Mehmet A. TAŞDEMİR, Surendra P. SHAH, Nilüfer ÖZYURT

Lif donatılı çimento esaslı kompozitlerde, lif dağılımının elektriksel bir yöntem ile tahribatsız ölçümü

Bu makalede lif donatılı çimento esaslı kompozitlerde lif dağılımı, reoloji ve mekanik özeliklerin ilişkilendirilmesi konulu tez çalışmasının bir bölümü anlatılmıştır. Lif donatılı çimento esaslı kompozitlerde (LDÇEK) lif dağılımının tahribatsız olarak ölçülebilmesi için elektriksel bir yöntem - Alternatif Akım – Empedans Spektroskopi (AA-ES) - kullanıldı. AA-ES kompozit malzemelerin çeşitli özeliklerinin araştırılması için yaygın bir şekilde kullanılan elektriksel bir yöntemdir. Son yıllarda bu yöntemden çimento esaslı malzemelerin çeşitli özeliklerinin araştırılması için faydalanılmaktadır. Bu çalışmada AA-ES’in taze ve sertleşmiş halde lif dağılımı karakteristiklerinin ölçülmesi için kullanılabilirliği araştırılmıştır. Son yıllarda yapılan araştırmalar sonucunda orta derecede iletken bir matris içerisinde yüksek iletkenlikli lifler kullanılması durumunda elde edilen empedans eğrilerinde çift-ark davranışı görüldüğü birçok araştırmacı tarafından belirtilmiştir. Bu çalışmada çift-ark özeliğinden faydalanılarak lif dağılımı karakteristiklerinin belirlenmesine çalışılmıştır. Lif yönlenmesi, lif segregasyonu ve lif topaklanması gibi farklı lif dağılımı sorunlarının tahribatsız olarak belirlenmesi için AA-ES ile ölçüm yapılmış ve elde edilen sonuçların değerlendirilmesi için bünyesel iletkenlik yaklaşımı kullanılmıştır. Üç farklı lif içeriği ve 2 farklı su/çimento oranı ile üretilen kübik numuneler üzerinde taze ve sertleşmiş halde AA-ES ölçümleri yapılmıştır. Taze haldeki ölçümler numunelerin üretilmesinden 25 dakika sonra başlatılmıştır. Sertleşmiş haldeki ölçümler ise 7. günde gerçekleştirilmiştir. Lif içeriği ve su/çimento oranının lif dağılımı karakteristikleri üzerindeki etkileri araştırılmıştır. Elde edilen sonuçlar AA-ES’in farklı lif dağılımı sorunlarının tahribatsız ve hızlı bir şekilde tesbit edilmesi ve ölçülmesi için kullanılabileceğini göstermiştir.

Non-destructive measurement of fiber dispersion in FRCs using an electrical method

A thesis study is conducted to correlate, fiber dispersion, rheology, and mechanical performance of fiber-reinforced composite (FRC) materials. In this paper, a part of this study on the use of Alternating Current – Impedance Spectroscopy (AC-IS) to monitor various fiber dispersion issues in FRC materials is given. The AC-IS is an electrical characterization method that can be used for studying various properties of composite materials. It is commonly used for characterizing micro-structural properties of cement-based materials. In this study research is conducted to develop AC-IS to monitor fiber dispersion in conductive fiber-reinforced cement-based materials. The use of AC-IS to detect various fiber dispersion phenomena, such as fiber orientation, fiber segregation and fiber clumping is investigated. Experiments are also performed on the fresh FRC specimens to understand the ability of AC-IS to be used in the fresh state. The use of AC-IS to characterize fiber dispersion is possible owing to a unique “dual-arc” behavior which occurs in the plots of impedance when highly conductive fibers are used in moderately conductive matrix materials. Conductive fibers act as insulating under low frequencies of alternating current, because of either an oxide film (e.g., on steel fibers) or a polarization layer (double layer/charge transfer resistance) forming at the fiber-electrolyte interface, while they are conducting under high frequencies of AC. This feature of conductive fibers makes it possible to obtain the conductivities of both the matrix material ($sigma_m$) and composite system (fiber-matrix) ($sigma$) by making only 1 AC-IS measurement. Conductivity of the composite system (σ) is dependent on the characteristics of fiber dispersion. By making use of this property, fiber dispersion characteristics of the FRCs are evaluated. An intrinsic conductivity approach is used for obtaining fiber dispersion characteristics from experimental data. Matrix-normalized conductivity ($sigma/sigma_m$) of a specimen with randomly dispersed fibers can be calculated using intrinsic conductivity of fibers $([sigma]_Delta$ - Intrinsic conductivity of a fiber can be calculated knowing only aspect ratio) and fiber volume content $(varphi)$ as given below: $sigma/sigma_m= 1 +[sigma]_Delta varphi$In this paper, firstly, the fundamentals of AC-IS, the dual-arc behavior and intrinsic conductivity approach are given. Then fiber dispersion issues are classified into 3 categories: fiber orientation, fiber segregation and fiber clumping. Cubic specimens are cast with three different fiber volume contents (% 1, 2, 4) and 2 different water-to-cement ratios (0.30 and 0.35). Then, experiments are conducted to study various dispersion issues. For fiber orientation AC-IS measurements from 3 reference directions of lab-size cubic specimens are obtained and the variations from random dispersion are calculated by comparing the results from the three directions. A tendency of fibers to be oriented in the plane (xy) vertical to the casting direction (z) is found for all the specimens. The severity of orientation is found to increase with increasing fiber content. For fiber segregation a new local-probe technique is used. A cubic specimen is cast with all the fibers in the bottom 1/3 of the specimen and AC-IS measurements are obtained from different locations. Finally, measured σ/σm profile of the specimen is plotted and used to evaluate fiber segregation. Measured $sigma/sigma_m $values of the specimen are found to increase in the fiber-rich end of the specimen. For fiber clumping, a dispersion factor (DF) is defined. The DF is based on comparing the normalized conductivity of and actual FRC (measured) to that of perfectly dispersed FRC (calculated – theoretical). The DF values are calculated for all specimens. AC-IS is also used in order to monitor fiber dispersion in the fresh state. Measurements obtained in the fresh and hardened states are compared. Results are found to be similar confirming the ability of AC-IS to be used in the fresh state. These results suggest that AC-IS can be a good candidate to be used in the field as a non-destructive and fast testing method.

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Akkaya, Y., (2000). Micro structural characterization of high performance fiber-reinforced cement composites, PhD thesis, PhD Thesis, Northwestern University, Evanston.
Balaguru, P. N. ve Shah, S. P., (1992). Fiber- Reinforced Cement Composites, McGraw-Hill, New York.
Banthia, N., Bindiganavile, V. ve Mindess, S., (2003). Impact resistance of fiber-reinforced concrete: A progress report, Proceedings, RILEM 4th International Workshop on High Performance Fiber-Reinforced Cement Composites (HPFRCC4), 117-131, Ann Arbor, USA.
Banthia, N., Moncef, A., Chokri, K. ve Sheng, J., (1994). Micro fiber-reinforced cement composites, I. Uniaxial Tensile Response, Canadian Journal of Civil Engineering, 21, 999-1011.
Benson, S. D. P. ve Karihaloo, B. L., (2003). CARDIFRC-Manifacture and constitutive behavior, Proceedings, High Performance Fiber Reinforced Cement Composites (HPFRCC4), 65-79, Ann Arbor, USA.
Bentur, A., (1989). Fiber-reinforced cementitious materials, Material Science of Concrete, J. P. Skalny, ed., The American Ceramic Society, 223-285.
Chermant, J. L., Chermant, L., Coster, M., Dequiedt, A. S. ve Redon, C., (2001). Some Fields of Applications of Automatic Image Analysis in Civil Engineering, Cement and Concrete Composites, 23, 157-169.
Christensen, B. J., Coverdale, R. T., Olson, R. A., Ford, S. J., Garboczi, E. J., Jennings, H. M. ve Mason, T. O., (1994). Impedance spectroscopy of hydrating cement-based materials: Measurement, interpretation, and application, Journal of American Ceramic Society, 77, 11, 2789-804.
Douglas, J. F. ve Garboczi, E. J., (1995). Intrinsic viscosity and the polarizability of particles having a wide range of shapes, John Wiley & Sons., New York.
Ferrara, L., Meda, A., Lamperti, T. ve Pasini, F., (2004). Connecting Fiber distribution, workability and mechanical properties of SFRC: An ıdustrial application to precast elements, Proceedings, 6th RILEM Symposium on Fibre-Reinforced Concretes - BEFIB, 493-504, Varenna, Italy.
Fleig, J. ve Maier, J., (1996). Surface conductivity measurements on AgCl single crystals using microeletrodes, Berichte der Bunsen-Gesellschaft- Physical Chemistry Chemical Physics, 100, 607-615.
Hixson, A. D., Woo, L. Y., Campo, M. A., Mason, T. O. ve Garboczi, E. J., (2001). Intrinsic conductivity of short conductive fibers in composites by ımpedance spectroscopy, Journal of Electroceramics, 7, 189-195.
Lawler, J. S., (2001). Hybrid fiber-reinforcement in mortar and concrete, PhD thesis, Northwestern University, Evanston.
Li, V. C., (2000). Large volume, high-performance applications of fibers in civil engineering, Journal of Applied Polymer Science, 83, 660-686.
Loche, J. M., Ammar, A. ve Dumargue, P., (2004). Influence of the migration of chloride ıons on the electrochemical ımpedance spectroscopy of mortar phase, Cement and Concrete Composites.
Mobasher, B., Stang, H. ve Shah, S. P., (1990). Microcracking in Fiber-Reinforced Concrete, Cement and Concrete Research, 20, 665-676.
Nawy, E. G., (2001). Fundamentals of highperformance concrete, John Wiley & Sons, New York.
Newman, J., (1966). Resistance for Flow of Current to a Disk, Journal of the Electrochemical Society, 113, 501-502.
Ozyurt, N., Woo, L. Y., Mason, T. O. ve Shah, S. P., (Accepted, 2005). Monitoring fiber dispersion in FRCs: Comparison of AC-Impedance spectroscopy and ımage analysis, ACI Materials Journal.
Peled, A., Torrents, J. M., Mason, T. O., Shah, S. P. and Garboczi, E. J., (2001). Electrical impedance spectra to monitor damage during tensile loading of cement composites, ACI Materials Journal, 98, 313-322.
Rapoport, R. J., (2002). Cellulose fiber-reinforced cement paste, mortar, and concrete, PhD thesis, Northwestern University, Evanston.
Reza, F., Batson, G. B., Yamamuro, J. A. ve Lee, J. S., (2003). Resistance changes during compression of carbon fiber cement composites, Journal of Materials in Civil Engineering, ASCE, 15, 414-421.
Shah, S., Kuder, K. ve Mu, B., (2004). Fiberreinforced cement-based composites: A Forty year odyssey, Proceedings, 6th RILEM Symposium on Fibre-Reinforced Concretes - BEFIB, 3-30, Varenna, Italy.
Shah, S. P., Sarigaphuti, M. ve Karaguler, M. E., (1994). Comparison of shrinkage cracking performance of different types of fibers and wiremesh, Fiber Reinforced Concrete; Developments and Innovations, J. I. Daniel, Shah, S.P., ed., American Concrete Institute, Michigan, 1-18.
Sun, W., Chen, H., Luo, X. ve Qian, H., (2001). The effect of hybrid fibers and expansive agent on the shrinkage permeability of high-performance concrete, Cement and Concrete Research, 31, 595-601.
Tasdemir, M. A. ve Bayramov, F., (2002). Yüksek performanslı çimento esaslı kompozitlerin mekanik davranışı, İTÜ Dergisi/d Mühendislik, 1, 2, 125-144.
Torrents, J. M., Mason, T. O. ve Garboczi, E. J., (2000). Impedance spectra of fiber-reinforced cement-based composites, Cement and Concrete Research, 30, 585-592.
Torrents, J. M., Mason, T. O., Peled, A., Shah, S. P. ve Garboczi, E. J., (2001). Analysis of the impedance spectra of short conductive fiberreinforced composites, Journal of Materials Science,, 36, 4003-4012.
Woo, L. Y., Wansom, S., Ozyurt, N., Mu, B., Shah, S. P. ve Mason, T. O., (2005). Characterizing fiber dispersion in cement composites using ACImpedance spectroscopy, Cement and Concrete Composites, 27, No. 6, 627-636.
Yang, Y., (2002). Methods Study on dispersion of fibers in CFRC, Cement and Concrete Research, 32, 747-750.