2016 aci apr18_wollastonite_v3

1. Use of Wollastonite as micro-
reinforcement in cementitious systems
Barzin Mobasher, Vikram Dey
School of Sustainable Engineering and the Built Environment
Arizona State University, Tempe, AZ
ACI Convention, Milwaukee, WI
April 18, 2016

2. Introduction
 Wollastonite is a naturally occurring calcium silicate mineral (CaSiO3)
 Primary constituents (about 90 %)– CaO.SiO2 , CS
 Minimal Carbon footprint, no calcination required
 Acicular (needle like) particle shape
 High brightness, low moisture & oil absorption, non-hazardous
 World reserves of 90 million tonnes
 Applications – Ceramics, Plastics and Paints
 Extension to macro-fibers in concrete, potential areas of opportunity:
 Reduced micro-cracking
 Improved mechanical properties
 Enhanced shrinkage resistance
 Low and Beaudoin (1993) showed improvements in mechanical properties
of cement-based binders with wollastonite microfibers with different
aspect ratios
HARRP 20x40 Fibers
(15x Magnification)
NYAD-G Fibers
(15x Magnification)

3. Presentation Outline
Wollastonite microfibers reinforcing
cementitious matrix at 50x magnification
 Mechanical Testing
 Different grades of wollastonite
 Closed loop COD controlled cyclic tests
 Wollastonite-Mortar, Hybrid-Mortar
 Restrained Shrinkage Cracking
 Experimental setup
 Specimen preparation
 Wollastonite-Paste Mixes
 Wollastonite-Textile Hybrid

4. Grades of Wollastonite
HARRP-20x40 (C2000)
Mean Particle Size - 2000 μm
Magnification – x38
HARRP-40 (C850)
850 μm
x38
NYAD-G (F55)
55 μm
x500
NYAD-MG (F33)
33 μm
x500

5. Specimen Preparation
Cementitious solids = cement + silica fume + wollastonite; W/CS=W/(PC+SF+WO)
Mix design
Wollastonite
Dosage
0% 5% 10% 15%
Portland Cement, PC 40% 38% 35% 33%
Fine Aggregates, FA 42% 42% 42% 42%
Silica Fume, SF 2% 2% 2% 2%
Wollastonite, WO - 2% 4% 6%
Water, W 16% 16% 16% 16%
Super Plasticizer 0.5 % - 2.5 % of cementitious solids
Mortar specimen nominal dimensions:
 Cube - 50 x 50 x 50 mm
 Moist cured for 7 days and 28 days
 Beam Size 1 - 25 x 75 x 325 mm
 Moist cured for 7 days and 28 days
 Volume fractions: Wollastonite 5%, 10%, 15 %
 Beam Size 2 - 100 x 100 x 450 mm
 Moist cured for 28 days
 Volume fractions:
 Wollastonite 10% (28 lb/yd3) and
 Forta-Ferro macrofibers 5 and 10 lb/yd3

6. Uniaxial Compression Test
 Effect of wollastonite is more pronounced after 28 days
 Increase in compressive strength in long term; by as much as 30 % compared to control
 Effect of aspect ratio of wollastonite microfibers is insignificant under compression

7. Cyclic Fracture Test
 Instron clip-on extensometer used for measuring crack opening displacement
 LVDT used for measuring axial deflection
 5 loading – unloading cycles up to 0.2 mm of crack opening.
 Loading under CMOD control; Unloading under Load Control
LVDT Clip Gage

8. Toughening Due to Fiber Bridging
 Modeling steps and requirements:
 Fiber de-bonding and pullout response
 Closing Pressure formulation for a single
isolated crack
 Crack face stiffness and crack closure
 Stress Intensity reduction
 Toughening and strength enhancements
x
s
Actual
Stress
LEFM
Stress
Traction Free
Crack Tip
Fracture
Process
Zone
Fictitious Crack Tip
Tensile Strength

9. Compliance Measurements
Load
CMOD
P
1
ein
Cel
Ceu
C0
b
S = 4b
P
t
CMOD
a a0
2 30
2
0.66
V( )=0.76 - 2.28 3.87 - 2.04
(1- )
a a
b
   

 
  
 Critical effective crack length, ac (ac = ao+Δa ):
 
2
6
u
c
c
EC b t
a
SV 

 Young’s modulus, E:
 
2
6 o o
i
Sa V
E
C b t


Source: Y.S. Jenq, S.P. Shah, “Two parameter fracture model for concrete”,J. Eng. Mech.,1985
0 0.04 0.08 0.12 0.16 0.2
CMOD, mm
0
200
400
600
800
Load,N
Load-CMOD
Loading zone
Unloading zone
Max points
Min points

10. R-Curve Methodology
R + n Rm 1
R + Rm n2
R + Rm n2
Rm
R + Rm n2
Rm
R
R + n Rm 1
R + Rm n2
a
Mobasher, B., “Mechanics of Fiber and Textile Reinforced Cement Composites”, CRC Press, 2012
 Fracture resistance curves represent the represents material's
resistance to initiation and propagation of cracks
 Parameters measured from loading-unloading cycles:
 Critical crack length:
 Loading and unloading compliance
 Load at onset of unloading
 Total inelastic deformation
 Represent: Unloading compliance vs. crack length and inelastic
deformation vs. crack length.
 Strain energy release rate:
 Stress Intensity Factor:
2
2 2
U in
R
dC dP P
G
t da t da

 
.R RK E G
(Nonlinear)
aac
Crack Extension,
Quasi-brittle Matrix
nnon
Instability
KI
KIC
KIC
s
Crack
Initiation
Ultimate
Failure
Self Propagating
Crack Growth
Zzone
Stable Crack
Growth
ZZZZsZoneZ
one
Brittle Matrix (Linear)
a0
c oa a a 

11. Fracture Parameters
 Two parameter fracture model proposed by Jenq and Shah to characterize fracture
resistance and energy dissipation of concrete in 1985
 Kinetics of crack growth was expressed as a function of unloading compliance
 Critical stress intensity factor
 Critical crack tip opening displacement
 Critical strain energy release rate (fracture toughness)
 Apparent flexural strength
 
  
  
2
max 32
2
1.99 1 2.15 3.93 2.7( ) ( )
3 ;
2 1 2 1
c c c cc c
IC c
c c
S a F
K P F
b t
    

  
   
 
 
 
    
1/22max 2 0
0 0 0 02
6
1 1.081 1.149 ;c c
C c
c
P Sa V a
CTOD
Eb t a

          
 
2
IC
IC
K
G
E

max
2
0
3
2 ( )
P S
MOR
t b a


Source: Y.S. Jenq, S.P. Shah, “Two parameter fracture model for concrete”,J. Eng. Mech.,1985

12. Effect of fiber dosage on fracture
response

13. Effect of different
grades of wollastonite
Dey, V., Kachala, R., Bonakdar, A., and Mobasher, B. (2015). “Mechanical properties of micro and sub-micron properties of wollastonite
fibers in cementitious composites.” Constr. Build. Mater., 82, 351–359.

14. Cyclic Fracture Test –
Hybrid Fiber Reinforcement

15. 0 5 10 15 20 25 300
1
2
3
4
5
6
Position, mm
Stress,MPa
0
dnq
b b
b
x
l
  
 s  s  
   
0 5 10 15 20 25 300
0.01
0.02
0.03
0.04
0.05
0.06
Position, mm
CrackOpening,mm
n
b
bb
l
x
uxu )()( 0

x
bridging zone
lb
crack
Crack Opening, Stress, Crack extension -
Sakai-Suzuki 1994
*(x)s
u(x)
0 0.01 0.02 0.03 0.04 0.05 0.060
1
2
3
4
5
6
u, mm
Stress,MPa
*(u)s

16. Back-calculated Tensile Response-
Hybrid Fiber Reinforcement
0
0
q
b b
b
x
w ( x ) w ( )
l

0
t0
b
E
s
  t-postpeak
b
g
w
L
 
0
0
1
n
q
b b
b
x
( )
l
 
s  s  
 
x
bridging zone
lb
crack
s
b
sb

17. SEM Micrographs of fractured
surface: HARRP 20 & NYAD-G
HARRP 20 Beam (Dosage – 10 %) NYAD-G Beam (Dosage – 10 %)

18. SEM Micrographs of fractured
surface: NYAD-MG
 Platelet – matrix interaction, fiber rupture, fiber pullout, and crack deflection mechanisms

19. Proposed setup for
2-D Shrinkage Cracking
Pressure
Gauge
Vacuum
Pump
Condenser
(Dry Ice and Alcohol)
Camera
Desiccator
Strain Gage Transducer Amplifier
and Computer Interface Unit
Data Acquisition System
Pressure
Regulator
Load
Cell
Sample
 Evaporation is simulated under low pressure condition.
M.Bakhshi, B. Mobasher, “Experimental observation of early age drying of PC paste under low-pressure”; CCC, vols.33, 2011

20. Plastic Shrinkage Cracking
Control Specimen NYAD-G Specimen

21. Restrained Shrinkage Cracking :
Crack Morphology
NYAD-G (15 %)
Control
HARRP 20 (15%)
Dey, V., Kachala, R., Bonakdar, A., Neithalath, N., Mobasher, B., “Quantitative 2D Restrained Shrinkage Cracking of Cement Paste with
Wollastonite Microfibers”, Journal of Materials in Civil Engineering, ASCE, 2016, manuscript in press.

22. Analysis algorithm of
shrinkage parameters
sample code:0602088
bit
Invert the
binary image
of crack pattern
Red points indicate detected intersection points
Detect cracks
intersection
points from
skeletonized
image of crack
Dilated intersection points
final processed image
Dilate the
intersection
points
Subtract
dilated
intersection
points from the
initial binary
image
Step 1 Step 2
Step 3 Step 4

23. Restrained Shrinkage Cracking :
Crack Properties
 Highlights
o Coarse fibers
• Crack width increased up to 51 %
• Crack length and area decreased by 41 % and 29 %
o Fine fibers
• Crack width and length decreased up to 40 % and 48 %
• Crack area decreased up to 69 %

24. Shrinkage Crack Growth
Dey, V., Kachala, R., Bonakdar, A., Neithalath, N., Mobasher, B., “Quantitative 2D Restrained Shrinkage Cracking of Cement Paste with
Wollastonite Microfibers”, Journal of Materials in Civil Engineering, ASCE, 2016, manuscript in press.

25. Effect of AR-Glass Textile Addition
on Shrinkage Cracking
 Highlights
o Control
• Crack length increased up to 9 %
• Crack width and area decreased by 36 % and 36 %
o Coarse fibers
• Crack length decreased up to 62 %
• Crack width and area decreased by 55 % and 26 %
o Fine fibers
• Crack width and length decreased up to 37 % and
59 %
• Crack area decreased up to 73 %
Control
ARG-TRC
F55
Textile + F55

26. Effect of Textile - Continued

27. Summary Findings and Conclusions
 Four different types of wollastonite fibers were evaluated as partial replacement of
cement in mortar, paste, and hybrid reinforcement mixes
 Wollastonite improves mechanical properties by reinforcing the brittle matrix at
the micro level
 Acicular shaped, randomly distribute wollastonite fibers, offer significant crack
bridging potential, and stress distribution capability.
 The smaller size wollastonite fibers (NYAD-G and MG) blend more efficiently
with the cementitious matrix than HARRP fibers, offer significant improvement in
flexural strength and fracture toughnes.
 Early age and long term gains include flexural strength, toughness and
compressive strength (up to 60%, 140% and 30% respectively) from the best
results obtained so far, when compared to plain cement mortars
 Early age plastic shrinkage cracking can be greatly reduced due to wollastonite
 Hybrid fiber reinforcements along with macro fibers, and textile bring synergistic
effect in reinforcing the matrix at both micro and macro-level.

28. Use of Wollastonite as micro-
reinforcement in cementitious systems
Thank You