ABSTRACT
The Hong Kong-Zhuhai-Macau (HZM) Sea Link is currently under construction and includes sea bridges of 28.8 km (four navig-able spans), two offshore artificial islands and an immerged tube tunnel of 6.8 km, with a total in-vestment of nearly 12 billion US dollars.
One of the technical challenges of HZM project was to achieve the service life of 120 years for the concrete structures in an aggressive marine environment. From a criticality analysis of the possible deterioration processes, the chloride-induced steel corrosion of RC/PC elements was identified as the controlling process of durability design.
The immerged tube tunnel is made of concrete, consisting of four 112.5 m in-place segments and 27 prefabricated segments of 180 m each. The cross dimension of the tunnel tube is 38 m × 11.4 m with wall thickness of 1.5 m. The concrete tunnel tube is exposed directly to sea water. The work presented refers specifically to the service life design of the precast concrete segments of immersed tunnel tube, the size and shape of which can be appreciated in Fig. 1. Shape of the full-section tunnel precast elements. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315207681/cd556cd4-4dcf-4efe-8e29-56fc67b8bfbd/content/fig16_1.jpg"/>
The main factors that determine the service life of these marine structures, exposed to chloride-induced corrosion, are the “penetrability” and thickness of the concrete cover that protects the steel reinforcement. In an initial design phase, these factors are defined by modeling the penetration of chlorides with analytical tools (based on the Duracrete approach).
In the construction phase, data are collected from the in-site laboratory for concrete chloride diffusivity, and from non-destructive tests for the concrete cover thickness and air permeability for the prefabricated segments of the immersed tube tunnel. On the basis of these data, two model-based approaches are used to predict the service life of the tube tunnel immersed in sea water. The first model is an analytical one of Fick’s model for chloride ingress into concrete using the corrosion initiation as durability limit state. The second model uses the “Exp-Ref” model, using the concrete cover thickness and air permeability as input parameters and calibrated by the EuroCode specifications on structural concretes in marine environments. Both approaches use Monte-Carlo simulations and consider the statistical properties of the input parameters. Fig. 2 presents the expected probability of reaching the “Corrosion Initiation Time” estimated by both methods. Probability chart of the corrosion initiation time for both Analytical and Exp-Ref Service Life Design methods. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315207681/cd556cd4-4dcf-4efe-8e29-56fc67b8bfbd/content/fig16_2.tif"/>
A comparison of the analytical and experimental predictions is made, showing compatibility with 120 years of service life. The convenience of verifying the analytical predictions with those obtained from site experimental data is discussed.
