UBC Theses and Dissertations
Study of some parameters affecting the measured flexural toughness of fiber reinforced concrete Islam, Shaikh Tashnuba
Fiber reinforced concrete (FRC) exhibits better performance not only under static and quasi-statically applied loads, but also under fatigue, impact, and impulsive loading. This energy-absorption attribute of FRC is usually termed “Toughness”. Experimental characterization of the toughness of FRC remains an actively debated topic. In this thesis, concerns with various available techniques were studied and better ways of characterizing the effects of fibers on the toughness of concrete were sought. For toughness characterization, beam tests which included standardized ASTM C1609 and C1399 tests were carried out both on lab-cast and site-cast specimens. In the first part of the study, the applicability of the initial loading rate described in ASTM C1609 was evaluated. Tests were conducted on specimens carrying two volume fractions of polypropylene fiber in two separate series with both the prescribed and proposed loading rates. A comparison between ASTM C1609 and C1399 was carried out later in the study. The Canadian Highway Bridge Design Code (CHDBC) technique prescribed for FRC was also studied. Based on the results of these tests, it can be concluded that the current loading rate specified in ASTM C1609-2010 is too high for normal strength concrete and it should be reduced to 0.001mm/min initially. It was also found that for calculating Residual Strength Index (Ri), ASTM C1609 procedure is more reliable than the ASTM C1399 as ASTM C1609 is performed in a feedback controlled mode (also called the closed-loop mode) which is very helpful for maintaining stability in specimens. Since energy absorption is one of the most effective criteria for characterizing FRC, a new method called Flexural Toughness Strength Method (FTSM) was proposed. Tests are carried out on beam specimens according to ASTM C1609 and load- deflection curves are analyzed using the FTSM method. The results demonstrate that the proposed FTSM leads to FRC attributes that are not susceptible to user errors and hence more reliable. The characterization of flexural toughness based on the FTSM approach is independent of the type of deflection measuring technique and no sophisticated instrumentation is required. The Flexural Toughness Factor calculated using this approach has consistently lower coefficient of variation.
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