Sustainable bridges - engineering a 300-year life

Sustainability objectives for bridges are best accomplished by ensuring durable bridges with long service life and low maintenance inputs that, on a whole-of-life basis, minimise material consumption over the long term. It is likely that such a bridge also has the lowest whole-of-life economic cost.

Such objectives were perhaps an innate characteristic of the ancient engineers whose structures still stand today as a testament to their sustainable design and construction practices, for example, the Ponte Vecchio in Florence, Italy (pictured right). This bridge was built around 1345 and is more than 600 years old.

Service life and design life

In targeting a service life performance for a structure, the asset owner needs to be aware not only of the initial cost of creating the structure, but its service life and the long-term cost of maintaining and repairing it over that service life, and finally its replacement cost.

A 300-year design life target is a new and interesting challenge. It requires the designer to explore outside the current codes, evaluate environmental loading and establish material performance over a long period, requiring extrapolation of current knowledge of climate and material properties as well as the extrapolation of material deterioration models.

Importantly, it also focuses the design and construction team to go beyond the standard durability response, to explore new means of achieving an extended service life and achieve a higher level of performance.

The Second Gateway Bridge durability

The Second Gateway Bridge is being built next to the original to duplicate the Gateway Arterial crossing of the Brisbane River. The new bridge is being built with a design life target of 300 years.

The 1627 metre-long Second Gateway Bridge is a pre-stressed and reinforced concrete bridge that sits in a range of environmental exposure conditions. The durability of the concrete comprising the bridge elements is the major factor in achieving a long service life.

In addition, several other requirements were mandated from the project scope and technical requirements. These were aimed at ensuring a minimum level of durability:

• Minimum B2 exposure classification (notwithstanding B2 exposure classification is described as within one kilometre of the coastline and the Gateway Bridge is approximately seven kilometres from the coast of Moreton Bay);
• Minimum 40 MPa concrete strength;
• Minimum 20 percent fly ash;
• Electrical connectivity of reinforcement in concrete piles, pile caps and piers for possible future installation of cathodic protection, and;
• Mix requirements for concrete in potential acid sulphate soil.

For the Second Gateway Bridge, the durability challenge is significant and varied. The bridge comprises a concrete structure with various elements in a range of exposure conditions:

• In the tidal/splash zone, where the main span piers are located in the Brisbane River which is essentially sea water;
• Permanently submerged, where the foundations close to the Brisbane River are below the tidal zone;
• In contact with the ground where the soil has, in places, high chloride content and acid sulphate potential, and;
• Air-exposed.

The philosophy adopted in meeting the extended design life is based on “building in” the required durability at the outset.

Concrete durability design

The exposure environments were evaluated for aggressivity towards concrete, due to deterioration mechanisms such as reinforcement corrosion caused by chloride ingress or carbonation, sulphate attack, microbiological attack and degradation resulting from acid sulphate soil exposure. In addition, other forms of deterioration and durability risks were considered such as alkali-aggregate reaction and thermal cracking.

In terms of chloride ingress, the greatest concern was for pile caps in the tidal/splash zone of the Brisbane River.

The river was determined to have a chloride concentration up to 18,000 ppm, which is similar to that of seawater. Testing of core samples from pile caps on the existing Gateway Bridge indicated surface chloride concentrations of 0.4 to 0.5 percent by weight of concrete. A “worst case” surface chloride concentration of 0.65 percent by weight of concrete was used in the analysis and is similar to published data for marine splash environments. Furthermore, the value of 0.65 percent also corresponds with the tendency for fly ash- and slag-modified concretes to have higher surface chloride concentrations.

The proposed pile cap design was to have 150mm minimum cover to black steel reinforcement and 75mm minimum cover to stainless steel reinforcement. The outer layer of concrete and stainless steel was regarded as sacrificial in the structural design.

Formwork to pile caps was to be kept in place for a minimum of 14 days to prevent early ingress of chloride ions to the concrete.

Another potential risk, particularly for concrete exposed to atmospheric conditions, is carbonation. This was primarily of concern for the superstructure elements of the bridge. The process of carbonation results in reduction of concrete pH. The passive iron oxide layer, which protects reinforcement from corrosion in concrete structures, is only maintained at higher pH levels. If the concrete becomes carbonated to the depth of reinforcement, the passive iron oxide layer is no longer stable and corrosion can occur in the presence of sufficient water and oxygen.

The presence of cracks permits local ingress of CO2 and can result in carbonation and subsequent corrosion ahead of the main carbonation front in sound concrete. The risk of reinforcement corrosion, initiation by the process of carbonation, can be reduced by using high quality concrete and sufficient depth of cover.

Construction control

The long life sought by the design will not be achieved without the necessary control applied by the construction team.

The first step is to supply a concrete mix with properties equal to or better than those assumed in the design. In this case trial concrete mixes were tested to ensure the chloride diffusion coefficients were achievable, and the concrete cast was tested during production to verify that the properties were achieved in the structure.

However, apart from this, the single most important element in achieving durable structures is to ensure the correct thickness of high quality cover concrete. The design must ensure the detailing is practical enough to prevent reinforcement congestion, but the construction must ensure the requisite cover is achieved, the compaction of the concrete achieves a good layer of dense cover concrete, and the concrete is sufficiently cured to prevent any early age cracking or incomplete hydration.

For the Second Gateway Bridge the contractor undertook an extensive education programme with a theme of three Cs (compaction, cover, curing) to encourage commitment of all workers to achieve a 300-year life.

• This article has been taken from a paper by John Connal, industry director, and Marita Berndt, principal engineer, Maunsell Australia, Melbourne, Australia, which was presented at the recent Austroads Bridge Conference in Auckland. For a copy of the full paper, visit www.austroads2009.co.nz.