Strength and reliability of carbon-fiber-reinforced cement composites

Cement & Concrete Composites 16 (1994) 15-21 © 1994 Elsevier Science Limited Printed in Great Britain. All fights reserved 0958-9465/94/S7.00 ELSEVIER

Strength and Reliability of Carbon-Fiber-Reinforced Cement Composites Houssam A. Toutanji, Tahar E1-Korchi & R. Nathan Katz Worcester Polytechnic Institute, Worcester, Massachusetts 01609, USA (Received 2 March 1993; accepted 26 May 1993)

Abstract

lure. The type of fibers currently used include steel, glass, polymers, natural and carbon fibers. Economic considerations have restricted the use of carbon fibers in cementitious composites on a commercial level. However, the qualifies of the resulting carbon-fiber-reinforced cement (CFRC) composites, such as high tensile strength and elastic modulus, chemical inertness and strong bond with the cement matrix, make carbon fibers an attractive reinforcement. Moreover, the steady reduction in cost due to advances in carbon fiber production technology2.3 has increased their usage in the construction industry. The present work focuses on the use of polyacrylonitrite-based (PAN) carbon fibers to improve the tensile and flexural strength of the cementitious composites. The tensile strength values are obtained using the cementitious composites axial tensile technique (CCATT). 4-7 This technique minimizes misalignment and stress concentrations at gripping and produces a uniform stress distribution along the tested specimen. Results have shown that stress created due to misalignment may reduce the measured tensile strength by as much as 80%. 4,6 The results obtained in this study are compared with those obtained by other investigators using the traditional uniaxial tensile technique for measuring the tensile strength of CFRC. Weibull statistics are used to determine the effect of carbon fibers on the reliability of tensile and flexural strength of cement paste specimens.

The effect of carbon fibers on the mechanical properties of cement paste composites is studied. The addition of polyacrylonitrite-based carbon fiber to a cementitious paste matrix results in a significant improvement in the tensile and flexural properties of the composites. The uniaxial tensile strength results are obtained using the novel cementitious composites axial tensile technique. The addition of l, 2 and 3 %vol. of carbon fiber to a cement matrix results in an increase in the uniaxial tensile strength of 32, 48 and 56%, respectively. The enhancement of the composite flexural strength was more significant, as compared to the uniaxial tensile strength. The flexural strength of cement matrix increased by 72, 95 and 138% with the addition of 1, 2 and 3 %vol. of carbon fiber, respectively. Weibull statistics indicate that reliability in flexure was not enhanced by fiber addition and there was no correlation between the percent fiber loading and the reliability of the composites. However, the reliability of the carbon-fiber-reinforced composite in tension was greatly improved and there is a positive correlation between fiber loading and the Weibull modulus, m. Keywords: Cement paste composites, carbon fiber, tensile strength, flexural strength, CCATT, reliability, CFRC composites. INTRODUCTION

CFRC COMPOSITES

The use of fibrous reinforcement to improve the mechanical properties of cementitious composites is currently being both studied and implemented by the construction industries. ! Fibrous composites, depending on fiber characteristics, will enhance ultimate strength and/or strain to fai-

CFRC composites have been investigated since the 1970s by numerous researchers, s-~2 The mechanical properties of CFRC have been evaluated for two types of composites, one prepared by hand laying the continuous carbon fiber in the 15

16

H . A . 7outanji, 1~ El-Korchi, R. N. Katz

tensile z o n e . 7-13 the other prepared by dispersing short fibers in a random two- or three-dimensional orientation in the body of the specimens. ~4-2" A summary of the results of the tensile and flexural strength properties of CFRC composites obtained from the above references is shown in Table 1. The data presented in Table 1 show some common trends despite the differences in specimen and test characteristics. These trends are (i) the tensile strength and flexural strength of cement paste are both improved by incorporation of carbon fiber, however, the percent gain in flexural strength is higher than the tensile strength; (ii) PAN-based carbon fiber is more effective in improving strength of CFRC than pitch-based carbon fiber; (iii) continuous fiber is a much more effective reinforcement than chopped fibers; (iv) the strength of CFRC is greatly improved with an

increase in carbon fiber content and by using carbon fibers with a length of 3 mm rather than 10 mm.

WEIBULL STATISTICS A certain degree of variation or scatter in strength is expected in brittle materials such as ceramics or cementitious materials. This is the result of both the large range of flaws and the stress intensity factor of these flaws (location and orientation effects). The strength analysis of brittle materials which is mathematically based on the weakest link concept can be easily evaluated using Weibull statistics. The Weibull method considers the body as a chain of elements and each element has a probability of failure. The strength of the specimen is the strength of the weakest

Table 1. Summary of the CFRC composite literature review Source

Ali et aL l'-

Akihama et al. ~4

Ohama et al? ~

Akihama et al. ~

Linton etal. ]"

Park et al. ~

Banthia et al. "

Fiber type

Fiber vol. (%)

Fiber length (rnm)

W/C ratio by wt

SF/C ratio by wt

Tensile strength increase (%)

Flexural strength increase (%)

PAN chopped PAN continuous PAN chopped PAN chopped PAN chopped Pitch chopped Pitch chopped Pitch chopped Pitch chopped Pitch chopped Pitch chopped Pitch chopped Pitch chopped Pitch chopped Pitch chopped Pitch chopped Pitch chopped Pitch chopped Pitch chopped PAN chopped PAN chopped PAN chopped Pitch chopped PAN chopped PAN chopped PAN chopped Pitch chopped Pitch chopped Pitch chopped Pitch chopped Pitch chopped Pitch chopped

3 3.7 2.1 4.2 5.3 2. I 4.2 5.3 1 3 5 1 11) 5 1.72 3.18 3-98 2.04 4.1)3 1-65 2 2 2 1 2 3 1 2 3 1 3 5

NA"

0'30

NA

N/A

3 3 3 3 3 3 3 3 3 10 0.30 10 111 10 117 10 10 6 6 6 6 3 3 3 3 3 3 6 6 6

I).47 0.47 0.47 0.47 0.47 (7.47 0.30 0.30 0.30 0.30 0.40' 0.30 (7.42 /).42 17.42 0.30 17.3(7 (7-37 17-37 (7-21 11.21 (7.3(I (7.3(7 0-3(7 0.3(7 0'30 0'30 0"30 (7.30 0.30

0'25 ~' 0' 25 ~' 0.25" 0'25 ~' 0.25 ~' 0.25 "' (7-40' (7-4(7' 0.40' 0-40' 125 0.40'

70 500 211) 225 270 30 61) 150 35 180 270 15 150 200 40 90 110 70 120

"NA, not available. hSilica fume in powder form. ' Silica fume in slurry form. aNS, silica fume is not specified.

NS J

0.195' 0-195' 0-195' 0' 195' 0.40' 0.40' 0.40' 0-40' 0-40' 0.40'

120 170 200 105 150 180

NS

NA

NA

350 500 510 125 210 271) 51) 261) 340 40 340 NA

165 165 917 917

NA

250 360 375

Strength and reliability of CFRC composites link. 21'22 Hence, the probability of failure of a specimen of a given size is:

ex{C° °u 'ldv,

(1)

where: Pf is the probability of failure; o is the applied tensile stress; cru is a constant which characterizes the stress below which no specimen fails; o 0 is a normalized constant which refers to the characteristic stress; m is the Weibull modulus; v is the volume under tensile stress The Weibull modulus, rn, is an indication of the scatter or dispersion of the data. The scatter in the data is inversely related to m, where a small Weibull modulus is indicative of a large dispersion in strength values. Often the variable or,,is set to zero and the two parameters of major interest are a,, and m. The probability of failure (Pf) is usually estimated by the use of ranking statistics. The ranking is based on strength from weakest to strongest. The probability of failure of each specimen, Pf, is then calculated based on its ranking, i, by: P, = - -

(n+l)

(2)

where n is the total number of specimens tested. The Weibull distribution function can be linearized into the following form: In In [1/(1 - Pf)] = m ha(o)- m In (oo)

(3)

which is of the form of a linear equation of y = rex. Therefore, by plotting I n [ l / ( 1 - Pf)] versus In(o) the Weibull modulus, m, is determined using the slope of the best fitting line through this data. The method of fitting the line is a matter of choice. The maximum likelihood estimator method 23 was selected for a best fitting line through the data.

17

sile strength of high-strength ceramics. 25 The first time this technique was utilized for cementitious composites was by the authors. 4 The technique that utilizes the hydraulic tensile tester has been referred to as CCATT. The main features of this technique are that: it minimizes misalignment and gripping problems, enables testing of large volumes and identifies the origin of fracture to study the fracture mechanics of the material. The CCATT hydraulic pressure chamber and specimen-piston assembly are shown in Fig. 1. The test specimens used are cylindrical bars 16 mm in diameter and 120 mm in length. The length that is exposed to the hydraulic pressure is 40 mm, since a 40 mm length of each specimen end is adhesively bonded to a steel piston. Description of the flexural test Four point bending tests were conducted on bar specimens using an Instron® testing machine. The dimensions of the specimens were 152 × 25 x 12.5 mm, the outer span being 20 mm and the inner span being 40 mm. The specimens were loaded at a crosshead speed of 0.5 mm/min. Load and displacement were digitally recorded at a rate of 10 data points per second. Materials The cement matrix consisted of 85% portland cement, ASTM Type II and 15% silica fume (in slurry form). The water-cement ratio was kept at 0.30 _+0.015 depending upon the percent volume of the fiber contents. In addition, 1% of superplasticizer (by the weight of the mix) was used to give the mix an appropriate workability. There are two main processes for making carbon fibers which are based on different starting materials. PAN-based carbon fibers have a higher hydraulic fluid iotak¢

chamber

N hydrauli~

hlgh pccssu~ seal

scn:w ~re~led caps

pressL~

EXPERIMENTAL P R O C E D U R E Description of the uniaxial tensile test The cementitious composites axial tensile technique has been presented 4-7 and is briefly described here. The hydraulic tensile tester was originally developed by Baratta and Driscoll as a simple tension test for brittle materials. 24 The design of the tester was later modified by ASCERA~, a Swedish engineering firm. The technique has been utilized to determine the ten-

[ Fig. 1. CCATT hydraulic pressure chamber with specimen-piston assembly.

18

H. A. "loutanji, 7: El-Korchi, R. N. Katz

strength and elastic modulus than petroleum and coal tar pitch (pitch) carbon fibers. PAN based carbon fiber was used in this work. The fiber consisted of tows, each tow having approximately 10 000 filaments. The tensile strength and modulus of elasticity of the carbon fiber are 3795 MPa and 235 GPa, respectively. The monofilament diameter is 7 mm.

Specimen preparation The carbon fibers were chopped into 10 mm lengths from continuous tows and dispersed in the cement paste. The chopped fibers were placed in an enclosed container, where a compressed air gun device was used to disperse the fibers into monfilaments. Mixes were prepared in a Horbart mixer and the carbon fibers were added to the cementitious paste gradually to assure homogeneous dispersion. Tensile and flexural specimens were cast in molds and compacted through external vibration. Cylindrical bar specimens of 16 mm in diameter and 120 mm in length with fiber contents of 1, 2 and 3 %vol. were cast vertically for the tensile test. For the flexural test, plates measuring 152 × ! 52 x 12.5 mm were prepared with a fiber loading of 1, 2 and 3 %vol. Before testing, the plates were cut into 152 x 25 × 12.5 mm beams. The specimens were moist cured for 65 days at 30°C and a relative humidity in excess of 95%.

RESULTS AND DISCUSSION

Tensile and flexural strength The results for the uniaxial tensile and flexural strength of CFRC and unreinforced cement paste specimens are presented in Fig. 2. The number of specimens, average tensile strength and the standard deviation for each composite are provided on the bar charts. The direct tensile strength of the cementitious composites increased by as much as 32, 48 and 56% with the addition of 1, 2 and 3 %vol. of PAN-based carbon fibers, respectively. The increase in the flexural strength of the cementitious composites was more significant as compared to the uniaxial tensile strength. The flexural strength increased by as much as 72, 95 and 138% with the addition of 1, 2 and 3 %vol. PAN-based carbon fibers, respectively. Statistical analysis using hypothesis testing at 95% confidence level indicates a significant increase in tensile and flexural strength with fiber addition. 6

17'

Fig. 2. Tensile and flexural strength values as a function of fiber loading.

The test results, as shown in Fig. 2, indicate that the uniaxial tensile strength and the flexural strength for the unreinforced composite have similar values. However, in the case of CFRC composites, the strength gain with fiber addition for flexural specimens is higher than the tensile specimens. As pointed out in previous works, 26,27 the tensile strength of unreinforced cementitious composites obtained using the traditional uniaxial tensile test is about two thirds of the modulus of rupture (flexural strength) because of the smaller stressed volume in the bending case. However, in this work the tensile strength results are obtained using the CCATT where the strength measured is higher than that obtained from the traditional uniaxial tensile test because of the minimization of misalignment.4.6 Hence, the tensile strength approaches the flexural strength. In the case of fiber-reinforced cement undergoing bending, the measured strains in the material on the tensile side where matrix cracking occurs increase at a greater rate than the compressive strains. The stress in a large part of the tensile zone will be roughly constant at the cracking stress and the neutral axis will move towards the compressive face. 28 This results in an increase in bending moment and can lead to a modulus of rupture higher than the ultimate tensile strength. In addition, the lower tensile strength of the CFRC specimens may be attributed to in-

Strength and reliability of CFRC composites

efficient fiber utilization. Since the flexural specimens are cast horizontally, vibration during processing may tend to align the fibers parallel to the longitudinal axis. This assertion was confirmed by observation during vibration and by examining the specimens after fracture. Typical load-deflection curves for the flexural specimens of CFRC and unreinforced cement paste specimens are shown in Fig. 3. The load-deflection curves of the composites show a linear behavior almost up to the maximum load. The first crack load of the CFRC is much higher than that of the unreinforced matrix. The composite behaves linearly up to the ultimate strength and does not exhibit any significant post-cracking behavior. This behavior was also observed by Akihama et a l ) 4 In addition, the stiffness of the cement paste composites tends to increase with fiber reinforcement as shown in Fig. 3. In general, it is difficult to compare the uniaxial tensile strength results of CFRC composites of different studies. This is due to the fact that in each study the factors that affect the strength of the composite are varied. Some of these factors include the following: testing method, fiber strength, fiber loading and fiber length, specimen geometry, and matrix properties. It seems that the uniaxial tensile strength results of CFRC specimens obtained in the present study are lower than those obtained by other investigators (see Table 1). This may be due to the inefficient utilization of fibers which is mainly attributed to the casting process of the cylindrical shaped specimens. Furthermore, the tendency of fiber breakage may be higher during placing the composite mix in cylinderical molds than plate molds, as tradi-

19

tionally done. The compaction required for casting vertical cylindrical molds is more rigorous as compared to the horizontal flexural plates. Therefore, the effective efficiency factor of the fibers in plate specimens which are used by other investigators is higher than that in the cylindrical specimens.

Reliability analysis The Weibull distributions for the tensile and flexural strength are shown in Figs 4 and 5, respectively. Each Figure shows both the CFRC composites and the unreinforced cement composite. For the tensile strength specimens, the Weibull modulus increases with the addition of carbon fibers. However, the increase is not proportional to the fiber loading, as shown in Fig. 6. For the flexural strength specimens, the Weibull modulus decreases with the addition of carbon fibers and the reduction is also not proportional to the fiber loading (see Fig. 6). Thus, the reliability of the reinforced composite is higher than the unreinforced cement paste in tension and lower in flexure. This behavior may be attributed to the difference in the flaw populations initiating failure, number of flaws present in each test specimen and the difference in failure mode because of the stress state in each test. Structural properties and average strength values are independent of reliability. One can have low average strength with high Weibull modulus and high average strength with low Weibull modulus. However, low scatter in strength values and higher Weibuli modulus will lead to more efficient and economical design. In our case the scatter in tensile strength values using the CCATT is lower than that of flexural strength

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Fig. 3. Flexural b e h a v i o r of C F R C specimens.

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Tensile Strength [~a}

Fig. 4. WeibuUtensile strength distributionfor CFRC and unreinforcedcementcomposites.

20

H. A. Toutanji, 72 El-Korchi, R. N. Katz

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matrix have similar values. In the case of CFRC composites, the addition of carbon fiber is more efficient in improving the flexural strength as compared to the uniaxial tensile strength. The addition of carbon fiber to the cement matrix increases the reliability of the composites in tension but decreases the reliability in bending. This improved reliability makes the tensile strength using the CCATT a more reliable parameter for design of CFRC composites.

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ACKNOWLEDGEMENTS

I ',exural Strerqt.'1 (l~Pa)

Fig. 5. Weibull flexural strength distribution for CFRC and unreinforced cement composites.

The authors would like to acknowledge the financial support of the National Science Foundation PYI grant No. MSS-9157050.

REFERENCES

10'

0'

Fig. 6. Weibull modulus as a function of fiber loading in tension and flexure.

with the addition of fiber loading. Therefore, tensile strength is a more reliable parameter for design of CFRC composites.

CONCLUSIONS

Significant improvement in the tensile and flexural strength of cementitious materials can be achieved with the addition of low volumes of carbon fibers. In this study, the average tensile strength using the CCATT and the average flexural strength of the unreirdorced cement paste

1. Beaudoin, JJ., Handbook of fiber-reinforced concrete, principles properties, development and applications. Noyes Publications, Park Ridge, N J, 1990. 2. Briggs, A., Bowen, D.H. & Kollek, J., International Conference on Carbon Fibers. Their Place in Modern Technology, Paper No. 17. Unwin, London, 1974, pp. 114-21. 3. Lubin, C. & Dastin, S., International Conference on Carbon Fibers. Their Place in Modern Technology, Paper No. 37. Unwin, London, pp.245-57. 4. EI-Korchi, T., Toutanji, H.A., Katz, R.N.. Leatherman, G.L., Lucas, H. & Demers, C., Tensile testing of fiber reinforced cementitious composites. In Proc. Materials Research Society, Vol. 211. Pittsburgh, PA, 1991, pp. 221-8. 5. Toutanji, H.A., EI-Korchi, T., Leatherman, G.L. & Katz, R.N., Tensile strength of carbon fiber reinforced cement composites. In Proc. Materials Research Society, Voi. 245. Pittsburgh, PA, 1992, pp. 359-64. 6. Toutanji, H.A., The Development of A Cementitious Composites Axial TensileTechnique and its Application to Carbon Fiber Reinforced Cementitious Composites. PhD Thesis, Worcester Polytechnic Institute, Worcester, MA, 1992. 7. Toutanji, H.A., EI-Korchi, T & Katz, R.N., Behavior of carbon fiber reinforcedcement composites in direct tension. J Cement Concrete Res., 23 (1993)618-26. 8. Briggs, A., Review: carbon fibre reinforced cement. J. of Mater. Sci., 12 (1977) 384-404. 9. Sarkar, S. & Bailey, M.B., Structural properties of carbon fibre reinforced cement. In Proc. RILEM Symposium 1975, Fiber Reinforced Cement and Concrete, ed. A. Neville. Construction Press, Lancaster, pp. 361-71. 10. Aveston, J., Mercer, R.A. & Silwood, J.M., Fiber reinforced cement--scientific foundations for specifications. In Proc.Composites Standard Testing and Design. National Physical Laboratory Conference, UK, 1974, pp. 93-103. 11. Waller, J.A., Carbon fibre cement composites. Civil Engng and Public Work Rev., 67 (1972) 357-61. 12. Ali, M.A., Majumdar, A.J. & Rayment, D.L., Carbon fibre reinforcement of cement. J. Cement Concrete Res., 2(1972)201-12.

Strength and reliability of CFRC composites 13. Akihama, S., Suenaga, T. & Banno, T., The behaviour of carbon fibre reinforced cement composites in direct tension. Int. J. Cement Composites & Light Weight Concrete, 6(3) (1984) 159-68. 14. Akihama, S., Suenaga, T., Nakagawa, T. & Suzuki, K., Influence of fibre strength and polymer impregnation on the mechanical properties of carbon fibre reinforced cement composites. In Proc. RILEM Symposium 1988, Development in Fibre Reinforced Cement and Concrete, Sheffield, 1988, paper 2.3. 15. Ohama, Y., Amano, M. & Endo, M., Properties of carbon fiber reinforced cement with silica fume. Concrete Int.: Design & Construct., 7 (3)(1985) 58-62. 16. Akihama, S., Suenaga, T. & Nakagawa, T., Properties and application of pitchbased carbon fibre reinforced concrete. Concrete Int.: Design & Construct., 10 (1988) 40-7. 17. Linton, J.R., Bernburg, P.L., Gartner, E.M. & Bentur, A., Carbon fiber reinforced cement and mortar. In Proc. Materials Research Society, Vol. 211. Pittsburgh, PA, 1991, pp. 255-64. 18. Park, S.B., & Lee, B.L., Fabrication of carbon fiber reinforced cement composites.In Proc. Materials Research Society, Vol. 211. Pittsburgh, PA, 1991, pp. 247-55. 19. Banthia, N., & Sheng, J., Micro-reinforced cementitious materials. In Proc.Materials Research Society, Vol. 211. Pittsburgh, PA, 1991, pp. 25-32.

21

20. Nishioka, K., Yamakawa, S. & Shirakawa, K., Properties and applications of carbon fibre reinforced cement composites. In Proc. R1LEM Symposium 1986, Development in Fiber Reinforced Cement and Concrete, Sheffield, paper 2.2. 21. Mclean, A.E, & Hartsck, D.L., Design with structural ceramics. In StructuralCeramics, (Treatise on Materials Science and Technology, Vol. 29). Academic Press, 1989, pp. 27-95. 22. Kingery, W.D., Bowen, H.K. & Uhlmann, D.R., Introduction to ceramic. John Wiley &Son, NY, 1976, 787pp. 23. Thoman, D.R., Bain, L.Z &Antle, C.E., Inferences on the the parameters of the Weibull distribution, Technometrics, 11(3) (1969) 445-60. 24. Baratta, EI., & Driscoll, G.W., A New Axial Tension Tester for Brittle Materials, (TR 69-02). Army Materials and Mechanics Research Center, 1969. 25. Hermonsson, L., Adlerborn, J. & Burstrom, M., Tensile testing of ceramic materials. In High Technology Ceramic, Elsevier, Amsterdam, 1987, pp. 1161-8. 26. Troxell, G.E. & Davis, H.E., Composition and Properties of Concrete. McGraw-Hill, NY, 1956, 177 pp. 27. Mehta, P. Kumar, Concrete Structure, Properties, and Materials, Prentice-Hall, NJ, 66 pp. 28. Hannant, DJ., Fibre Cements and Fibre Concretes. John Wiley & Son, NY, 1978, 35 pp.