Page 50

2016SummerShotcreteEMag

Goin’ Underground Presented at the World Tunneling Conference, 2016 (San Francisco, CA). Paper reprinted with permission from The Society for Mining, Metallurgy & Exploration Inc. (SME), copyright holder (www.smenet.org). Nondestructive Approach for Shotcrete Lining Strength Monitoring By Vishwajeet Ahuja and Benoit Jones Shotcrete lining forms an integral part of conventional tunneling and is widely applied for underground excavations. Early strength gain of the shotcrete is a crucial aspect for ground support and safety of operatives. Strength requirements are dependent on various factors such as lining thickness, ground type, excavation size, and tunnel depth. The early strength gain is typically monitored using destructive tests, such as needle penetration, stud driving or coring samples for uniaxial compressive strength testing in the laboratory. Being destructive, these tests cannot be directly performed onto the lining without causing damage that must be repaired, which is a particular problem for permanent linings. For this reason and to avoid the need for operatives to work near exposed ground and/or fresh shotcrete, these destructive tests are often performed on panels, which are sprayed at the same time as the tunnel lining. All current testing methods are also very local, testing only a small part of the lining or a panel, which may not be representative because the temperature history could be significantly different. Therefore, these tests do not provide an accurate or complete picture of the lining strength gain. New testing methods that are non-destructive and can scan the whole lining remotely would be extremely desirable. This paper describes a new method, using thermal imaging techniques, that achieves these aims. It also discusses the real-time on-site application of the method, providing insight into the experience gained and conclusions derived. Shotcrete Shotcrete used for tunnel linings requires immediate strength development. The strength development is a direct result of the hydration reaction of cementitious materials present in it. A progressive sequence of the hydration reaction changes it from a solid suspension (typically referred to as fresh concrete) to a solid skeleton with a porous network and thereafter into a solid with predominantly discontinuous pores (Byfors 1980). In the case of shotcrete, early strength is needed to support the self-weight and then continuing early age strength gain is required to begin to support ground loads. These strength requirements, along with other workability needs, are met by careful concrete mixture design, the use of admixtures, such as accelerator and superplasticizer (BS EN 934-5 2007), and supplementary cementitious materials, such as silica fume. Strength Development The strength gain in concrete is known to be linearly proportional to the amount of cement hydration reactions that have taken place (Byfors 1980) and can be represented as shown in Fig. 1. If this relationship is known for a given concrete mixture, then concrete compressive strength (fc) may be estimated if degree of hydration (ξ) is known. Like many other chemical reactions, the rate of hydration (dξ/dt) for a given concrete mixture is dependent on temperature as well as the degree of hydration, as shown in Fig. 2. Thus, it is widely accepted that the degree of hydration and, in turn, strength development is dependent on its temperature history (Byfors 1980). Various maturity functions have been developed, such as those presented in ASTM C1074 (2011), which can be used to estimate strength development 48 Shotcrete • Summer 2016


2016SummerShotcreteEMag
To see the actual publication please follow the link above