CEMENTITIOUS COMPOSITES REINFORCED WITH VEGETABLE FIBERS

M-A. Arsène1, H. Savastano Jr.2, S. M. Allameh3, K. Ghavami4 , W. O. Soboyejo3

1 Département de Chimie- UFR SEN, Université des Antilles et de la Guyane, campus de

Fouillole, 97157 Pointe-A-Pitre, Guadeloupe (FWI) 2 Faculty of Animal Sciences and Food Engineering, University of São Paulo, Pirassununga,

SP, Brazil 3 Department of Mechanical and Aerospace Engineering (MAE), Princeton University,

NJ 08544-5263, USA 4 Department of Civil Engineering, PUC-Rio, 22453-900, Rio de Janeiro, Brazil

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. KEYWORDS Vegetable fiber, pulp fiber, fiber cement composite, BFS/fiber composite, strength, toughness, microstructure, affordable housing. 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. 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 �Fiber Selection and Composite Preparation�. 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 �Microstructure�. Some physical and mechanical characteristics and the effects of natural fiber reinforcement on composite strength and durability are presented in Section �Macrostructure Behavior�. Fracture toughness and resistance-curve of natural fiber-reinforced composites are analyzed in Section �Fracture�. Finally, emerging/potential applications are discussed in Section �Application of Asbestos-Free Fiber Cement in Roofing Tiles�, before presenting the salient conclusions arising from this work in Section �Concluding Remarks�. FIBER SELECTION AND COMPOSITE PREPARATION 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.

• 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 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).

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. 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. 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 �Composite cement/fiber elaboration�. 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 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. 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). MICROSTRUCTURE

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 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.

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

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). Composites

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 (Mechanical and physical properties). 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.

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. 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

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

MACROSTRUCTURE BEHAVIOR

Fiber and Pulp

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, 25oC and relative humidity

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). 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

0 400 800 1200 1600

40

70

80

0

Lognormal base e

Percent

Per

cent

100 1000100 100

9595

99 99

506050

20

30201010

5511

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. 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).

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.

Test methods The test methods explained in this section were applied to the composites produced as mentioned in Composite BFS/fiber elaboration 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.

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 (oC) 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. 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

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 ± 0.04 E. 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

MO

R (M

Pa)

1 month

13 months25 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

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 (k

J/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.

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.

FRACTURE Raw pads of fiber cement based on BFS binder reinforced with 8% sisal kraft were produced as described in �Composite cement/fiber elaboration� and �Composite BFS/fiber elaboration�. 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 ~25oC. 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. 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

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):

dxx

VKKKKL

yfmssbmc ∫

+=∆+=

05.0

5.02 σα

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).

dxxahVKL

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 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)

Stre

ss In

tens

ity F

acto

r, K

(MPa

m0.

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. 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. 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 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 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.

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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