## 3 Method
### Calculation Model of Bridge Carbon Emissions
#### 3.1.2 Bridge Carbon Emissions Calculation Method  
To calculate the carbon emissions in the materialization stage of the bridge (as shown in Figure 1), the bridge construction stage is decomposed and analyzed according to the analysis results of the carbon emission factors during the construction period; from there, the entire bridge construction stage is divided into processes that are independent and have a unified quantitative expression of carbon emissions. Based on the LCA modeling method, a unit process carbon emission measurement model is established. Then, it is necessary to calculate the total carbon emissions in the bridge construction stage according to the inverse decomposition method [31]. This process yields the structural carbon emission universal measurement model for the construction stage of the bridge project, as shown in Figure 2:  
Figure 1: Calculation boundary of carbon emissions of bridges (the dashed line is the research content of this paper). Figure 2: Modeling idea of the bridge engineering materialization stage. (1) The bridge infrastructure projects are classified, and the unit process is divided according to the composition and construction characteristics of each infrastructure project. (2) The materials of the unit process within the boundary are analyzed and studied, and the lists of materials, off-site processing machinery and equipment, transportation vehicles, and on-site construction machinery are determined. (3) Based on the above list, a determination is made regarding whether upstream traceability is needed. If necessary, the corresponding substance list is updated according to the unit process traceability, and then the unit process list data are collected and calculated to obtain the demand for each substance in the list; if not, the unit process inventory data are directly collected and calculated to obtain the demands of each substance in the inventory. (4) The demands of each substance in the list are combined with the basic theory of carbon emission measurement to calculate and obtain the life cycle inventory data of the unit process. (5) The life cycle list data of each subproject, division, unit and construction phase in the construction stage from the bottom up are summarized and obtained. The bridge carbon emission calculation model during the construction stage is as follows:  
\[E=E_{cl}+E_{gs}+E_{gg} \tag{1}\]  
In Equation (1), \(E\) is the total carbon emissions, and \(E_{d}\), \(E_{gs}\), and \(E_{gg}\) are the carbon emissions in the bridge material production, transportation and on-site construction stages, respectively. (1) Carbon emission calculation model in the material production stage  
Bridge engineering requires many building materials to construct and produce abutments, beams, piers, piles and other components, and the production and processing of these materials produces considerable CO\({}_{2}\). The carbon emissions calculation model of building materials is as follows:  
\[E_{cl}=\sum\limits_{j=1}^{\mu}\sum\limits_{i=1}^{n}m_{ij}\cdot Q_{i}\cdot Z_{i} \tag{2}\]  
In Equation (2), \(m_{ij}\) is the quantity of type \(i\) building materials required for the construction of type \(j\) subsystem; \(Q_{i}\) is the fixed consumption of type \(i\) materials; \(Z_{i}\) is the carbon emission factor of type \(i\) building materials produced per unit mass; and \(\mu\) is the number of abutments, and beams of subsystems.