## 1 Literature Review
### Global Warming Potential, GWP, Carbon Dioxide Equivalence, and CO\({}_{2}\)eq  
The greenhouse gas inventory is more complete if all GHGs are included rather than only CO\({}_{2}\). Global warming potential (GWP) is a term that is used to describe the relative potency of a greenhouse gas (molecule for molecule) considering how long it remains active in the atmosphere [20]. The global warming potential (GWP) of a gas is the amount of warming that a gas causes over a specific period of time (normally, this is over 100 years). Greenhouse gas equivalent (CO\({}_{2}\)e) is used to describe the different GHGs in a common unit. CO\({}_{2}\)e indicates the amount of CO\({}_{2}\) that will have an equivalent global warming impact [21]. The commonly observed GHGs are CO\({}_{2}\), CH\({}_{4}\), N\({}_{2}\)O HFCS, PFCS, and SF6. According to IPCC inventory guidelines, the GWP of CO\({}_{2}\) is set to a standard value of 1. For a 100-year evaluation period, the GWP value for CH\({}_{4}\) is 25, N\({}_{2}\)O is 298, HFCS is 1430, PFCS is 12,200, and SF6 is 22,800 [22]. With CO\({}_{2}\) having an index value of 1, the GWP for all other gases is the number of times that they cause more warming than CO\({}_{2}\), e.g., 1 kg of Methane = 25 kgCO\({}_{2}\)eq [21]. Some researchers [22] also presented a table that indicated the GWP for a 20-year evaluation period. It was reported that in a construction project, the emissions of hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride are extremely low and negligible. CO\({}_{2}\), CH\({}_{4}\), and N\({}_{2}\)O are considered the primary emitted gases. During the construction stage for precast piles, the carbon emissions of construction machinery are primarily caused by energy consumption such as electricity, diesel, and gasoline. The emissions are calculated based on the carbon emission factors of machinery and equipment and energy consumption using diesel, gasoline, and electricity. The scholars [22] expressed the carbon footprint as follows:  
\[\text{Carbon footprint }=\text{ Activity level data }\times\text{ Carbon emission factor}\]  
Referencing a previous work, the EPA [19] stated that 8887 gCO\({}_{2}\) emission/gallon of gasoline consumed was the agreed upon common conversion by some stated agencies. This is approximately 2.35 kgCO\({}_{2}\)/L of gasoline. For diesel, 10,180 gCO\({}_{2}\)/gallon was mentioned. This is approximately 2.69 kgCO\({}_{2}\)/L of diesel. For passenger vehicles in 2019, the ratio of carbon dioxide to total greenhouse gas emissions (including carbon dioxide [CO\({}_{2}\)], methane [CH\({}_{4}\)], and nitrous oxide [N\({}_{2}\)O]) expressed as carbon dioxide equivalents for passenger vehicles is 0.994. Carbon dioxide equivalence for a gasoline-powered passenger vehicle will be = (8887 gCO\({}_{2}\) per gallon)/0.994 = 8.94 kgCO\({}_{2}\)e/gallon. This is approximately 2.36 kgCO\({}_{2}\)eq/L for gasoline. The interest in GHG emission reduction is not only observed in the transportation sector. There is an increase in the industry's interest in carbon emissions during construction stages [22]. However, transportation cuts across various industries as, often, there is a need for the movement of goods and services from one point to the other. A previous study used life cycle analysis (LCA) to study the carbon footprint of the precast concrete pile during the construction of a building. The goal of an LCA is to assess the environmental impact of a system or product. The study divided the carbon footprints of precast concrete piles into three sources: Material transportation, construction equipment, and office area [22]. The reduction of GHG emissions from the exhaust of construction equipment and emissions that are associated with the operation and maintenance of various construction offices will result in a reduction of GHG emissions in the construction industry.