The lnter ~ational Journal of Cement Composites and Lightweight Concrete, Volume 1 I, Number I February 1989

T o u g h n e s s d e t e r m i n a t i o n for f ibre r e i n f o r c e d c o n c r e t e

Youjiang Wang* and Stanley Backer*

S y n o p s i s Methods currently used to evaluate the toughness of fibre reinforced concrete (FRC) are reviewed. Parameters representing tha fracture energy de.'.ermined from the area under the Ioad- displacem3nt curve obtained from conventional laboratory te,,;ts are discussed. An improved toughness parameter is proposed based on the secant compliance (the displacement/load ratio), rather than judging by load or displacement alone as by conventional methods.

The new parameter is shown to represent the toughness properties of FRC in a more consistent manner than other similar methods when applied to a wide range of idealiseo load-displacement curves. However. like other toughness parameters determined from the area under the Ioad-di~placement curve, its use should be limited to comparisons among results ob ta in~ under the same test conditions.

K e y w o r d s Fibre reinforced concrete, touEhnc3s, energy absorption, metal fibres, synthetic fibres, composite materials, toughness index, fracture properties, compliance, displacement, fracture tests, quality control, construction materials, tension tests.

I N T R O D U C T I O N Fibre reinforcement can significantly improve the properties of concrete. Strength increases, if any due to fibre reinforcement, are typically a few percent, but the toughness, or energy-absorbing ability, of fibre rein- forced concrete (FRC) is frequently higher than that of plain concrete by orders of magnitude. In the process of FRC fracture, fibres bridging the cracks in the matrix, before being pu~lc~ out or stressed to rupture, can provide resistance to crack propagation and opening. Consequently, FRC materials can be insensitive to the presence of notches, depending on the ~ffectiveness of the reinforcement. The material zone which undergoes ;nelastic detormation associated with fibre bridging actions is often called the process zone. The process zone of FRC can be extremely large and its size is dependent on the structural geometry and loading condi- tion. Great amounts of energy are absorbed in the. process zone during FRC fracture process. Extensive studies have been carried out to characterise the toughness of FRC, however further work is still neces- sary for development of the 'best way' for determination of FRC toughness.

ideally, a parameter for FRC toughness should be a material characteristic independent of test specimen

* ~epartment of IV:,,~.nanical Engineering, Massachusetts Institute of Technology. Cambr ~.ge. Massachusetts 02139. USA.

Received 16 M.ly ; ~,88 Accepted 13 July 1988

~) Longman Group L,K Ltd 1989 0262-50751891111(,2011/$G2.00

geometry, dimensions, and loading conditions. For econ- omic reasons, it should be easily obtained from simple tests, and be expressed in terms of a single parameter for ease of comparison and quality control. In practice, however, the methods often used to assess toughness of FRC do not meet these requirements.

The use of conventional fracture mechanics to characterise FRC toughness i~as been discussed exten- sively [1-5]. Becaus~ of the high ductility of FRC. parameters in linear el3stic fracture mechanics (LEFM) such as the fracture cough.hess (K~c} ,~nd fract~re energy (Gc~. and the parameters in elasto-plastic fracture mech- anics (EPFM) such as the critical J-integral (Jc) and the critical crack opening displaceme~tt (CODc). are not directly applicable to FRC. In order for LEFM to .ae valid, the relevant d=mensions of the specimen must o,~ much larger than the process zone size in FRC. and even for EPFM the specimen size has to be large enough to allow the full development of the process zone. The process zone size in FRC can easily exceed several me, ters [6} and so LEFM and EPFM cannot be applied to laboratory-sized specimens. Likewise, although a valid fracture toughness parameter (e.g. K1¢ or Jc) for FRC P.r)uld be obtained in principle by full scale test or by tndirect testing [4], it would still not be applicable to structures that do not meet the size requirement. The R-curve in EPFM, which is a functional relationship between the crack resistance and the crack extension, aiso depends on specimen geometry and dimensions [4].

Hillerborg [2] has shown that the fracture process of an FRC structure can be characterised by the stress- crack separation curve which could be obtained from a direct tensile test. The ~=rea under the curve corresponds

11

Toughness determination for fibre reinforced concrete Wang and B~c~'er

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$ 0,2 0 .4 O. =

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,, n ~ J f

0.8 , .0 1.2 1.4 1o6 I .S

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Figure 8 Load-displac6.-nent cu rve;~ for CTS tests. Percentages n parentheses indicate the ,ibre volume fractions (Vf)

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It is noted [n~t the proposed toughness index is" nondimensionali~¢d by the LOP toughness (To) which reflects the effect of the specimen size, geometry, matrix properties, and the test conditions. However in some cases when the matrix of FRC is weakened by fibre inclusion (for example, in directions perpendicular to fibres), the LOP toughness (To) may be drastically reduced, thus causing the toughness index !DTIn) to become unreasonably high. In such a case both the DTIn and To should be used so as to avoid misleading conclusions.

C O N C L U S I O N Methods of linear elastic fracture mechanics and elastic- plastic fracture mechanics are not applicable to labora- tory-sized specimens of fibre reinforced cencrete (FRC). FRC fracture behaviour can be characterised by the stress-crack separation curve, but for routine quality control and toughness comparisons, a simple method to ovaluaEe the toughness of FRC is still needed.

In this paper, a new toughness index is proposed based on the secant compliance criterion. Unlike conven- tional methods which use either load or displacement criterion, this index is related to both the strength and serviceability requirements for construction materials in a single parameter. This toughness index can be easily determined from the load-displacement curve and it has been shown to give a better description of the toughness behaviour of FRC in comparison with the generally used standard methods proposed by ACI [9] and ASTM 1121 when applied to a wide range of idealised test curves. This index is proposed for toughness comparison and quality control. However, like other indices, it may be expected to depend on test specimen geometry and loading conditions. Such a drawback car~ be minimised if standard test methods and specimen dimensions are used.

A C K N O W L E D G E M E N T The authors would like to thank Professor V.C. Li for his

valuable comments. Partial support for this research project was provided by the E.I. du ,~ont de Nemours Co., Inc.

REFERENCES 1. Mindess, S. 'The fracture of fibre-reinforced and

polymer impregnated concretes', The International Journal of Cement Composites, Vol. 2, No. 1, February 1980, pp. 3-11.

2. Hillerborg, A. 'Analysis of fracture by means of the fictitious crack model, particularly for fibre reinfor- ced concrete', The International Journal of Cement Composites, Vol. 2, No. 4. November 1980, pp. 177-84.

3. Hibbert, A. P. and Hannant, D. J. 'Toughness of fibre cement composites', Col~positr=s, Vol. 13, April 1982, pp. 105-11.

4. Li, V. C. 'Fracture resistance parameters for cementitious materials and their experimental determination', in Application of Fracture Mechan- ics to Cementitious Composites, (ed. S. P. Shah), Martinis Nijhoff Publishers, Dordrescht, 1985, pp. 431--49.

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Toughness determination for fibre rein forced concrete ~t, ~:ng and Backer

10. Johnston, C. D. 'Definition and measurement of flexural toughness parameters for fibre reinforced concrete', Cement, Cor~crete and Aggreg~t~ ~", 4, No. 2, Winter 1982, pp. 53--60.

11. Johnston, C. D. 'Precision of flexural strength and toughness parametcrs for steel fibre reinforced concrete', Cement, Concrete. and Aggregates, Vol. 4, No. 2, Winter 1982, pp. 61-7.

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13. Barr, B. I. G. and Liu, K. 'Fracture of GRC materials', The International Journal of Cement Composites and Lightweight Concrete, Vol. 4, No. 3, August 1982, pp. 163-71.

14. Barr, B. I. G., Liu, K., and Dowers, R. C. 'A toughness index to measure the energy absorption of fibre reinforced concrete', The International Journal of Cement Composites and Lightweight Concrete, Vol. 4, No. 4, November 1982, 221-7.

15. Barr, B. I. G., Hasso, E. B. D. 'A study of toughness indices', Magazine of Concrete Research, Vol. 37, No. 132, September 1985. pp. 162-74.

16. Henager, C. H 'A toughness index of fibre con- crete', in RILEM Symposium 1978: Testing and Test Methods of Fibre Cement Composites, The Construction Press, Lancaster, 1978, pp. 79-86.

17. Wang, Y., Backer, S., and Li, V. C. 'An experimental study of synthetic fibre reinfor_'ced c~mentitious composites', Journal of Materials Science. Vol. 22. No. 12, December 1987, pp. 4281-91.

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