E V A L U A T I O N OF C O M M E R C I A L W O O D - C E M E N T

C O M P O S I T E S F O R S A N D W I C H - P A N E L F A C I N G

By Gebran N. Karam ~ and Lorna J. Gibson, 2 Associate Members, ASCE

ABSTRACT; Within the general framework of developing a cement-based sandwich panel for housing construction, wood-cement composites were identified as a prom- ising facing material. Five commercially available wood-cement and natural-fiber- cement board products were studied in an experimental program. The modulus of elasticity, the limit of proportionality stress, the modulus of rupture, and the post- cracking behavior were measured in three-point bending. The compressive strength and failure mode were characterized from uniaxial compression tests on small cubic specimens. The microstructure of these composites was characterized using a scan- ning electron microscope. The effect of the fiber reinforcement on the mechanical properties was related to the observed behavior.

INTRODUCTION

New technologies with the potential of improving the quality and afford- ability of housing construction have been investigated as part of the Inno- vative Housing Construction Program at Massachusetts Institute of Tech- nology (MIT). The advantages of using sandwich panels, made by bonding two thin stiff faces to a thick lightweight core, for folded plate roofing systems, walls, and floors were established: their use can simultaneously achieve the structural support, thermal insulation, and space enclosure re- quirements (Kucirka 1989; Dentz 1991; Maalej 1991; Tonyan 1991). Tonyan (1991) analyzed the behavior of sandwich panels made from different com- binations of materials satisfying the structural and thermal requirements defined in the Uniform Building Code. Wood-cement composites were iden- tified as promising materials for the faces of sandwich panels for housing construction.

Wood-cement composites fall in two families of products (Karam 1990): wood or natural-fiber-reinforced cements and mortars (WFRC), and wood- particle cement-bonded products (WPCB). W F R C has been developed as a replacement for asbestos cement and is manufactured in a similar manner. Cement, water, fibers obtained by wood pulping, and fillers are mixed into a slurry, formed into thin sheets, dewatered by vacuum, and accumulated to the desired thickness and shape. The fresh product is then cured under controlled temperature, humidity, and pressure. Autoclaving is sometimes used to speed the curing process and increase the strength of the product. WFRC typically contains 5 to 15% fibers per volume and can be made into fiat boards, pipes, and other shapes. WPCB is produced by a process akin to that of particleboards and resin-bonded reconstituted wood products. Wood flakes and particles are formed into mats with portland cement as a binder. The mats are stacked, pressed, clamped, and heated between steel

~Ph.D. candidate, Room 1-235, Dept. of Civ. and Envir. Engrg., Massachusetts Inst. of Technol., 77 Massachusetts Ave., Cambridge, MA 02139.

2Assoc. Prof., Room 1-274, Dept. of Civ. and Envir. Engrg., Massachusetts Inst. of Technol., 77 Massachusetts Ave., Cambridge, MA.

Note. Discussion open until July 1, 1994. To extend the closing date one month, a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on June 5, 1992. This paper is part of the Journal of Materials in Civil Engineering, Vol. 6, No. 1, February, 1994. �9 ISSN 0899-1561/94/0001-0100/$1.00 + $. 15 per page. Paper No. 4201.

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cauls in a hydraulic press. The clamped stacks are then cured under con- trolled temperature and humidity. Finally the stacks are declamped and the individual boards trimmed and air cured. WPCB contains from 20 to 45% wood particle per volume and is only made into boards.

REVIEW OF PREVIOUS WORK

Interest in WFRC was spurred in the late 1970s and early 1980s by the search for a substitute for asbestos fibers in asbestos-cement products. The potential and feasibility of using wood and other natural cellulosic fibers as a reinforcement in cement-based composites was identified and investigated by different researchers; their efforts resulted in a series of commercial products destined to replace asbestos cement (Coutts and Campbell 1979; Campbell and Coutts 1980; Cook 1980; Krenchel and Jansen 1980; Harper 1982; Wells 1982; Coutts and Michell 1983; Studinka 1989). To date, most of the published research work has dealt with the effects of wood-fiber type, processing techniques, and fiber loadings on the bending strength and frac- ture toughness of WFRC products (Coutts and Campbell 1979; Mai et al. 1983; Coutts 1984; Courts and Warden 1985; Coutts 1987a; Thomas et al. 1987; Coutts 1989; Coutts and Warden 1990; Michell and Freischmidt 1990; Vinson and Daniel 1990). The microstructure and the role of the fiber- matrix interface have been investigated (Mindess and Bentur 1982; Coutts and Kightly 1982, 1984; Morrissey et al. 1985; Coutts 1987b; Souroushian and Marikunte 1992) as well as the durability and weathering resistance (Sharman and Vautier 1986; Akers and Studinka 1989; Akers et al. 1989; Bentur and Akers 1989a, 1989b; Tait and Akers 1989; Souroushian and Marikunte 1992). But modeling of the mechanical properties has received only limited attention (Andonian et al. 1979; Karam 1991). Coutts (1988), Gram (1988), Fordos (1988), Bentur and Mindess (1990) and Karam (1990) have reviewed and summarized the current state of the art.

On the other hand, WPCB has been known for a long time, the earliest products dating back to 1930. It has, however, received a limited interest m the research literature. Recently published works have concentrated on state of the art reviews and technical assessments (Dinwoodie and Paxton 1984; Sorfa 1984; Sarja 1988; Dinwoodie and Paxton 1988).

WFRC and WPCB boards have been confined to nonstructural and sem- istructural applications; in rare occurrences of structural uses these com- posites have been integrated with other products in well defined building systems. Despite its volume, the published literature provides limited and incomplete information: it describes the variations in the mechanical prop- erties as a function of the fiber volume and the processing methods for experimentally produced composites; it does not contain studies or com- parisons between specific commercially produced products. The product literature made available by manufacturers with the commercial products is at best insufficient for attempting any safe structural application of wood- cement composites: it lacks some important information such as the stress at the limit of proportionality, the cracking strain and the work of fracture. The present papei" presents the first evaluation of commercially available wood-cement composites for structm al applications. Five products covering all the types commercially available at the time and location of the present study have been obtained and tested. The microstructure of the products has been studied to elucidate its role in the mechanical behavior of the composites. The mechanical behavior and properties were investigated in

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an experimental program to complement the manufacturers data and are reported in the rest of this work.

MATERIALS

Four different wood-fiber-reinforced cement boards and one wood par- ticle cement board were evaluated for potential use as sandwich panel fac- ings. Their microstructure and mechanical properties were characterized in an experimental program. The products are

�9 Eflex (Eternit Inc., Reading, Pa.), a high-density [1.76 Mg/m 3 (1-10 pcf)], fully autoclaved panel, composed of portland cement, natural (wood and plant) fibers, and mineral fillers ("Eternit" 1989). The boards obtained were 7.5 mm thick.

�9 Eterboard (Eternit Inc.), a medium-high-density [1.53 Mg/m 3 (96 pcf)], autoclaved panel, composed of portland cement, natural (wood and plant) fibers, and mineral fillers ("Eternit" 1989). The boards obtained were 6 mm thick.

�9 Eterspan (Eternit Inc.), a medium density [1.01 Mg/m 3 (63 pcf)], autoclaved panel, composed of portland cement, treated natural (wood and plant) fibers, and mineral fillers ("Eternit" 1989). The boards obtained were 12 mm thick.

�9 Hardiflex (James Hardie Industries Ltd., Australia), an autoclaved panel of 1.47 Mg/m 3 (92 pcf) density, composed of portland cement, ground silica sand, cellulose (softwood) fibers, and selected additives ("JHBP" 1989). The boards obtained were 6.35 mm thick.

�9 Bison cement board (Bison Werke, Germany), of 80 pcf density, composed of wood flakes and portland cement ("Bison-Werke" 1989). The boards obtained were 16 mm thick.

The exact composition of each of these products and the pro- cessing details was not available from the manufacturers at the time of the present study.

EXPERIMENTAL PROGRAM

Microstructural Investigation The microstructure of the wood cement composites was studied under

the scanning-electron microscope. The specimens were all gold plated under vacuum. Sawn and fractured surfaces of the wood fiber cement products specimens were characterized in the following manner:

First, plane surfaces were carefully prepared by cutting wafers with a diamond wafer saw from the composites; the wafers were used to investigate the general microstructure, the fiber-cement interface, and the geometry of the fibers and voids. The volume fraction of fibers present in the composites was determined by linecount measurements (Underwood 1970) on a grid superimposed on each micrograph. The volume fraction of fibers is given by the ratio of the number of points of the grid that fall on a fiber to the total number of the points in the grid.

Second, small samples were cut out from the fracture surfaces of failed three-point bending test specimens; these were used to study the fractured surfaces and to determine the state of the fibers at fracture. The fracture mechanism was assessed by investigating whether fiber pullout or fiber breakage took place.

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Three-Point Bending Test Three-point bending tests were performed on specimens from each avail-

able type of board to determine the flexural load-deflection relationship of each product, the modulus of rupture, the stress at the limit of proportion- ality and the modulus of elasticity. Test method ASTM D-1037-87 (Standard Methods 1987) for evaluating the properties of wood-based fiber and particle panel materials was used. The manufacturing process of WFRC imparts to the fibers a preferential orientation along the production direction of the sheets, introducing anisotropy in the material properties. 51-mm-wide, 254- mm-long specimens were cut on a diamond saw with their length parallel or perpendicular to the production direction corresponding to the prefer- ential alignment direction of the fibers; five specimens were cut in each orientation. The specimens were tested in an Instron machine (model 4201, Instron, Canton, Mass.) in three-point bending with a span of 203 mm. The thickness of the specimens ranged from 6 to 16 mm, corresponding to the thickness of the commercial boards. The specimens were tested at room temperature (21~ and at ambient humidity (20% RH). In the case of Eflex and Eterboard, additional specimens from each board, five parallel to the production direction and five perpendicular, were tested wet after soaking in water at room temperature for 24 hours. The crosshead speed on the Instron machine was set to 5 mm/min. A 5 kN load cell was used. The X-Y plotter of the machine yielded a load-displacement curve for each test. The maximum load was noted separately from the machine's digital control board. The modulus of rupture was computed as

3 P L

~b = 2 b d 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (1)

the stress at the limit of proportionality as

3PlopL Crlop - 2 b d 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (2)

and the modulus of elasticity as

E - (~P)L3 (AS)4bd3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (3)

where b = width of specimen; d = thickness (depth) of specimen; E = stiffness or modulus of elasticity; = length of span; P = maximum load; Plop -- load at proportional limit; orb = modulus of rupture or ultimate bending stress; ~op = stress at proportional limit or first crack stress; Ap = change in the load measured on the linear part of the load deflection curve; and A5 = change in deflection measured on the linear part of the load deflection curve.

The work to fracture for three-point bending test specimens was also computed as the total area under the load-deflection curve divided by the cross section of the specimen. The dimensions of the specimens used for the particle cement board were smaller than those previously described because of the smaller sizes of the sample boards received: their dimensions were 150 mm length, 25.4 mm width, and 16 mm thickness or depth. The testing span used was 120 ram. The particle cement board manufacturing process produces randomly oriented wood particles in the board; for this reason the five specimens were cut at random orientations. All other ex-

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perimental conditions and the computation of the results were as previously described.

Compression Testing The compressive strength of the commercial wood cement boards was

determined from compressive tests on small cubic specimens. Tests were run in accordance with ASTM C170-87 Standard Test Method for Com- pressive Strength of Natural Building Stone (1990). The size of cubic spec- imens obtainable from flat boards was limited to the thickness of the boards, i.e., to values ranging from 6 to 16 mm. The cutting of the specimens was done on a high-precision diamond-tip wafering saw. When cubes were dif- ficult to obtain, prisms with parallel faces were cut with an aspect ratio (height to width) not exceeding 2:1. Five specimens from each type of board were tested with the load applied parallel to the board's surface and five others were tested with the load applied perpendicular to the board's surface. The tests were performed on an Instron machine with a load cell of 5 kN capacity. The crosshead displacement speed was set at 5 mm/min. The compressive strength of the specimen was determined as

P crc = ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (4)

where crc = compressive strength of the specimen; P = maximum load of the specimen at failure; and A = initial area of the bearing surface.

When the ratio of height to lateral dimension of the specimen differed from unity by more than 25%, the calculated compressive strength was multiplied by a correction factor to adjust it to the corresponding cube strength. The correction factor given by ASTM C170-87 is

1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (5)

where a = diameter or lateral dimension; and h = height.

EXPERIMENTAL RESULTS AND COMMENT

Morphology of Fibers and Voids Micrographs of the sawn plane surfaces of the different wood cement

composites were analyzed. A typical micrograph is shown in Fig. 1. It was noted that frequently the fibers appear in clusters as opposed to single fibers. The voids in the matrix can be spotted as darker regions; single air bubbles can be identified. The final density of the composite is directly related to the porosity of the matrix. A portion of the surface of each fiber in a cluster is overlapped by its neighbors, eliminating contact with the matrix and reducing the efficiency of the fibers in the cluster. Some voids are associated with clusters of fibers; no cement grains or hydration products have pene- trated inside the cluster. A significant fraction of the surface of the fibers, whether single, paired, or in a large clump, is adjacent to voids. The rein- forcing efficiency of the fibers in contact with voids is reduced compared to fibers fully embedded in the matrix.

Morphology of Fiber-Matrix Interface The interface between the fiber and the matrix was investigated to de-

termine the type of bond that is developed. The wood fiber cements as well

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FIG. 1. Micrograph of Hardiflex Showing Microstructure

as the particle cement board were studied and it was noted that the fiber- matrix interface is cracked and discontinuous in almost all cases, with a longitudinal crack running between the fiber and the matrix. Cracking may be due to the differential shrinkage upon drying between the two compo- nents. As well, the matrix does not form a continuous envelope around the fibers. Discontinuities and voids appear in the matrix immediately surround- ing the fiber. This was also reported by Coutts (1987b). Finally, the nature of the fiber-matrix bond is predominantly mechanical. The stress transfer at the interface takes place by mechanical keying of the fiber into the matrix. It is held in the cement at discrete locations or anchors. This will generate a stick-slip type of behavior in pullout. The interface shear strength is con- trolled by the local stress concentrations at the anchor points (Morrissey et al. 1985).

Results of Three-Point Bending Tests The results of the three-point bending tests are presented in Table 1. The

modulus of elasticity (MOE), the modulus of rupture (MOR), and the stress at the limit of proportionality (~op) are given. In addition, the strain at the limit of proportionality was calculated as the stress ~iop over the modulus of elasticity. The values given are for specimens tested at room temperature and ambient humidity. The values quoted by the manufacturers in the prod- uct literature have been reproduced in the last column when they were available. The results in Table 1 agree reasonably well with the published manufacturers data. Discrepancies can be explained by the differences in the testing methods and the statistical variations due to the size and number of the tested specimens. The standard deviation of each result was calcu- lated; it is given in Table 1. The standard deviations vary from 5% to 10% of the mean values, showing the consistency of the properties measured and the homogeneity of the composites. Note the difference between the prop-

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erties of the WFRC boards measured parallel and perpendicular to the direction of production. The modulus of elasticity measured parallel to the direction of production is 5% to 15% greater than that measured perpen- dicular to the direction of production, and the modulus of rupture is greater by 20% to 40%. The stress at the limit of proportionality is of the order of 70% to 80% of the ultimate stress. The limit of proportionality stress is important in design, because if it is exceeded at any time in the service life of the structural composite component, cracking and irreversible damage will result.

Load-deflection curves for WFRC and WPCB boards in three-point bend- ing are shown in Figs. 2 and 3. The area under the load deflection curve gives the work of fracture of the composite, a measure of the toughness of the composite, or its capacity to absorb energy. The shape of the curve gives insight into the mechanism of fracture.

Eflex and Eterboard, tested dry at ambient humidity (Fig. 2), show a relatively brittle fracture mode: after the limit of proportionality is reached the load-deflection relationship becomes nonlinear over a very short interval until the ultimate capacity is reached. Then, the load drops almost instan- taneously to less than 20% of the ultimate and continues to decrease. The brittle character of the fracture is due to the strong bond between the fiber and the matrix. As the load increases beyond the cracking strength of the matrix, the fibers start resisting the load until their ultimate tensile capacity is reached. At the ultimate load the fibers break and the composite fails suddenly. In the samples of Eflex and Eterboard tested wet, the modulus of elasticity, the strength at limit of proportionality, and the ultimate bending strength decreased in the same proportion (Fig. 2); this decrease varied from 20% to 30%. The major change in the mechanical behavior is the appearance of a pseudoductile plateau after the limit of proportionality, increasing the deformation before fracture relative to the dry state; this moisture effect has been observed by Coutts and Kightly (1984), Vinson and Daniel (1990), and Souroushian and Marikunte (1992). The water weak-

500

400

300

200

100

0

FIG. 2.

I I I ! I

Eflex dry Eflex wet

-- -- -- Eterboarddry . . . . . Eterboard wet

I I ""--I-- ~ ~ - - , I

0 1 2 3 4 5 6

Midspan defleclion (ram)

Load-Deflection Relationship of Efflex and Eterboarfl in Bending

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400

300

i I I I I I - - Hardiflex l _ ~ D B i s o n

200

-, 100 " - ~

0 0 2 4 6 8 10

Midspan deflection (mm)

FIG. 3. Load-Deflection Relationshp of Eterspan, Harfliflex and Bison in Bending

ens the fiber-matrix interface and acts as a lubricant increasing the tendency of the fibers to pullout rather than break after the matrix has cracked. The fiber pullout mechanism is responsible for the high ductility and toughness shown in this case.

The behavior of Hardiflex board (Fig. 3) shows a pseudoductile behavior with a large plateau before the ultimate bending stress capacity. The inter- facial bond allows the fibers to pull out when the matrix cracks. The resulting frictional slippage at the fiber-matrix interface absorbs energy and allows the composite to pick up more load in a pseudoplastic manner until the ultimate capacity is reached. At the ultimate load, some of the fibers break causing a drop in the stress. The continuation of pullout allows a gradual failure with the load dropping slowly to zero. Eterspan shows a different load-deflection relationship (Fig. 3). The nonlinear load-deflection interval is small and there is no clear plateau. The load at the limit of proportionality and the ultimate load are very close. The fibers do not carry much load before fracture, a sign of a very weak interfacial bond. The pullout of most of the fibers results in a gradual failure of the composite as seen from the long tail of the load-deflection curve. The elastic strains reached by Eterspan are also higher than those measured for the other wood fiber cements. The Bison wood particle cement board (Fig. 3) exhibits a behavior similar to the brittle wood fiber reinforced cements, Eflex and Eterboard. The load- deflection curve shows a sharp maximum because of the discrete failure of large wood flakes. Some flake pullout takes place as well as some breakage. This behavior is due to the small number of wood particles reinforcing the cracked section.

The work of fracture, a measure of toughness of the composites, is given in Table 2. The standard deviation of each result is given between paren- theses. These values are in agreement with similar measurements reported by Coutts (1984), The work of fracture shows the same trends reported for the results in Table 1. For dry and wet WFRC specimens at ambient relative humidity, the work of fracture measured parallel to the direction of pro-

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TABLE 2. Work of Fracture in Bending

WORK OF FRACTURE

Parallel Perpendicular

Standard Standard Mean deviation Mean deviation

Product (N. m/m a) (N. m/m a) (N. m/m 3) (N- m/m 3) (1) (2) (3) (4) (5)

Eflex, dry Eflex, wet Eterboard, dry Eterboard, wet Eterspan Hardiflex Bison

1,164.5 1,949.0 1,141.8 1,597.0 1,735.4 1,824.7

614.7

61.29 373.0 59.54 82.30

196.13

704.0 735.5 570.9 707.5

1,092.7 162.86 161.1

1,152.3 614.7

19.26 57.89 82.30 52.53 35.02

211.89 161.1

TABLE 3. Compression Test Results and Reported Manufacturers' Values

Product Orientation (1) (2)

Eflex Para. Eflex Perp. Eterboard Para. Eterboard Perp. Eterspan Para. Eterspan Perp. Hardiflex Para. Hardiflex Perp. Bison Para. Bison Perp.

Ultimate Compressive Strength

Mean Std. dev. (MPa) MPa)

(3) (4)

63.4 13.78 84.06 17.23 49.7 11.02 86.8 4.13 18.6 2.07 20.7 2.36 31.0 7.37 55.8 2.99 16.5 4.82 18.6 2.07

Note: Para. = Parallel; Perp. = Perpendicular; A = not available.

Strain at failure ( - ) (5)

0.0095 0.011 0.034 0.33 0.025 0.075 0.037 0.15 0.039 0.153

Std. dev.

Ultimate compressive strength reported

by manufacturer (no direction specified)

(MPa) (6)

72 72

106 106 13 13

N/A N/A 14.5 14.5

= Standard deviation; N/

duction is about 50% and 200% higher respectively than the work of fracture measured perpendicular to the production direction. The contribution of the fibers to the work of fracture is more important for loading parallel to the direction of production than perpendicular to it due to their preferential orientation. The humidity increase in wet specimens increases the contri- bution of the fibers to the work of fracture leaving that of the cement matrix at about the same level causing the observed increase in anisotropy.

Results of Compression Tests The results of the compression tests are given in Table 3. The standard

deviation of each result is given in parentheses. Deviat ions vary from 5% to 20% of the mean values. The compressive strength reported by the manufacturer was found to be close to that measured from the small cubic specimens loaded perpendicular or parallel to the faces of the board. Typical

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compressive stress-strain relationships, those of Eterspan loaded parallel and perpendicular to the production direction, are given in Fig. 4. All WFRC products exhibit qualitatively the same type of stress-strain relationships in compression. The WFRC specimens loaded parallel to the faces failed at loads 10-40% smaller than those loaded perpendicular to the faces. Spec- imens loaded parallel to the faces of the board failed by delamination or tensile splitting along vertical planes while specimens loaded perpendicular to the faces of the board failed in shear, as shown in Fig. 4. The delamination failure mode is due to the presence of weak planes, resulting from the processing (Karam 1990). The fresh boards are produced by accumulating laminae (1-3 mm thick) on a roller up to the desired finished thickness (4-16 mm). If the boards are used as faces in a sandwich structural panel such as in wall components or folded plate roofs, they are likely to be loaded on edge resulting in compressive stresses parallel to the faces of the boards. The lower compressive strength will control; and if compressive failure takes place, it will be by delamination or splitting. Note that the Bison cement board exhibits the same anisotropy because the manufacturing process ori- ents the wood flakes in horizontal planes with their faces parallel to the faces of the board. The strains at failure, calculated as the shortening at failure over the initial height, are also reported in Table 3. These values show a marked anisotropy.

Fracture Surfaces Fracture surfaces have been studied to investigate the failure mode of

the fibers (Figs. 5-8). The fracture surface of Eflex tested dry shows the great majority of the fibers to be broken (Fig. 5). In contrast, the fracture surface of Eflex tested wet, shows intact fibers that have pulled out of the matrix, undamaged, with their tapered ends visible to the eye (Fig. 6). The load-deflection behavior of Eflex, tested dry, shows a brittle mode of frac- ture while that tested wet shows some pseudoplasticity. In the dry state,

,,i,o , ,

20 ~ ~ ' - - - - . ~

5 (parallel) ~ Etcrspan (perpendicular)

I 0 �9 I I I I I

0 2 4 6 8 10 12 14

Strain (%)

FIG. 4. Compressive Stress-Strain Relationship of WFRC Small Cubes and Modes of Failure

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FIG. 5. Fracture Surface Micrograph of Eflex Dry

FIG. 6. Fracture Surface Micrograph of Eflex Wet

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FIG. 7, Fracture Surface Micrograph of Eterboard Dry

FIG. 8. Fracture Surface Micrograph of Hardiflex

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most of the fibers are well bonded to the matrix while in the wet state, the fibers pullout and break close to the surface of the crack. Wetting appears to reduce the strength of the fiber-matrix bond inducing the characteristic pseudoplastic behavior and increasing dramatically the work of fracture (Table 2).

The fracture surface of Eterboard tested dry shows a mixture of pulled out and broken fibers (Fig. 7), explaining the mixed behavior shown in the load-deflection relationship: a short plateau corresponding to some fiber pull out is followed by brittle fracture due to the failure of the well bonded fibers in tension (Fig. 2). Again, pullout of the lubricated fibers dissipates frictional energy increasing the work of fracture of wet specimens (Table 2).

Eterspan and Hardiflex, the most ductile of the wood fiber cement com- posites, show fracture surfaces with almost all the fibers pulled out from the matrix. Fiber breakage takes place in the case of damaged fibers and strongly anchored ones in Hardiflex (Fig. 8). The dominant micromechan- ism, fiber pull out, controls the load-deflection relationship presented in Fig. 3, and is at the origin of the pseudoplastic behavior of Hardiflex. In the case of Eterspan with very weak fiber-matrix interfaces, the only effect of the fibers is to introduce a ductile and slow failure mechanism. The work of fracture measured for Eterspan and Hardiflex is very close to that mea- sured for Eflex and Eterboard at 100% RH (Table 2). This is in agreement with the fact that fiber pull out is the dominant failure mechanism in all these cases.

The observations made in the present paper in the study of the fracture surfaces agree well and consistently with previous research work by Coutts and Kightly (1982, 1984), Mindess and Bentur (1982), and Souroushian and Marikunte (1992).

SUMMARY AND CONCLUSIONS

In view of their potential use as facing materials for sandwich panels, five commercial wood-cement composites were investigated in an experimental program. The modulus of elasticity, modulus of rupture, and compressive strength were determined along with other relevant mechanical measures, and the load-deflection behavior in bending was established. The micro~ structure of the composites was investigated and related to their mechanical properties. The following conclusions can be drawn from the present study:

1. All the board products evaluated have shown a marked anisotropy in their mechanical properties, whether in bending or compression. This an- isotropy results from the manufacturing process and has to be carefully considered in any structural application.

2. Water absorption and moisture can decrease the modulus of elasticity and the modulus of rupture by up to 30%, while increasing the work of fracture by at least 50%. This effect is very important especially in weather exposed applications of wood-cement composites.

3. The limit of proportionality stress in bending controls the use of these composite boards. Irreversible damage and cracking result when this load is exceeded.

4. Products with high work of fracture and pseudoductile behavior after first crack will result in safer structural assemblies. This property has to be

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investigated as not all products show the same stress-strain characteristic behavior.

5. The fiber-matrix interface condition controls the stress-strain behavior, toughness, and ductility of the wood-cement composites and it is moisture sensitive.

6. The durability and long-term moisture resistance of these products has not been investigated in the present study. Accelerated aging and durability testing results from the literature point to a good strength and properties retention. Further research in this area is needed before a final evaluation of wood-cement composites can be made.

7. Wood-cement composites are a promising class of new materials. The present study has contributed some basic information to allow for the pre- liminary design of structural systems with some commercially available board products.

ACKNOWLEDGMENTS

The authors would like to acknowledge the financial support of Alcan, GE-Plastics, ITW, Mobay, US-Gypsum, and Weyerhaueser through the Housing Construction Technologies consortium at MIT.

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