Enhanced Fracture Toughness and Impact Resistance in Nanocellulose-Modified Concrete

This study evaluated cellulose nanocrystal (CNC)- and cellulose nanofibril (CNF)-modified concrete as an admixture-assisted mixture-optimization system, with emphasis on fracture toughness, impact resistance, and the microstructural features associated with the measured performance. Eleven mixtures, including a control and ten nanocellulose-modified concretes containing 0.05-0.25 wt.% nanocellulose relative to cement, were investigated through fresh-property tests, compressive and flexural strength tests, three-point bending and wedge-splitting fracture tests, repeated drop-weight impact tests, and multiscale characterization by SEM, TEM, XRD, FTIR, TGA, and MIP. Because superplasticizer and defoamer dosages were adjusted to maintain castability and dispersion, the results are interpreted as practical mixture-optimization evidence rather than as a fully isolated causal test of nanocellulose alone. Under this design, both nanocellulose types improved fracture and impact performance up to 0.15 wt.%; the CNC-0.15 mixture showed the highest wedge-splitting fracture toughness (1.16 MPa·m¹/², +31.8%), fracture energy (195 N/m, +52.3%), and impact energy (308 J, +55.6%) relative to the control. These gains coincided with a denser hydration-related microstructure, smaller average pore diameter, lower harmful-macropore fraction, and improved matrix-aggregate interfacial compactness. The findings support an interpretation based on hydration regulation, pore refinement, interface stabilization, and crack-path modification, while also identifying the need for matched admixture-only control groups in future studies.