Fracture and resistance-curve behavior in hybrid natural fiberand polypropylene fiber reinforced composites

T. Tan • S. F. Santos • H. Savastano Jr. •

W. O. Soboyejo

Received: 16 August 2011 / Accepted: 8 November 2011 / Published online: 29 November 2011

� Springer Science+Business Media, LLC 2011

Abstract This article presents the results of a combined

experimental and theoretical study of fracture and resis-

tance-curve behavior of hybrid natural fiber- and synthetic

polymer fiber-reinforced composites that are being devel-

oped for potential applications in affordable housing.

Fracture and resistance-curve behavior are studied using

single-edge notched bend specimens. The sisal fibers used

were examined using atomic force microscopy for fiber

bundle structures. The underlying crack/microstructure

interactions and fracture mechanisms are elucidated via in

situ optical microscopy and ex-situ environmental scanning

microscopy techniques. The observed crack bridging

mechanisms are modeled using small and large scale

bridging concepts. The implications of the results are then

discussed for the design of eco-friendly building materials

that are reinforced with natural and polypropylene fibers.

Introduction

In recent years, there has been increasing interest to use of

the natural fiber-reinforced cementitious composites for

affordable infrastructures [1, 2]. Significant study has also

been carried out on natural fiber-reinforced composites that

are produced by processes to be used in developing

countries [3, 4]. The durability of natural fiber-reinforced

composites has been examined by Savastano [4]. However,

there have been fewer studies of the fracture properties of

synthetic polymer and vegetable fibers. These will be

explained in this article using model composites consisting

of synthetic polymer and vegetable fibers in different

matrices containing carbonate filler, phylite or metakaolin.

This article presents the results of a combined experi-

mental and theoretical study of the R-curve behavior in

some hybrid natural and polypropylene fiber-reinforced

cementitous composites. The motivations for elected

materials are multifold. Sisal and polypropylene fibers can

be used as low modulus fibers to improve composites

toughness. Eucalyptus or sisal pulps [4] can improve

composite processing and also the mechanical reinforce-

ment of the fragile matrices. Mineral additions in these

cemetitious composites, such as carbonate filler, phylite or

metakaolin, can contribute with decrease of clinker con-

sumption and pozzolanic activities involving portlandite

consumption.

The article examines the effects of commercial poly-

propylene and sisal fibers. Resistance-curve behavior is

elucidated using edge notched bend specimens (SENB) in

three-point bending experiments. The fiber bridging pro-

cess was revealed by in-situ crack bridging images

obtained from optical microscope. Subsequently, single-

fiber experiments were conducted to measure the tensile

strength of these reinforcing fibers. Based on these

T. Tan � W. O. Soboyejo

Department of Civil and Environmental Engineering,

Princeton University, Princeton, NJ 08544, USA

T. Tan � H. Savastano Jr. � W. O. Soboyejo (&)

Princeton Institute for the Science and Technology of Materials,

Princeton University, Princeton, NJ 08544, USA

e-mail: soboyejo@aol.com

S. F. Santos � H. Savastano Jr.

Construction and Ambience Group, Faculty of Animal Science

and Food Engineering, University of Sao Paulo,

Sao Paulo, Brazil

W. O. Soboyejo

Department of Mechanical and Aerospace Engineering,

Princeton University, Princeton, NJ 08544, USA

123

J Mater Sci (2012) 47:2864–2874

DOI 10.1007/s10853-011-6116-1

experimental results, the measured resistance curves are

then compared to predictions from small- and large-scale

bridging (SSB/LSB) models that include microstructural

and crack/microstructure interactions by using Environ-

mental scanning electron microscopy (ESEM). The impli-

cations of these results are discussed for the development

of hybrid components reinforced with natural and synthetic

fibers.

Materials

Materials processing

The matrices used in this study include carbonate filler,

metakoalin, and phylite in Brazil. The control binder was

the Brazilian Portland cement CPV-ARI [5] of high initial

strength and low amount of mineral additions in its com-

position. The sisal pulp and eucalyptus pulp are produced

through the pulping process described by [6, 7]. Two kinds

of fibers, polypropylene fibers (PP), and sisal fibers (SS),

were used to reinforce the matrix bases. The primary

properties of macro fibers and pulps that were used in this

study are summarized in Table 1.

The microstructural features of the sisal fiber bundles

were characterized using a Dimension 3100 Atomic Force

Microscope (AFM) (Dimension, Bruker Instruments,

Kennewick, WA) [11, 12]. Prior to AFM analyses, fiber

samples were stabilized in Permatex 84101 adhesive

(Permatex, Hartford, CT). They were then cut along their

cross sections, which were polished to 0.5 lm surface

finish. The surface morphologies of the samples were then

characterized using the AFM instrument in tapping mode.

Two-dimensional and three-dimensional images were

obtained, as shown in Fig. 1a and b. The two- and three-

dimensional images reveal a cell structure that consists of

sisal fiber bundles. The surface topography of the region

that was examined had a root-mean squared (RMS)

roughness of *21.3 nm.

Composite processing

The raw materials that were used in the production of fiber-

reinforced composites are listed in Tables 2 and 3. The

slurry vacuum de-watering technique [6, 13] was applied to

produce these composites in laboratory scale as a crude

reproduction of the industrial large scale processes such as

Hatschek (or wet) method [14] commonly used for fiber

cement production. The specimens were produced in the

laboratory by slurrying the raw-material in water solution

(20% of solids) before a vacuum drainage of the excess of

water and pressing at 3.2 MPa during 5 min. After press-

ing, the thickness of the pad was around 15 mm. Upon

completion of consolidation, the pads were sealed in a

plastic bag to cure in saturated air at ambient temperature

for 2 days. In order to optimize hydration, the curing was

continued by placing the pads in a water bath at room

temperature for an additional 26 days. The pads were then

air-cured in a laboratory environment until they were pre-

pared and tested after 3 years.

Experimental procedures

Resistance-curve experiments

The resistance-curve experiments were performed on

SENB specimens [15, 16] with the widths, W, of 15 mm,

thicknesses, B, of 15 mm and the length, L, of 65 mm. The

initial notch-to-width ratio, a0/W, were 0.25. The speci-

mens were prepared by starting from the raw pads that

were produced using techniques discussed in Sect. 2.1. The

sides of the specimens were then diamond polished to a

0.5 lm finish using a diamond cutoff wheel.

Table 1 Pulp and fiber properties

Property Sisal fiber Polypropylene fiber Sisal pulp Eucalyptus pulp

Kappa numbera – – 20 ± 0.8 1.8 ± 0.1

Canadian standard freenessb (mL) – – 220 220

Fiber length (length weighted)c (mm) 5.20 ± 0.42 6.00 ± 0.10 1.13 ± 0.05 0.78 ± 0.01

Fiber width average (lm)d 120.0 ± 22.7 12.0 ± 0.05 18.7 ± 0.2 17.3 ± 0.3

Aspect ratio 40 500 60 45

Notice Fiber Fiber Pulp Pulp

a Appita P201m-86 [8]b AS 1301.206s-88 [9]c Kajaani FS-200 [10]d Average of 20 determinations by scanning electron microscopy (SEM)

J Mater Sci (2012) 47:2864–2874 2865

123

The resistance-curve experiments were performed under

three-point bending tests. The loads were applied across a

supporting span of 36 mm using an Instron Model 8501

(Instron, Canton, MA) servo-hydraulic testing machine that

was operated under displacement control at a displacement

rate of 10-4 mm/s. A Celestron digital microscope (Cele-

stron, Torrrance, California) was used to record the in situ

videos of crack growth and crack/material interactions.

This was continued until unstable crack growth/fracture

occurred during incremental loading. The experiments

were carried out in a laboratory environment with a relative

humidity of 45% and a temperature of 20 �C.

The stress intensity factor K in the three-point bending

test was obtained by the following expression in the ASTM

E399-90 fracture toughness testing code [17]:

K ¼PS f a

W

� �

BW1:5ð1Þ

where

fa

W

� �

¼3 a

W

� �0:51:99� a

W

� �1� a

W

� �2:15� 3:93 a

W þ 2:7 a2

W2

� �h i

2 1þ 2 aW

� �1� a

W

� �1:5

ð2Þ

where P is the applied load, S is the supporting span, B is

the beam thickness, W is the beam depth, and a the crack

length. Details on stress intensity expression of the three-

point bending test can be found in the ASTM E399-90 [17].

Three tests were performed for each type fiber composite.

Data of each sample were recorded with the crack exten-

sion to 6 mm.

Fracture surface characterization

After the SENB experiments, the fracture surfaces of

specimens were characterized using an FEI Quanta 200

Environmental Scanning Electronic Microscope (ESEM)

(FEI, Hillsboro, Oregon) [18]. This enabled high quality

topographic images to be obtained under low vacuum with

controlled water vapor pressure. The fracture surfaces were

examined using the ESEM at pressures ranging from 8.3 to

9.8 Torr (1106 to 1307 Pa). Point-wise Energy Dispersive

Fig. 1 Atomic force microscopy (AFM) images of sisal fiber bundle

structure: a Two-dimensional surface topography and b Three-

dimensional image of sisal fiber bundle structure

Table 2 Polypropylene fiber- reinforced composites, with different

cellulose pulps contents (% by volume of solid raw materials)

Raw materials Formulations (% by volume)

PP ? EU PP ? Sisal

Cement CPV-ARI 56.99 56.99

Carbonate filler (CF) 25.45 25.45

Silica fume (Silmix) 3.58 3.58

Polypropylene fiber (PP) 3.56 3.56

Sisal pulp – 10.42

Eucalyptus pulp (EU) 10.42 –

Table 3 Sisal fiber- reinforced composites, with different cellulose

pulps contents (% by volume of solid raw materials)

Raw materials Formulations (% by volume)

Sisal ? Met Sisal ? Phylite

Cement CPV-ARI 58.12 58.12

Metakaolin (Met) 31.38 –

Phylite – 31.38

Sisal fiber 3.57 3.57

Eucalyptus pulp 6.93 6.93

2866 J Mater Sci (2012) 47:2864–2874

123

Spectroscopy (EDS) analyses were carried out at various

spots to distinguish the local differences in chemical

compositions and the mapping of element distributions.

Single-fiber tensile tests

The mechanical properties of the polypropylene and sisal

fibers were measured using micro-testing techniques

improved from previous study [19–22]. This was done to

provide measurement of strength for use in crack bridging

models. Hence, the experimental techniques will only be

summarized here.

The polypropylene and sisal fibers were aligned and

bonded to the clamps using LOCTITE 495 super bonder

(Henkel Corp., Westlake, OH). The loading fixture is

schematically shown in Fig. 2. The clamps were then fas-

tened using screws before the super bonder cures. After

12 h, single-fiber tensile tests were conducted in an Instron

model 5878 electromechanical machine (Instron, Canton,

MA). The fibers were loaded continuously to fracture at a

displacement rate of 0.01 mm/s. A Celestron digital

microscope (Celestron, Torrrance, California) was used to

record in situ images of specimen lengths that were used to

determine the strains [15]. The resulting stress–strain

curves were obtained from load-position curves for these

fibers. The variations in the stress–strain behavior were

characterized by performing triplicate tests on each type of

fiber.

Modeling

Many efforts have been devoted to study the fiber bridging

models, such as Budiansky et al. [23], Bennett and Young

[24, 25], Li and Soboyejo [26], and Kung et al. [27]. The

role of crack bridging was modeled using an idealized

elastic–plastic spring model, proposed originally by

Budiansky et al. [23] to study toughening due to small-

scale bridging. Similary, a theoretical model developed by

Bloyer et al. [28, 29], and used in earlier study by Lou et al.

[30] and Savastano et al. [4], was used to estimate the

toughening due to large scale bridging.

Small scale bridging models

For small scale bridging, in which the size of the bridging

zone is much smaller than the crack length (bridge

length B 0.5 mm) [23], the extent of toughening due to the

crack bridging may be expressed in terms of the maximum

stress intensity factor the material can sustain before fail-

ure. The fracture toughness of the composite, Kc, can be

expressed as the sum of the matrix fracture toughness, Km,

and the toughening component due to small-scale crack

bridging, DKSSB, this gives:

Kc ¼ Km þ DKSSB ¼ Km þffiffiffi2

p

r

aVf

ZL

0

ryffiffiffixp dx ð3Þ

where a is the constraint/triaxiality factor (typically

between 1 and 3 and taken to be 1 in this study) [23], Vf is

the volume fraction of bridging phase, L is the length of the

bridging ligament, ry is the uniaxial yield stress, and x is

the distance from the crack-tip.

Large scale bridging models

For large scale bridging, the toughening due to ligament

bridging is given by [28, 29]:

DKLSB ¼ Vf

ZL

0

aryh a; xð Þdx ð4Þ

where h(a, x) is the weighting function for the bridging

tractions given by Fett and Munz [31]. This is given by:

hða; xÞ ¼ffiffiffiffiffiffi2

pa

r1ffiffiffiffiffiffiffiffiffiffi1� x

a

p 1þX

t;lð Þ

AtlaW

� �

1� aW

� � 1� x

a

� �mþ1

0

@

1

A

ð5Þ

where the coefficients Aml are given in Ref. [31] for a

single-edge notched bend (SENB) specimen. The overall

composite fracture toughness for large scale bridging is

now given byFig. 2 Schematic of clamping system of the single-fiber tensile

experiment

J Mater Sci (2012) 47:2864–2874 2867

123

Kc ¼ Km þ DKLSB ¼ Km þ Vf

ZL

0

aryh a; xð Þdx ð6Þ

where the terms in the above expression have the same

meanings as previous definition.

Results and discussion

Resistance-curve behavior

The resistance-curve obtained for the fiber-cement com-

posites are presented in Figs. 3, 4, 5, and 6. The crack

initiate at stress intensity factors, K0, of 0.12 MPaffiffiffiffimp

(SS

fibers and EU pulp in OPC-based matrix with metakaolin)

and 0.22 MPaffiffiffiffimp

(SS fibers and EU pulp in OPC-base

matrix with phylite). For composites with carbonate filler

and silica fume, the crack initiation stress intensity factors

are 0.27MPaffiffiffiffimp

(PP fibers and EU pulp in OPC-based

matrix with carbonate filler) and 0.35 MPaffiffiffiffimp

(PP fibers

and SS pulp in OPC-based matrix with carbonate filler).

For the sisal fiber based composites, the use of phylite

results in higher levels of toughening than those in the

composites with metakaolin addition. However, in the case

of the formulation with PP fibers, the reinforcement with

SS pulp is more effective than reinforcement with EU pulp.

Details of the fiber properties are presented in Table 1,

where it can be checked the higher length and aspect ratio

for SS pulp in comparison to EU pulp. Toughening

mechanism will be discussed in this section after present-

ing the results of the single-fiber tensile tests and fracto-

graphic analyses.

Single-fiber tensile experiment

The stress–strain curves obtained from the single-fiber

tensile tests are presented in Fig. 7a and b. From Fig. 7a, it

shows that the average ultimate tensile strength of poly-

propylene fibers is 1019.1 ± 32.7 MPa. Also the total

strain to fracture of the polypropylene fibers is

22.0 ± 1.0% strain. Note that the stress–strain curves are

comparable from tests on three fiber samples each. In the

case of the sisal fibers, the average ultimate tensile strength

is 989.0 ± 23.0 MPa, while the strain to fracture is

14.0 ± 1.4%. As for the polypropylene fibers, the stress–

strain curves obtained for the sisal fibers are reasonably

consistent (Fig. 7b).

Fig. 3 Experiment R-curves for the composites with carbonate filler

and silica fume reinforced by PP fibers and SS pulps. Predictions from

SSB and LSB models are also presented

Fig. 4 Experiment R-curves for the composites with carbonate filler

and silica fume reinforced by PP fibers and EU pulp. Predictions from

SSB and LSB models are also presented

Fig. 5 Experiment R-curves for the composites with phylite

reinforced by SS fibers and EU pulp. Predictions from SSB and

LSB models are also presented

2868 J Mater Sci (2012) 47:2864–2874

123

Comparison of measured and predicted toughening

The measured resistance curves obtained for the different

fiber composites are compared with predictions on the

micromechanical models in this section. The small scale

bridging model (SSB) was used for the modeling resistance-

curve behavior for crack extension, Da, less than 0.5 mm

Fig. 6 Experiment R-curves for the composite with metakaolin

reinforced by SS fibers and EU pulp. Predictions from SSB and LSB

models are also presented

(a)

(b)

Fig. 7 Stress-strain curves for fibers used in reinforced composites

a the polypropylene fibers b the sisal fibers

(a)

(b)

Fig. 8 In situ ‘‘fiber bridging’’ images of composites with carbonate

filler and silica fume reinforced by PP fibers and SS pulp: a t = t*;

b t = t ? 80s

Table 4 Fiber mechanical properties used in SSB and LSB models

Matrix MET PHYLITE CF ? Silimix CF ? Silimix

Fiber SS fiber SS fibers PP fiber PP fiber

a 1.0 1.0 1.0 1.0

ry 400 MPa 400 MPa 600 MPa 600 MPa

EU pulp EU pulp EU pulp SS pulp

Vf 10.6% 10.7% 11.8% 12.2%

Table 5 Intrinsic fracture toughness levels for different composites

Sisal fiber Polypropylene fiber

Metakaolin Phylite Eucalyptus

pulp

Sisal

pulp

Intrinsic fracture

toughness (MPaffiffiffiffimp

)

0.30 0.50 0.60 0.70

J Mater Sci (2012) 47:2864–2874 2869

123

[23], while the large scale bridging model (LSB) was used

to model resistance-curve behavior for crack extension Da,

greater than 0.5 mm [28, 29]. As in Figs. 3, 4, 5, and 6, the

SSB predictions (based on Eq. 3) are represented with solid

star symbols; while the LSB predictions (based on Eq. 4)

are represented dash dot lines (experimental notch length)

and solid cross symbols (infinite notch length).

From the single-fiber tensile tests, the yield strengths of

the PP fibers and SS fibers were measured to be 600 and

400 MPa, respectively. Also, the volume fractions of fibers

were estimated from in situ images of the crack profiles,

such as those presented in Fig. 8a and b. These images,

which were obtained, respectively for each composite type,

show clearly that these fibers toughen the composites by

crack bridging. The red marks in both Fig. 8a and b

showed the single-fiber extending condition, which is

approximately 30% in strain for large scale bridging

regime. From Fig. 7a, we know that the ratio between

actual stress and yield stress is around 1.0, which means ain Eq. 4 should be taken to 1 in the LSB model prediction.

The data that were used in the predictions of crack

bridging are presented in Table 4. The resulting predictions

in the small- and large- scale bridging regimes are gener-

ally in good agreement with the trends in the experimental

data. This is in agreement with the results from prior study

for natural fiber composites [4].

Finally in this section, it is important to obtain tough-

ening estimates that do not depend on specimen geometry.

These can be obtained by determining the asymptotic

values of h(a, x) from Eq. 5, as the specimen widths

approach toward infinity. The resulting toughening levels

can then be substituted into Eq. 6 along with the initiation

fracture toughness levels to obtain estimates of the intrinsic

fracture toughness levels. The resulting values are pre-

sented in Table 5. These show that OPC-based composites

with carbonate filler reinforced by PP fibers and SS pulps

have the highest intrinsic fracture toughness. However,

OPC-based composites with metakaolin reinforced by SS

fibers and EU pulp have the lowest intrinsic fracture

toughness. The combination of fibers and pulps with higher

Polypropylene fiber

Eucalyptus pulp Carbonate filler + Silica + Cement CPV-ARI

(a)

(b)

Fig. 9 Microstructure

characterization of composites

with carbonate filler reinforced

by PP fibers and EU pulp

a SEM image of the composite

microsturctures; b EDS analysis

curves of various spots in the

microstructure

2870 J Mater Sci (2012) 47:2864–2874

123

length and aspect ratio (as in Table 1), such as PP fibers

and SS pulp, is providing better anchorage of the rein-

forcing elements. In the case of LSB, PP fiber also pre-

sented higher elongation (Fig. 7a, b) which could also help.

Both formulations of matrices with carbonate filler and

silica fume were efficient providing good interfacial fiber-

matrix linkage and/or avoiding fiber degradation.

Fracture modes analysis

The fracture modes analyses by Energy Dispersive Spec-

troscopy (EDS) are presented in Figs. 9, 10, 11, and 12 for

two composites, i.e., the composite with carbonate filler

reinforced by PP fibers and EU pulp and the composite

with metakaolin reinforced by SS fibers and EU pulp. It is

indicated that (with arrows for example) the deformations

and pull-out in the corresponding micrographs. In Fig. 9a,

the debonding of PP fiber and the matrix can be detected

for the composite with carbonate filler reinforced by PP

fibers and EU pulp. It shows that not too much hydration

products are left on the surface of the debonded PP fiber,

indicating a poor adhesion between the fiber and matrix,

due to the length and aspect ratio of the EU pulp, some are

completely pulled out from the matrix. In Fig. 9b, the

point-wise EDS results show that the incidence of C in

spectrum 1 is consistent with the location of the PP fiber.

Element mapping distributions from the EDS are integrated

into one image shown in Fig. 10a; while the individual

element mappings are also presented in Figs. 10b–g. The

matrix shows that concentration of C in the PP fiber area. A

small variation of Ca and Si spreads over the scanned area;

while the presence of Mg and Al are detected.

The fracture surface of composite with metakaolin

reinforced by SS fibers and EU pulp are shown in Figs. 11

and 12. In Figs. 11a and 12a, the delaminations between

the SS fiber and the matrix are presented with some matrix

stick to the surface of the deboned SS fiber. Some EU pulp

fibers are also completely pulled out from the matrix. The

incidence of C in spectrum 1 corresponds to the location of

the SS fiber. Element mapping distributions were also

integrated into one image shown in Fig. 12a; while the

individual element mappings are also presented in

Fig. 12b–g. In the matrix, the distributions of Al, Si, and

Ca varied a little over the scanned area. The concentrated

areas of C indicate the location of SS fiber and EU pulp.

EDS analyses on the fracture surfaces were also per-

formed for the other two composite types. Similar results

were obtained between the composites with carbonate filler

and silica fume reinforced by PP fibers and SS pulps and

the composites with carbonate filler and silica fume rein-

forced by PP fibers and EU pulp, as well as between the

composites with phylite reinforced by SS fibers and EU

pulp and the composites with metakaolin reinforced by SS

fibers and EU pulp.

Implications

The implications of the above results are important. First,

they suggest that the fracture toughness/resistance-curve

behavior of composites reinforce with natural and synthetic

Fig. 10 EDS mapping of element distributions in with carbonate filler reinforced by PP fibers and EU pulp a Integration of the element

mappings; b C; c O; d Mg; e Al; f Si; g Ca

J Mater Sci (2012) 47:2864–2874 2871

123

fibers can be estimated from the properties of their con-

stituents. This suggests that such hybrid composites can be

designed using micromechanical models of the dominant

toughening mechanisms (in this case crack bridging). Such

design approaches could greatly facilitate the design of

composite microstructures that can resist crack growth

under a whole range of scenarios that are relevant to eco-

friendly materials for affordable housing.

In the case of the materials considered in this study, the

current study suggests that robust composites can be

designed with vegetable and polypropylene fibers. Conse-

quently, the systems may be useful in long term implica-

tions in which the natural fibers would be expected to

degrade due to chemical and mechanical interactions

between the fibers and matrix materials.

Further work is clearly needed to study the chemical

interactions between the fiber and matrix materials. The

long term effects of exposure to natural weathering (with

moisture and heat) should also be explored to provide

insights into how environmental exposure can degrade the

mechanical properties of hybrid composites. Nevertheless,

the current results are important since they suggest the

potential for the future development of robust and afford-

able composite materials for use in the future development

of affordable housing in the world.

Conclusions

This article presents the results of a combined experimental

and theoretical study of resistance-curve behavior of hybrid

composites reinforced with synthetic polymeric (polypro-

pylene) fibers and natural (sisal) fibers. These were used to

reinforce cement-based matrices with carbonate filler and

silica fume, phylite and metakaolin. The salient conclu-

sions arising from this study are summarized below:

1. Carbonate filler and silica fume added pastes rein-

forced with polypropylene fibers exhibit initiation

Eucalyptus pulp

Sisal fiber

Metakaolin + Cement CPV-ARI

(a)

(b)

Fig. 11 Microstructure

characterization of composites

with metakaolin reinforced by

SS fibers and EU pulp a SEM

image of the composite

microsturctures; b EDS analysis

curves of various spots in the

microstructure

2872 J Mater Sci (2012) 47:2864–2874

123

fracture toughness levels of *0.35 MPaffiffiffiffimp

for the

composites with sisal pulp and *0.27 MPaffiffiffiffimp

for the

composites with eucalyptus pulp. For phylite and

metakaolin added pastes reinforced with sisal fibers

and eucalyptus pulp, the initiation fracture toughness

levels are *0.22 MPaffiffiffiffimp

and *0.12 MPaffiffiffiffimp

,

respectively. These values are not significantly differ-

ent from those of the plain cement paste (0.2–0.3

MPaffiffiffiffimp

) [32] suggesting that the reinforcement of

these fibers and pulp did not influence significantly the

behavior of the cementitious matrix at this stage of

fracture performance.

2. Toughening in these fiber-reinforced composites

occurs largely as a result of small- and large scale

bridging effect. Predictions from the SSB and LSB

models are in good agreement with measured resis-

tance curves.

3. The intrinsic toughness of fiber-cement composites

reinforced with polypropylene and sisal fibers was

estimated to between *0.3 and 0.7MPaffiffiffiffimp

. This

represents the true specimen-independent fracture

toughness value of materials.

4. The current results show that the bridging models

developed earlier for the modeling of crack bridging

can be extended to the prediction of resistance-curve

behavior in hybrid composites reinforced with natural

and synthetic fibers. The results also show that the

dominant toughening mechanism is provided by the

natural or synthetic fibers.

Acknowledgements This study was supported by the Princeton

University Grand Challenges Program, the Division of Civil and

Mechanical Science Foundation (Grant number CMS 0303492) and

the Division of Materials Research of the National Science Foundation

(Grant number DMR 0231418). The authors are grateful to the Dr.

Clark Cooper, Dr. Jorn Larsen-Basse, and Dr. Carmen Huber for their

encouragement and support. Brazilian agencies Fapesp and CNPq also

provided financial support and grants (processes Fapesp 2010/16524-0

and CNPq. 305792/2009-1, respectively) that were very much

appreciated for the involvement of the researchers from the University

of Sao Paulo. Appreciation is also extended to Dr. Nan Yao and Mr.

Gerald Poirier of the Imaging and Analyses Center of Princeton

Institute for the Science and Technology of Materials (PRISM) for

their assistance with microstructure characterization techniques.

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