Innovations in Pulp Fiber-Cement Combinations Kim Kurtis School of Civil and Environmental Engineering Georgia Institute of Technology Atlanta, GA
March 19, 2007 1
Collaborators Hiroki Nanko, Institute of Paper Science and Technology
at Georgia Tech Ben Mohr, Tennessee Tech University Nabil El-Ashkar, Arab Academy for Science, Technology, and Maritime Transport Joe Biernacki, Tennessee Tech University Marc Millard, Georgia Institute of Technology
2
Motivation for Research Pulp fiber-cement composites have been called “tomorrow’s growth product”1
Many potential advantages over traditional building materials: - Pulp fibers are renewable and non-hazardous - highly tailorable composition and structure - simple, low-cost production is possible - more decay resistant than wood
Many current markets for thin-sheet composites (e.g., backerboard, siding, shinges)
Emerging markets for pulp fiber reinforced concrete cast-in place and precast sections 1 Kurpiel,
F., “Fiber-cement siding is tomorrow’s growth product”, Wood Technology, November (2000).
3
Motivation for Research
Challenges with dispersing pulp fibers in cement-based matrices result in limited production/processing methods
Influence of exterior exposure (i.e., heat/rain or wet/dry cycles) on composite performance has not been well-established, but must be understood to ensure long-term performance
4
Simplified representation of a Hatschek machine for fiber-cement production (Wilden, 1986).
Background: Pulp Fiber Properties Fiber Length (mm)
Average Tensile Strength (MPa)
Average Fiber Elastic Modulus (GPa)
Kraft pulp
4-8
800-1000
Thermomechanical pulp (TMP)
1-2
Steel (microfiber)
Polypropylene
Asbestos
Density (g/cm3)
Cost ($/kg)
18-43
1.5
0.5
Unbleached: 3-8% lignin Bleached: 0-1% lignin
400-700
--
0.5
0.4
23-31% lignin
2-5
700-2000
200
7.9
>2
High resistance to impact and abrasion, corrosive
varies
300-500
6
0.91
1.5-2.5
Chemically inert, noncorrosive
5-40
200-1800
--
2.55
n/a
Carcinogenic
Notes
5
Background: Structure of Pulp Fibers
Layered structure • P (primary wall) • S1 (outer layer) • S2 (middle layer) • S3 (inner layer)
Variations with species, growth very thin thin thick thin
Lumen 6
Objectives To improve dispersion of pulp fibers in cement-based
matrices To characterize effects of pulp fiber addition on mechanical behavior To characterize effects of pulp fiber addition on shrinkage To improve understanding of the durability of pulp-fiber cement composites
7
Improved Fiber Dispersion in Cement-Based Composites by Surface Coating
Cellulosic (paper pulp) fibers
have potential for economical reinforcement of concrete and other cement-based composites _ SiO 2+ + In cement, these fibers tend to “clump”, reducing strength
Economical treatment with carbohydrates and nanoparticles facilitates their dispersion and binding in cement-based composites
Cellulosic microfibers
(net negative charge)
+
+ + + + +
_
SiO2
SiO2-
+
Cationic carbohydrate coupling agent
Silica-rich nanoparticles (net negative charge)
8 US Patent 6,933,038 B2, Nanko and Kurtis.
Improved Fiber Dispersion in CementBased Composites by Surface Coating
Silica Fume
Class F Fly Ash
9 US Patent 6,933,038 B2, Nanko and Kurtis.
Improved Fiber Dispersion in Cement-Based Composites by Surface Coating
Without fiber coating, clumping is readily apparent
With fiber coating, good dispersion and improved performance are achieved 10
Objectives To improve dispersion of pulp fibers in cement-based
matrices To characterize effects of pulp fiber addition on mechanical behavior To characterize effects of pulp fiber addition on shrinkage To improve understanding of the durability of pulp-fiber cement composites
11
Flexural Behavior: Mortars P 1.5 in
Longer fibers (softwood vs. hardwood) result in increased toughness, as expected As expected, post-peak toughness increases with increasing Vf, although peak strength generally decreases
300
3 in
300
Plain Mortar
Plain Mortar HW Fibers SW Fibers
HW Fibers
250
250
SWFibers
200
200
Load (lb.)
Load (lb.)
ASTM C 293, 328
150
100
150
100
Vf=1.2%
50
Vf=2%
50
0
0
0
0.01
0.02
0.03
Deflection (in)
0.04
0.05
0.06
0
0.01
0.02
0.03
Deflection (in)
0.04
12
0.05
0.06
Behavior in Compression Use of fiber increases concrete ductility Strength and E of concrete are decreased, particularly as Vf increases Concrete ASTM C 39
5000
Control
W/C= 0.5
Vf=0.5% Vf=1%
4000
Stress (psi)
Vf=2% 3000 W/C = 0.7 2000
1000
0 0
0.005
0.01
0.015
0.02
0.025
Strain
0.03
0.035
0.04
0.045
0.05
Mortar ASTM C1019 13
Behavior in Compression Fiber dispersion greatly influences compressive strength
7 day 14
Objectives To improve dispersion of pulp fibers in cement-based
matrices To characterize effects of pulp fiber addition on mechanical behavior To characterize effects of pulp fiber addition on shrinkage To improve understanding of the durability of pulp-fiber cement composites
15
Free Shrinkage Free shrinkage is greater with non-uniform fiber distribution 2000
no fiber surface treatment
Shrinkage (microstrains)
1800 1600 1400 1200 1000 800
control
600
Vf=0.25% (T)
400
Vf=0.25% (UB-UT)
200 0 0
20
40 60 Time (days)
80
100
16
Restrained Shrinkage Pulp fibers can mitigate cracking in shrinkage-prone cement-based materials
shrinkage crack in unreinforced concrete 1
pulp fiber reinforced concrete is uncracked
Crack width (mm)
0.8
0.6
0.4
ASTM C1581
0.2
0 0
10
20
30 Age (days)
40
50
17
Field Tests: Slab Shrinkage Cracking Cracks in unreinforced slab (NF)
1200 1000
Age at
1st
12
crack
10 Crack Area (in. 2)
Age First Crack Observed (min.)
Cast 7 slabs, varying fiber type and Vf
800 600 400
8 6
2
0
0 PFA
PFB
PFC
OFA
Slab Type
OFB
OFC
Pulp fibers mitigate shrinkage cracking
4
200
NF
Crack surface area, 20h
NF PFA-(low PFB(control) pulp) (med pulp)
PFC- OFA-(low OFB(high poly) (med pulp) poly)
OFC(high poly)
18
Shrinkage What is/are the mechanism(s) by which pulp fibers improve resistance to shrinkage cracking? mechanical - improved crack bridging (i.e., more pulp microfibers
vs. polymeric macrofibers for same Vf)? physical - microcrack vs. macrocrack arrestment? chemical - “internal curing”?
water-filled pores
surface treatment controls moisture release rate
empty pores
water-filled fiber or powder
shrinkage
Prior to self-dessication
Shrinkage due to self-dessication
Internal curing
19
Shrinkage: Background Cement-based materials are prone to shrinkage: chemical shrinkage – volume of reactants is greater than volume of products autogenous shrinkage – due to self-dessication and generation of tensile stress in small capillaries drying shrinkage – due to moisture loss to surrounding environment
When external or internal restraint to this deformation exists, micro and macrocracking can result, which compromises mechanical properties and durability Internal curing can offset autogenous shrinkage
20
Internal Curing Autogenous shrinkage measured in sealed, corrugated tubes of paste,
Autogenous Deformation (microstrain)
from final set Superabsorbent polymers (SAPs) have been shown to reduce autogenous shrinkage by the release of internal curing water 400 0 -400 -800 -1200 -1600 0.1 Control
1 0.25% SAP
Time (days) 0.50% SAP
10
0.75% SAP
100 1.00% SAP
21
Internal Curing: Pulp Fibers Some wood-derived materials appear to be effective internal
curing agents Current research focuses on determining the chemiphysical properties which are advantageous for this application
Autogenous Deformation (microstrain)
400 0 -400 -800 -1200 -1600 0.1 Control
1 0.75% TMP
Time (days) 1.50% TMP
10 2.25% TMP
100 3.00% TMP
22
Objectives To improve dispersion of pulp fibers in cement-based
matrices To characterize effects of pulp fiber addition on mechanical behavior To characterize effects of pulp fiber addition on shrinkage To improve understanding of the durability of pulp-fiber cement composites
23
Load
Effect of Wet/Dry Cycling Percent mass change
35
P 1.5 in
30 Fiber Fiber Fiber Fiber
25 20 15
A B C D
ASTM C 293, 328
10 5
3 in
0 0
4
8
12 16 Time (hr)
20
First Crack/Peak Strength Post-cracking Toughness
24
Load Peak Strength First Crack Strength 24
Deflection
Deflection
Average Peak Strength (lbf)
Effect of Wet/Dry Cycling: Strength 350 300 250 200 150 100 50 0
5
10 15 20 Number of Cycles Kraft Fiber A (never-dried, unbleached, unbeaten) Kraft Fiber B (once-dried, bleached, beaten) Kraft Fiber C (once-dried, bleached, unbeaten) Kraft Fiber D (never-dried, bleached, unbeaten) Thermomechanical Pulp (TMP)
25
Mohr, B.J., Nanko, H., Kurtis, K.E. “Durability of Kraft Pulp Fiber-Cement Composites to Wet/Dry Cycling.” Cement and Concrete Composites 2005; 27(4): 435-448 Mohr, B.J., Nanko, H., Kurtis, K.E. “Durability of Thermomechanical Pulp Fiber-Cement Composites to Wet/Dry Cycling.” Cement 25and Concrete Research, 2005; 35(8): 1646-1649.
Effect of Wet/Dry Cycling: Toughness Average Post-cracking Toughness (lbf-in)
12 10 8 6 4 2 0 0
5
10
15
20
25
Number of Cycles Fiber Fiber Fiber Fiber TMP
A B C D
(never-dried, unbleached, unbeaten) (once-dried, bleached, beaten) (once-dried, bleached, unbeaten) (never-dried, bleached, unbeaten)
Mohr, B.J., Nanko, H., Kurtis, K.E. “Durability of Kraft Pulp Fiber-Cement Composites to Wet/Dry Cycling.” Cement and Concrete Composites 2005; 27(4): 435-448 Mohr, B.J., Nanko, H., Kurtis, K.E. “Durability of Thermomechanical Pulp Fiber-Cement Composites to Wet/Dry Cycling.” Cement 26and Concrete Research, 2005; 35(8): 1646-1649.
Effect of Wet/Dry Cycling: Fracture Surfaces
Fiber A-0 cycles (50X)
Fiber A-5 cycles (50X)
Fiber A-25 cycles (50X)
27
Composite Degradation Mechanism Based on mechanical testing results and some preliminary microscopy, a three-part progressive degradation mechanism was proposed1: 1. Initial fiber-cement or fiber interlayer debonding (accompanied by alkali leaching), 2. Destabilization of monosulfate and reprecipitation of secondary ettringite in void space created by (1), and 3. Fiber embrittlement due to calcium hydroxide (Ca(OH)2) reprecipitation within fiber cell wall structure. With mechanical testing, ESEM and SEM were performed with EDS to further understanding of progressive degradation during wet/dry cycling. 1
Mohr BJ, Nanko H, Kurtis KE, Durability of kraft pulp fiber-cement composites to wet/dry cycling, Cement and Concrete Composites 27;2005:435-448.
28
Prior to initial drying
0 cycle mechanical results
Average Post-cracking Toughness (lbf-in)
Degradation Mechanism: Initial Debonding (1) 12 10 8 6 4 2 0 0
5
10 15 Number of Cycles
20
25
Initial drying
fiber-cement debonding KOH and Ca(OH)2 present on fiber surface Pore solution expelled through ends and pits
1st wetting 1 cycle mechanical results
29
Degradation Mechanism: Initial Debonding (1) Kraft pulp fiber
Kraft pulp fiber ~135% lateral reduction
Swollen state
Shrunken state
TMP fiber ~10% lateral reduction TMP fiber
30
Recall: Pulp Fiber Properties Fiber Length (mm)
Average Tensile Strength (MPa)
Average Fiber Elastic Modulus (GPa)
Kraft pulp
4-8
800-1000
Thermomechanical pulp (TMP)
1-2
Steel (microfiber)
Polypropylene
Asbestos
Density (g/cm3)
Cost ($/kg)
18-43
1.5
0.5
Unbleached: 3-8% lignin Bleached: 0-1% lignin
400-700
--
0.5
0.4
23-31% lignin
2-5
700-2000
200
7.9
>2
High resistance to impact and abrasion, corrosive
varies
300-500
6
0.91
1.5-2.5
Chemically inert, noncorrosive
5-40
200-1800
--
2.55
n/a
Carcinogenic
Notes
31
ESEM EDS Analysis
A Monte Carlo simulation was run to determine an appropriate accelerating voltage (i.e., 10 kV) as to only account for chemical elements on the fiber surface and in the fiber cell wall.
Samples were examined in a water vaporous environment.
No sample coating was necessary.
EDS quantization was standardized against a previously known slag composition and verified against synthetic ettringite.
B.J. Mohr, J.J. Biernacki, and K.E. Kurtis, “Microstructural and Microchemical Changes in Pulp Fiber Cement Composites During Wet/Dry Cycling”, Cement and Concrete Research, V.36( 7):1240-1251, July 2006.
32
Degradation Mechanism: Initial Debonding (1)
0.3
Kraft pulp fiber composites CH replaces C-S-H, MS, and ettringite as the dominant phase on fiber surface 0 cycles
TMP fiber composites no observable changes in surface composition 0 cycles 1 cycle CH + Ettringite CH + Monosulfate
0.3
1 cycle CH + Ettringite CH + Monosulfate
0.2
Al/Ca
Al/Ca
0.2
0.1
0.1
0.0
0.0 0.0
0.1
0.2 S/Ca
0.3
0.0
0.1 S/Ca
0.2 33
0.3
1st Wetting
Leaching of alkalis (K+) 1 cycle mechanical results
Average Post-cracking Toughness (lbf-in)
Degradation Mechanism: Secondary Ettringite Precipitation (2) 12 10 8 6 4 2 0 0
5
10 15 Number of Cycles
20
25
Re-wetting Drying
Precipitation of secondary ettringite at
Full swelling restricted Complete swelling
fiber/cement interface
restriction by 10 cycles
34
Degradation Mechanism: Secondary Ettringite Precipitation (2) Kraft pulp fiber composites 0 cycles 1 cycle 2 cycles CH + Ettringite CH + Monosulfate
0.5 0.4 0.3
0.4 0.3
0.2
0.2
0.1
0.1
0.0
0.0 0.0
0.1
0.2
0.3 S/Ca
0.4
0.5
0 cycles 1 cycle 2 cycles CH + Ettringite CH + Monosulfate
0.5
Al/Ca
Al/Ca
TMP fiber composites
0.0
0.1
0.2 0.3 S/Ca
350.4
0.5
Degradation Mechanism: Secondary Ettringite Precipitation
Kraft pulp fiber composites
TMP fiber composites
36
Degradation Mechanism: Secondary Ettringite Precipitation
Ettringite observed at sites of fiber-cement interfacial debonding
37
Degradation Mechanism: Secondary Ettringite Precipitation No radial microcracking was observed around the kraft pulp fibers after 25 wet/dry cycles.
Thus, ettringite formation does not appear to be exerting any tensile stresses against the cement matrix.
Since the pulp fibers are more compliable, ettringite formation only exerts pressure on the fiber, eventually restricting fiber swelling. 38
Wet State
Average Post-cracking Toughness (lbf-in)
Degradation Mechanism: Fiber Embrittlement (3)
Complete swelling restricted
12 10 8 6 4 2 0 0
5
10 15 Number of Cycles
20
25
Embrittlement Subsequent Drying of Restricted Fibers
Pore solution must diffuse through fiber cell wall, doposting Ca(OH)2
Ca(OH)2 occupies space between cellulose microfibrils Fibers are “mineralized”
39
Degradation Mechanism: Fiber Embrittlement Kraft pulp fiber composites
TMP fiber composites
0 cycles 1 cycle 2 cycles 25 cycles CH + Ettringite CH + Monosulfate
0.5 0.4
0.4
Al/Ca
Al/Ca
0.3 0.2
0.3 0.2
0.1
0.1
0.0
0.0 0.0
0.1
0.2
0.3 S/Ca
0.4
0.5
0 cycles 1 cycle 2 cycles 25 cycles CH + Ettringite CH + Monosulfate
0.5
0.0
0.1
0.2 0.3 S/Ca
400.4
0.5
Degradation Mechanism: Fiber Embrittlement 25 cycles (50X) Areas of fiber swelling; rest of fiber is presumably mineralized
Initial ESEM micrograph at 10% RH
ESEM micrograph after 30 minutes at 41 98% RH
Degradation Mechanism: Fiber Embrittlement Kraft pulp fiber composites Ca(OH)2 and organics present on fiber surface and in cell wall Fiber is mineralized by filling of void space in cell wall with new precipitates
TMP fiber composites Ca(OH)2 found only in lumen Lignin might suppress movement of pore solution through cell wall, protecting fiber from mineralization
42
Summary of Proposed Mechanism The higher lignin contents of the TMP and unbleached kraft pulp fibercement composites is believed to lead to greater toughness, particularly for low numbers of cycles.
Initial kraft fiber-cement or fiber interlayer debonding is observed after 1 wet/dry cycle. TMP fibers exhibit minimal shrinkage.
Reprecipitation of ettringite occurs within the void space around/within the kraft fibers after 2 wet/dry cycles. No ettringite is observed around TMP fibers.
Once fiber swelling/shrinkage is fully restrained due to the ettringite, CH reprecipitates within the kraft fiber cell wall. No reprecipitation within the TMP fiber cell wall is observed, most likely due to the presence of lignin. 43
Mitigation Matrix modifications in kraft fiber-cement composites were evaluated by replacing portland cement with the following supplementary cementitious materials (SCMs): Silica fume (10, 30, 50%) Slag (10, 30, 50, 70, 90%) Class F and C fly ash (10, 30, 50, 70%) Metakaolin (10, 30%) Diatomaceous earth / volcanic ash blend (10, 30, 50%) Two types of metakaolin and DEVA blend were used, each varying in particle size only. Ternary and quaternary blends were also investigated to optimize cost and performance. 44
Mitigation: SCMs
C3 S + H C2 S + H
FAST
FAST
C-S-H + CH C-S-H + CH
Pozzolan + CH + H
SLOW
C-S-H
45
Mitigation: Silica Fume
Average Post-cracking Toughness (lbf-in)
100
No matrix modifications 10% silica fume replacement 30% silica fume replacement 50% silica fume replacement
10
1
0.1 0
5
10
15
20
25
Number of Wet/Dry Cycles 46
Mitigation: Silica Fume After 25 wet/dry cycles 0.50
0.30
0.4 0.3 0.2
0.20 0.10
0.1
0.00
0.0
0.00
No SCMs 10% silica fume 50% silica fume CH + Ettringite CH + Monosulfate
0.5
Al/Ca
Al/Ca
0.40
No SCMs 10% silica fume 50% silica fume CH + CSH Monosulfate + CSH CSH
0.40 0.80 Si/Ca
1.20
0.0
0.1
0.2 0.3 S/Ca
0.4
0.5 47
Migitation: Slag No matrix modifications 10% slag replacement 30% slag replacement 50% slag replacement 70% slag replacement 90% slag replacement
Average Post-cracking Toughness (lbf-in)
100
10
1
0.1 0
5
10
15
20
25
Number of Wet/Dry Cycles 48
Migitation: Slag After 25 wet/dry cycles
0.50 0.40
0.20
0.4 0.3 0.2
0.10
0.1
0.00 0.00
0.0
0.40 0.80 Si/Ca
1.20
No SCMs 10% slag 50% slag 90% slag CH + Ettringite CH + Monosulfate
0.5
Al/Ca
Al/Ca
0.30
No SCMs 10% slag 50% slag 90% slag CH + CSH Monosulfate + CSH CSH
0.0
0.1
0.2 0.3 S/Ca
0.4
0.5 49
Mitigation: Mechanism Appears to be related to pozzolanic reaction and concomitant changes in pore solution chemistry, rather than reductions in permeability TMP (primary) - w/c=0.26 TMP (primary) - w/c=0.3 TMP (primary) - w/c=0.35 TMP (primary) - w/c=0.4 TMP (primary) - w/c=0.5 TMP (primary) - w/c=0.6
1
0.1
0.01 0
5
Average Post-cracking Toughness (lbf-in)
Average Post-cracking Toughness (lbf-in)
10
10 15 Number of Wet/Dry Cycles
Kraft pulp - w/c=0 Kraft pulp - w/c=0 Kraft pulp - w/c=0
10
20
25
0
5
1
0.1 10
15
Number of Wet/Dry Cycles
5020
2
Summary of Mitigation Strategies Generally, all SCM replacements led to improved wet/dry cycling
composite toughness durability. The following SCM replacements completely mitigated composite degradation: • 30, 50% silica fume • 30% metakaolin • 90% slag • 10% metakaolin / 70% slag • 10% silica fume / 70% slag SCM replacement minimized ettringite reprecipitation due to reduction in available calcium ions through formation of supplementary C-S-H. Calcium hydroxide reprecipitation minimized due to formation of supplementary C-S-H. 51
Summary Pulp fiber reinforcement of cement-based materials can improve ductility and toughness while retaining strength IF good fiber dispersion is achieved
An economical method for surface treatment to improved dispersion has been developed
In addition to their use in thin sheet composites, there is a potential large new market for pulp fiber reinforcement of concrete for shrinkage crack mitigation
In addition to mechanical and physical effects of pulp fiber reinforcement in concrete, internal curing likely also plays an important role 52
Summary Wet/dry cycling has been shown to lead to progressive degradation of fiber/cement composites, when fibers which show shrinkage on initial drying are used
Use of more dimensionally stable, and lignin-rich pulp fibers (e.g., TMP) and/or incorporation of SCMs can mitigate this degradation
However, degradation may be inconsequential for certain applications (i.e., mitigation of early-age shrinkage)
53
Ongoing Activities Quantifying effects of
Percent Expansion
pulp fiber reinforcement for mitigation of cracking due to expansive reactions in concrete (e.g., alkali silica reaction (ASR), freeze/thaw) Tailoring fiber chemical composition and morphology for internal curing Development of coating methods to facilitate internal curing
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0
7
14 21 28 Exposure Time (days) control 1% kraft 3% kraft 1% TMP 3% TMP 1% 1/4" polypropylene 3% 1/4" polypropylene
Commercialization??? 54
Acknowledgements
Georgia Tech/IPST seed grant IPST PATHWAYS research program IPST AMRC research funds Georgia-Pacific, Weyerhaeuser, Abitibi U.S. National Science Foundation (CMS-0122068), funded through U.S. Housing and Urban Developments Partnership for Advancing Technology for Housing (PATH) program
Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the sponsors. 55
Effect of Wet/Dry Cycling Despite the economic advantages of the use of pulp fibers in cement-based materials to improve ductility and resistance to shrinkage cracking in both thin section composties and concrete, the durability of these materials must be assessed. Research goals here include:
Characterization and quantification of the nature of the fiber-cement fracture surface prior to and after wet/dry cycling
Use of this characterization to better understand the process of degradation (i.e., relative contributions of fiber pull-out and fiber fracture to damage propagation and toughness)
Assessment of potential mitigation strategies 56
Methods With laser scanning confocal microscopy (LSCM): Samples can be studied without
drying, epoxy impregnation, coating or other preparation which may alter structure No depth of field limits (good for rough surfaces) Volumetric or surface characterization of microstructure. Quantification of surface structure
57
LSCM Applications in Cement-based Composites
Air void
Hydration product
Phase contrast
Through-aggregate imaging K.E. Kurtis, N.H. El-Ashkar, C.L. Collins, and N.N. Naik, “Examining Cement-Based Materials by Laser Scanning Confocal Microscopy”, Cement & Concrete Composites, 25(7);2003:695-701.
58
Method: Imaging Imaging
“Stacks” of images for each location
• • • • •
9 locations/surface 400x magnification Imaged at 495 V Ar-laser 5 μm incremental focal planes
3D representation of microstructure
Montage of focal plane images
59
Method: Roughness Number
A ∑ RN = ∑ Ap i
Ai = triangulated surface area Api = corresponding projected area
i
K.E. Kurtis, N.H. El-Ashkar, C.L. Collins, and N.N. Naik, “Examining Cement-Based Materials by Laser Scanning Confocal Microscopy”, Cement & Concrete Composites, 25(7);2003:695-701.
60
Results: Roughness Number Roughness Number
160 Slag Class C fly ash Silica fume
120
y = 11.637x + 12.963 R2 = 0.9663
80
y = 12.011x + 17.238 R2 = 0.944
40
y = 11.805x + 17.783 R2 = 0.8908
0 0
2
4 6 8 Post-cracking Toughness (lbf-in)
10
12
Very good correlation between toughness and RN Inherent surface roughness due to particle roughness and porosity causes RN >1
when theoretical toughness is 0. Essentially no difference in surface roughness for various matrix compositions. Thus, matrix particle size distribution has no significant mechanical toughening 61 effect at the observed scale.
Method: Fractal Dimension
A user-defined line placed on the LSCM topographic images describes the z-directional crack path through the composite.
Using Image Pro Plus, the fractal dimension was calculated for each fracture profile.
This method allows for the isolation of matrix roughness from the net fracture surface roughness. Thus, the effect of fiber addition rate on the composite toughening can be evaluated.
Matrix cracking only
Cracking in reinforced composite 62
Results: Fractal Dimension Fractal Dimension
1.1 Without fibers With fibers
1.08 1.06
y = 0.0029x + 1.0535
y = 0.0014x + 1.0518
1.04 1.02 1 0
2
4 6 8 Post-cracking Toughness
10
12
Slope of matrix (measured in subregions without fibers) is indicative of fiber
contribution to crack deflection and matrix fracture surface coarsening. If m=0, composite failure would be completely brittle with no frictional energy dissipation (i.e., pure fiber pull-out). 63
Results: Fractal Dimension
Area between trendlines (blue region) indicative of contribution of fibers to
composite toughening. An increase in the slope of the net surface line indicates that the fibers have an increasing effect on dissipating energy through matrix crack deflection. The relative slopes of these lines may be used to quantify the extent of fiber-cement bonding and the degree of fiber pull-out versus fiber fracture during composite 64 failure
Summary
LSCM was advantageous for the quantification of fracture surfaces in fiber-cement composites and useful for understanding the mechanisms of failure.
A strong correlation between composite post-cracking toughness and surface roughness was observed for fiber-cement composite fracture surfaces.
A degree of inherent surface roughness, likely due to the inhomogeneous and porous microstructure of the hydrated cement, which does not contribute to toughness, was measured by both the roughness number and fractal dimension.
Partial replacement of portland cement with silica fume, slag, or Class C fly ash did not have a noticeable influence on the pulp fiber-cement composite fracture behavior as measured by the fracture surface roughness number. 65
Summary
The fractal dimension of the fracture surfaces showed that matrix cracking was a contributing factor for increased toughness. Composite failure was shown to be a combination of fiber pull-out and fiber fracture.
Increased toughness of the composites was attributed to increased fiber pull-out, as compared to samples with minimal toughness which primarily failed by fiber fracture.
Fractal dimension calculations indicated that the fibers contributed to composite toughening by fiber pull-out and crack deflection. However, crack deflection was not as significant as previously thought.
66
Ongoing Research
More research is necessary to elucidate the mechanisms of fracture behavior as measured by the fractal dimension.
In addition, by measuring the differences in fractal dimension as a function of the number of wet/dry cycles, the influence of environmentally-induced degradation on fracture behavior can be assessed.
67
Acknowledgements
Support from
the National Science Foundation (CMS0122068), the Institute of Paper Science and Technology (IPST)/Georgia Tech seed grant program, and IPST PATHWAYS program is gratefully acknowledged.1
The authors also gratefully acknowledge the contributions of Dr. Hiroki Nanko, of IPST, to this study.
1
Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the sponsors.
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Questions?
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