CONCRETE FOR UNDERGROUND STRUCTURES

بتن سازه های زیرزمینی

To thoroughly understand the information presented in this book, it is important for the reader to have a basic knowledge of concrete components, how concrete is placed underground, and how the underground environment affects concrete mixture design overall. Many readers will already possess this knowledge, but some may not. This chapter aims to provide a brief and reasonably basic foundation in these topics for readers who are not completely versed in concrete material science. For a more detailed description of concrete basics and concrete mixture design, many textbooks are available, such as Design of Reinforced Concrete (McCormac and Brown 2009). Underground projects such as tunnels, shafts, and caves almost always incorporate concrete elements.
The most significant use of concrete underground is as a lining that provides initial and/ or final ground support and, if needed, protection from corrosive environments. Initial and final linings may be cast-in-place (CIP) concrete, precast concrete segments, shotcrete, or combinations thereof. CIP concrete uses forms into which the concrete is placed and allowed to set until it attains a specified strength and the forms can be removed. Precast concrete segments are manufactured at a segment manufacturing plant and installed in the tunnel behind tunnel boring machines (TBMs). Shotcrete is transported, similar to CIP concrete, to the point of application before being sprayed directly onto the tunnel surface using a spray nozzle without the need for formwork. All three applications of concrete raise construction issues underground that differ from considerations aboveground. The biggest differences arise from the confined nature of underground construction, distance from point of delivery to point of placement, and the atmosphere or environment underground. These issues will recur again and again as we discuss in later chapters the construction and specification considerations related to each of the concrete applications. Different methods of construction require different applications of concrete, whether excavating in rock or soft ground and whether using drill-and-blast or mechanical methods of excavation. Combinations of CIP concrete, shotcrete, and precast concrete segments are applied in almost all underground excavations in either primary or secondary linings or in one-pass lining systems. The following information about concrete mixture design will not be new to anyone with design
or construction experience. Rather, it is intended to provide context for briefly introducing the aspects of concrete mixture design that are specific to the use of concrete underground, which will be discussed in greater detail in later chapters of this book. It is important to note that in a welldesigned
concrete mixture all the ingredients are properly proportioned for the specific purposeئof the concrete (typical volumetric proportions are shown in Figure 1.1). CIP mixtures are not the same as precast segments or shotcrete mixtures. A simple example of the interrelations of concrete mixtures is the gradation of aggregates. Reducing the maximum size of coarse aggregate—for example, in a shotcrete mixture—requires an increase in the proportion of fine aggregate, consequently requiring a higher proportion of cement to adequately cover the increased surface area of aggregate with paste. This increased surface area of aggregate and more cement means that more energy is required for proper mixing prior to application. There are further consequences of increasing cement content with regard to water content, water-to-cementitious-materials (W/C) ratio, and many other factors. Suffice to say, all aspects of a concrete mixture are related and can rarely be taken in isolation if the most efficient application of concrete is desired. Most frequently, there is more than one correct answer when so many variables exist.

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