Dynamic mechanical behavior and energy dissipation characteristics of alkali-activated ultra-high strength concrete after high temperature

Alkali-activated ultra-high strength concrete (AA-UHSC) exhibits advantages in strength and sustainability; however, its mechanical behavior after exposure to high temperature remains unclear. Therefore, quasi-static compressive tests, split Hopkinson pressure bar (SHPB) tests, mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM) were conducted on AA-UHSC specimens at room temperature and after exposure to high temperature. The quasi-static and dynamic mechanical properties of AA-UHSC at room temperature and after exposure to high temperature were investigated. The results indicated that both the quasi-static and dynamic compressive strengths decreased with increasing temperature, but 35.5 MPa of quasi-static compressive strength was retained at 600 °C. Moreover, higher loading velocities mitigated the strength loss. In addition, within the temperature range of 200–600 °C, AA-UHSC exhibited a more pronounced strain rate effect compared with room temperature, and the influence of the strain rate effect was similar across this temperature range. Three impact failure modes were defined, with their distribution and corresponding response characteristics clarified. The correlation between failure modes, loading rate, and reflected waveforms was revealed. The energy partitioning characteristics under impact load was clarified, and the effective energy range was determined based on the reflection waveform characteristics. The relationship between energy dissipation and failure modes, as well as its variation with increasing temperature and impact velocity, was established. The ultimate fragmentation energy dissipation was identified as an important indicator for assessing the impact resistance of structural elements in engineering applications.