Life Cycle Assessment of Portland Cement and Concrete Bridge : Concrete Bridge vs. Wooden Bridge

Abstract: Today global warming mitigation, natural resource conservation and energy saving are some of the significant concerns of different industries, such as cement and concrete industries. For that reason, a streamlined life cycle assessment (LCA) model of one ton of a Portland cement, CEM I produced in Cementa AB’s Degerhamn plant, has been developed by using the LCA software KCL-ECO. LCA is a tool that identifies in which stages of a product’s life cycle the most environmental burdens occur. The environmental analysis was limited to identify total energy consumption and total carbon dioxide (CO2) emissions per ton of Portland cement. Results show that the most significant energy consumption and CO2 emissions are related to clinker kiln, due to the process of calcination of limestone and fuel combustion in the kiln. Of total CO2 emissions, 52 % and 46 % result from the calcination process and fuel combustion respectively.  One of the applications of CEM I is in construction of concrete bridges. Therefore an LCA model of a concrete bridge located north of Stockholm was developed in KCL-ECO. Environmental indicators calculated are: total CO2 emissions and energy consumption through the entire life cycle of the bridge. CO2 uptake or carbonation of the concrete during the service life of the product and end of life treatment is one of the advantages of concrete products. During the carbonation process, some of the total CO2 released from calcination will be absorbed into the concrete. Results indicate that production of raw materials and transports during the life cycle of the concrete bridge, are main contributors to total CO2 emissions. Among raw materials, cement production has the highest CO2 emissions. Energy consumption is mainly related to concrete and concrete products production. CO2 uptake during the use phase of the bridge is small compared to total CO2 emissions from calcination. Furthermore, the results show that different waste handling practises result in different CO2 uptake behaviours. The total CO2 uptake from crushing and storing of the demolished concrete (scenario 1) and landfilling of the demolished concrete (scenario 2) is 10 % and 5 % of the total CO2 emissions from calcination respectively.  Since comparison of different construction materials from an environmental point of view is always desirable, the LCA tool was used to compare the total energy consumption and the CO2 emissions from a concrete bridge and a wooden bridge. The functional unit was defined as 1 square meter of bridge surface area, since the bridges were of different sizes and shapes. In this comparison the total emissions and energy consumption were much higher for the concrete bridge than for the wooden bridge.  In order to show how different assumptions could affect the results, a virtual concrete bridge with the same shape and size as the wooden bridge was designed and compared with the wooden bridge. The functional unit selected for this case was one bridge. In this case the virtual concrete bridge requires less energy, while the wooden bridge emits less CO2 to the atmosphere. For the wooden bridge, CO2 in growing forests was included, which could be debated. Overall, a comparison of the environmental performance of the wooden bridge and the concrete bridges was more complex than initially expected and great care is recommended in choosing material and application. With concrete, the design (and quantity of material used) seems to be a very sensitive parameter and may result in much larger energy used and CO2 emissions than a wooden bridge. On the other hand, the virtual bridge comparison showed that concrete advantages such as higher durability and lower maintenance may be theoretically combined with a comparable energy and climate performance as a wooden alternative.