## 3 Sources of Discrepancies in LCA Stages
### Stage 1: LCA Scope
#### 3.1.2 Functional Unit Selection
As seen in Figure 8, when the same GWP results were modelled using a FU of kg eq CO\({}_{2}\)/m\({}^{3}\)/MPa instead of a volume-based kg eq CO\({}_{2}\)/m\({}^{3}\), the optimum replacement percentage dropped to only 40%. Smaller gaps between both FU results were found when examining the results for GGBS from [PERSON] et al. [19], as seen in Figure 8. This could be due to the fact that the results for the compressive strength were tested after 90 days of curing instead of the common 28, at which the pozzolanic reaction would mature and most BCC mixes would achieve comparable strength to that of OPCC [70]. According to [PERSON] et al. [71], premature concrete deterioration, due to carbonation and chloride penetration, is responsible for a loss of 2.2 trillion USD, which is equivalent to 3% of the world's gross domestic product (GDP). This means that durability is a more detrimental factor to the performance of concrete than that of the compressive strength. Thus, the durability of concrete is an essential factor that needs to be included when the LCA of reinforced BC concrete is compared to reinforced OPC concrete. [PERSON] et al. [56] defined a FU where the volume of the BC concrete is multiplied by its compressive strength and the chloride ion penetration resistance, and is compared to the FU of an equivalent OPC concrete. Similarly, [PERSON] et al. [21] and [PERSON] et al. [69] used experimental data of the different BCC mixes in terms of compressive strength and chloride penetration to compare the performance of BCC with OPCC. However, for the absolute environmental impact values to be credible, these durability properties need to be translated into the service life to describe a performance parameter of concrete [59]. [PERSON] and [PERSON] [47] accounted for a 100 year timeframe as the service life of concrete, but only carbonation was used to determine the service life. [PERSON] et al. [57] devised a FU that divides the volume of cement by the number of years of durability from both chloride penetration and carbonation. Furthermore, an accurate methodology was proposed by [PERSON] et al. [30], where the test results of thirty different BC concrete mixes were incorporated into a FU (A-indices) that converts carbonation and chloride penetration parameters into expected service-life predictions. However, in all of the aforementioned, concretes with more than 100 years of durability will have a better environmental impact using both indices, while the specified service life for the mix is only 100 years. The same applies to compressive strength. Not capping the performance nor the durability of the concrete being studied, though it maximizes the sustainability potential according to [PERSON] et al. [72], impinges upon the performance base specifications of the concrete. Instead, performance-based specifications similar to those in the framework proposed by [PERSON] et al. [55] should be adopted. Finally, according to [PERSON] et al. [57], the comparison between the environmental impacts of two construction materials can be reliable, only after considering the combined effects of mechanical and durability characteristics. Figure 8: A comparison between the impact of blended cement concrete (BCC) using a volume-based FU and a FU normalized to strength. The user's choice for a unit-based LOD could be based on a personal preference or absence of the necessary details about the project, such as the quantity of concrete per member or per a whole structure. However, the benefit of studying the member or a whole structure in which the concrete mix would be a part adds is that it another important factor to the equation, which is the optimization of the total volume. For example, increasing the prescribed strength requirement for concrete from 25 MPa to 50 MPa in a solid slab building would decrease the volume of concrete needed by around 15%, especially with columns [28].