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Flexural toughness of fibre reinforced concrete Chen, Lianrong

Abstract

Fibres are added to concrete to improve energy absorption and apparent ductility. Flexural toughness, derived from the load-deflection response, is often used to describe these improvements. There are, however, a number of uncertainties regarding how FRC flexural toughness should be measured, interpreted, or used. Various flexural toughness test methods have been developed in different countries. ASTM-C1018, JSCE-SF4, and NBP No. 7 are three of the most commonly used test methods. Based on a large number of experiments, the present work studies effects of testing and material variables on both the load-deflection response and the flexural toughness of FRC, investigates the reproducibility of the flexural toughness test methods of FRC (both within-laboratory and between laboratories), discusses difficulties in toughness evaluation, defines various flexural toughness parameters, assesses relative advantages and disadvantages of different methods of characterizing toughness, and develops recommendations for a more suitable procedure for FRC flexural toughness evaluation. It has been found the load-deflection response of FRC is an experimental property and so are the flexural toughness parameters derived from the response. They depend not only on material variables but also on testing factors. Of the various testing variables, the deflection measuring system, the loading system, and specimen size/geometry are the three most important factors; while amongst the various material variables, fibre content and fibre type/profile are the most important. The major difficulties in measuring the load-deflection response, caused by either extraneous deformations, first crack determination, or instability, has much more significant effects on ASTM toughness parameters than on JSCE toughness parameters. For this reason, ASTM parameters may not be particularly sensitive in distinguishing either amongst different fibre contents for SFRC and PFRC or between SFRC and PFRC with similar low fibre contents. JSCE parameters are more sensitive in this regard. On the basis of assessing the reliability and efficiency of various toughness parameters, both a suite of toughness parameters and toughness performance levels are proposed. The full suite of toughness parameters provide more complete useful information, while Toughness Performance Levels offer sufficient information for toughness evaluation.

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