Christian Munch-Petersen, Danish Technological Institute, Concrete Centre
One of Scandinavia’s largest investments in infrastructure—the Fixed Link across the Øresund Strait between Denmark and Sweden was opened on July 1, 2000. The link includes a two-track railway and a four-lane highway. The crossing consists of an immersed tunnel 2.2 miles (3.5 km) long, an artificial island 2.5 miles (4.1 km) long, a western approach bridge 1.3 miles (2.0 km) long, a cable-stayed high bridge 0.7 miles (1.1 km) long with a free span of 1608 ft (490 m), and an eastern approach bridge 2.3 miles (3.7 km) long. The immersed tunnel and the cable-stayed bridge are the largest of their types in the world carrying both road and rail traffic.
The Concrete Strategy
In 1994, the link’s owner—Øresundskonsortiet—appointed an expert concrete group including specialists from the Danish Technological Institute and the Swedish Lund Institute of Technology. The group’s first task was the development of the following strategy:
- Owner defines and controls concrete quality.
- Quality is defined by the requirements for concrete production including concrete mix proportions (Materials) and requirements for execution including curing (Workmanship).
- Quality is controlled by requirements for inspection, testing, and documentation as part of a quality system in accordance with EN ISO 9001.
- Requirements must be established by the owner and owner’s consultants based on well-known technology. The requirements must ensure a service life of 100 years with proper maintenance but without any major repair work. Corrosion of reinforcement is not permitted to start within the 100-year service life.
- Strategy is enforced through the preparation of a comprehensive concrete specification as part of the bid documents.
- Ensure open competition between contractors, without compromising quality.
- Specifications must leave as much freedom as possible for the contractors to choose concrete mix proportions, but thorough attention must be given to the risk of failing to obtain the defined quality.
The concrete had to meet high performance requirements. However, the term “high performance concrete” (HPC) is not used in Denmark. Requirements can be high or low, but performance can only be “yes” or “no.” Therefore, per the Danish definition, there is no such thing as HPC. Nevertheless, in reality, concrete for the link would be described as HPC according to USA terminology.
Well-Known Technology
Well-known technology is defined as technology that is well tried with positive results under similar environmental conditions. Often, the owner would rather use well-known technology with the above definition than try a new (and maybe unsafe) technology in order to try to cut costs. When preparing the specification, questions about this principle arose. Which technologies are wellknown and by whom? If the common and “well-known” technologies are regarded (and maybe proved by experience) as unsafe by experts, it may be sensible to use a new and—according to experts—safe technology. This is called an innovation.
The main innovations in the specifications were the use of:
- European constituent material standards
- Defined conformity procedures
- Stress calculations for early-age cracking
- Service life calculations including workmanship
Main Types of Concrete
The single crucial factor in the durability design of a concrete structure is the concrete cover to the reinforcement. The effectiveness of the cover depends on its thickness and the concrete quality. The concrete quality is primarily a function of the water-cement (w/c) ratio; therefore, durability design must include a minimum cover and a maximum w/c ratio. The specified values for the minimum cover and the maximum w/c ratio depend on the required service life and the aggressiveness of the environment, which again is somewhat dependent on the detailed geometric design of the structure and varies very much along the structure’s surface.
It is sensible to define only a few different types of concrete with regard to w/c ratio. The thickness of the concrete cover can then be varied depending on required service life and aggressiveness of the environment. Thus, the total number of concrete types is reduced. For Øresund, only two types of concrete were defined:
- Type A with a maximum w/c ratio of 0.40
- Type B with a maximum w/c ratio of 0.45
Both types of concrete existed in a frost-resistant and a non-frost-resistant version and both types can be used with a cover of 2 or 3 in. (50 or 75 mm) depending on the environment. This way eight different environmental classes were addressed with only two concrete types. For all eight classes, the possibility of a 100-year service life was feasible.
Early-Age Crack Control
Temperature differences and autogeneous shrinkage can cause early-age cracking in the first few days of a structure’s life. The requirements for early-age crack control are of great importance to the service life of a structure. Therefore, the Øresund requirements stated that the contractor had to calculate a cracking risk (P).
The calculation of P was performed using a finite element method. The input data were the variation of the properties of the hardened concrete with time and other data necessary to describe the contractors’ planned execution, such as type of formwork, concrete temperature at placement, internal cooling system, and external insulation. The stresses calculated by the program were divided by the axial tensile strength of the concrete to give the cracking risk, P. The maximum acceptable risk of cracking (P) was specified as 0.7 for all water-retaining structures and splash zones and 1.0 or 1.3 for structures where some cracking is acceptable.
Frost Resistance
Guaranteed frost resistance was a must for the exposed structures. This was achieved through the selection of a concrete mix with an air-entrained system of high quality and stability. Through comprehensive pre-testing of the concrete, the necessary air contents were determined for the fresh and hardened concretes. The pre-testing included tests for salt-scaling and internal frost resistance. In production, only salt-scaling tests were made.
The Result
Defining a concrete strategy and following it has been a great success. The contractors were left with the possibility of deciding their own work procedure and concrete mixes while the quality was maintained because of the clear and strict specification. However, some problems arose where the contractor left out the air entrainment in some mixes or where the air content was too low. In these cases, the routine tests showed non-conformity of the mixes for frost resistance. No cracks have been found in the immersed tunnel. Some early-age cracks have been found in some of the structural elements. In all cases, the cracks were due to mistakes in the contractor’s planning and execution. These were documented by calculation using actual construction data. Stress and strain analysis of the early-age cracking, therefore, gave a true picture of the cracking risk.