Self-healing is recognized as a promising technique for increasing the durability of concrete structures by healing cracks, thereby reducing the need for maintenance activities over the service life and decreasing the environmental impact. Various self-healing technologies have been applied to a wide range of cementitious materials, and the performance has generally been assessed under ‘ideal’ laboratory conditions. Performance tests under ideal conditions, tailored to the self-healing mechanism, can demonstrate the self-healing potential. However, there is an urgent need to prove the robustness and reliability of self-healing under realistic simulated conditions and in real applications before entering the market. This review focuses on the influence of cracks on degradation phenomena in reinforced concrete structures, the efficiency of different healing agents in various realistic (aggressive) scenarios, test methods for evaluating self-healing efficiency, and provides a pathway for integrating self-healing performance into a life-cycle encompassing durability-based design. GRAPHICAL ABSTRACT

The steel–concrete interface (SCI) is a complex, multi-phase and multi-scale system. It is widely known to influence the performance and long-term durability of concrete structures. However, a fundamental understanding of its properties and effects on corrosion initiation of embedded reinforcing steel remains elusive. This is attributed to its complicated heterogeneity and time-dependent nature, exacerbated by the lack of suitable techniques for systematic and detailed characterisation. This paper, prepared by members of the RILEM Technical Committee 262-SCI, critically reviews available information regarding current methods (laboratory or field-based) for characterising local properties of the SCI that have been identified as governing factors affecting corrosion initiation. These properties include characteristics of the steel such as mill scale and rust layers, and characteristics of the concrete such as interfacial voids, microstructure and moisture content. We evaluated over twenty methods and summarised their advantages, applications and limitations. The findings show a severe lack of well established, non-destructive techniques that are suitable for direct monitoring of the SCI at a representative scale with sufficiently high resolution (spatial, temporal), particularly for moisture related aspects. Several promising novel techniques with significant potential for further development and application were identified and discussed. Finally, we provide several recommendations for future research needs that are required to advance this critically important topic.

Ultrahigh-performance fiber-reinforced concrete (UHPFRC) features outstanding durability properties in uncracked state, although its composition based on very low water/cement ratios may result in a higher risk of early-age cracking due to increased shrinkage. This paper, based on research performed in the framework of the H2020 project ReSHEALience, studied aggressive chemical exposure conditions. A UHPFRC reference mix with 0.8% crystalline admixtures (CAs) and two other mixes, one containing 0.25% alumina nanofibers (ANFs) and one containing 0.15% cellulose nanocrystals (CNCs), both by weight of cement, were investigated. This study analyzed the synergy between CA and the nanoconstituents (ANF and CNC) on the physical, chemical, mechanical, and durability properties the and self-healing capacity of uncracked and cracked UHPFRC. The results showed that the presence of nanoadditives improves the mechanical properties independently of environmental curing conditions, refines the pore structure, favors cement hydration, and improves the intrinsic durability with a Cl and water transport reduction. Moreover, in the cracked state, nanoconstituents improved the self-sealing and self-healing capacity of specimens immersed in geothermal water.