Corrosion Destruction of a metal by chemical, electrochemical, or electrolytic reaction with its environment
Crack A complete or incomplete separation of concrete into two or more parts produced by breaking or fracturing. It is caused by tensile forces exceeding the tensile strength of concrete
Crown The highest point of the internal curved surface of a tunnel cross-section
Delamination Delamination occurs in reinforced concrete structures due to reinforcing bar corrosion, in which case the oxidised metal of the reinforcement is greater in volume than the original metal. The oxidised metal therefore requires greater space than the original reinforcing bars, which causes a wedge-like stress on the concrete. This force eventually overcomes the relatively weak tensile strength of concrete, resulting in separation (or delamination) of the concrete above and below the reinforcing bars.
Efflorescence Water flowing through concrete dissolves some of the readily soluble calcium hydroxide and other solids and in time may cause serious disintegration of concrete. Many concrete structures bear evidence of this action by the presence of white deposits or efflorescence on their surface. This results from leaching of the calcium hydroxide and subsequent carbonation and evaporation.
Extrados The outer curve of an arch or circular/semicircular element, which in the case of a tunnel lining may be, but is not always, in direct contact with the adjacent ground
Intrados The inner curve of a circular or semicircular element. In the case of a tunnel lining defines the internal space
Invert (tunnel invert) The bottom surface of a tunnel
Joint A physical separation in concrete, whether precast or cast-in-place, including cracks if intentionally made to occur at specified locations
Leakage Can occur at joints where sealing material has failed or through cracks and other discontinuities.
Lining Permanent or temporary cover to the rock or soil surface at the wall of an excavation for a tunnel. This work is limited to the permanent concrete lining of a tunnel.
Maintenance of tunnels In this work it is taken to be synonymous with intervention that includes structural maintenance, repair, rehabilitation and retrofit and is used interchangeably with these words.
Map cracks Interconnected cracks that vary in size and form networks similar to that of sun cracking observed in dry areas. They vary in width from barely visible, fine cracks to well defined openings. They are found in both slabs and walls.
Reactive maintenance Maintenance that is carried out in response to inadequate performance
Resistance Resistance is the capacity of a member or component, or a cross-section of a member or component of a structure, to withstand actions without mechanical failure
Segment An arc-shaped preformed component that forms part of the lining of a tunnel
Service life The period of duty after which replacement or major renewal/refurbishment, rather than continued use, is anticipated to be justifiable on an economic or operational basis
Sidewall The vertical or near-vertical internal surfaces of a tunnel, forming a curved or straight plane that defines its sides. In an arched tunnel the sidewalls extend up to the springing where they support the arch. In earlier tunnels the sidewalls were constructed to be near vertical, but later were often curved to provide a more structurally efficient ovoid cross-section
Spalling Flaking and loss of concrete material from an exposed surface caused by corrosion of embedded metallic reinforcement, or frost, salt action or mechanical action. It is a roughly circular or oval depression in the concrete

Spring line
Line or plane from which an arch or vault springs, located at the junction between the supporting sidewalls/abutment and the arch or vault
Staining A discoloration of the concrete surface caused by the passing of dissolved materials through cracks and deposited on the surface when the water emerges and evaporates. Staining can be of any colour although brown staining may signify the corrosion of underlying reinforcing bar.
Structural Reliability The probability that the cross-section under study will not fail structurally within a given time period

This project is funded by the European Union