Mechanical and permeability properties of the interface between normal concrete substrate and ultra high performance fiber concrete overlay

► Newly overlay UHPFC is able to achieve high bond strength with the NC substrates. ► Sand blasted surface give the best interfacial mechanical bonding in comparison to other surface preparation methods. ► UHPFC overlay significantly improves the impermeability characteristics of the repaired NC substrate. As a rule of thumb, the interfacial bonding between deteriorated concrete structures with a newly overlay repair material is one of the most important factors for structural functionality and safety as well as durability performance. In order to acquire an enhanced resistance against penetration of harmful substances, a good and effective bonding is necessary at the concrete interfaces. The objective of this study is to examine experimentally the mechanical properties and permeability characteristics of the interface between normal concrete (NC) substrate which represents old concrete structures and an overlay of ultra high performance fiber concrete (UHPFC) as a repair material. The mechanical interfacial bond characteristics were assessed using the slant shear and splitting tensile tests to quantify the influence of the differently roughened substrate surfaces. On the other hand, the permeability characteristics were evaluated by means of the rapid chloride permeability, gas and water permeability tests. The results show that the newly overlay UHPFC achieves high bond strength and bonds efficiently with the NC substrates. The specimens with sand blasted substrate surface give the best interfacial mechanical bonding in comparison to other types of surface preparation. The permeability tests proved that the interfacial bonding is very good and efficient which significantly improve the impermeability of the composites, and this was clearly shown by the SEM micrograph of the interface. Hence, it is envisaged that the use of UHPFC concomitant with appropriate surface preparation of the substrate should be able to provide effective and durable concrete repair.