Mechanics of Cracking in Fiber Reinforced Cementitious Composites Eduardo B. Pereira 2011, October...

Mechanics of Cracking in Fiber ReinforcedCementitious Composites

Eduardo B. Pereira

2011, October 24 - 27

Supervisors:

Joaquim A. O. Barros, Gregor Fischer

Escola de Engenharia

Semana da Escola de Engenharia October 24 - 27, 2011

2|Mechanics of Cracking in Fiber Reinforced Cementitious Composites Eduardo B. Pereira



Semana da Escola de EngenhariaOctober 24 – 27, 2011

Outline of the Presentation

• Introduction to the material:• Definitions;• Some applications.

• Assessment of tensile performance:• Tensile stress-strain behavior: pseudo-strain hardening and multiple cracking;• Tensile stress-crack opening characterisation: PVA, PAN, PP.• Relevant features of the tensile stress-crack opening behavior in SHCC.

• Mechanics of cracking: initiation and propagation: • Investigation of crack initiation and propagation in cementitious composites: CT speciments;• Observation of cracking process using digital image analysis;• Analysis of the influence of aggregate size and fiber reinforcement;• Measurement of crack profiles and analysis of FPZ.

• Conclusions





• FRC: Fiber Reinforced Concretes;

• FRCC: Fiber Reinforced Cementitious Composites;

• SHCC: Strain Hardening Cementitious Composites;

• HPFRCC: High Performance Fiber Reinforced Cement Composites;

• UHPFRCC: Ultra HPFRCC;

Definitions

• Composite = Fibers + Matrix

• Matrix = cement + other fine additions + chemical admixtures + aggregates...

in Naaman and Reinhardt, 2005





• FRC: Fiber Reinforced Concretes;

• FRCC: Fiber Reinforced Cementitious Composites;

• SHCC: Strain Hardening Cementitious Composites;

• HPFRCC: High Performance Fiber Reinforced Cement Composites;

• UHPFRCC: Ultra HPFRCC;

Definitions

• Composite = Fibers + Matrix

• Matrix = cement + other fine additions + chemical admixtures + aggregates...





Some applications

In Rokugo et al. Materials and Structures 2009http://ace-mrl.engin.umich.edu/NewFiles/webapp_files/frame.htm

• Extruded elements• Continuum bridges• Retrofitting of dams, viaducts,

retaining walls and irrigation channels• Seismic aplications: coupling beams

in high-rise buildings;

ECC link slab





- s e .

From Li, V.C. 2005

Material characterization

Material design - s w

PART 1: Assessment of the tensile performance of SHCC





Single Crack Tension Test - SCTT





Analysis of crack formation and propagation: DIC





E = 20 GPa

n = 0.2

Analysis of crack formation and propagation: FEM





Analysis of crack formation and propagation: FEM

Load increment 10s = 2.34 MPa

w = 0.0029 mm

Load increment 37s = 4.57 MPa

w = 0.0075 mm





Tensile strength

Length Diam Fiber reinforcement (%vol) Fiber

MPa mm m C1 C2 C3 C4 C5 C6

PVA 15 1600 8 40.0 1 2 0 0 1 1

PP 900 12 40.0 0 0 2.5 0 1.25 0

PAN 1.5 826 6 12.7 0 0 0 2 0 1

Long plates

Casting formwork shape:600 mm

120 mm

(20 mm)

Materials:





0.0 0.5 1.0 1.5 2.00

1

2

3

4

5

6

7

Ten

sile

Str

ess

(MPa

)

CMOD (mm)

average curve

experimental results

0.0 0.5 1.0 1.5 2.00

1

2

3

4

5

6

7

average curve

Ten

sile

Str

ess

(MPa

)

CMOD (mm)


0.0 0.5 1.0 1.5 2.00

1

2

3

4

5

6

7

Ten

sile

Str

ess

(MPa

)

CMOD (mm)

average curve


0.0 0.5 1.0 1.5 2.00

1

2

3

4

5

6

7

Ten

sile

Str

ess

(MPa

)

CMOD (mm)

average curve


0.0 0.5 1.0 1.5 2.00

1

2

3

4

5

6

7

Ten

sile

Str

ess

(MPa

)CMOD (mm)

average curve


0.0 0.5 1.0 1.5 2.00

1

2

3

4

5

6

7

Ten

sile

Str

ess

(MPa

)

CMOD (mm)

average curve


1% PVA

2% PVA

2% PAN

1% PVA + 1% PAN

2.5% PP

1% PVA + 1.25% PP

Results:





• Importance of the assessment of the s - w law:• Material design;• Structural analysis / modelling / design.

• Tailoring of the tensile properties of SHCC:• Distinction of characteristic behaviors of different types of fibers and/or matrices;• Explicit quantification of possible synergistic effects resulting of the combination of different

types of fibers. • Test results are very sensitive to the main composite parameters; slight differences are detected,

important to guiding efficiently the material design process.

Remarks:





PART 2: Mechanics of cracking - initiation and propagation• Investigation of cracking mechanisms near the crack tip of propagating cracks:

• Study the application of digital image analysis;• Composites with different aggregate sizes and matrices;• Effect of fiber reinforcement in the fracture process zone and crack profiles.





• Specimen geometry: CT specimen.

From Sanford, R. “ Principles of Fracture Mechanics”.

Mechanics of cracking in Cementitious Composites






SPECIMEN GEOMETRY





Cement Fly ash Fine sand(0.170 mm)

Quartz Powder

Water

420 g 850 g 150 g 150 g 100 cm3

MORTAR and FRCC

Fiber Tensile strength Length Diameter

PVA 1600 MPa 8 mm 40 mm

Cement Water

1360 g 570 cm3

CEMENT PASTE (w/c =0.42)

Cement Aggregates (0-4 mm) Water

660 g 1341 g 275 cm3

CONCRETE (w/c = 0.42)

DISPLACEMENT RATE: 5 mm/s

Depth = 12 mm

Notch width = 0.5 mm

Materials and test procedure:





0

50

100

150

200

250

300

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6CMOD [mm]

Lo

ad [

N]

cement paste

mortar

concrete

FRCC - initial peak

FRCC - ultimate peak

CEMENT PASTE MORTAR

CONCRETE

FRCCMechanical results:





Example:concrete

Digital Image-based monitoring setup:





Entire loading sequence. Example:

concrete specimen

Digital Image-based monitoring results:





Real surface(B&W)





Facet overlay





0

50

100

150

200

250

300

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Loa

d [N

]

cement paste

mortar

concrete

FRCC - initial peak

FRCC - ultimate peak

90% of peak load

(before peak) peak load 50% of peak load

(after peak)

Cement

paste

concrete

mortar

FRCC

Digital Image-based monitoring results:





Measurement of the crack profiles:

• Virtual clip gage length:2 mm• Longiturinal spacing: 1 mm• Total length covered: 30 mm

• Virtual clip gage length:10 mm

• Longiturinal spacing: 1 mm• Total length covered: 30 mm

Specimens analyzed: Mortar and FRCC

x

Morphology of FPZ





x

-0.005

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0 10 20 30

d(m

m)

x (mm)

measured resultspolinomial trendline

P-10%

P

P-20%

P-50%

P-10%

x

-0.005

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0 10 20 30

d(m

m)

x (mm)


P-20%

P-50%

P-10%

P

P-50%

P-10%

2 mm

10 mm

2 mm

10 mm

2 mm 10 mm

P P - 20%

Morphology of FPZ - mortar





-0.005

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0 10 20 30

d(m

m)

x (mm)


MORTAR FRCC

• Alteration of crack profiles;• Formation of a compression zone ahead of the crack tip;• Smaller crack lengths for the same CTOD;• Sharper transition from the intact bulk material to the open crack.

-0.005

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0 10 20 30

d(m

m)

x (mm)


P-10%

P

P-20%

P-50%

P-10%

2 mmMorphology of FPZ – influence of fiber reinforcement





• The compression zone extends to the material surrounding the crack, including the open crack region.

-0.010

-0.005

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0 10 20 30d

(mm

)

x (mm)


-0.005

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0 10 20 30

d(m

m)

x (mm)


P-20%

P-50%

P-10%

P

P-50%

P-10%

MORTAR FRCC10 mm

Morphology of FPZ – influence of fiber reinforcement





Remarks:

• The initiation and propagation of cracks can be traced in cementicious composites with appreciable resolution using the digital image analysis;

• Different crack smearing or branching features were identified, deppending of aggregate size and matrix composition; secondary shrinkage cracks were also detected; the analysis of these features of the cracking processes helped to explain the mechanical results observed.

• The precise measurement of the crack profiles showed that the fibers in the FRCC alter the shape and the micromechanics of the fracture process zone. For the same crack opening, crack shape and crack length are modified, as well as the strains surrounding the fracture process zone. Therefore the fiber contribution is not confined to the fiber bridging region of the fracture process zone.





Conclusions:

• The complex interaction between phases in cementitious matrix composites requires a thorough knowledge of the underlying mechanics of cracking;

• The test-setup proposed during the first part of this presentation contributes to an effective assessment of the tensile performance of SHCC. It is sensitive to important parameters of the composite, therefore it is useful both to the optimal design of the material and to the accurate constitutive modeling with SHCC;

• The better understanding of cracking process, crack morphology and the mechanics of the fracture process zone contribute to the efficient design and the full exploration of SHCC special features; the use of digital image analysis to study the surface of specimens while undergoing cracking showed addequate resolution and potential in the present research context.





Thank you!

Mechanics of Cracking in Fiber Reinforced Cementitious Composites Eduardo B. Pereira 2011, October...

Documents

Transcript of Mechanics of Cracking in Fiber Reinforced Cementitious Composites Eduardo B. Pereira 2011, October...