MECHANICAL PROPERTIES OF FIBER REINFORCED ...

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Thin section fiber reinforced concrete (FRC) cladding panels are widely used in the construction ... using a water cooled diamond blade saw. Flexural test ...
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Pergamon

Cementand ConcreteResearch, Vol. 24, No. 6, pp. 1121-1132,1994 Copyright© 1994ElsevierScienceLtd Printedin the USA. All rights reserved 0008-8846/94 $6.00+.00

0008-8846(94) 00056-5 MECHANICAL PROPERTIES OF FIBER REINFORCED LIGHTWEIGHT CONCRETE COMPOSITES

M. Perez-Pena* and B. Mobasher ** *Senior Member of Technical Staff, USG Corporation, Libertyville, IL 60048 **Assistant Prof. of Civil Eng., Arizona State Univ., Tempe, AZ 85287-5306 (Refereed) (ReceivedSeptember16. 1993;in rmal formMarch 15, 1994)

ABSTRACT

Hybrid composites with variable strength/toughness properties can be manufactured using combinations of brittle or ductile mesh in addition to brittle and ductile matrix reinforcements. The bending and tensile properties of thin sheet fiber cement composites made from these mixtures were investigated. Composites consisted of a woven mesh of either polyvinyl chloride (PVC) coated E-glass or polypropylene (PP) fibers for the surface reinforcement. In addition, chopped polypropylene, acrylic, nylon, and alkali-resistant (AR) glass fibers were used for the core reinforcement. It is shown that by controlling fiber contents, types, and combinations, design objectives such as strength, stiffness and toughness, can be achieved. Superior post-cracking behavior was measured for composites reinforced both with glass mesh and PP mesh. Load carrying capacity of PP mesh composites can be increased with the use of 1% or higher chopped PP fibers. Glass mesh composites with short AR glass fibers as matrix reinforcement indicate an increased matrix cracking strength and modulus of rupture. Combinations of PP mesh/short AR glass did not show a substantial improvement in the matrix ultimate strength. An increased nylon fiber surface area resulted in improved post peak response. Introduction

Thin section fiber reinforced concrete (FRC) cladding panels are widely used in the construction industry. Compared to conventional precast concrete, the production, transportation, and installation costs of these material are significantly lower [1]. The first FRC composites were made by the Hatschek process [2]. In theses composites, various fibers types are used as the filtering media in the retention of cement particles during manufacture. The fiber length used is hence shorter than the optimum length, resulting in brittle products. 1121

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M. Perez-Pena and B. Mobasher

Vol. 24, No. 6

Development of a new class of cementitious materials based on thin sheet hybrid composites is necessary to address the needs of construction industry. Areas of application include exterior and interior walls, roofing products, floor underlayment, and retrofit projects. Woven meshes have been utilized in the manufacture of thin sheet concrete products[3] [4] [5]. Placement of fiber reinforcement at the surface of the composites results in higher matrix and composite strengths. This is partly due to the length effect of the continuous fibers which allows for full stiffness utilization. In comparison, large volume fractions (> 3%) of chopped fibers are required to effectively reinforce concrete products. A woven mesh facilitates panel production in a continuous manufacturing line. Since the primary mode of loading is due to bending (especially during installation), the fibrous mesh provides a continuous reinforcement in the tensile region. Use of short fibers in addition to surface reinforcement is advantageous for both short term handling, installation, and long term performance. Due to panel weight considerations, lightweight concrete mixtures with densities around 1280 Kg/m 3 (80 pcf) are desirable which result in a matrix with relatively low bending strength. One possible way to increase ductility is to use fiber reinforcement. The composites under study consist of woven or fibrillated mesh of both ductile and brittle continuous fibers. Use of a low fiber volume fraction moderately increases the first-crack strength, ultimate tensile, and flexural strengths in addition to better postpeak response. Fibers control the cracking process by preventing localization and generate a homogeneous microcracking state to dissipate energy over the entire volume [6]. The objective of this work was to study the interaction among combinations of brittle and ductile mesh in conjunction with brittle and ductile matrix reinforcements. Desired strength/toughness properties can be obtained using a variety of mesh and matrix reinforcements. It is shown that by controlling fiber contents, types, and combinations, design objectives such as strength, stiffness and toughness, can be optimized. Two types of polymer coated woven E-glass, and fibrillated polypropylene (PP) mesh were used. Chopped fibers (acrylic, A-R glass, polypropylene and nylon) were used. Materials and Experimental Procedures The Composite Formulation A lightweight-aggregate concrete matrix was used. Its major components were a high early strength portland cement, class C fly ash and expanded clay lightweight aggregate in a proportion of 25:19:56. Air entrainment dosage was 1.25 fl. oz. per 100 Ibs. of cementitious materials. The superplasticizer was a sodium salt of a sulfonated naphthalene condensate at a dosage of 25 fl.oz, per 100 Ibs. of cements. A constant water:cementitious solids ratio of 0.37 was used throughout the study. Proprietary additives were used to accelerate strength development. Two types of surface reinforcement were studied, an E-glass mesh with a polyvinyl chloride (PVC) coating, and a polypropylene mesh. A single layer of mesh was embedded on the surface of specimens except for the glass/pp mesh composites

Vol. 24, No. 6

FIBER COMPOSITES,MECHANICALPROPERTIES,TOUGHNESS

1123

Table 1. Summary of the combinations for the hybrid composites

Short fiber core reinforcement no fiber

Acrylic

Nylon

AR-glass

No Mesh

PP 0.38 % 0.75 % 1.12% 1.48 %

PP Mesh

yes

Glass Mesh

yes

PP/Glass Mesh

yes

1.0 %, L~ = 12.7mm

0.62 % L~ = 12.7mm

1.7 % D=5 or 32 deniers

1.0 % I_f = 12.7mm

0.75 % Lf=12.7mm

which contained a layer of each. In addition, four types of short fibers were used in the matrix. These fibers were: acrylic, PP, AR glass, and nylon; short and continuous fiber composites evaluated in this study are presented in Table 1. Sheets with nominal dimensions of 83.8 cm x 30.4 cm x 1.3 cm (33 x 12 x 0.5 in.) were cast in the laboratory and subjected to 14 days of curing at 90% RH at a temperature of 32.2 °C (90"F). Dog bone specimens for the tension test were cut using a water cooled diamond blade saw. Flexural test coupons with nominal dimensions of 30.4 cm x 10.1 cm x 1.3 cm (12 in x 4 in x 0.5 in) were tested in four point bending. Mechanical Testing Tensile experiments were conducted under a closed-loop strain controlled condition. Test procedures for measurement of post-peak responses of FRC materials used in this study were based on the techniques developed previously [7] [8] [9]. The elongation of the tensile specimens were measured using the response of a clip gage mounted across a gage length of 152.4 mm (6 in). The first crack load is measured at the onset of the deviation of the load-deformation response from linear behavior. The maximum load is dependant on the tensile strength of reinforcing mesh in addition to the pullout response of the fibers. This load was used to compute the nominal tensile strength of the composite based on the original cross sectional area. Four point flexural tests were conducted. Deflections at the two loading points

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M. Percz-Pena and B. Mobasher

Vol. 24, No. 6

were averaged by means of a jig which allowed for continuous deflection monitoring up to values as high as 24.5 mm (1 inch). This response was subsequently used as the feedback signal in the control of the test. Toughness of the composites was obtained by integrating the load versus deflection at loading points during the entire loading cycle. Additionally, toughness up to the first cracking load was also measured. Test data were collected at a frequency of 1 Hz using a 12 bit resolution data acquisition system. Digital data analysis software were developed for graphical analysis of the data. The values reported in Tables 2 through 5 are averages of at least three replicate samples. Standard deviations are reported in parentheses (), and the coefficient of variation in square brackets [ ].

B)

Figure 1. SEM micrographs of fractured lightweight concrete matrix. Bar indicates 13.5 #m in (A) and 21.0 #m in (B).

Vol. 24, No. 6

FIBER COMPOSITES,MECHANICALPROPERTIES, TOUGHNESS

1125

Results and Discussion Microscopic Analysis Densities of the composites ranged between 75 to 83 pcf, indicating a significantly higher degree of porosity as compared to normal weight FRC materials. A scanning electron microscope (JEOL 840) was used to study the fracture surface. Figure 1 shows the photomicrographs of the lightweight core. Typical pores, produced by air entraining admixtures were estimated to be several hundred microns as shown in FIG.1(a). A second source of porosity was the expanded clay aggregate as shown in FIG.I(b). The bonding mechanism of fibers to the matrix in such a porous microstructure may be quite inferior to the conventional FRC composites, significantly reducing the efficiency of the short fibers. Such a microstructure benefits from the use of continuous fiber mesh as the external reinforcement.

Tensile Response of Hybrid Composites As shown in Figure 2, the tensile stress-strain response of a composite with glass mesh and short acrylic fibers exhibits the characteristic response of strong continuous fibers in a weak brittle matrix. The linear portion of stress strain response, terminates by the failure of the matrix at the Bend Over Point (BOP). After the BOP, load is transferred to the fiber mesh and short fibers at the crack sites. As the load increases, additional cracks form at regular spacing and reduce the stiffness. The maximum load occurs due to the failure of glass fiber strands. The post peak response is attributed to the pullout response of short fibers. The energy dissipated 2.0

1.5

m~ m (D

1.0

.,~ rY]

0.5

0.0 0.000

0

~

0.005

,

~

0.010

,

0.015

Strain, rnrn/rnm Figure 2.

Tensile stress strain curve of lightweight concrete thin sheets reinforced with a PVC coated glass mesh and chopped Acrylic fibers in matrix.

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M. Perez-Pc'na and B. Mobasher

Vol. 24, No. 6

is due to both the mesh reinforcement which results in distributed cracking and the pullout of short fibers. The area under the stress-strain curve was used as a measure of ductility of the composite. Polypropylene and Glass Mesh Surface Reinforcements Flexural load deflection responses of glass and PP mesh composites are shown in FIG. 3. The flexural stress, associated with the first cracking point is referred to as the proportional elastic limit (PEL) and the elastically equivalent stress parameter measured at the maximum load is referred to as the modulus of rupture (MOR). Summary of the test results are provided in Table 2. The composites with glass mesh carry loads beyond the PEL to an MOR level of 6.5 MPa, exceeding the performance of the PP mesh composites with their MOR of 4.4 MPa. The toughness of these composites are comparable at around 2.1 to 3.6 Nm. The crack spacing was larger for samples with PP mesh as compared to the glass mesh. This is due to the lower stiffness of the PP fibers, a weaker matrix/fiber mechanical bond, and a less than optimum load transfer mechanism. The effect of the combined use of PP and glass mesh is also shown in FIG.3. Specimens with glass/PP mesh were able to sustain higher first crack loads as compared to samples with either glass or PP. The ultimate strength is significantly higher than single PP mesh composites. In these composites, after the glass fails, about half of the load carrying capacity is carried by the PP mesh. The toughness increases to 5 Nm which is as much as twice the toughness of a single glass mesh composite.

500|~

~

400~ Z

3 0 0 t~

~

200

o

Figure 3.

~~~~~ Glass

~ ~( ~ ~

'

~

PP/Glass PP

~~C ~O CC~

'

Deflec[ion, mm Flexuralload deflexion response of glass and ~ continuous mesh composites.

Vol. 24, No. 6

FIBER COMPOSITES, MECHANICAL PROPERTIES, TOUGHNESS

Table 2.

Mesh

1127

Flexural response of continuous fiber glass, PP and combination glass/PP mesh. (no matrix fibers)

Deflection, mm

Young's Modulus, GPa

Strength,

Toughness,

MPa

Nm

at 1st crack

at max load

E

PEL

MOR

at 1st crack

max

Glass Mesh

0.35 (0.09) [25]

11.4 (0.51) [4]

8.0 (1.7) [22]

3.4 (.45) [25]

6.5 (.23) [4]

0.06 (0.02) [32]

2.14 (.16) [7.6]

PP Mesh

0.41 (.05) [12]

23.3 (2.9) [12]

9.6 (1.6) [17]

4.0 (.31) [8]

4.38 (.03) [1]

0.09 (0.01) [13]

3.65 (.51) [13.8]

PP & Glass Mesh

0.33 (0.07) [20]

8.56 (0.74) [9]

10.6 (2.2) [20]

5.1 (.27) [5]

7.9 (. 16) [2]

0.1 (0.02) [21 ]

5.02 (.7) [ 14]

Values for the standard deviation are shown in parentheses ( ) and for the coefficient of variation in square brackets [ ].

Polypropylene Chopped Fiber-Reinforced Composites Load-deflection response of composites with 0.38, 0.75, 1.12 and 1.48 Vol% of 0.5 in. long PP fibers are shown in FIG. 4. Summary of the test results are also provided in Table 3. An increase in load carrying capacity and postcrack strength is observed by increasing the fiber content. Volume fractions around 1% or higher are needed for the load carrying capacity to be equal or exceed the first crack strength. By comparing the flexural response of the mesh vs. the fibers in Figures 3 and 4, it is observed that the load deflection curve of samples with 1.12 and 1.48 Vol% PP fibers reached similar or even higher load levels as samples with PP mesh. The toughness provided by the mesh however is about three times higher than the chopped fiber composites. Similar behavior was observed for tensile PP-FRC specimens, i.e. the load carrying capacity increases with fiber volume fraction. At relatively low fiber content, the stiffness drops after first matrix cracking. Increasing fiber volume fraction causes composites to carry increasing loads after matrix cracking. Matrix cracking load is sustained in the post cracking zone at volume fractions larger than the 1.12% level.

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M. Perez-Pena and B. Mobasher

Vol. 24, No. 6

3OO

~

200

~ 4 /

/I,

0 Figure 4.

,,

*~,-'-~* 0.75~ V,

=l.12

~ DeflecLion,

v, v,

1'0 mm

15

Flexural response chopped PP fiber composites.

Mesh/Chopped Fibers Reinforcement Acrylic and PP Fibers

Chopped acrylic and PP fibers, 12.7 mm (0.5 in) in length were used to reinforce the core. Flexural test results are tabulated in Table 4 and are compared with composites with the glass mesh without fibers. The maximum feasible loading of PP fibers that would still allow embedment of glass mesh on the composite surfaces was 0.75% by volume. At this volume fraction level of 12.7 mm ( 0.5 in) long PP fibers, the PEL and MOR strength of the composites are marginally affected. Similar results are observed with 12.3 mm (0.5 in) long Acrylic fibers. When used at the maximum practical volume fraction of 0.62% (from a mixing point of view), acrylic fibers provided marginal strength enhancement, and increased first crack toughness, but similar maximum toughness. The degree of variation observed in the first crack toughness values may be partially due to the method used in determining the first crack point [10]. A-R Glass Fibers

Results shown in Table 5 suggest that stiffness of panels with glass mesh may benefit by addition of 1.0 vol% AR glass fibers. This is indicated by a 16% increase in the modulus of elasticity and matrix cracking strength (PEL). Addition of AR glass fibers did not show a definite improvement in the matrix strength for the composites with PP mesh. However, a 13% increase in the modulus of elasticity indicated higher stiffness for samples with the PP mesh and A-R fibers.

Vol. 24, No. 6

FIBER COMPOSITES,MECHANICALPROPERTIES,TOUGHNESS

1129

Table 3. Flexural response of PP mesh versus PP chopped fibers. Mesh

Fibers

Deflection mm

PP Mesh

None

Young's Modulus GPa

Strength

Toughness

MPa

Nm

at 1st crack

at max load

E

PEL

MOR

1st crack

max

No Fibers

0.41 (.05) [12]

23.3 (2.9) [12]

8.6 (1.2) [15]

4.0 (.31) [8]

4.38 (.03) [1]

0.09 (0.01) [13]

3.65 (.51) [13.8]

PP 0.38%

0.54 (.29) [54]

1.09 (.584) [54]

6.2 (2.3) [37]

3.28 (.24) [7]

3.28 (.24) [7]

0.11 (0.01) [66]

.62 (.156) [37]

PP 0.75%

0.45 (.06) [12]

0.89 (.11) [12]

7.8 (1.8) [24]

3.61 (.15) [4]

3.61 (.14) [4]

0.08 (0.12) [15]

.568 (0.01) [10]

PP 1.12%

0.47 (.01) [3]

0.99 (.076) [7]

6.0 (.46) [7]

3.52 (.14) [4]

3.57 (.19) [5]

0.09 (.005) [6]

1.09 (.013) [21 ]

PP 1.48%

0.32 (.28) [43]

0.64 (.28) [43]

6.4 (.32) [5]

3.95 (.14) [4]

3.96 (.14) [4]

0.08 (.054) [66]

1.23 (.075) [9]

Values for the standard deviation are shown in parentheses ( ) and for the coefficient of variation in square brackets [ ]. Figure 5 shows the effect of short brittle glass fibers on composites with two different meshes. The increase in the flexural strength for the AR/Glass mesh composites is accompanied by a lower maximum deflection at failure, in addition to a decrease in crack spacing. For PP mesh composites, the first cracking load was not surpassed in the post cracking zone, however the overall load carrying capacity and the toughness are significantly increased.

Nylon Fibers Using two different fiber deniers, the effect of nylon fiber size was studied on composites with glass mesh. Flexural test results in Table 5 show the significance of

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M.Perez-PenaandB.Mobasher

Table 4. Mesh

Flexural response of glass mesh and chopped acrylic and PP fibers. Fibers

Deflection mm

Glass Mesh

Vol.24.No.6

Young's Modulus GPa

Strength

Toughness

MPa

Nm

at 1st crack

at max load

E

PEL

MOR

1st crack

max

No Fibers

0.35 (.09) [25]

11.4 (0.51) [4]

8.0 (1.7) [22]

3.44 (.45) [25]

6.45 (.23) [4]

.06 (.02) [32]

2.14 (.16) [7.6]

PP 0.75%

0.45 (.14) [32]

11.6 (.685) [6]

8.0 (2.1) [27]

3.56 (.11) [3]

7.44 (.21) [3]

0.07 (.02) [32]

1.67 (.15) [28]

Acrylic 0.62%

0.52 (.12) [22]

10.7 (.94) [9]

8.8 (2.6) [29]

4.13 (.11) [3]

7.2 (.05) [1 ]

0.1 (.02) [57]

1.19 (.18) [9]

Values for the standard deviation are shown in parentheses ( ) and for the coefficient of variation in square brackets [ ]. small fiber denier. The glass mesh and the nylon fibers, used as a secondary reinforcement showed superior behavior compared to samples without reinforcement in the core matrix. The smaller denier (5.0) composites showed higher post crack 5001

~ 400t

~ ~

Glass Mesh 1.0~ V~ AR Fibers

/

o

g

ib

DeflecUon, Figure 5.

1'5 mm

Flexural response of hybrid composites with glass fibers.

Vol. 24, No. 6

Table 5.

Mesh

Glass Mesh

FIBER COMPOSITES, MECHANICAL PROPERTIES, TOUGHNESS

Influence of chopped AR glass fibers on the flexural response of composites with glass and PP mesh. b) Flexural response of glass mesh composites with chopped nylon fibers of two different deniers. Fibers

Deflection mm

Young's Modulus GPa

Strength MPa

at I st crack

at max load

E

PEL

MOR

at 1st crack

max

No Fibers

0.35 (.09) [25]

11.4 (.51) [4]

8.0 (1.7) [22]

3.4 (.45) [25]

6.5 (.23) [4]

.06 (.02) [32]

2.14 (.16) [7.6]

AR Glass 1.0%

0.33 (.03) [7]

7.7 (.64) [8]

9.3 (.62) [15]

4.0 (. 17) [4]

5.8 (.34) [6]

.12 (.01) [11 ]

2.23 (.02) [1 ]

No Fibers

0.41 (.05) [12]

23.3 (2.9) [12]

8.6 (1.2) [15]

4.0 (.31) [8]

4.4 (.03) [1]

0.09 (0.01) [13]

3.65 (.51) [14]

AR Glass 1.0%

0.32 (.04) [14]

23.1 (.34) [1.5]

9.8 (.36) [7]

3.9 (.19) [5]

3.9 (.19) [4]

0.09 (.01) [11]

4.79 (.74) [15]

Nylon 1.7% D= 5

0.32 (.01) [4]

12.3 (.04) [8]

8.9 (.8) [9]

3.35 (.04) [1 ]

7.8 (.25) [3]

0.08 (.007) [9]

4.37 (0.05) [1.2]

Nylon 1.7% D = 32

0.34 (.06) [19]

13. (.02) [5]

10.5 (1.3) [12.3]

4.03 (.25) [6]

6.9 (.16) [2]

0.09 (0.004) [5]

3.96 (.21) [3.88]

PP Mesh

Glass Mesh

1131

Toughness Nmm

Values for the standard deviation are shown in parentheses ( ) and for the coefficient of variation in square brackets [ ]. loads compared to composites with the larger den er (32.0) fiber. These results confirm the importance of increasing the specific surface area on the strength of the composites as shown by Romauldi and Batson [11], and others.

Conclusions Thin sheet lightweight concrete composites were studied. By using various polymeric and synthetic fibers, the properties of the sheet lightweight concrete

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M. Perez-Pena and B. Mobasher

Vol. 24, No. 6

composites can be directly engineered. PP mesh is an effective reinforcing material for thin concrete products. However, a higher degree of mechanical bonding and interlocking between the matrix and the mesh can improve the post-peak loading behavior. Fiber reinforced composites were fabricated with significant post-peak toughness by using PP fiber contents of 1% or higher (1.12 and 1.48%). Combination of glass mesh/AR glass fiber increased matrix cracking strength and loading capacity compared to composites without fibers. Combinations of PP mesh/AR glass show an increase in first cracking strength and toughness values with no improvement in the matrix ultimate strength. Increased fiber surface area in composites with a combination of glass mesh/nylon fibers resulted in improved post peak response.

Acknowledgements Authors acknowledge the help of Mr. M. R. Alfrejd for conducting the flexural tests. We would like to thank B.R. Link and R. S. Blancett for their support and encouragement while working on this paper. We would like to thank Dr. K.C. Natesaiyer for discussions on the paper. B.M. is thankful to the Research Initiation Award from the National Science Foundation (Grant No. 82-MSS9211063, Program Director Dr. Ken Chong)

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Hiendl, H., Asbestos Cement Machinery CL. Attenkofer, Ludwigsplatz 30, 8440 Straubing/Germany, p 128.

3.

Schupack, M., ACI, SP-124-21, 1990, pp 421-436.

4.

Balaguru, P. N., and Shah, S. P., Fiber-Reinforced Cement Com.Dosites, pp. 365412, McGraw-Hill, New York, 1992.

5.

Odler, I., Fiber-Reinforced Cementitious Materials eds, S. Mindess & J. Skalny, Mat. Res. Soc. Symp. Proc., 211, pp.265-273, Pittsburgh, PA 1991.

6.

Mobasher, B., Castro-Montero, A., and Shah, S. P., Exp. Mech., 30, 90, pp. 286294.

7.

Gopalaratnam, V. S., and Shah, S. P., ASCE J. of Eng. Mech. Div., Vol. 113, No. 5, May 1987, pp.635-652.

8.

Mobasher, B., and Shah, S. P., ACI SP 124-8, 1990. pp137-156.

9.

Mobasher, B., and Shah, S. P., ACI Mat. J. Sept-Oct. 1989, pp. 448-458.

10.

EI-Shakra, Z.M. and Gopalaratnam, V.S., Cem. Concr. Res. 23, 1455-1466, 1993.

11.

Romualdi, J. P., and Batson, G. B., J. of Eng. Mech. Div., ASCE, 89, No. EM3, June 1963, pp.147-168.