Cyclic compressive response and stress-strain modeling of nano-metakaolin concrete: Effects of dosage and curing age

Bridge decks, airport runways, and other transportation infrastructures commonly experience cyclic compressive loads throughout their service life. Despite its potential advantages, the performance of NMK concrete under such cyclic compression remains insufficiently understood, thereby limiting its practical application. In this study, uniaxial cyclic compression tests were performed on 36 prismatic specimens to evaluate the effects of NMK dosage (0%, 3%, 5%, and 7%) and curing age (7, 14, and 28 d) on the mechanical response. A novel brittleness index ( BI NMK ), formulated from the post-peak envelope curve, was proposed to quantify material toughness under cyclic loads. By combining hysteresis loop analysis with digital image correlation (DIC), the damage evolution processes in NMK concrete throughout cyclic compression were comprehensively characterized. The results indicate that the incorporation of NMK significantly enhances both the peak compressive strength ( f cu ) and the initial stiffness ( E is ) of concrete. At 28 d, the incorporation of 7% NMK(NM7) increased the f cu and E is of concrete by 40.8% and 68.2%, respectively, compared with ordinary concrete (OC). During the pre-peak stage, the presence of NMK effectively delayed damage development and further limited the accumulation of internal damage. Compared with OC, NM7 exhibited reductions of 48.8% and 31.0% in the increase rate of residual strain at 14 d and 28 d, respectively, whereas the stress ratio at the onset of stiffness degradation increased by 35.2% and 7.5%, respectively. Analyzing from the energy perspective, the total dissipated energy of NM7 was 25.7% and 30.0% lower than that of OC at 14 and 28 d, respectively. During the post-peak stage, NMK limited the further propagation of internal cracks. In the stable stiffness degradation stage, the stiffness degradation rate of NM7 was 31.8% and 41.2% lower than that of OC at 14 and 28 d, respectively. Finally, a modified cyclic compression stress-strain model considering NMK dosage and curing age was proposed, and the relationship between the model correction coefficients and the BI NMK was established. Comparisons with the experimental results showed that the proposed model accurately captured both the pre-peak and post-peak stress-strain responses (R 2 >0.95). This study systematically elucidates the effects of curing age and NMK content on the cyclic compressive behavior of concrete, highlighting the beneficial influence of NMK on pre-peak damage evolution. The findings provide design guidance for the application of NMK concrete in transportation infrastructures such as bridge decks and airport runways.