Rock cavern sealing
CO2
STEEL
Operational period
Ca2+
GROUNDWATER CONCRETE
•
aggressive ions and leaching of
cement paste cause physical and
chemical ageing of concrete
Steel reinforcement corrosion
Disintegration of concrete
INTRODUCTION
MODEL
EXPERIMENTAL WORK
SIMULATIONS
CONCLUDING REMARKS
Transport processes into concrete
•
Diffusion through pores is a predominant phenomenon during both of the
exposure periods
Schematic diagram of the processes in a concrete pore
INTRODUCTION
MODEL
EXPERIMENTAL WORK
SIMULATIONS
CONCLUDING REMARKS
Thermodynamic model
•
Modelling based on thermodynamic equilibrium offers a versatile and
theoretically robust approach for predicting physiochemical interactions
Flowchart of the thermodynamic model introduced in the study
INTRODUCTION
MODEL
EXPERIMENTAL WORK
SIMULATIONS
CONCLUDING REMARKS
Concrete specimens
•
Concrete specimens were fabricated* in 1997 and stored in constant
environmental conditions in air or submerged in exposure solutions
Binders
Binder
CEM
BFS SF
S1
I 42.5 N-SR (SR) 100%
S2
II/A-M(S-LL) 42.5 N (Yleis) 90%
10%
I 42,5 R (Mega) 20%
S3
75% 5%
Concrete mixes
Concrete Binder a/b ratio w/b ratio
B1
4.0
0.35
B2
S1
5.0
0.43
B3
6.0
0.50
B4
4.0
0.35
B5
S2
5.0
0.43
B6
6.0
0.50
B7
4.0
0.35
B8
S3
5.0
0.43
B9
6.0
0.50
*) A joint research programme by Teollisuuden Voima Oyj (TVO) and Fortum Power and Heat Oy
INTRODUCTION
MODEL
EXPERIMENTAL WORK
SIMULATIONS
CONCLUDING REMARKS
Environmental conditions of the specimens
Exposure solutions
Solution Chemical solution Aggressive component (mg/l)
L1
20
L2
Na2SO4
500
L3
1000
L4
50
L5
NaCl
1000
L6
10000
L7
SO4 - 20, Cl - 50, Mg – 5
Na2SO4+NaCl+
L8
SO4 - 500, Cl - 1000, Mg - 100
MgCl2*6H2O
L9
SO4 - 1000, Cl - 10000, Mg - 300
•
Specimens were stored also in ground water and in atmospheric conditions
INTRODUCTION
MODEL
EXPERIMENTAL WORK
SIMULATIONS
CONCLUDING REMARKS
Laboratory analyses for model validation
• Energy-dispersive
X-ray analyser
• Scanning electron
microscope
• Steel-die method
• Ion chromatography
Pore
solution
composition
• pH value tests for concrete
suspension and extracted
pore solution
Porosity
properties
Element
distribution
Model
validation and
initial values
pH value
• Capillary water uptake test
• Mercury intrusion porosimetry
• Nitrogen gas adsorption
• Indirectly test for tortuosity
Penetration
of aggressive • Titrations
species
Analyses of
cement
hydrates
• X-ray diffraction
• Thermogravimetric
and differential
thermal analysis
INTRODUCTION
MODEL
EXPERIMENTAL WORK
SIMULATIONS
CONCLUDING REMARKS
Result of the comparisons
•
The thermodynamic model reasonably describes variable time-dependent physiochemical phenomena in concrete justifying its use in the long-term simulations
Distributions of Ca(OH)2 and CaCO3 and pH value
in the B3 concrete after 13 years of exposure
Chloride concentrations in the B2 concrete after
13 years of exposure
INTRODUCTION
MODEL
EXPERIMENTAL WORK
SIMULATIONS
CONCLUDING REMARKS
Operational period of 100 years
•
Carbonation changes composition of concrete affecting the initial conditions at the
beginning of the post-closure period
Cement hydrates in the B2 concrete after 100a
Total porosity and pH in the B2 concrete after
100a
INTRODUCTION
MODEL
EXPERIMENTAL WORK
SIMULATIONS
CONCLUDING REMARKS
Post-closure period of 400 years
•
Concrete performed satisfactorily during the simulated period of 500 years, but
the steel reinforcement corrosion cannot be totally excluded
Cement hydrates in the B2 concrete after 500a
Simulated Cl-/OH- ratio in the B2 concrete after
500a
INTRODUCTION
MODEL
EXPERIMENTAL WORK
SIMULATIONS
CONCLUDING REMARKS
Concluding remarks
• The thermodynamic model introduced offers a plausible basis to
understand and estimate the ageing phenomena of concretes
• The model based simulation reveals the latent factors of deterioration
supporting the optimal selection of concrete composition or finding
proper corrective measures
• Generally the test concretes have performed well under the conditions of
the experiments
• Future challenges in the research of concrete deterioration include the
role of aggregates and the long-term alteration of the calcium-silicatehydrate gel as well as the initiation of steel reinforcement corrosion