Study on effects of flowability on steel fiber distribution patterns and mechanical properties of SFRC

Steel fiber reinforced concrete (SFRC) is a multiple-composite material developed during the early 1970s. In SFRC, short steel fibers are randomly distributed in concrete. Steel fibers can prevent the development of micro-cracks inside the concrete and reduce the expansion and development of the macro-cracks, thus enhance mechanical performance of SFRC. However, there is lack of studies on the influence of flowability of fresh SFRC on the steel fiber distribution patterns and mechanical properties of hardened SFRC.<br><br>In this research, steel fibers made by the thin-plate shearing method are used. Standard specimens are cast in which steel fibers are added to the concrete mix. The slumps ranging from 80 mm to 200 mm are employed as the parameter to reflect the flowability of SFRC.<br><br> The main research work is as follows: <br>(1) By cutting the specimens in three directions (transverse, horizontal and vertical sections) and quantizing the steel fibers in each section, effects of flowability on steel fiber distribution patterns are assessed. Distribution rate, distribution coefficient and orientation coefficient are the three factors used for describing steel fiber distribution patterns in this research. Calculated results of these factors of different flowability SFRC are summarized and compared.<br> (2) Basic mechanical properties tests including compressive strength, splitting tensile strength and flexural strength tests are conducted for different flowability SFRC. The splitting tensile tests along three directions of specimens of SFRC are carried out in view of the different orientation of steel IV fibers in these directions. Load-deflection curve of flexural toughness test is plotted and analyzed. <br>(3) Two commonly used methods, i.e., ASTM C1018 (Standard Test Methods for Flexural Toughness and First Crack Strength of Fiber Reinforced Concrete) method and the Chinese Standard JG/T472-2015 (Steel Fiber Reinforced Concrete), are used to access flexural toughness of SFRC. Fracture energy is also calculated.<br> (4) Formulas for calculating moment of inertia and flexural stress of flowable SFRC are proposed. The results show that an increase of flowability has no influence on the orientation of steel fibers and leads to a decrease of sectional uniformity. Steel fibers orientated in a longitudinal direction of higher flowability SFRC tend to precipitate towards the bottom layer of the specimens. This resulted in much better flexural performance including flexural toughness and fracture energy. This would indicate that, instead of studying the entire cross section, the distribution rate and distribution coefficient of steel fibers in tensile zone of specimen should be considered as the main factor determining flexural performance of SFRC. Calculations for bending stiffness and flexural stress based on the distribution rate of high flowability SFRC are recommended.<br><br>Moreover, due to the layering effect of steel fibers, traditional test methods are not suitable for determining basic mechanical properties such as compressive strength, splitting tensile strength and flexural strength of SFRC, which require further investigations.