Besides residential and commercial buildings, construction of infrastructure such as roads, bridges, viaducts, metros, etc. also contribute substantially to the emission of greenhouse gases, mainly carbon dioxide. The latter is broadly divided into operational and embodied carbon. As far as bridges and viaducts are concerned, their operational carbon is generally lower than the embodied carbon and is mostly related to the energy required for their operation and maintenance works. The use of renewable energy in the form of solar/wind/hybrid systems augur well for this purpose.
There are quite a few examples in recent years of the use of renewable energy to cater to the operational energy requirements of bridges and viaducts in different parts of the world. One such example from India includes the 250-m long Akota Dandiya Bazar Bridge in Vadodara, Gujarat, where solar panels have been installed on top of the bridge.

Since substantial quantities of energy-intensive materials like steel and concrete are used in the construction of bridges and viaducts, the embodied carbon from these structures is often an order of magnitude higher than that from highways and buildings. While reduction of embodied carbon from these structures is certainly welcome and in fact is an urgent necessity, it would first be essential to know where we stand today with regards to the average carbon footprints of the existing bridges/viaducts. Once the current benchmark levels are known, one can then plan appropriate strategies for gradually reducing the carbon footprints of the new bridges/viaducts.

It is indeed an arduous task to conduct the exercise of estimating the average footprints of the existing bridges/viaducts, which among others involves obtaining the material quantities used in old structures and analysing the same. Thanks to Dr David Collings, Technical Director, Arcadis, U. K. who has painstakingly collected and analysed the data on some 200 bridges, mostly from the U.K. A recent paper by Dr Collings¹ on this topic highlights some interesting trends. It would be appropriate to provide a glimpse of the same here.

Estimation of embodied carbon involves consideration of all five service life cycle categories, which according to the EN 15978 includes the product stage (A1-3), construction stage (A4-5), use stage (B1-7), end-of-life stage (C1-4) and beyond life cycle (D). It is reported that for medium-size residential buildings, nearly 50% of the embodied carbon is generated during the product stage itself, which involves raw material extraction, transport and manufacturing². In the case of bridges and viaducts, this percentage could be higher as these structures use a substantial amount of energy-intensive materials like concrete and steel. Further, significant embodied carbon may be generated during the construction stage (A4-5) , especially if the fabricated materials are to be hauled over longer distances and it becomes essential to construct longer diversion roads, causeways, temporary bridges, etc.

Dr Colling’s data analysis of 200 bridges includes the estimation of embodied carbon from the product and construction stages (A1 to 5). The combined embodied carbon from these two stages is termed as ‘Capital Carbon’. The data of 200 structures reportedly include a variety of bridge types – ranging from culverts to suspension bridges. The data are grouped into footbridges, highway and railway bridges, and by the main material type of the deck.

The data analysis conducted by Dr Collings indicates that there is clear correlation between the embodied carbon content and three main parameters i.e. bridge length, bridge area and cost. Normalized capital carbon data analysis was also performed. It was observed that while there was a significant scatter in the capital carbon from 1 to 7 tCO_2e/m², the average normalised value of the same was 2.42 tCO_2e/m². The range of normalized capital carbon appeared broadly similar across the range of the length of structure. Also, a positive correlation was observed between the normalized carbon and the span. The type of loading was found to have an influence on embodied carbon – railway bridges tend to have more carbon than highway and footbridges.

Dr Collings observed that construction method-related carbon content needs careful evaluation. He opines that if detailed evaluation is not possible at the planning stage, then extra 25% should be added to the embodied carbon footprint to allow for the carbon emissions emanating from wastage, transportation, temporary works and the method of construction.

Based on the data analysis performed, Dr Collings suggests improvements in two main areas – firstly, reduce the amount of materials and aim towards the lower range of the theoretical carbon content and secondly, minimise the foundations and substructures, as these appear to be a major part of the capital carbon on many bridges.

Incidentally, it may not be out of context to draw reader’s attention to another interesting article by five well known structural engineers – Will Arnold, Mike Cook, Duncan Cox, Orlando Gibbons and John Orr – who have proposed the use of a Structural Carbon Rating Scheme (SCORS) to compare high-carbon and low-carbon design decisions and options. The SCORS rating is based on the estimated A1–5 emissions of the primary structure (superstructure plus substructure), calculated in accordance with IStructE’s guide “How to calculate embodied carbon” ⁴. The embodied carbon from A1-5 is then divided by the gross internal area (GIA) of the completed structure.

The structural engineers can assess the carbon impact of their work and also the progress that must be made in the near future to mitigate the carbon emissions. The authors advocate that all structural engineering firms should set in-house science-based SCORS targets for average structural A1–A5 emissions across all projects and then target year-on-year reductions. It is also suggested that the SCORS embodied carbon count in terms of kgCO_2e/m² may be uploaded to the RICS Building Carbon Database so that the same is freely available to all to maintain transparency across the profession.

India is currently experiencing substantial progress in the infrastructure sphere involving besides others, construction of bridges and viaducts. At such a juncture, it will be appropriate if the structural engineering fraternity in the country, including their professional associations, undertake the work of creating and analysing the database of the existing bridges/viaducts to find out the average value of the embodied carbon of Indian bridges/viaducts on the one hand also evolve structural carbon rating scheme for indigenous structures.

References