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Recently, nano silica (NS) has gained the attraction of academic researchers and the construction industry because of enhancing the properties of cementitious composites. Although there have been some related reviews, the comprehensiveness and advancedness need to be further improved. This paper is a detailed review of previously conducted studies to explore the influence of NS in cementitious composites for evaluating mechanical properties and durability. The impact of NS on the fresh state, i.e. setting time and workability, and in the hardened state, i.e. compressive, flexural, and split tensile strengths is considered. Besides, the long-term durability is discussed that include permeability, resistance against acid and base attack, abrasion resistance, and carbonation resistance. Furthermore, volume stability and microstructure of concrete with NS are presented. A huge number of studies showed the positive effect of NS with optimized content for improving the concrete properties, while a negative effect was observed with the use of excess NS content. The inclusion of NS in cementitious composites substantially enhances the mechanical properties, durability and microstructure. Meanwhile, better dispersibility is the key to ensure the strengthening effect of NS, which can be improved by changing the morphology/size of NS, optimizing the stirring method, and adding surfactants. Further investigation of the application of NS in special concrete is its development direction, and the effect of NS on the microstructure of main hydration products needs to be further explored. [ABSTRACT FROM AUTHOR]

The performance of any repaired concrete structure, and thus its service life, depends on the quality of the interfacial transition zone of the composite system formed by the repair material and the existing concrete substrate. In this work, the properties of the interfacial transition zone between normal concrete (NC) substrate as an old concrete and ultra-high performance fiber-reinforced concrete (UHPFC) as a repair material was investigated. Pull-off and splitting cylinder tensile tests were performed to quantify the bond strength in direct and indirect tensions, respectively. The microstructure of the interfacial transition zone was also studied using scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM/EDS). Different types of NC substrate surface preparation methods were used. An optical three-dimensional surface metrology device was used to estimate the substrate roughness parameters. Based on the results, high interfacial bond strength was achieved on the 3rd, 7th, and 28th days. The pull-off test results revealed that all failures occurred in the substrate, regardless of the substrate surface roughness. The majority of failures in the split tensile test also occurred in the substrate. SEM/EDS proved that the use of UHPFC as a repair material chemically, physically, and mechanically improved the repaired interfacial transition zone to become stronger and denser, as well as more uniform, and durable. Moreover, the use of UHPFC increased the service life of repaired structures and minimized the number and extent of interventions to the lowest possible level. [ABSTRACT FROM AUTHOR]