This paper presents the results from an experimental study on the optimum amount of polypropylene fibres to be used in lightweight high-strength concrete to prevent spalling when exposed to hydrocarbon fire, taking into consideration the characteristics of the lightweight aggregate, the water-to-cement ratio (W/C) of the mixtures, and the length and thickness of the fibres.Twelve different concrete mixtures were made. One block, 610x425x770 mm in size, was cast from each mixture and tested for fire resistance under hydrocarbon fire exposure. The temperature in the blocks during the test was recorded. After the test, the condition of the blocks was evaluated, and cores were taken for determining the residual compressive strength of the concrete.Results from the study show that close to 3.5 kg of the 20-mm polypropylene fibres per cubic meter of concrete is required to prevent the spalling of a low W/C lightweight concrete made with a silica fume-blended cement when subjected to hydrocarbon fire but that only 1.5 kg of the finer 12.5-mm fibres per cubic meter is sufficient.The amount of 20-mm fibres required to prevent spalling for a higher W/C of 0.42 is significantly less: of the order of 1.5 kg per cubic meter of concrete.The susceptibility of the concrete to spalling increases with the degree of absorption of the lightweight aggregate used in concrete.
In this study, sidewalk sections were made in the field using seven concrete mixtures, applying a finishing and curing practice that is commonly used in Montréal, Canada. For each of the sidewalk sections, large slabs (1.2 m × 1.2 m) were cast from which specimens were cored and tested in the laboratory for determining their basic mechanical properties and deicing salt scaling resistance following ASTM C672 test procedures. Also, during the casting of the sidewalk, companion specimens were cast on site, using concrete from the same batch, and were subjected to the same tests as the “cored” specimens. The resistance to deicing salt scaling of these “laboratory specimens” was evaluated according to ASTM C672 and to BNQ NQ 2621–900 (2002 standard of the province of Quebec, Canada) test procedures. The results were compared with the performance of the sidewalk sections after four winters of outdoor exposure. The visual evaluation of the sidewalks after four winters has confirmed the severity of the ASTM C672 procedure and the adequateness of the BNQ procedure to better evaluate the deicing salt scaling resistance of concrete made with supplementary cementing materials (SCMs). The field evaluation should, however, continue for a longer period of time to increase the confidence in the BNQ test or to allow for changes as needed.
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