## 1 Introduction  
Fiber-reinforced concrete has gained wide popularity due to the beneficial effect of fibers in improving toughness and ductility, crack resistance, tensile strength, impact and fatigue resistance, and durability [1; 2; 3; 4; 5]. The use of macrofibers, such as steel fibers, as reinforcement in concrete is well-established in applications such as industrial pavements, precast structural elements, tunnel lining, etc. [3]. Several guidelines and specifications for steel-fiber reinforced concrete (SFRC) are currently available to support the design of elements and structures with such material [6; 7; 8; 9]. However, in applications such as the precasting of thin elements, the use of rigid steel fibers is not the most suitable option, due to the reduced concrete workability, difficulties in placement and finishing, as well as hazards in handling due to the stiffness of the fibers. Therefore, there is a high demand for alternative fibers that can effectively overcome such limitations in fiber-reinforced concrete (FRC) for structural applications. Self-compacting concrete (SCC) was first developed in 1988 to achieve more durable concrete structures [10], and since then it has been increasingly adopted by the construction industry. Due to its high flowability, self-compacting concrete is compacted by its self-weight without any vibration or added compacting effort [11]. The self-compacting properties also have an impact on timescales and production efficiency, improving working conditions, and the health of construction workers [12]. The self-compacting properties are beneficial in a wide range of applications, from reinforced-concrete columns in high-rise buildings to structural upgrade and retrofit [13]. Another application of this technology is in the precast concrete industry. Precast elements often require the use of high-strength concrete to provide sufficient early-age strength development to allow for early demolding, as well as lifting and transportation to the construction site. High-strength concrete is more brittle than normal concrete, and this limitation can also be ameliorated with the addition of fibers to increase its ductility [4; 14]. The use of fiber-reinforced high-strength self-compacting concrete seems to be a viable solution for the precast concrete industry, as the concrete can be delivered straight into the molds, without any vibration or compacting effort, increasing production quotas and cost savings, allowing the production of slender elements, and ultimately improving quality and durability of structures. The performance of SCC with steel fibers [2; 15; 16; 17; 18; 19; 20; 21] and synthetic fibers [22; 23], in hybrid and non-hybrid mixtures, has been extensively reported in the literature. However, in the recent years, new synthetic fibers have become available in the market--for instance, twisted-bundle macro-synthetic fiber--and yet limited information is available on the performance of SCC reinforced with such fibers. Twisted-bundle macro-synthetic fiber is a blend of two types of fibers: (i) Fibrilated polypropylene fibers to reduce and control shrinkage and temperature cracking, and (ii) very heavy-duty twisted-bundle monofilament fiber made of a robust synthetic copolymer, to increase load-transfer and post-crack performance. A recent study [24] reported the effects of twisted-bundle macro-synthetic fibers on the mechanical properties of high-strength concrete. However, this study did not consider the flexural performance and the post-cracking flexural performance of the twisted-bundle macro-synthetic fiber-reinforced concrete. Another study [25] reported the effect of twisted-bundle macro-synthetic fibers on fresh and mechanical properties of ultra-high performance self-compacting concrete. However, the composites reported in this work did not include coarse aggregate, and again, the post-cracking behavior was not investigated.