celulose

1. CEMENTITIOUS COMPOSITES
REINFORCED WITH VEGETABLE
FIBERS
Marie-Ange Arsène1
, Holmer Savastano Jr.2
, Seyed M.
Allameh3
, Khosrow Ghavami4
, Wole O. Soboyejo5
1
Laboratoire COVACHIMM , Département de Chimie- UFR
SEN, Université des Antilles et de la Guyane, campus de
Fouillole, 97157 Pointe-A-Pitre, Guadeloupe (FWI).
marsene@princeton.edu
2
Rural Construction Group, Faculty of Animal Sciences and
Food Engineering, University of São Paulo, C.P. 23, 13635-900,
Pirassununga, SP, Brazil. holmersj@usp.br
3
Princeton Materials Institute and the Department of
Mechanical and Aerospace Engineering (MAE), Princeton
University, P.O. Box CN5263, Princeton, NJ 08544-5263, USA.
allameh@princeton.edu
4
Department of Civil Engineering, PUC-Rio, 22453-900, Rio de
Janeiro, Brazil. ghavami@civ.puc-rio.br
5
Princeton Materials Institute and the Department of
Mechanical and Aerospace Engineering (MAE), Princeton
University, P.O. Box CN5263, Princeton, NJ 08544-5263, USA.
soboyejo@princeton.edu
ABSTRACT
This paper presents a review of research on the development of cementitious matrix
composites reinforced with vegetable fibers for non-conventional construction. The study
emphasizes the possibility of recycling wastes from agriculture and industry in the production
of building materials. Following a brief review of the material selection, details of the
hierarchical functionally graded composite microstructures of natural materials and
composites are presented. The effects of composite reinforcement on composite strength are
then discussed. This is followed by a review of recent work on the fracture toughness and
toughening of natural fiber-reinforced composites. The effects of aging are described before
highlighting the potential emerging applications of vegetable fiber-reinforced composites for
roofing tiles in affordable housing.
1

2. KEYWORDS
Vegetable fiber, pulp fiber, fiber cement composite, BFS/fiber composite, strength, toughness,
microstructure, affordable housing
1. INTRODUCTION
The global need for affordable housing has stimulated extensive research on cementitious
matrix composites. Unlike synthetic fibers, natural fibers offer a cheap and sustainable
approach that can be used to reduce the overall costs of construction materials (Swamy,
1988). Nevertheless, the construction industry continues to be the main consumer of energy in
a world in which energy issues remain at the forefront of human conflict and global
political/economic stability (Plessis, 2001). Furthermore, the global production of cement
contributes about 3.4% of the total CO2 into the earth’s atmosphere (CDIAC, 2003). This has
motivated efforts by researchers to develop alternative materials that reduce the amount of
CO2 and other toxic gases that are released into the environment.
Hence, the need for sustainable, energy efficient construction materials has oriented extensive
research on alternative materials that can reduce the cost and environmental impact of
construction processes. Two approaches have been explored: one includes the use of intrinsic
modification (change in internal composition) to reduce the emissions associated with cement
production, and the production of other construction materials. Examples of such approaches
include the use of admixtures, limestone substitution and pozzolanic cements (Taylor, 1997).
The other approach includes the use of extrinsic modification (reinforcement with fibers) in
the design of composites with attractive combinations of strength, stiffness, fracture
toughness/resistance-curve behavior and durability (Swamy, 1988, Tolêdo Filho et al., 2003
and Lhoneux et al., 2002).
The second approach will be the focus of this paper. Within this context, the composites may
be reinforced with synthetic polymers such as polypropylene, rayon, nylon, polyester, Kevlar
and carbon fibers (Bentur and Mindess, 1990). However, such fibers are generally not readily
available in developing countries. Also, in cases where they are readily available, they may be
too expensive to use in construction materials for affordable housing. This has stimulated
extensive research into the design of composites reinforced with natural fibers such as
bamboo, sisal, coconut husks, sugar cane, banana leaf and wood fibers (Ghavami, 1984,
Sobral, 1990 and Barbosa et al., 2000). Most of the initial efforts in this area have focused on
the replacement of hazardous asbestos fibers with alternative natural fibers that are often
readily available as agricultural by-products (Savastano Jr. et al., 2000) or industrial wastes
(Savastano Jr. et al., 2001), with little or no current economic value. The incorporation of
such natural fibers into cements and earth-based materials offers significant potential for the
development of low-cost construction materials for affordable housing. However, there are
significant challenges that must be overcome before natural fibers can be incorporated into
building materials.
First, the existing knowledge on natural fibers and natural fiber composites must be
disseminated into the educational system and technical literature available in both developing
and developed countries. The lack, or limited access to such information promotes the use of
conventional construction materials that lead to many of the problems identified above.
2

3. Hence, one objective of this review is to provide an overview of the natural fibers and natural
fiber-reinforced systems that are being developed for potential applications in infrastructure.
Second, most natural vegetable fibers consist of lignin, hemicellulose and cellulose that react
with available cement and non-cement matrices. In particular, the interfacial reactions
between the cement matrix and the lignin can lead to significant degradation in the composite
strength (Gram, 1988). There is, therefore, a need for studies designed to extract lignin, or
coat the natural fibers in ways that limit/control the interfacial reactions. There is also a need
for efforts to control the matrix composition by doping with materials that limit/control the
extent of interfacial reactions that can occur. Preliminary ideas for such interfacial design will
be identified later in this review. The existing knowledge of the effects of composite
reinforcement with natural fibers will also be reviewed. The review will go beyond simple
considerations of the effects of reinforcement on composite strength and stiffness (Savastano
Jr. et al., 2000 and Soroushian et al., 1995). Hence, it will also consider the effects of natural
fiber reinforcement on fracture toughness/resistance-curve behavior, durability,
environmental-induced damage and aging (Savastano Jr. et al., 2003b, Tolêdo Filho et al.,
2003 and Lou et al., 2003).
This review presents an overview of the current understanding of cementitious matrix
composites reinforced with natural fibers. The review is divided into seven sections.
Following the introduction, materials selection criteria are presented in Section 2. These
provide compelling support for the current focus on natural fibers. A multi-scale
characterization of vegetable fiber microstructure and natural fiber/composite microstructures
is then presented in Section 3. Some physical and mechanical characteristics and the effects of
natural fiber reinforcement on composite strength and durability are presented in Section 4.
Fracture toughness and resistance-curve of natural fiber-reinforced composites are analyzed in
Section 5. Finally, emerging/potential applications are discussed in Section 6, before
presenting the salient conclusions arising from this work in Section 7.
2. FIBER SELECTION AND COMPOSITE PREPARATION
2.1 Fiber Selection
The ligno-cellulosic residues can be classified, following the selection criteria below:
• General identification of agricultural production, which generates residues.
• Residues identification: correlation with main products and production processes.
• Available amount of residues: to point out other possible uses with actual demands.
• Local availability: in order to choose between transportation or local processing.
• Market value of the residue.
• Physical and mechanical properties of composites and materials.
Based on these criteria, some different types of residues were selected by Savastano Jr. et al.
(1999) in Brazil from sisal and coir fibers and from eucalyptus pulp industry. All the wastes
listed in Table 1 are already available for immediate use in civil construction:
• Sisal field by-product presents large availability at the processing sites and low
commercial interest. A good option as a complementary income for rural producers. This
residue needs simple cleaning by passing into a manual cylindrical rotary sieve.
3

4. • Waste of eucalyptus pulp has almost no commercial value and great availability. Their
disadvantages are the very short fibers (average length = 0.66 mm) and their high
moisture content in the origin.
• The residual short coir fibers represent low commercial value, great potential of
production and almost no use at present time. Before the use of this waste fiber, powder
separation (about 50% by mass) and drying are required.
TABLE 1. RESIDUES OF THE FIBER PROCESSING.
Fibrous residues Sisal field by-
product
Waste of
eucalyptus pulp
Residual short coir
fibers
Original humidity (%) 10 61 32
Market price
(US$/ton)
Zero 15 90 (maximum)
Amount (ton/year)
- source
30,000
- 1 cooperative
17,000
- 1 large industry
7,500
- 2 large industries
MAIN PRODUCT Commercial fiber
before drying
Pulp for paper
production
Fibers longer than
100 mm
RELATION
RESIDUE/MAIN
PRODUCT (%)
300 0.5 200 - 2880
TABLE 2. PHYSICAL AND MECHANICAL PROPERTIES OF VEGETABLE AND POLYPROPYLENE
FIBERS.
Properties Density
(kg/m3)
Water
absorption
(% by mass)
Elongation at
break (%)
Tensile
strength (MPa)
Young's
modulus
(GPa)
Sisal (Agave
sisalana)
1370 110.0 4.3 a
458 a
15.2 a
Coir (Cocos
Nucifera)
1177 93.8 23.9 -51.4 a
95 - 118 a
2.8 b
Malva (Urena
lobata)
1409 182.2 5.2 c
160 c
17.4 b
Disintegrated
newsprint (P elliottii
& E citriodora)
1200 - 1500 a
400 a
na 300 - 500 a
10 - 40 a
Bamboo (Bambusa
vulgaris)
1158 a
145 a
3.2 a
575 a
28.8 a
Piassava (Attalea
funifera)
1054 a
34.4 - 108 a
6.0 a
143 a
5.6 a
Polypropylene 913 - 22.3 - 26.0 250 2.0
Note: a
Agopyan (1988), b
Guimaraes (1984) and c
Oliveira and Agopyan (1990).
Obs.: na = non available information.
In Table 2, the most suitable Brazilian vegetable fibers are presented, based on their physical
and mechanical properties, cost, durability in natural wet environment and production. As
4

5. they are natural products, the fibers are heterogeneous so the coefficient of variation in some
properties can be as high as 50%. Only as a comparison, the characteristics of polypropylene
fibers are included in the table.
The availability was also the main motivation of the Guadeloupean laboratory for studying
sugar cane bagasse, banana and coir fiber (Arsène et al., 2001, Bilba et al., 2002, 2003).
2.2 Composite Preparation
Natural fiber reinforced cement (NFRC) is a generic name covering different realities,
because of the nature of the matrix, the nature of the fiber, but also because of the treatment
applied to the fiber.
The studied matrices were based on ordinary Portland cement (OPC), blast furnace slag (BFS)
or mortar produced with OPC.
In the reported results on composites, the fibers are from sisal, banana, bagasse or eucalyptus.
The fibers were pre-treated either by pulping, chemical kraft process and chemi-
thermomechanical pulping (CTMP) (Savastano Jr. et al., 2003 a, b), or by pyrolysis.
2.2.1. Composite cement/fiber elaboration
Cement composite pads measuring 125 x 125 mm and reinforced with 12% by mass of
Eucalyptus grandis waste kraft or 8% of either CTM-pulp were prepared in the laboratory
using a slurry vacuum de-watering technique. The selection of fiber contents was based on the
optimum levels found in a similar study published elsewhere (Savastano Jr. et al., 2000). Pads
of each formulation were prepared in groups of three, pressed simultaneously at 3.2 MPa for 5
min, and sealed in a plastic bag to cure in saturated room temperature air for the following
seven days.
On completion of the initial saturated air cure, pads destined for testing at a total age of 28
days were wet diamond saw into three 125 x 40 mm flexural test specimens. Specimen depth
was the thickness of the pad, which was in the region of 6 mm. The specimens were then
allowed to air cure in a laboratory environment of 23 ± 2°C and 50 ± 5% relative humidity
prior to the conduct of mechanical and physical tests.
2.2.2. Composite BFS/fiber elaboration
When using BFS as matrix, ground agricultural gypsum and construction grade hydrated lime
were used as activators in the proportions 0.88:0.10:0.02 by mass. This mixture was
employed for the composite elaboration following the same process as described in Section
2.2.1.
Three pads of each formulation were allocated for exposure to 3, 12 and 24 month periods of
weathering in temperate Australian and tropical Brazilian environments. Additional sets of
pads were stored continuously in the laboratory over the same periods to provide specimens
for the determination of reference properties at the different ages. On removal from their bags,
these pads were allowed to air cure in the laboratory environment to 28 days of age before
being exposed to weathering or further aging in the same environment. At total ages of 4, 13
5

6. and 25 months, test specimens were prepared as previously described and stored in laboratory
conditions for seven days to achieve equilibrium moisture content prior to testing.
2.2.3 Composite mortar/fiber elaboration
The elaboration process used for the mortar corresponds to simple mixing of the elements
sand, cement, fiber and water according to AFNOR norm NF P15-301. The fiber used in case
of bagasse-cement mortar, were submitted to pyrolysis treatment and the ratio of fiber was up
to 10% by mass. The homogeneous mixture of the composite was cast in a 40 mm x 40 mm x
160 mm specimen mould. The moulds were shacked on a shock table and stored in a tropical
room environment (28°C - 80% humidity).
3. MICROSTRUCTURE
3.1 Fibers
Many fibers have been used in fiber cement composites including polymer, glass and
vegetable fibers. Natural fiber cement reinforced composites are the focus of interest here.
They were obtained from different vegetable fiber, banana leaf, coconut husk, sugar cane,
sisal, and eucalyptus wood. The matrix was either based on Ordinary Portland Cement (OPC)
or Blast Furnace Slag (BFS).
As reported in the literature by Coutts (1992), Bledzki and Gassan (1999) and Li et al. (2000),
vegetable fibers contain cellulose, lignin and hemicellulose. The compositions of natural
fibers discussed in this article are reported in Table 3.
TABLE 3. CHEMICAL COMPOSITION OF THE FIBERS
Nature of the fiber Chemical composition
Lignin (%) Cellulose (%) Hemicellulose (%) Extractives (%) Ash (%)
Bagasseb
21.80 41.70 28.00 4.00 3.50
Banana leaf a
24.84 25.65 17.04 9.84 7.02
Banana trunka
15.07 31.48 14.98 4.46 8.65
Coconut coir a
46.48 21.46 12.36 8.77 1.05
Coconut tissue a
29.70 31.05 19.22 1.74 8.39
Eucalyptus 25.4 41.57 32.56 8.20 0.22
Sisal c
11.00 73.11 13.33 1.33 0.33
Note: a
Bilba et al. (2002), b
Ouensanga and Picard (1988) and c
Bledzki and Gassan (1999).
As mentioned by Li et al. (2000) these compositions vary with the location of the fiber in the
plant and with the age of the plant. The composition of the vegetable fiber may even vary
6

7. with the climate and soil conditions of the region in where the plants grow. Table 4 brings the
geometrical details for some fibers from Guadeloupe.
TABLE 4. MICROSTRUCTURE OF SOME UNITARY FIBERS - GEOMETRICAL DETAILS
Nature of the fiber Geometrical characteristic
Large diameter D
(µm)*
Small diameter
d (µm)*
D/d * External wall
(µm)*
Unitary fiber
density
(1/mm2
)*+
Bagasse 6.6 - 26 3.3 - 15.7 2.5 - 1 1.25-3.3 3240-5600
Banana leaf 11 - 5.6 3.5-5.5 2.4 - 1.5 2.2 11574
Banana trunk 8.5 - 20 5 - 15 3 - 1 1.25 - 2.5 5185 - 6670
Coconut coir 3 - 13 3 - 6 2 - 1 1.25 - 4 10300 - 10700
Coconut tissue 4 - 12 4 - 8 2 - 1 2 14500
The data presented in the table, have been determined by SEM images on basis of 15 images.
*The minimal and maximal experimental values observed are reported.
+
Unitary fiber density = (mean number of unitary fibers)/(surface area of the macro-fibers).
The cellulose, a natural polymer, is the main reinforcement material. The chains of cellulose
form microfibrils, which are held together by hemicellulose and form fibrils. The fibrils are
assembled in various layers to build up the structure of the fiber. Fibers or cells are cemented
together in the plant by lignin. Then the usual denomination for fibers is in fact a reference to
strands of fibers with some important consequences on durability studies, as discussed in a
following section. This multi-level organization appears clearly on SEM observations of raw
fibers. The example of banana fiber observed at two different magnifications is reported in
Figures 1a and 1b.
FIGURE 1a (MAG. X200) FIGURE 1b (MAG. x1200)
FIGURE 1 a-b. BACK-SCATTERING ELECTRONIC IMAGE (BSEI) OF BANANA TRUNK FIBER.
In case of pulp fibers, after pulping process, the external aspect of the fiber is modified.
Figures 2, 3 and 4 are micrographs of sisal, banana and eucalyptus pulps with different levels
of fibrillation.
7

8. FIGURE 2. BY-PRODUCT SISAL CTMP. HIGH MAGNIFICATION SHOWING THE EXTERNAL
FIBRILLATION OF THE FILAMENT.
FIGURE 3. BANANA CTMP.
FIGURE 4. EUCALYPTUS GRANDIS WASTE PULP AFTER HOT WATER DISINTEGRATION. GENERAL
VIEW.
Examination of Figure 2 of sisal mechanical pulp illustrates the extent of primary wall
fibrillation. Similar behavior was observed with banana pulp (Figure 3), which seems to be
8

9. quite sensitive to mechanical treatment, probably due to the lower wall thickness of this fiber
(1.25 µm) when compared to that of sisal (12.5 µm) (Mukherjee and Satyanarayana, 1984).
The beneficial effects of refining are thought to be due to the better flexibility and external
fibrillation of fibers (Coutts, 1988). As a consequence of refining, composites experience
better retention of cement particles by the fiber network during vacuum drainage, adequate
pad packing during pressing and more effective fiber-matrix and fiber-fiber bonding.
However, some undesirable effects can also take place during beating, such as the generation
of fines and the fiber shortening.
Examination of Eucalyptus grandis kraft pulp revealed the presence of short fibers and the
absence of fibrillation (Figure 4). However, the combined physical and morphological aspects
of this particular fiber suggest that it may provide an acceptable performance as cement
reinforcement. The fibers appeared flexible, twisted and with irregular surfaces (drying
phenomena) in keeping with other studies (Soroushian et al., 1995). They also displayed
evidence of lateral shrinkage as an irreversible characteristic of recycled fibers (McKenzie,
1994). As the fibers work by bridging micro-cracks developed during loading of the brittle
cement matrix, the strength and toughness of the composite material is directly related to fiber
content and bond frictional stresses (Coutts, 1988).
3.2 Composites
3.2.1 Microstructure properties of fiber cement composites
The morphology of fracture surface was analyzed for most of the fiber cement composites
using scanning electron microscopy. This visual parameter permits to appreciate the effect of
most parameters. The following examples light up to illustrate the impact of aging and
processing on microstructure.
A fracture surface of non-aged banana fiber reinforced BFS, in which numerous broken
filaments can be observed, is shown in Figure 5. The inclusion of these long and supposedly
strong fibers (tensile strength of individual fiber ~700-800 MPa, comparable to that of Pinus
radiata as reported by Zhu et al., 1994 and Coutts, 1990) could be expected to give rise to
considerable fiber pullout. The predominance of fiber fracture suggests that either the fibers
were damaged during pulping or the fibrillation imparted resulted in sufficiently improved
anchorage in the matrix to significantly reduce the critical fiber length (Beaudoin, 1990). The
high incidence of fiber fracture led to the composite absorbing relatively little energy in the
post-cracking stage despite possessing a flexural strength similar to that of the sisal composite
(Section 4.2.3).
Brittle fracture could be expected in air-cured OPC-based composites after aging (Bentur and
Akers, 1989). Examination of the fracture surfaces of composites exposed for 12 months to
the Melbourne climate revealed still no evidence of fiber petrifaction and confirmed that the
fibers remained in good condition. Fiber pullout rather than fracture was predominant in the
composites, as would be expected in light of their toughness values. A fracture surface of 8%
banana CTMP in BFS is shown in Figure 6. In contrast to the surface of the non-aged
specimen (Figure 5), the fibers remain largely intact. This suggests that the fiber-matrix
interface or the matrix itself has weakened, in turn weakening the composite but improving its
toughness by allowing greater dissipation of energy through the mechanism of fiber pullout.
9

10. FIGURE 5. SEM IMAGE OF THE FRACTURE SURFACE OF BANANA CTMP IN BFS. HYDRATION AGE:
32 DAYS.
FIGURE 6. SEM IMAGE OF THE FRACTURE SURFACE OF BANANA CTMP IN BFS AFTER 12
MONTHS EXPOSURE TO A TEMPERATE CLIMATE.
Cracked fracture surfaces can appear and be related to the preparation process of the material.
This phenomenon illustrated in Figure 7, corresponding to mortar reinforced bagasse
composite, is related to the presence of voids, air bubble causing high porosity in the material.
3.2.2 Fiber-matrix transition zone
The so called transition zone, defined as a region of the cement paste close to the fiber, with
thickness from 10 to 100 micrometers, presents different characteristics from the bulk matrix.
In cement composites, low porosity and portlandite (calcium hydroxide crystals)
concentration on transition zone must improve the fiber-matrix bonding. With the fiber-matrix
bonding increase, the elastic tensile strength also increases and sometimes the ductility
reduces (Savastano Jr. and Agopyan, 1999).
Figure 8 presents a general view (low magnification backscattering electronic image - BSEI)
of sisal and eucalyptus fibers in blast furnace slag (BFS) based mortar, showing also the
10

11. location of some energy dispersive spectroscopy (EDS) analysis (spots 1 - 3 in the image).
Large dark areas (spot 1) represent the cross-sections of sisal strands with traces of Si and Ca.
It is possible to identify a ring involving the sisal fiber formed by hydration products as a
transition zone, as previously described by Savastano Jr. and Agopyan (1999). Spots 2 and 3
indicate hydration product and anhydrous BFS areas, respectively. Small dark areas
throughout the image could be related either to eucalyptus fibers or to pore concentrations
near the fiber strands.
FIGURE 7. BSEI OF MORTAR WITH BAGASSE FIBER.
FIGURE 8. BSEI OF BFS MORTAR WITH EUCALYPTUS AND SISAL FIBERS. SPOT 1 – SISAL FIBER;
SPOT 2 – HYDRATION PRODUCTS; SPOT 3 – ANHYDROUS CEMENT GRAIN.
4. MACROSTRUCTURE BEHAVIOR
4.1 Fiber and Pulp
4.1.1 Fiber
The mechanical properties of the composite present variations. The experimental results
obtained on tensile test are reported with their standard deviation in Table 5. The tests were
performed in normal environment condition of the laboratory, 25o
C and relative humidity
11

12. 40%-50%, with the test machine TCD 200 model from CHATILLON using the software
NEXYGEN for measurement. The load was applied monotonically at a speed of 12.7
mm/min (0.21 mm/s). Different authors using tensile and micro-tensile tests determined the
mechanical properties of natural fibers. Table 5 also synthesizes the experimental values and
the mechanical values available in the literature.
TABLE 5. FIBER MECHANICAL PROPERTIES.
Fiber Experimental results Literature data d
Nature of the fiber Diameter
(mm)
Tensile
strength
(MPa)
Elongation
at break (%)
Young
modulus
(GPa)
Tensile
strength
(MPa)
Elongation
at break
(%)
Coconut coir 0.299 182
[± 43]
7.0
[± 0.7]
4.0 - 6.0 175a,
200 b
11.4
Coconut tissue 0.354 265
[± 174]
9.2
[± 1.8]
Bagasse 0.275 426
[± 335]
8.6
[± 1.7]
290 b
Banana trunk or
stem
0.116 351
[± 227]
8
[± 1]
95 b
Banana leaf* 0.215
0.967
22.42
[± 7]
Sisal 511 - 635 a c
1100 b
2 - 2.5
Sources: a
Bisanda (1992), b
Ministry of Urban Development and Poverty Alleviation (Government of India),
c
Netravali and Chabba (2003) and d
Bledzki and Gassan (1999).
Between bracket is reported the standard deviation of the results.
* As the banana fibers present an elliptic shape two diameters are given.
The experimental measurements can be analyzed in terms of statistical distribution. Figure 9a
presents the statistical distribution of the strength of bagasse fibers. The probability
distribution appears to be lognormal (Figure 9b).
4.1.2 Pulp
Regarding their geometry fibers are heterogeneous materials. Savastano Jr. (2001) studied
these variables reported in Table 6 for three pulped fibers.
TABLE 6. PULP AND FIBER PHYSICAL PROPERTIES.
Fiber Freeness (ml) Fines (%) 1
Length (mm) 2
Width (µm) 3
Aspect ratio
(length/width)
E. grandis 685 7.01 0.66 10.9 61
Sisal 500 2.14 1.53 9.40 163
Banana 465 1.55 2.09 11.8 177
1
Arithmetic basis, 2
length-weighted basis, 3
average of 20 determinations by SEM
12

13. 0 400 800 1200 1600
40
70
80
0
Lognormal base e
P
er
c
e
nt
Percent
1001000100100
9595
99
99
50
60
50
20
30
20
10
10
5
5
1
1
FIGURE 9.a) DISTRIBUTION CURVE FIGURE 9.b) LOG NORMAL SIMULATION
OF THE STRENGTH RESULTS FOR BAGASSE FIBER.
The experimental difficulties explain the few mechanical characteristics available. The main
physical attributes of the pulps produced are summarized in Table 6. The Canadian Standard
Freeness (CSF) of each pulp was determined in accordance with AS-1301.206s-88. CSF is an
arbitrary measure of the drainage properties of pulp suspensions and is associated with the
initial drain rate of the wet pulp pad during the de-watering process (Coutts and Ridikas,
1982). Fiber length and fines content were determined using a Kajaani FS-200 automated
optical analyzer.
4.1.3 Durability
Vegetable fibers are affected by the environmental temperature and humidity, and also by the
medium in which they are immersed, due to the hemicellulose and lignin decomposition.
These components are present in the intercellular layers and their decomposition reduces the
reinforcement capacity of the individual fibers (cells). Tensile strength of sisal and coir fibers
decreases up to 50% if immersed in saturated solution of calcium hydroxide (pH about 12) for
28 days (Agopyan, 1988).
To avoid aging effects in the composites, some approaches are available: a) protection of the
strand fibers by coating or sealing the dry composite to avoid the effect of alkaline water; b)
high casting compaction and high-pressure steam curing for providing matrix carbonation, if
necessary adding silica fume; c) alternative binders based on industrial and agricultural by-
products such as blast furnace slag (BFS) and fly ash (Guimarães, 1990, John et al., 1990 and
Soroushian et al., 1996).
4.2 Composite
The results here reported were obtained by Savastano Jr. (2001) from Eucalyptus grandis
waste kraft or 8% CTMP pulp from sisal or banana fiber. The experimental procedure is
reported below.
13

14. 4.2.1 Test methods
The test methods explained in this section were applied to the composites produced as
mentioned in Section 2.2.2 by slurry de-watering process. A three point bend configuration
was employed in the determination of modulus of rupture (MOR), modulus of elasticity
(MOE) and fracture energy. A span of 100 mm and a deflection rate of 0.5 mm/min were used
for all tests in an Instron model 1185 universal testing machine. Fracture energy was
calculated by integration of the load-deflection curve to the point corresponding to a reduction
in load carrying capacity to 50% of the maximum observed. For the purpose of this study, the
fracture toughness (FT) was measured as the fracture energy divided by specimen width and
depth at the failure location. Nine flexural specimens were tested for each formulation and
condition of exposure. The mechanical test procedures employed have been described in
greater detail by Savastano Jr. et al. (2000).
Water absorption and bulk density values were obtained from tested flexural specimens
following the procedures specified in ASTM C 948-81. Six specimens were used in the
determination of these physical properties.
4.2.2 Weathering conditions
28 days after manufacture the allocated series of composites of each formulation were placed
in a rack facing the Equator at an angle of inclination of 45o
to age naturally in the temperate
environment of Melbourne, Victoria, Australia (37o
49' S of latitude). Exposure commenced
in April 1999 for those composites reinforced with E. grandis pulp, and July 1999 for the
remaining composites. Corresponding series of composites were exposed in a like manner to
the tropical environment of rural Pirassununga, São Paulo state, Brazil (21o
59' S of latitude).
Exposure of these series began in July 1999. Table 7 lists the main long term climate averages
for the Australian and Brazilian exposure sites.
TABLE 7. CLIMATE AVERAGES IN AREAS OF WEATHERING TESTS.
Temperature (o
C) Relative humidity (%)Place
Ave max /
month
Ave min /
month
Ave max /
month
Ave min /
month
Average
rainfall
(mm/year)
Melbourne, Vic, AU 1
25.8 / Jan 5.9 / July 82 / June 60 / Jan - Dec 654
Pirassununga, SP, BR 2
30.1 / Jan - Feb 9.5 / July 77 / Jan - Feb 63 / Aug 1363
Source: 1
Bureau of Meteorology, Australia; 2
Air Force Academy, Defense Ministry, Brazil.
4.2.3 Mechanical and physical properties
Table 8 and Figures 10 and 11 depict the mechanical and physical properties of the various
composites. The property means are indicated with the corresponding standard deviations.
Non-aged composites presented flexural strengths in excess of 18 MPa, representing a 130%
improvement over a plain BFS matrix of similar formulation. As shown in Figure 10, two
years of external exposure to tropical or temperate weather resulted in a considerable
reduction in strength, which had fallen to 4.9 MPa in the case of the 8% banana CTMP
formulation exposed in Australia. The loss in mechanical strength of composites subjected to
either natural weathering or aging in the controlled environment is attributable to both fiber
degradation in alkaline environment (Gram, 1988) and matrix carbonation predominantly.
The mechanism of carbonation (Wang et al., 1995 and Taylor, 1997) consumes calcium ions
14

15. from hydration products and hence causes the weakening of composites. Qualitative
evaluation using an indicator solution of 2% phenolphthalein in anhydrous ethanol revealed
that the aged composites were completely carbonated. The greater severity of the natural
environment on composite properties can also be attributed to interfacial damage resulting
from volume changes of the porous and hygroscopic vegetable fibers inside the cement matrix
(Savastano Jr. and Agopyan, 1999).
TABLE 8. MECHANICAL AND PHYSICAL PROPERTIES OF COMPOSITES AT 28 DAYS.
Fiber
Type Content
(% by
mass)
Binder MOR (MPa) FT (kJ/m2
) MOE (GPa) Water
absorption
(% by mass)
Density
(g/cm3
)
Nil - BFS 8.1 ± 2.2 0.03 ± 0.01 11.6 ± 1.7 17.6 ± 0.9 1.84 ± 0.03
12 BFS 18.2 ± 2.8 1.25 ± 0.20 5.0 ± 0.6 32.3 ± 1.7 1.33 ± 0.04E. grandis
12 OPC 22.2 ± 1.3 1.50 ± 0.18 8.0 ± 1.1 24.8 ± 0.8 1.47 ± 0.02
Sisal 8 BFS 18.4 ± 1.4 0.85 ± 0.10 5.9 ± 0.5 32.9 ± 0.6 1.33 ± 0.01
8 OPC 21.7 ± 1.1 0.79 ± 0.17 9.9 ± 0.3 22.9 ± 1.2 1.51 ± 0.02
Banana 8 BFS 18.9 ± 1.9 0.51 ± 0.10 6.2 ± 0.6 31.7 ± 0.6 1.36 ± 0.02
8 OPC 20.9 ± 2.0 0.51 ± 0.10 9.8 ± 0.5 23.6 ± 0.9 1.50 ± 0.02
100.0%
69.0%
64.7%
77.9%
45.6%
46.7%
68.1%
32.3%
26.0%
67.5%
0.00
5.00
10.00
15.00
20.00
25.00
Lab Exp ext Victoria AU Exp ext Sao Paulo BR
Environment
MOR(MPa)
1 month
13 months
25 months
4 months
FIGURE 10. BANANA CTMP IN BFS. VARIATION IN COMPOSITE MOR WITH AGE AND CONDITIONS
OF EXPOSURE.
Non-aged BFS (one month curing) based composites possessed modulus of elasticity (MOE)
values between 5.0 and 6.2 GPa, approximately 50% of that of the plain BFS matrix. The
reduction is associated with the low modulus of the cellulose fibers employed and the
15

16. additional porosity resulting from their inclusion. In the case of banana CTMP in BFS, MOE
dropped to the interval of 3.2 – 4.8 GPa after 13 months of aging, and to the range of 1.2 – 4.0
GPa after 25 months of aging. The higher values of MOR and MOE related to OPC based
composites (Table 8) could signalize the necessity of better levels of hydration for the
alternative binder in further steps of work.
100.0%
255.6%227.9%
196.1%
192.5%
207.1%
215.5%
153.2%
162.9%
269.7%
0.000
0.200
0.400
0.600
0.800
1.000
1.200
1.400
1.600
1.800
Lab Exp ext Victoria AU Exp ext Sao Paulo BR
Environment
FT(kJ/m
2
)
1 month 13 months 25 months4 months
FIGURE 11. BANANA CTMP IN BFS. VARIATION IN COMPOSITE FT WITH AGE AND CONDITIONS OF
EXPOSURE.
Fracture toughness (FT) is the matrix property most often enhanced by the presence of fibers,
which, in these materials, produced a 17-fold or even greater increase. Eucalyptus grandis
composites showed better results than the others in the initial age of 28 days. The higher
content of eucalyptus pulp employed and also its lower anchorage length (Table 6) seem to
provide the consequent higher absorption of pullout energy.
As shown in Figure 11, after a period of weathering or laboratory aging, composites
demonstrated ductility similar to or even higher than those at 28 days. 8% banana fiber in
BFS, exposed in any of those environments, possessed a toughness of at least ~0.8 kJ/m2
which corresponds to a significant increase in comparison with the short term value (~0.5
kJ/m2
). The improvements in ductility can be linked to the losses in MOR and MOE,
confirming the expected compromise between strength and ductility in such composites.
FT values after weathering indicate that the integrity of the fibers within the BFS matrix has
not been significantly reduced by decomposition. In a previous study of sisal, malva and coir
strands in OPC, Savastano Jr. and Agopyan (1999) reported reductions of at least 50% in
ductility after only six months in a laboratory ambient. Tolêdo Filho et al. (2000) and Bentur
and Akers (1989) noted similar embrittlement in aged vegetable fiber-OPC composites and
found that it could be directly attributed to the petrifaction of the reinforcement through the
migration of hydration products to the fiber lumens and pores.
16

17. Short term water absorption (WA) and bulk density values of the composites were in the
ranges of 23 - 33% by mass and 1.3 - 1.5 g/cm3
respectively, regardless of the fiber type
(Table 6) or content. Plain BFS matrix, produced with an analogous process to that reported
in the present study for composites, was found to have a WA of 18% by mass and density of
1.8 g/cm3
, confirming the influence of the vegetable fibers on the volume of capillary voids in
fiber-cements. The lower permeability of OPC based composites is connected with the higher
values of mechanical strength and confirms the idea of improved hydration of this binder in
the short term.
5. FRACTURE
Raw pads of fiber cement based on BFS binder reinforced with 8% sisal kraft were produced
as described in Sections 2.2.1 and 2.2.2. After the pulping process in laboratory scale, the
sisal fiber presented average length and width of respectively 1.65 mm and 13.5 µm as
previously determined in correlated work by Savastano Jr. et al. (2000).
On completion of the initial saturated air cure during the initial seven days, the raw pads were
allowed to air cure in a laboratory environment until they were tested, approximately nine
months after production.
The resistance-curve experiments were performed on single-edge notched bend (SENB)
specimens with thickness (B) of ~7.5 mm and width (W) of ~12.5 mm. The initial notch-to-
width ratio (ao/W) was ~0.25.
Experiments were conducted under three-point bend loading, with a span of 50 mm. The
resistance-curve experiments were performed in an Instron model 8872 servohydraulic testing
machine, after pre-cracking under far-field compression as applied by Soboyejo et al. (1993).
The tests were conducted with the specimens in a laboratory environment with a relative
humidity of ~40 – 50% and a temperature ~25o
C.
The specimens were loaded monotonically in incremental stages that corresponded to a stress-
intensity-factor range (K) increase rate of 0.05 MPa m . This was achieved at a ramp rate of
2 N/s. The specimens were then unloaded to examine their sides for evidence of possible
crack growth. This was continued until crack growth was detected in an ex-situ optical
microscope. Subsequently, the above process was continued, in an effort to study the
crack/microstructure interactions that give rise to resistance-curve behavior. This was
continued until unstable crack growth/fracture occurred during incremental loading. The
calculations of K were obtained from an expression in the ASTM E399-90 code of the
American Society for Testing and Materials.
5.1 Micromechanical modeling
An energy approach (Soboyejo, 2002) may be used to explain the toughening due to crack
bridging by ductile fibers or ligaments. The toughening of the brittle matrix due to small-scale
bridging by ductile fiber reinforcement may be idealized using an elastic-plastic spring model
(Figure 12a), as proposed by Budiansky et al. (1988), and Li and Soboyejo (2000). For small-
scale bridging, in which the size of the bridging zone is much smaller than the crack length
(Kung et al., 2001), the extent of ductile phase toughening may be expressed in terms of the
17

18. maximum stress intensity factor the material can sustain before failure (fracture toughness).
Hence, 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 crack bridging, ∆Kssb. The
fracture toughness of the ductile-reinforced composites may thus be estimated from Eqn. 1, as
stated by Soboyejo (2002):
dx
x
VKKKK
L
y
fmssbmc ∫





+=∆+=
0
5.0
5.0
2 σ
α
π
(1)
Where α is the constraint/triaxiality factor (typically between 1 and 3) (Kung et al., 2001, Lou
and Soboyejo, 2001), Vf is the volume fraction of ductile phase, L is the length of the bridging
ligament, σy is the uniaxial yield stress, and x is the distance from the crack tip (Figure 12a).
For large-scale bridging conditions (where the length of the bridging zone is comparable to
the overall crack size), the toughening increment (∆Klsb) is given by Eqn. 2 (Li and Soboyejo,
2000, Lou and Soboyejo, 2001, Bloyer et al., 1998, 1999).
dxxahVK
L
yflsb ),(∫=∆ σα (2)
Where Vf is the volume fraction of ductile phase, L is the length of the bridging ligament, α is
the constraint/triaxiality factor, σy is the uniaxial yield stress, x is the distance from the crack
tip and h(a,x) is the weighting function for the bridging tractions (Figure 12b) (Fett and Munz,
1994). The method of calculation for this function is given by Fett and Munz, 1994.
FIGURE 12a. SCHEMATIC ILLUSTRATIONS: SPRING MODEL OF CRACK BRIDGING.
FIGURE 12b. SCHEMATIC ILLUSTRATIONS: WEIGHTED DISTRIBUTION OF TRACTIONS ACROSS
LIGAMENTS.
18

19. 5.2 Resistance-curve
The experimental resistance-curve obtained for the fiber-cement composite is shown in Figure
13. Stable crack growth initiated at a stress intensity factor, K0, of ~0.7 MPa m . The amount
of stable crack growth (∆a) was considered around 1.8 mm (a/W ~0.4), and the stress
intensity factor reached ~1.0 MPa m .
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Crack Growth, ∆a (mm)
StressIntensityFactor,K(MPam0.5
)
Experimental Small scale bridging
Large scale bridging Intrinsic toughness
FIGURE 13. COMPARISON OF MEASURED AND PREDICTED RESISTANCE-CURVES USING SMALL
AND LARGE SCALE BRIDGING MODELS FOR THE SISAL FIBER-REINFORCED CEMENT COMPOSITE.
The presence of short vegetable fibers has a significant effect on the fracture toughness, and
on the extent of stable crack growth observed in this cement-based material. It is of interest to
compare the results of the current study to prior reports of fracture toughness/resistance-curve
behavior in cementitious materials.
Similar resistance-curve behavior was observed in cement matrices reinforced with different
types of fibers, such as carbon, steel and polypropylene, in studies by Eissa and Batson (1996)
and by Banthia and Sheng (1996). Banthia and Sheng (1996) used the R-curve experiment to
study the effects of polypropylene fibers (4 µm diameter, 6 mm long, E = 1.41 GPa, tensile
strength = 32 MPa) as reinforcements in a cement paste based matrix. Following
reinforcement with up to 3% fiber content, the improvement of the composite toughness was
significant, compared with the un-reinforced matrix. At this level of reinforcement, the
effective final crack length, aeff, (measured using compliance method) varied from 8 – 8.45
mm (W ~25 mm) and values for KI reached ~0.55 MPa m . Ouyang and Shah (1992) and
Visalvanich and Naaman (1981) proposed values of 0.8 and 1.3 MPa m , respectively, for
the critical stress intensity factor (KIc) for plain cement mortars.
19

20. 5.3 Bridging Predictions x Experimental Results
The measured resistance-curve behavior in Figure 13 can be compared with the predictions
based on the micromechanical models presented earlier. As in previous work reported by Lou
and Soboyejo (2001), small-scale bridging (SSB) was presumed to occur for crack growth,
∆a, less than ~0.5 mm, and large-scale bridging (LSB) was assumed for ∆a ≥ 0.5 mm. Typical
values of crack bridging parameters such as bridge length, L, and fiber volume fractions were
extracted by quantitative image analyses of side profiles.
The predictions for small-scale bridging (based on Eqn. 1) are compared with the measured
resistance-curve in Figure 13. This shows that the value reached for the overall increment,
∆Kssb = 0.136 MPa m , seems to be an underestimation of the experimental measurements.
This behavior is in agreement with a previous work by Kung et al. (2001) regarding the
predicted resistance-curve.
A comparison of the predictions obtained from Eqn. 2 and the measured LSB resistance-curve
is also presented in Figure 13. The LSB model also predicts the overall trends as an
underestimation (~0.9 MPa m ) although much closer of the experimental data.
Different resistance-curves would be expected from different specimens due to differences in
the heterogeneous microstructures. The measured toughness levels are also dependent on
crack length. It is, therefore, important to obtain estimates of fracture toughness that do not
depend so strongly on crack length or specimen geometry. The estimates would represent the
true intrinsic toughness of the composites, which would normally require the testing of very
large specimens. This can be achieved by artificially increasing the specimen width, W, such
that W→ . The function h(a,x) is found to approach an asymptotic value when this is done
(Figure 13).
∞
The resulting value of ∆Klsb corresponding to the above asymptotic value is ~0.09 MPa m .
From Eqn. 2, this gives an estimated intrinsic fracture toughness value of ~0.84 MPa m .
The maximum K in the R-curve is around 1.0 MPa m . The difference between the
simulations with real value of W and with W ∞→ shows the influence of the specimen
dimensions especially for higher values of ∆a (Figure 13).
It should be noted that although both SSB and LSB models capture the general trends of the
measured resistance-curves, the values predicted by the models are always somewhat lower
than the corresponding experimental measurements. The explanation is believed to be
connected to the heterogeneity of the material, the random distribution of the reinforcement
and the degradation of ligaments bridges between the cement matrix and the natural fibers.
6. APPLICATION OF ASBESTOS-FREE FIBER CEMENT IN ROOFING TILES
The interest for vegetable fibers as substitutes of asbestos for popular building is mainly
justifiable by their competitive prices and origin from renewable sources. As observed by
Coutts (1988), developed societies have achieved high performance cellulose-cement
products by adopting elaborated technologies with high-energy consumption in processes. On
the other hand, researches in developing countries (Agopyan, 1988) concentrated mostly in
20

21. the use of strand fibers and simple production process linked to important concerns about
durability (Tolêdo Filho et al., 2000).
The following application can be point out as example of cement based materials reinforced
with plant fibers produced at very low-cost and with high potential for buildings in poor areas.
Savastano Jr. et al. (1999) developed roofing tiles that were fabricated using the Parry
Associates, UK- equipment, for molding and compaction by vibration. The formulations
varied as shown in Table 9. The dimensions of tiles were 487 x 263 x 6 mm (frame measures)
with consumption of 12.5 pieces per m2
of roofing and format very similar to ceramic Roman
tiles. After 48 h, the tiles were de-molded and submitted to saturate air curing during seven
days followed by air curing in laboratory ambiance until tested.
A three-point bend configuration (major span = 350 mm, deflection rate = 55 mm/min)
adapted from Gram and Gut (1994) was employed for determination of maximum load and
specific energy at 28 days of total age on tiles previously immersed in water for 24 h. The
mechanical tests were performed in an Emic model DL 30000 universal testing machine.
Specific energy is proposed here as the total energy dissipated up to 70% of load reduction
and divided by the cross section area. Physical properties (warping, water tightness and
absorption) were also determined in compliance with Brazilian standards for concrete roofing
tiles (ABNT NBR-13852-2). The main results are summarized in Table 9.
TABLE 9. PHYSICAL AND MECHANICAL PROPERTIES OF ROOFING TILES.
Fiber (Vf%) Slag : lime :
gypsum : sand;
w/c
Warping
(mm)
Water
absorption (%
by mass)
Dry mass at
100 °C (g)
Thickness
(mm)
Maximum
load (N)
Specific
energy
(kJ/m2
) *
Reference
(no fibers)
0.86 : 0.04 :
0.10 : 1.5 ; 0.40
0.91 14.1 2101 9.37 672 0.442
Eucalyptus
pulp (2%)
0.86 : 0.04 :
0.10 : 1.5 ; 0.48
2.01 17.6 1833 9.15 629 0.527
Sisal (1%) +
eucalyptus
pulp (1%)
0.86 : 0.04 :
0.10 : 1.5 ; 0.48
2.52 16.7 1867 8.59 556 0.498
Coir (2%) 0.86 : 0.04 :
0.10 : 1.5 ; 0.48
1.47 17.1 1993 10.9 454 0.802
(*) Test stopped when load decreased 70% in relation to maximum load.
The warping was always less than 3 mm, which constitutes a favorable point for the adopted
fabrication process. This property is concerning the capacity of one tile to adjust with others
in the roof. All series presented no wet marks during the tightness test, after 24 h under 250
mm of water column pressure. The water absorption was always less than 20% by mass after
immersion for 24 h. These results are acceptable in compliance with Brazilian standards for
corrugated sheets of fiber-cement for roofing purposes (ABNT NBR-12800).
During flexural tests the tiles reinforced with vegetable fibers presented specific energy
higher than that of plain tiles. All tested series (with six tiles each one) satisfied the minimum
flexural load of 425 N (85% of 500 N, for saturated tiles), as quoted by Gram and Gut (1994),
in spite of better results with plain material.
Similar studies carried out by Pimentel (2000) employed mortars based on OPC and
reinforced with Pinus caribaea residues from pencil manufacture. The main result was the
21

22. production of roofing tiles using the same Parry Associates device as presented above. The
mechanical behavior of tiles at short term demonstrated to be comparable to that of the plain
mortar used as reference. The flexural load was of at least 490 N and the toughness of tiles
produced with the composite material was up to 124% superior to the control. Several other
cement-based composites containing vegetable fibers or particles were extensively studied by
the same research group (Lopes et al., 2000 and Beraldo, 1997) for rural construction
applications.
7. CONCLUDING REMARKS
The cementitious composites reinforced with natural fibers represent one way of recycling
waste that is of energetic and economic interest for developing countries.
The cement composite is a material with interesting potentiality:
• The cementitious-fiber composites do not present health hazard.
• The price of the material could be as 30% cheaper than usual construction materials.
• The mechanical properties of cement-reinforced composites can equal usual construction
materials ones.
It is an interesting substitute to cement asbestos panels and corrugated sheets. Their
application can concern different part of housing, as roof, ceiling and boarding partition. Their
wide spreading is still limited because of durability and environmental resistance, which
represent the nearest goals in this field.
ACKNOWLEDGEMENTS
The authors would like to thank the National Science Foundation (Inter-Americas
Collaboration in Materials) and the Princeton Materials Institute (PMI), USA. The authors are
also grateful to the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq
– Ciam Program) and to the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
(Capes - Procad), Brazil. The first author appreciates the interest and financial support of La
Région Guadeloupe for the fiber reinforced cement composite project. The second author is
grateful to the Financiadora de Estudos e Projetos (Finep) – Habitare Program, and to the
Fundação de Amparo à Pesquisa do Estado de São Paulo (Fapesp - Pite), Brazil. He would
also like to thank the Commonwealth Scientific and Industrial Research Organization,
Forestry and Forest Products, Australia.
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