Experimental study on dynamic compressive dimension effect and damage evolution law of fly ash concrete under seismic strain rate

Many scholars have obtained preliminary findings regarding the study of the dynamic size effect of concrete; however, a unified explanation for the development law of internal damage in fly ash concrete caused by this dynamic size effect has not yet been achieved. Compression tests were conducted on cylindrical specimens of fly ash concrete with varying sizes under dynamic loads ranging from 1.0 × 10^–5(s⁻¹) to 1.0 × 10^–2(s⁻¹) seismic strain rate. The experimental findings indicate that the peak stresses were increased by 29.07%, 38.19% and 48.18% for the three sizes of specimens, large, medium and small, respectively, under the condition that the strain rate was increased from 1.0 × 10^–5 (s⁻¹) to 1.0 × 10^–2 (s⁻¹). From the overall trend analysis, the impact of strain rate on fly ash concrete gradually increases as the size decreases. The size effect of fly ash concrete can be attributed to the internal heterogeneity of specimens, which results in varying degrees of damage development. Similarly, the strain rate effect of meso-components is also caused by uneven damage development within fly ash concrete. The damage development law of fly ash concrete is then investigated by analyzing the changes in the 3D-DIC strain cloud map, using advanced technology known as 3D digital image correlation (3D-DIC). At strain rates of 1.0 × 10^–4(s⁻¹), 1.0 × 10^–3(s⁻¹), and 1.0 × 10^–2(s⁻¹), the full-stage damage degree factor (D_f1) in the pre-loading phase is 76.47%, 54.90%, and 25.49% of the static strain rate (1.0 × 10^–5(s⁻¹)), respectively. At strain rates of 1.0 × 10^–5(s⁻¹) and 1.0 × 10^–2(s⁻¹), the slopes of post-peak damage change (D_f2) for specimen sizes L, M, and S are 2.09, 2.27, and 2.5, and 2.25, 7.6, and 10.62, respectively. This suggests that in smaller specimens, damage development is primarily concentrated in the post-peak phase. Finally, the uniform static and dynamic size effect law of compressive strength in fly ash concrete is established based on the influence mechanism of damage development on dynamic strength and size effect. The research findings provide a theoretical foundation for the application and advancement of fly ash concrete engineering.