Material characteristics of 3D printed concrete subjected to highly dynamic loading

In this study the quasi-static, low- and high-dynamic compressive properties, and ballistic resistance of two types of 3D printed concrete have been investigated and compared to normal strength cast concrete (NSC). For the printed materials, an unreinforced commercially available mortar and a PVA fiber-reinforced strain-hardening cementitious composite (SHCC) mixture were used. The compressive strength and its dynamic increase factor, strain at peak stress, elastic modulus, and energy absorption capacity have been determined. For the high-dynamic compressive tests, a Split Hopkinson Pressure Bar apparatus has been used. For all compressions tests cylindrical specimens cored out in three orientations from the printed and cast concrete have been used.
At quasi-static strain rates, clear differences in compressive properties between the three materials were found. The behavior of the NSC and the unreinforced mortar was brittle and isotropic, while that of the SHCC was ductile. However, at dynamic strain rates, these differences disappeared and the failure mode of the SHCC became brittle too. This is likely caused by a change in failure mode from fiber pull-out to fiber breakage in the SHCC.
The ballistic tests were performed with an armor-piercing projectile impacting rectangular specimens at 630 – 760 m/s, resulting in significantly different responses. Most importantly, the SHCC specimens stayed intact even after four closely spaced shots, while both the NSC and unreinforced mortar specimens fractured completely. It is suggested this is due to the high fracture toughness of SHCC. Thus, the experiments suggest that even though the materials show similar behavior under dynamic strain rates, the SHCC behavior differs under localized, highly dynamic loading.