Shotcrete in repair and rehabilitation projects

Shotcrete, also termed as sprayed concrete, refers to a cement-based mixture that is proiected pneumatically at high velocity onto a target surface. The material component of shotcrete is essentially concrete or sometimes mortar, but the process of shotcrete application is unique. The first equipment for pneumatically applying a fine aggregate cement mixture (named as gunite) was developed in the early 20th Century in the United States. Shotcrete has traditionally been used all over the world in mining and tunnelling for the temporary constructions to ensure the workers safety. In the 100 years since the shotcrete process was developed, shotcrete has gradually played a valuable role in repair and rehabilitation projects mainly in North America. Compared to cast in place concrete, shotcrete is particularly advantageous in situations when formwork is cost prohibitive or impractical and where forms can be reduced or eliminated, access to the work area is difficult, thin layers or variable thicknesses are required or normal casting techniques cannot be employed. Moreover, the excellent bonding of shotcrete to surface of other damaged materials such as aging concrete, masonry surface, wood, steel structure is another useful property to perform effective and durable remedial work. The force of the impact of this pneumatically propelled material on the surface causes compaction of the shotcrete paste matrix into the fine surface irregularities and results in good adhesion to the surface. Within limits, the material is capable of supporting itself in vertical or overhead applications. Properly applied shotcrete is nowadays a proven and durable repair material characterized by high strength, low absorption, good resistance to weathering and resistance to some forms of chemical attack. Many of the physical properties of shotcrete are comparable or superior to those of conventional concrete or mortar having the same composition. This paper describes the two different application techniques of shotcrete, namely the dry-mix process and the wet-mix process, and discusses their respective advantages and disadvantages. The drawbacks of conventional shotcrete (only based on cement and aggregate mixtures) are reviewed and high performance shotcrete are presented. These novel shotcrete materials are produced through the addition of new generations of chemical admixtures, supplementary cementitious materials and reinforcement fibres. Cases histories of a successful use of the high permormance shotcrete in the rehabilitation and repair projects are described.