Antwerp Dry Docks – Restoration of a dry dock pt. 1
The Antwerp dry docks in the Hansadok are together with the city dry docks near the city the only remains of 200 years tradition of ship building and restoration in the port of Antwerp. After years of decay, EDR hopes to transform the site into a new and modern ship yard, together with the port authorities. SBE oversees the entire renovation of the site with 3 wet docks, 6 dry docks, warehouses, offices, workshops, cranes, etc., from master plan through to detailed civil and mechanical engineering. We already discussed the renovation of the dock gate in a previous post; discussing the importance of a proper test campaign before starting renovation works. In this post a case study is discussed.
Dry dock 5, the second largest and newest dock, is the first to be restored. Its construction dates to the sixties and the dock is 240 m long, 40 m wide and 9 m deep. To accommodate the typically large vessels that are docked here, a brand-new haul in system will be installed at the dry dock. A modern dry dock requires all handling to be operated by the dry dock operation crew and not by the vessel’s own winches. As soon as the vessel is positioned in front of the lock’s entrance, electrically driven winches installed at the front of the dry dock take over command and haul the ship towards its position above the dock blocks. A chariot moves along a steel guiding rail throughout the process. The maximum winch capacity has been doubled compared to the initial winch capacity at the dry dock’s conception in the sixties.
This posed an extreme challenge for the concrete design team. The team was asked to find a structural design that was able to anchor these enormous forces in the original concrete structure, which was not designed for these loads. Some factors that needed consideration were the fact that as concrete ages, its compression strength increases. Durability issues such as carbonation and chlorides endanger the structural health of the structure on the other hand. To quantify these effects, SBE proposed a semi-NDT campaign. NDT stands for non-destructive testing. The goal of this testing was to optimise the design parameters and choose an optimal restoration technique for the concrete, allowing for another 50 years life.
Several advanced techniques were applied in this test campaign. Firstly, concrete cylinders were extracted at well-studied locations. These concrete cylinders were tested for compressive strength. Statistical processing allows for specifying a 5%-characteristic value of material strength. This result proved to be more beneficial than the design strength that had been adopted at the conception of the dry dock. The location of these concrete cores had been chosen at the wall dock-side, at the wall soil-side and at the concrete upper plate. This way, the team could differentiate between the concrete’s health at the soil side, where it had been influenced by ground water mainly; at the dock side, where it had been influenced by the dock water mainly; and at the roof, where it had been influenced by surface runoff mainly. Also, a difference in casting technique for the walls and the roof led to different concrete health.
Another important measure for the concrete’s health includes the carbonation front and the chloride content. Due to ongoing reaction with the carbon dioxide, the protective pH changes over time. By the time the carbonation front reaches the steel reinforcement, corrosion is initiated. A lab test with phenolphthalein was conducted to measure the location of the carbonation ingress. It appeared that differences existed between mid-section locations and the expansion joint. Also, at crack location there was a notable influence.
The chloride content is of particular interest as it poses an immediate threat for the steel reinforcement. A concrete core is divided into three parts of well-defined length. This way a chloride profile with increasing depth can be deduced. The speed of chloride ingress is an indication of the remaining design life. Furthermore, in earlier times, chlorides were sometimes part of the concrete mix due to raw materials extracted in marine environments.
Another technique that has been applied at the dry dock to assess the concrete health consists of an electrical potential measurement. The conductivity measurement gives an indication of the locations with fastest corrosion degradation. These zones require specific attention in the restoration design.
The steel reinforcement location, amount and cover have been confirmed by special equipment. This allowed the team to draft location of reinforcement anchoring with minimal conflict towards existing reinforcement.
In another blog post [next week], the conclusions of the NDT campaign will be presented as an example of how comprehensive inspection results in an economical design.