AbstractThere is strong demand for ductile cement-based materials in various rehabilitation systems of concrete infrastructure. This study investigated the flexural performance of nanomodified cementitious composites incorporating a novel class of basalt macrofibers, basalt-fiber pellets (BFP), in an overlay system. The cementitious composites comprised 50% fly ash or slag replacement with 6% nanosilica and 4.5% BFP. The cementitious composites [top (overlay) layer] were evaluated based on mechanical compatibility with intact and precracked concrete substrate (bottom layer) through flexural loading. In addition, three-dimensional finite-element models were established for these configurations to analyze the effect of overlay thickness and substrate compressive strength on the performance of the overlay systems. The results showed effective bonding of the composites with the substrate (without transverse delamination) and highlighted their contribution to enhancing the postcracking performance of the overlay system, in which the intact system of Specimens N-F-4.5 and N-G-4.5 had toughness 324% and 471%, respectively, higher than that of reference concrete specimens. Numerical analysis showed the influence of overlay thickness on the ductility of both systems (intact and precracked), whereas the substrate strength affected only the first-cracking strength of the intact system (approximately 25%, increasing the compressive strength of the substrate concrete from 40 to 50 MPa).