qingke nie changjun zhou huawei li xiang shu baoshan huang...
TRANSCRIPT
Qingke Nie1
Changjun Zhou2
Huawei Li1
Xiang Shu3
Baoshan Huang3
3Hebei Research Inst. of Construction & Geotechnical Investigation Co., Ltd.
3Harbin Institute of Technology
3The University of Tennessee, Knoxville
At
International Symposium on Systematic Approaches to Environmental
Engineering in Transportation
Sulfate Attack
Soils in Xinjiang Autonomous Province,
China
Mechanism of Sulfate Attack
HSAC 16H HS2C HSAC
(aq) 2OH HSC (aq)SO CH
32362124
-
2
-2
4
β’ Improve strength, workability, durability;
β’ Generally less expansive than cement;
β’ Green. Many are industrial by-products, like
fly ash and silica fume.
SCMβs Advantages
Laboratory Tests
Part 1
Laboratory Tests
Objectives: to find an optimal concrete
Resisting the sulfate attack;
Meeting compressive strength, workability,
etc;
Utilizing the locally available materials
adequately, especially the SCMs.
Laboratory Tests
β’ Chemical Analysis
β’ XRD
β’ Calorimetry test
β’ Cube compressive strength test
β’ Mortar bar expansion exposed to a sulfate
solution
β’ Chloride ion penetration test
Chemical Analysis
SiO221.34% Al2O3
4.06%
Fe2O3(T)5.45%
MnO0.30%
MgO2.20%
CaO63.22%
Others 3.43%
Sulfate Resisting Cement
SiO223.14%
Al2O36.62%
Fe2O3(T)3.93%
MnO0.12%
MgO1.94%
CaO60.63%
Others 3.63%
Portand Cement
SiO260.09%
Al2O319.51%
Fe2O3(T)6.69%
MnO0.09%
MgO2.23%
CaO4.88%
Others 6.52%
Fly Ash 1
SiO250.52%
Al2O334.46%
Fe2O3(T)4.23%
MnO0.04%
MgO0.56%
CaO2.83%
Others 7.36%
GN Admixture
FAII
XRD
Quartz
58%
Diaoyudaoite
15%
Mullite 15%
Na2Al22O34Β·2H2O
6%
Tobermorite 4% Other 2%
C3S 52%
C2S 27%
C3A 5%
C4AF 12%
Other 4%
SRC
Calorimetry test
OPC OPC: FAI OPC:FAII OPC: S75 OPC: S95 OPC: SF
1 0.7:0.3 0.7:0.3 0.7:0.3 0.7:0.3 0.9:0.1
OPC:
CM OPC: GN
OPC: FAI:
CM OPC: FAII OPC: FAII
OPC: FAII:
S95
1:0.1 1:0.03 1:0.11:0.1 0.55:0.45 0.6:0.4 0.6:0.2:0.2
SRC SRC: CM SRC: GN SRC: FAI:
CM
Type I
Cement
Type I
Cement: FAI
1 1:0.1 1:0.03 1:0.11:0.1 1 0.7:0.3
w/cm=0.4
OPC
70%OPC+30%F
A I
70%OPC+30%FA II
70%OPC+30%
S75
70%OPC+30%
S95
Time since started (h)
Cube Compressive Strength & Mortar Bar Expansion
Rapid Chloride Permeability Test
Conclusions based on Tests
β’ SCMs decreased hydration rate and C3A content of cementitious materials;
β’ Compressive strength of combined cement mortar with SCMs met requirement;
β’ OPC+SCMs mortar and SRC mortar with/without CM admixture performed better than OPC mortar under sulfate environment;
Conclusions based on Tests
β’ According to chemical analysis, GN admixture
is similar to fly ash, while CM admixture is
similar to slag.
β’ OPC+fly ash concrete is a better choice than
SRC concrete in an environment enriching
both sulfate and chloride in soil.
β’ The recommended percentage of fly ash added
into concrete would be 25-35%.
Part 2
Numerical Simulation of
Sulfate Attack on Concrete
Acknowledgement
o Dr. Barzin Mobasher from Arizona State University
o Dr. Kimberly E. Kurtis from Georgia Technology of
Institute
o U.S. Bureau of Reclamation (USBR)
Chemical
components
change in cement
paste
Volume
expansion Cracks
Service
Life
Decrease
Sulfate
Diffusion
Accelerate Repeat Steps
Process of Sulfate Attack on Concrete
ππ’ππππππ π πππ’πππ‘πππ π€πππ ππππππ€ π‘βππ ππππ€ πβπππ‘.
Reactions in harden cement paste causing expansion
πΊπ¦ππ π’π
πΆπ ππ» 2 + ππ2ππ4 β 10π»2π β πΆπ2ππ4 β 2π»2π + 2ππππ» + 8π»2π
π΄πΉπ‘ πΆ4π΄π»13 + 3πΆπ π»2 + 14π» β πΆ6π΄π 3π»32 + πΆπ»
πΆ4π΄π π»12 + 2πΆπ π»2 + 16π» β πΆ6π΄π 3π»32
residual πΆ3π΄ + 3πΆπ π»2 + 26π» β πΆ6π΄π 3π»32
(ππ+βππ)/ππ
2. 48
1.51
2.26
More complicated considerations
due to fly ash Pozzolanic reactions
3πΆπ» + 2π = πΆ3π2π»3 3πΆπ» + π΄ + 3π» = πΆ3π΄π»6
πΆπ(ππ»)2 πππ2 π΄π2π3 π»2π
Good Things Bad Things
CH from cement hydration consumed
CaO in fly ash added
Makes concrete less permeable πΆ3π΄π»6 prone to sulfate attack
Concrete curing
Reactions in harden cement paste causing expansion
π΄πΉπ‘ πΆ4π΄π»13 + 3πΆπ π»2 + 14π» β πΆ6π΄π 3π»32 + πΆπ»
πΆ4π΄π π»12 + 2πΆπ π»2 + 16π» β πΆ6π΄π 3π»32
residual πΆ3π΄ + 3πΆπ π»2 + 26π» β πΆ6π΄π 3π»32
πΆπ΄ + ππ β πΆ6π΄π 3π»32
Extra Reactions due to Fly Ash under Sulfate Attack
Gypsum is from:
β’ residual gypsum after cement hydration;
β’ Sulfates reacts with CH.
πΆ3π΄ + 3πΆπ π»2 + 26π» β πΆ6π΄π 3π»32
πΆπ ππ» 2 + ππ2ππ4 β 10π»2π β πΊπ¦ππ π’π + 2ππππ» + 8π»2π
πΆ4π΄π»13 + 3πΆπ π»2 + 14π» β πΆ6π΄π 3π»32 + πΆπ»
πΆ4π΄π π»12 + 2πΆπ π»2 + 16π» β πΆ6π΄π 3π»32
residual πΆ3π΄ + 3πΆπ π»2 + 26π» β πΆ6π΄π 3π»32
Reactions in OPC concrete
As consumed by pozzolanic reactions from fly ash, the CH may be not enough to
produce enough gypsum to support the formation of ettringte, which can restrain
the expansion of concrete.
Available CH in Concrete?
Cement hydration produces CH
Fly ash contains CaO, which can be partially
converted into CH
Pozzolanic reactions consumes CH
πΆπ»ππ£ππ = πΆπ»βπ¦πππ + πΆπ»πππ¦ππ β β πΆπ»πππ§π§
Diffusion of sulfate ions
a saturated concrete, unsteady state:
Molecular transport=convection+accumulation+reaction rate
combination of Fickβs diffusion, convection transport, and chemical reaction
π·β2π = π’π»π +ππ
ππ‘+ π
Where u is velocity; c is concentration; t is time; D is diffusion coefficient.
Diffusion of sulfate ions
In dilute solution, if no pressure and temperature gradients exists:
π·β2π = π’π»π +ππ
ππ‘+ π
ππ
ππ= π·
π2π
ππ2β πππΆ
ππΆ
ππ= β
πππΆ
π
Define: π = π β ππΆ
ππ
ππ= π·
π2π
ππ2 Only one unique variable, Z
Boundary conditions
Boundary conditions
let L be the thickness of the slab, X=xL, T=L2t/D, u=U/U0, z=Z/U0, and c=C/U0
ππ§
ππ‘=π2π§
ππ₯2
ππ
ππ= π·
π2π
ππ2 Written as
ππ’
ππ‘=
π2π’
ππ₯2β ππ’2 + ππ’π§ π =
ππΏ2π0ππ·
where:
boundary and initial conditions: for all t, at x=0 and x=1: u=1; for t=0,
0<x<1: u=0.
Numerical solution of the diffusion-reaction equation
truncated Taylor series
π’π,π+
1
2
= π’π,π + βπ2 π’π,π β ππ’π,π
2 + ππ’π,ππ§π,π (βπ‘
2)
π’π,π+
1
2
= π’π,π + βπ2 π’π,π β ππ’π,π
2 + ππ’π,ππ§π,π (βπ‘
2)
βπ2 π’π,π =
π2π’
ππ₯2=π’π+1,π β 2π’π,π + π’πβ1,π
βπ₯ 2 where:
π’π,π+1 β π’π,π
βπ‘=1
2βπ2 π’π,π + π’π,π+1 β π
π’π,π + π’π,π+1
2π’π,π+
12+ ππ’
π,π+12π§π,π
Crank-Nicolson formula for π’π,π+
1
2
:
Cracking affects diffusion
(D1)max/ D2=10
Numerical solution of the moving boundary diffusion-reaction equation
1. Solve the equation for the fixed boundary (composite medium);
2. Solve the moving boundary problem for the diffusion equation with no reaction (2nd Fickβs law with moving boundary), for the two cases: discontinuous and continuous diffusivity.
3. Apply the method devised for the previous step to the moving diffusion-reaction equation.
Crystallization pressure of ettringite
Riecke principle:
π =π π
ππ πΏπ(
πΆ
πΆπ )
where R is the ideal gas constant, T is temperature, ππ is molar volume, C is
actual concentration of the solute during concentration, and πΆπ is saturation
concentration.
For ettringite, at temperature 25OC, with a molar weight of 1252g and a
specific gravity of 1.78g/cm3, P = 2.4 - 8.1 MPa for a degree of
supersaturation πΆ
πΆπ of 2 - 10.
Effect of crystallization pressure of ettringite
When crystallization occurs in pores at a distance
comparable to the size of a pre-existing crack,
and if the crystallization pressure is high enough, this
crack can propagate.
Tensile stress-strain response of concrete
πΈ = πΈ0
πΈ = πΈ0 1 β Ο
πΈ = π ( π β π0)
π0 = ππ β ππ‘ πΈ0
Modeling of expansion
π = ππ(1
πΈππ£π,β1
πΈ0)
where ππ is the residual stress in the specimen
before sulfate attack due to shrinkage; πΈππ£π, is
the average modulus over the cross-section.
Expansion:
Effect of porosity : πππππππππ‘ππ = ππ β πΞ¦
ππ: volumetric strain
π is the fraction of capillary porosity being filled, and Ξ¦ is the capillary porosity
Reaction βππ/ππ
AFm to AFt 0.51
C3A to AFt 1.26
C4AH13 to AFt 0.48
Validation on the extended model
A long term observation database on linear expansion of concrete
from US Reclamation Bureau was utilized to validate the extended
model.
Ordinary portland cement concrete and concrete with 25% cement
replaced with fly ash were selected.
Inputs
Parameters OPC concrete 75%OPC+25%FA concrete
L (m) 0.067 0.067
H (m) 0.152 0.152
D2 (m2/s) 4e-13 4e-13
D1/D2 (>1) 10 10
U0 (mol/m3) 9 9
Cement content (kg/m3) 360 270
MVC 3.12 3.12
wc 0.48 0.48
DRcement 0.9 0.9
phi_frac 0.45 0.40
CC3Ai 0.09 0.057
Gypsum 0.05 0.05
DRC3A 0.9 1
k (m3/molΒ·s) 1e-7 1e-7
E0 (MPa) 30000 30000
ft (MPa) 3 3
residual_s (MPa) 10 10
Fly ash dosage (kg/m3) 0 90
CaO content in fly ash (%) 0.14 0.14
Al2O3 content in fly ash (%) 0.19 0.19
SiO2 content in fly ash (%) 0.44 0.44
C3S content in cement (%) 0.433 0.433
C2S content in cement (%) 0.317 0.317
Should pozzolonic reactions considered or not?
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0 5 10 15 20 25 30 35 40
Lin
ear
Exp
ansi
on
(%
)
Time Since Sulfate Attack (Year)
Concrete Cylinders with 25%
Cement Replaced with Fly Ash
Tixer-mobasher Model
Extended Model
It is not enough to just consider the dilution effect and the permeability change in concrete.
Pozzolanic reactions are necessary to be considered in sulfate attack on concrete.
Porosity fraction can be filled by expansion products?
0
0.05
0.1
0.15
0.2
0.25
0.3
0 5 10 15 20 25 30 35 40
Lin
ear
Ex
pan
sio
n (
%)
Time Since Sulfate Attack (Year)
Concrete Cylinders with 25%
Cement Replaced with Fly Ash
Fraction of Porosity Filled by
Expansive Products=0.38
Fraction of Porosity Filled by
Expansive Products=0.40
Fraction of Porosity Filled by
Expansive Products=0.42
Penetration of sulfate ions in concretes
The dilution effect of fly ash makes the C3A concentration in concrete smaller,
thus postpone the transition from AFm to AFt, decrease the expansive products.
Penetration of sulfate ions in concretes
Since the same diffusion coefficient was utilized in the two concretes,
the penetration speeds were supposed to be the same in the two concretes
Conclusions
The addition of fly ash make concrete less permeable, therefore slows down
the penetration of sulfate ions in concrete.
Compared to OPC concrete, the fly ash concrete has better sulfate
resistance. The linear expansion of concrete with fly ash is greatly smaller than
the OPC concrete at the same moment.
The addition of fly ash dilutes the concentration of C3A and CH. the
pozzolanic reactions change the chemical components and their concentrations in
concrete, therefore slow down the transition from AFm to AFt.
Conclusions
The pozzolanic reactions due to the addition of fly ash into concrete should
be considered when numerical simulation methodologies are utilized to
investigate the sulfate resistance of concrete.
The proposed model was validated by the measured linear expansion of
concrete under sulfate attack by USBR. The model successfully reflects the
consumption of CH in concrete and gives reasonable prediction on the linear
expansion of concrete in 10 to 20 years.
Thank You!