## 3 Experimental Work
### Split Hopkinson Pressure Bar Tests and Discussion
This is one of the reasons for using 10 mm diameter samples, although some aggregate sizes are greater than 20 mm. The samples used were obtained by water jet cutting. Therefore, the strength conditions at cutting edges may differ. In order to eliminate these differences, 28 tests were performed. However, results from 21 tests were used since the other 7 tests appeared not to reflect the expected homogeneous behavior of the concrete. As stated above, in the tests, three groups of specimens were used. The test pressure was selected from the range of 0.1 to 0.3 MPa, while the striker speed was in the range of 7.89-13.48 m/s. The test results of 21 specimens are given in Figure 6.1 In these tests, the standard deviations concerning the strength, strain and strain rate values are 30.34 MPa, 0.009 and 262.68 s\({}^{-1}\), respectively. A high-speed camera was also used in this study to visualize the dynamic behavior of the ultra-high-strength concrete. The high-speed camera recorded the test process at a frame rate of the microsecond order. Figure 9a shows the instantaneous states of a sample at 6 different time instances, while the corresponding stress-strain relation is given in Figure 9b. The numbered labels in Figure 9b correspond to the 6 frames given in Figure 9a. Figure 6.2 UHSC-16 (Ultra High Strength Concrete—Specimen number 16) SHPB test results: (**a**) true strain-time; (**b**) true stress-time; (**c**) true strain rate-time; (**d**) true stress–strain relations. In the first frame, the sample seems to behave elastically. The stress-strain curve is also linear in the period of 0 to 40 \(\upmu\)s. During this stage, the strains are reversible. The first failure behavior, the first fracture, is observed in the second frame. The specimen is now possessing a non-linear stress-strain relation, which means that after this point the strains are irreversible. In the third frame, the spreading of cracks and the deeper fractures are observed to occur in the regions close to the location with the first failure. It has been considered that at this stage the specimen well exceeds its compressive strength since the local failures are spread out. The spreading of the fractured regions due to decrease in stress level are observed in the fourth frame. In the last two frames, the stress level decreases, cracks and fractures are now everywhere. Finally, the dynamic load carrying capacity of the ultra-high-strength concrete specimen reaches its lowest point after breaking away of the pieces from the specimen. Figure 6.3 The true stress–true strain behavior of specimen UHSC-16 during the maximum strain rate of about 960 s\({}^{-1}\): (**a**) High-speed test visualization of the sample UHSC-16; (**b**) Stress–strain relation for the specimen UHSC-16. In the dynamic compression tests, the length to diameter ratio ranging from 0.5 to 1.0 is acceptable. In order to diversify the strain rates and to see the size effects, the specimens having the length/diameter ratios varying in the range of 0.56 to 1.18 were used. As it is seen in Figure 10, As it is seen in this figure, under the same test conditions, the strain rates measured in specimens with lower length/diameter ratios are higher than those obtained for the specimens having a relatively higher ratios. This was an expected result for a specimen cut from a homogenous and linear isotropic material. Although, the concrete specimens used in these tests were not made of a homogeneous material, the strain rate are calculated from Equation (4) as  
Figure 6.4  
where \(v_{1}\) and \(v_{2}\) are the output and input velocities, respectively, and \(\ell\) is the length of the specimen.