## 3 Sources of Discrepancies in LCA Stages  
### Stage 1: LCA Scope  
#### 3.1.1 System Boundary  
More than 75% of the studies (107) we reviewed, in which an LCA study was actually conducted, used a cradle-to-gate system boundary, as shown in Figure 5. A cradle-to-gate system boundary would limit the scope of the processes and resulting emissions and energy studied in the LCA up until the  
Figure 4: Number of LCA studies per (**a**) type of concrete, (**b**) published year and (**c**) country. production stage, excluding the use and end-of-life stages. As stated by [PERSON] et al. [24] the ISO 24067, released in 2013 to provide a benchmark for the LCA methodology, specifies that for a user to exclude the use and end-of-life stages while conducting an LCA, there needs to be enough evidence that the results will not be affected by this. Hence, it would not be acceptable to cut-off the use and end-of-life phases from the scope due to the following reasons (i-iii):  
(i) Throughout its \"use\" phase, concrete exhibits carbonation, a process by which carbon dioxide is absorbed by the concrete from the exposed environment reacting with the calcium compounds in its matrix, forming carbonates [62]. A justifiable cut-off percentage is when the processes affects less than 1% of the total environmental impact according to [PERSON] et al. [16] and 5% according to [PERSON] et al. [63]. However, through the carbonation process, concrete can absorb, throughout its whole service life, 13-48% of the carbon dioxide it emitted during the production phase [62]. This value of the captured carbon dioxide, denoted as carbon sequestered, varies depending on the concrete type, exposure conditions and geometry [44]. Out of the 107 references, only seven included the sequestered carbon in the LCA study, with varying values, as seen in Table 1. OPCC can capture up to 47% of its embodied carbon during use and end-of-life phases, while BCC can capture only up to 22% [39]. In all cases, it is apparent that the sequestered carbon ought to be included in an LCA study to allow for its value to be deducted from the carbon emissions in the remaining processes. (ii) By omitting the use phase, the user is also assuming that all concrete mixes being compared will sustain the required service life. However, the findings in Table 2 show proof from the literature that this assumption is not true. Depending on the exposure conditions, concrete cover and the concrete mix, there is a high probability that a reinforced green concrete mix is unable to fulfil its required service life, especially one above 60 years [61]. Hence, according to [PERSON] et al. [56], there should be at least a 20% increase in the environmental impact of concrete to account for the potential maintenance \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**References**} & **First Author’s Last** & \multirow{2}{*}{**Year**} & \multirow{2}{*}{**Kg CO\({}_{2}\)/m\({}^{3}\)**} \\  & **Name** & & \\ \hline
[62] & [PERSON] & 2010 & 5 \\
[29] & [PERSON] & 2013 & 61 \\
[64] & [PERSON] and [PERSON] & 2016 & 172 \\
[65] & [PERSON] & 2013 & 10 \\
[46] & [PERSON] and [PERSON] & 2010 & 30 \({}^{*}\) \\
[66] & [PERSON] & 2019 & 39 \({}^{*}\) \\
[44] & [PERSON] & 2017 & 60 \\ \hline \hline \end{tabular}  
* Calculated based on a percentage of the 320 eq CO\({}_{2}\)/m\({}^{3}\) average value.  
\end{table} 
Table 1: The different values for carbon sequestration from the papers reviewed. Figure 5: Meta-analysis of the selected system boundary for each of the LCA studies reviewed in this paper. that allows it to fulfil its service life requirements. Nevertheless, [PERSON] el al. [15] set the expected CO\({}_{2}\) emissions for concrete at 0.60 eq kg CO\({}_{2}\)/m\({}^{3}\)/year in service.