## 1 Introduction  
Concrete and other cement-based materials are widely used as construction materials due to their remarkable economic efficiency and durability [1; 2; 3; 4]. However, they are prone to cracking due to their low tensile strength and low strain capacity at fracture, and the inherently brittle nature may cause unexpected failure at the ultimate state. These drawbacks are traditionally overcome by introducing the concept of reinforced concrete with embedded reinforcing steel bars, which are continuous and designed to be specifically located in the structure to optimize its performance. Fiber reinforced concrete (FRC) with discontinuous short fibers was also developed as an alternative for the same purpose. FRC is a competitive and useful construction material because fibers in the FRC resist crack propagation with the help of stress transfer from the matrix to the fiber and furthermore its fabrication process is flexible and simple. Since the first major investigation of steel fiber reinforced concrete (SFRC) was conducted in the early 1960s in order to improve its brittleness [5], numerous studies on the SFRC and its applications have been carried out [6; 7; 8; 9]. Among various kinds of SFRCs developed to date, ultra-high-performance fiber-reinforced concrete (UHPFRC) exhibits excellent mechanical performance [10; 11; 12; 13]. It has extremely high compressive strength over 150 MPa with a very low water-to-binder ratio as well as excellent toughness and energy absorption capacity by adding less than 2 vol% of steel fibers [14; 15; 16; 17; 18]. The mechanical properties of general SFRC are dependent on the fiber properties (geometry, volume fraction, tensile strength, orientation, etc.) [19; 20; 21; 22; 23], the concrete properties (strength, elastic modulus, aggregate size, etc.) [24; 25; 26; 23], and the properties of the interface between the fiber and the matrix [27; 28; 29]. When the concrete properties are constant, the tensile performance of SFRC is primarily dependent on the geometry, volume fraction, and distribution of fiber. The general tensile strength of steel fibers used for SFRC is sufficiently high to avoid rupture, and thus the effect of tensile strength is usually negligible. Consequently, the analytical models for the tensile strength or principal characteristics of the tensile behavior of SFRC are generally expressed as a function of the fiber volume, the aspect ratio (defined as the ratio of the fiber length to the diameter), and the fiber orientation [23; 30; 31; 32]. The first two parameters are determined in the step of mix design, whereas the other is related to the form geometry and the placing method. With regard to ultra-high-performance fiber-reinforced concrete (UHPFRC), there have been extensive studies on the effect of fiber orientation on the tensile performance [20; 21; 33; 34; 35; 36; 37]. The effect of fiber orientation is much more significant in UHPFRC than in normal SFRC. This is mainly due to its high homogeneity with a composition of fine solid particles with a maximum particle size of 0.5 mm and sufficient flowability enough for self-consolidating. To produce highly flowable UHPFRC with a very low water to binder ratio, water reducing admixture is inevitably required in the mix. The rheological properties of UHPFRC are controlled by to the amount and type of the water reducing admixture (WRA) [38; 39], and they have a significant influence on the fiber distribution determined during the process of placing and flowing of UHPFRC [40; 41; 42]. This sequential influence is eventually connected to the mechanical performance of the composites [35; 36; 37]. Meanwhile, fresh UHPFRC shows considerably higher viscosity than normal SFRC or plain concrete, which may have a positive effect of improving the fiber distribution as well as a negative effect of worsening workability such as handling, delivering, and placing. This means that it is necessary to control the rheological properties of UHPFRC for balancing a better fiber dispersion with an acceptable workability.