CO2 emissions and energy consumption during the construction of concrete structures

Comparison between prefab and insitu concrete viaducts

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Introduction Contractors and clients in the construction industry are increasingly interested in possibilities to reduce CO2 emissions and energy consumption. In the civil engineering industry most CO2 emissions are due to the production of construction materials and construction of a structure. The research focuses on the differences between insitu and prefab concrete construction and whether or not there is a significant difference in energy consumption and CO2 emission between the two construction processes. The research will cover the entire production and construction process; from the winning of the primary material to the delivery of the structure. Quick scan tool To determine and quantify the differences between the two construction methods a tool is developed, called the quick scan tool (QST). This tool is focussed on the construction of a viaduct. The QST defines four elements in a viaduct: Deck/beams, columns, abutments and foundation. There are 5 construction methods defined to nuance the results of the tool. From insitu construction in its most basic form, to fully automated produced prefab elements. With the definition of each construction method, calculation values are appointed. Emissions factors are gathered from multiple sources. The production and construction process is divided in four phases of emissions: Material, transport, factory and onsite. Results The QST shows that reductions in CO2 emissions and energy consumption are possible when constructing with prefab. The reductions originate from three sources. 1) Prefab structures are constructed with high strength concrete (C53/C65), therefore less construction material is required. Due to the reduction of construction material less CO2 is emitted. 2) The process of prefab construction is more efficient than insitu. Especially the emissions onsite are reduced, because less equipment is required onsite and project time is shorter. The emissions due to transport, will in general, be higher. 3) A prefab deck is constructed with box beams, this results in less force on other load bearing elements and reduces their size. All comparisons in the research are made to “the worst case scenario”, a predefined case which is an insitu concrete structure, constructed with the least environmental friendly electricity. With prefab concrete construction a total reduction of 23% can be obtained in comparison to “the worst-case scenario”. Sensitivity analysis The sensitivity analysis shows there are a number of possibilities to reduce the energy consumption and CO2 emission of a project. Reducing construction weight and reducing construction material have the most significant influence on the emissions of a project. The implementation of green electricity in factories and onsite is one of the easiest ways to reduce CO2 emissions, especially combined with the implementation of (electrical) tower cranes. Especially with prefab construction optimizing transport routes is beneficial. Other CO2 reducing measures, like carpooling or hybrid cars have less effect on the CO2 emission of a project. The influence of each of these measures on the energy consumption and CO2 emission of a project depends on the type of construction method which is researched. Discussion The results of the tool should be put in the right perspective, due to considerable differences in calculation values and emission factors found in literature. These differences originate from differences in; scope, used data and assumptions. Due to the great array of data used by research institutes the data is difficult to assess. Future Construction companies have a number of tools at their dispense to reduce CO2 emissions in construction industry. Constructing light, reducing the quantity of construction material and utilizing the more efficient process of prefab construction are the most important. Other more well-known options like using green electricity and carpooling have less impact. To which extend construction companies are going to implement these measures depends on how the matter is incorporated in tenders. Governments have two possibilities to reduce CO2 emissions on a project level. A maximum CO2 emission can be defined or companies could get reimbursed by reducing CO2 emissions. The willingness of the government to tackle environmental problems is important in this matter.