Guide for Specifying Underground Shotcrete (ACI 506.5R-09)

Description

Shotcrete, the method of spraying concrete onto a surface ,is ideally suited for the support and construction of under-ground excavations in earth and rock structures. The pneumatic projection of shotcrete onto a surface at high velocity provides specific quality enhancements that interact with the ground surface and prepared substrates, providing superior bond characteristics; increased density; and resultant strength, durability, and toughness. These qualities are desirable inground support and lining applications. In North America, shotcrete is conventionally defined as pneumatically applied concrete, while in Europe it is more commonly referred to ass prayed concrete. Although concrete plays a very important role in under-ground construction, shotcrete has many advantages over concrete. Final concrete linings for tunnel and underground structures are typically composed of cast-in-place or precast concrete sections that provide a structurally sound and durable subsurface infrastructure.

The principal disadvantage with these systems is that they are difficult to install during the excavation phase of tunnel construction. They require formwork orassembly systems that simply cannot be employed in tunnel openings where the immediate concern is ensuring a safe andstable excavation. Accordingly, the cast-in-place lining is oftenplaced after excavation is complete, which results in a muchlonger time to complete the tunnel.In contrast, shotcrete in combination with other supportelements can provide early and effective ground support and,therefore, a means of ground control necessary for safe,viable, and economic tunnel construction under a wide rangeof geological conditions.

Shotcrete technology has been broadly developed throughoutthe construction industry over the last century. The evolutionof mining and civil tunneling methods has placed uniquedemands on the materials, equipment, and personnel thatcomprise current concepts of a shotcrete system for under-ground support. With this gradual evolution in technologyand trial and error, came acceptance, adaptation, and newmeans and methods of successful shotcrete application.Thedesign and placement of shotcrete underground is muchmore challenging than shotcreting above ground.