Interfacial bond between modified micro carbon fiber and high-strength cement paste in UHPC: Bond-slip tests and molecular dynamic simulation

The use of carbon fiber (CF) as the reinforcement in ultra high performance concrete (UHPC) can avoid the steel fiber corrosion problem in harsh environment. The interfacial bonding between CF and matrix play a key role on the composite performances. However, the interfacial bonding between CFs with different surface treatments and the high strength paste in UHPC are not yet clear. According to micro CF pull-out tests in this study, the chemical debonding energy of anodized CF and epoxy sizing CF increased by 117.0% and 41.8%, respectively, compared with that for non-modified CF. Correspondingly, the CF/matrix bond strength enhanced by 88.6% and 29.1%, respectively. More attachments were found on the surface of the anodized CF pulled out from the matrix, and its interfacial friction bond strengths was improved by 67.8% after anodizing. Molecular dynamic simulation shows that the oxygen-containing groups (-OH, =O and –COOH) on the surface of CF can form a strong van der Waals force with C–S–H gel, and hydroxyl and carboxyl groups can also form hydrogen bonds with oxygen atoms in C–S–H gel, which is even stronger than van der Waals force. Moreover, the hydrogen bonding ability of carboxyl groups is 1.5 times higher than that of hydroxyl groups. Therefore, it can be more effective to improve the bonding between CFs and paste matrix by grafting carboxyl groups to the CF.