Figure 1 (b) shows the numerical results that illustrate the behavior of stress wave propagation in a concrete specimen bar. After the compressive stress wave propagation, the reflected tensile stress waves are superposed at the center of the specimen bar and the tensile stress region is developed. The blue denotes the compressive stress wave and region, and the red the tensile ones. By making use of this process of stress wave propagation, a simple measuring method for impact tensile strength of concrete materials was proposed. The impact tensile experiment is conducted by means of Hopkinson bar technique. MECHANICAL PROPERTIES OF CONCRETE SPECIMEN Concrete and mortar specimens were used for this study. In this paper, however, the results only concerning concrete specimens (with coarse aggregate) are presented. The concrete mix proportions are shown in Table 1. An ordinary Portland cement with fine and coarse aggregates was employed for the fabrication of concrete specimens. The maximum size of coarse aggregate was 10 mm. Concrete specimens cured for 10 months were used for this study. In the impact tensile strength experiment, cylindrical bar specimens of concrete with 750 mm length and 50 mm diameter were used. Static tests of splitting tension (Brazilian), 3-point bending and compression were also carried out to examine the static mechanical properties of the concrete specimen. TABLE 1 MIX PROPORTIONS OF CONCRETE FOR A CUBIC METER TABLE 2 MECHANICAL PROPERTIES OF CONCRETE Material Quantity Water/cement ratio 0.5 Portland cement 370 kgf Water 185 kgf Fine aggregate 874 kgf Coarse aggregate 874 kgf Water-reducing admixture 0.74 kgf Total weight per cubic meter 2303 kgf Mechanical Properties Value Splitting tensile strength ( σts) 2.9 MPa Bending strength ( σb) 5.4 MPa Compressive strength ( σc) 27.8 MPa Elastic modulus (E) 35.1 GPa Mass density ( ρ) 2315 kg/m3 Poisson ratio ( ν) 0.2 Velocity of stress wave (c0) 3894 m/s In the static tests of splitting tension and compression, pieces of specimens with a length of 100 mm were cut from the long cylindrical bar specimens. To avoid some difficulties associated with the direct tension test such as specimen holing and proper alignment, the splitting tension test was carried out. The static splitting tensile test is an acceptable indirect method for determining tensile strength of concrete materials [6,10]. In the static bending test, the span was set to be 700 mm. All the static tests were conducted using an INSTRON (model 5586) material test machine at loading rates of 0.05 - 0.5 mm/min. Table 2 shows the static mechanical properties of the mortar specimens obtained by the static tests. Each strength is the mean value of cumulative fracture probability, which is discussed later. Here, c0 is the velocity of stress waves, given by c0 2 = E/ ρ, which almost corresponded to the experimental observation of elastic waves through the concrete specimen bar. EXPERIMENTAL TECHNIQUE AND ARRANGEMENT The Hopkinson bar technique is a widely used technique to determine the mechanical properties of materials at high loading rates. In this experiment, the setup consists of an air gun, a stainless-steel striking bar (SUS306), an aluminum alloy input bar (JISH4040), and a set of recording devices. The striking bar with 500 mm length and 20 mm diameter is chosen in order to initiate concentration of tensile stress at the center, C, of a specimen bar, shown in Fig. 2. The input bar is of length 1500 mm
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