## 1 Introduction
[PERSON] and [PERSON] [7] and, [PERSON] and [PERSON] [8], obtained testing data for strain rates above 1 s\({}^{-1}\) by using hydraulic testing machines. [PERSON] and [PERSON] [9], [PERSON] [10], and [PERSON] and [PERSON] [11] achieved strain rate levels on the order of 10 s\({}^{-1}\) using a drop-weight impactor. Especially, in last two decades, it has become possible to observe the dynamic compressive behavior of the concretes under strain rates of 10\({}^{2}\)-10\({}^{3}\) s\({}^{-1}\) using the Split Hopkinson Pressure Bar (SHPB) test setups. Additionally, in some high strain rate studies on fiber-reinforced normal and high-strength concrete samples that had compressive strength values of 35-110 MPa, strain rates in the range of 10 to -700 s\({}^{-1}\) were measured by using SHPB test setups [12; 13; 14; 15; 16; 17]. In these experimental studies, the specimens having radii varying in the range of 50 to 75 mm, and a length of 37.5 mm, were used. It is well known that it is almost impossible to produce concrete specimens that have uniform composition due to difficulties involved in controlling the mixture content of a concrete. On the other hand, there exists a small number of experimental studies in which specimens with a radii in the range of 10 to 15 mm were used [18; 19]. In these works, the attained strain rates were relatively high, varying in the range of 700-1700 s\({}^{-1}\). Nowadays, to improve, especially, the tensile properties of concretes, different types of fibers have been used in the UHSC productions [20; 21]. Using reinforced concrete specimens with the fiber volume ratios of 0.5%, 1%, and 1.5%, the energy dissipation, the crack initiation capacities and the dynamic behavior under the high strain rates were studied both experimentally by using SHPB setups and numerically [22; 23; 24; 25]. By using the SHPB test results, a number of numerical verification studies was also reported in the literature [26; 27; 28]. In recent years, studies on the dynamic behavior of concrete materials has intensified on Reactive Powder Concretes (RPC), which show high strength, durability and fracture properties. The static and dynamic strengths, the dynamic increase factor (DIF), the effects of strain rate and the effects of the geometric size of the specimens were discussed in a recent review paper [29]. In the available literature, there exist some deficiencies in the discussion of the data that may be extracted from the SHPB tests so that they can be transferred into the numerical data base required in numerical modeling of concrete materials subjected to extreme loading conditions. So, there is a need to extend the data, especially on the parameters generated by making use of the SHPB data. Therefore, using such numerical data, fed into available computer codes, may ease both the experimental and numerical studies of the behaviors of structures made of high or ultra high-strength concretes. The RPC is the most popular ultra strength type of concrete but RPC requires special production methods: its cost of construction is relatively higher than that of constructions based on conventional methods. For this reason, the concrete which is conventionally (low water/cement ratio, use of GGBF slag to fill the gaps) produced in this work was preferred in spite of the fact that its workability is low. In this study, an ultra-high-strength concrete with a compressive strength of 135 MPa has been designed and produced. To determine the behavior of the high-strength concrete under dynamic loading conditions, specimens of different sizes were produced by using a water jet cutting machine. These specimens were cut from the same concrete test samples that were prepared for the quasi-static tests. The SHPB data were utilized to determine the parameters of the Johnson-Holmquist-Cook material model by using an in-house developed computer code on the MATLAB environment. The Johnson-Holmquist-Cook model is one of the material models available in most computational software, such as LS-DYNA.