A comparative study of life cycle carbon emissions of two commonly used simple-supported beam bridges

Carbon emissions from bridge engineering are an important component of the carbon emissions in the construction industry. The life cycle carbon emissions (LCCE) of two commonly used simple-supported beam bridges, hollow slab bridge and T-beam bridge, are studied and compared. Firstly, in order to establish the criteria for the comparison of different bridge types, the principle of equal stiffness is proposed for the superstructures, and the principle of matching load effect with bearing capacity is proposed for the substructures. Based on the above two principles, 6 bridges of 2 types and 3 spans are designed for comparative analysis. Then, a calculation model of the LCCE of simple-supported beam bridges is established, in which four stages, production, construction, operation and demolition stages are included, and the carbon emission factors of each stage are established. Finally, the carbon emissions of 6 bridges designed above are calculated, and the main factors affecting the LCCE of simple-supported beam bridges are discussed. The calculation results show that (1) For the same span, the LCCE of T-beam bridges are about 8-10% lower than those of hollow slab bridges. The reasons for this are that T-beam bridges use 22-32% less concrete and 4.5-11.5% less reinforcement than hollow slab bridges, which reduces carbon emissions during the production and demolition stages, and that the durability of the lateral connections of T-beam bridge is better than that of hollow slab bridge, which reduces carbon emissions from the maintenance and repair of T-beam bridge in the operation stage by about 30%. (2) Carbon emissions in the production stage of simple-supported beam bridges account for 83-84% of the LCCE, 11-12.6% of the LCCE in the operation stage, 3.8-4.5% of the LCCE in the construction stage, and 1% of the LCCE in the demolition phase. Therefore, the reduction of carbon emissions is most effective in the production and operation stages of these bridges. (3) Steel and concrete are the two materials that have the greatest impact on carbon emissions of simple-supported beam bridges. 100% recycled steel can reduce the carbon emissions of bridges by 17.7 -19.2% compared with 50% recycled steel. 50% and 100% recycled coarse aggregate concrete can reduce the carbon emissions of bridges by approximately 2.9% and 5.7%, respectively. (4) The carbon emission of the superstructure of the T-beam bridge is 12.5-14.2% less than that of the hollow slab bridge in the same span, and the differences in carbon emissions of the substructures are very small due to the small differences in the loads they are subjected to. With the increase of bridge span, the carbon emission per unit area of the superstructure of simple-supported beam bridge increases, that of the substructure decreases, and that of the whole bridge decreases firstly and then increases, which makes it possible to choose appropriate bridge span to decrease the carbon emission.