73 State-of-the-Art Review for the Application of Shotcrete in Tunnel Lining Rehabilitation H. El Naggar 1,  , A.R. Sabouni 2 1 Department of Civil Engineering, ALHOSN University, P.O. Box: 38772, Abu Dhabi, UAE 2 Vice Chancellor, ALHOSN University, P.O. Box: 38772, Abu Dhabi, UAE ABSTRACT: There are numerous constraints associated with the rehabilitation of concrete linings in subway systems. First, there is limited clearance for placement of additional lining materials to reinforce the tunnel lining. Second, the time available to rehabilitate and to place additional lining materials is restricted. Shotcrete can be placed relatively quick, this rapid application makes shotcrete one of the most suitable materials for tunnel lining rehabilitation were there are strict time limitations. Shotcrete is a concrete product that can be applied to any surface as a pneumatic spray. The main advantages of shotcrete are the elimination of formwork, the ability to place it in unreachable (inaccessible) areas, and its relatively rapid application. The main focus of this paper is a state-of-the-art review of the application of shotcrete in tunnel linings. The review considers research conducted over the past few decades in the fascinating field of enhancing the properties of the shotcrete and shotcrete design considerations. Keywords: Shotcrete, Tunnel lining, rehabilitation, pneumatic spray, subway system 1. INTRODUCTION There are numerous constraints associated with the rehabilitation of concrete linings in subway systems. First, there is limited clearance for placement of additional lining materials to reinforce the tunnel lining. Second, the time available to rehabilitate and to place additional lining materials is restricted. For the Toronto Transit system, there is generally only 2hrs/night for work and clearance is limited to 50 mm. Thus, there is a need for fast applied materials for the rehabilitation of subway tunnel linings. Shotcrete is a multipurpose material that can be used in various types of applications including structural repairs. The major advantages of shotcrete are the elimination of formwork, the ability to place it in unreachable (inaccessible) areas, and its relatively rapid application. The main focus of this paper is a state-of-the-art review of the application of shotcrete in tunnel lining. The review considers research conducted over the past few decades in the field of enhancing the properties of the shotcrete and shotcrete design considerations. 1.1 Definition of Shotcrete Shotcrete is defined by ACI Committee 506 (1995) as mortar or concrete that is pneumatically applied at a high speed. Its relatively rapid mode of application (spraying) makes shotcrete a desirable repair material for applications such as tunnelling, mining, slope and rock stabilization and structural repair. More conventional repair methods, such  Corresponding Author. Tel: + 971 2 407 0529; fax: + 971 2 407 0799 E-mail address: h.elnaggar@alhosnu.ae AHU J. of Engineering & Applied Sciences 2 (2/3) : 73-83 (2009) © 2009 ALHOSN University