## 3 Sources of Discrepancies in LCA Stages
### Stage 1: LCA Scope
#### 3.1.2 Functional Unit Selection  
The second part of the LCA scope that the user selects is the FU. According [PERSON] et al. [56], a functional unit is the element that dictates how the inputs and outputs of the LCA are quantified. There are four main levels for functional units of concrete: a whole structure or a building, which was selected by around 6% of the 107 studies reviewed, as seen in Figure 6; or a component of a structure such as a beam, column or a bridge girder, and only 16% of the studies opted for that. The most famous level of detail (LOD) studied in concrete LCA is the material unit for concrete, which was selected by around 70% of the references. A unit-based FU for concrete is divided into: volume-based, volume-based while considering the strength of concrete and volume-based while considering both \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Reference** & & [36] & & & & [46] & \\ \hline
**Concrete Type** & **OPCC** & **25\%GGBS** & **50\%GGBS** & **15\% FA** & **35\%FA** & **50\%FA** \\ \hline \multicolumn{2}{c}{**Cover**} & & & & & \\
**(mm)** & & & & & & & \\ \hline \multirow{4}{*}{Carbonation} & 65 & 62 & 124 & 200 & - & - & - \\  & 50 & 124 & 200 & 200 & - & - & - \\  & 35 & 200 & 200 & 200 & - & 100 & 100 \\ \hline \multirow{2}{*}{Chloride Penetration} & 65 & 33 & 68 & 114 & - & - & - \\  & 50 & 67 & 138 & 200 & 51 & - & 60 \\ \cline{1-1}  & 35 & 91 & 186 & 200 & - & - & - \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Service life predictions for different concrete types from the literature. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**References** & **First Author’s Last** & **Year** & **kg CO\({}_{2}\)/m\({}^{3}\)** \\  & **Name** & **Year** & **kg CO\({}_{2}\)/m\({}^{3}\)** \\ \hline
[28] & [PERSON] & 2017 & 25 \\
[29] & [PERSON] & 2013 & 5 \\
[67] & [PERSON] & 2015 & 13 \\
[62] & [PERSON] & 2010 & 5 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Some values for the environmental impact of demolishing concrete from the literature reviewed. the strength and service life of concrete. [PERSON] et al. [56] claims that it is not accurate to call a unit volume a FU since it is not indicative of enough comparable functional properties. It should be called a declared unit instead. However, as seen in Figure 6, 65% of the references reviewed in this paper where a unit-based FU was selected did not consider strength nor durability; 25% considered strength and only 10% considered both. According to [PERSON] et al. [68], depending on the type of unit-based FU selected, the results for the LCA study might vary up to 30%. In order to further investigate this through our systematic review, the following examples were prepared. In terms of the difference in strength between the concrete alternatives in question, for RAC, maintaining the same binder, replacing fresh aggregates with coarse and/or fine recycled aggregates from CDW, will decrease the strength of the resulting mix (coarse aggregate (RC) and fine aggregate (RF) respectively) [58]. This decrease in strength of the concrete incorporating the recycled aggregates can sometimes be larger than the associated decrease in environmental impact. Hence, as shown in Figure 7, it is clear that when a FU of kg eq CO\({}_{2}\)/MPa was used instead of just kg eq CO\({}_{2}\), the environmental impact of the mixes with the recycled aggregates turned out to be larger than that of OPCC, not less [69]. For BCC types, the results from [PERSON] et al. [21] suggested that the optimum replacement of OPC with FA in terms of minimizing environmental impact in a BCC mix is 70%. However, adding FA  
Figure 6: The division of the FU in the LCA studies reviewed based on (**a**) Level of Detail and (**b**) type of unit volume. Figure 7: A comparison between the impact of a recycling aggregate concrete (RAC) using volume-based FU and a FU normalized to strength [69]. beyond a certain threshold would significantly decrease the compressive strength of concrete [35].