# Life Cycle Assessment for the Production Phase of Nano-Silica-Modified Asphalt Mixtures
## 4 Results and Discussion
### Computation of Normalised Score  
Table 6 and Figure 4 show the normalized score in each impact category of nano-modified asphalt materials. According to Ref. [46], by inspection, large values of normalized scores as compared to the total are classified as worse performing impact categories, while those with small normalized scores of approximately less than 2% of the total are classified as better performing impact categories. Table 6 shows that nano-modified asphalt materials only perform significantly better in four impact categories: photochemical oxidation (0.0217 pts/FU), ecotoxicity (0.0634 pts/FU), ozone depletion (0.2323 pts/FU), and global warming (0.3102 pts/FU). However, to fully understand when and how an impact category is classified as either better or worse performing, a logarithmic scale criterion was used. This was especially useful in situations where there existed large variation in the normalization scores. It is argued by Ref. [47] that dimensionless data is more appropriately plotted on an arithmetic scale to clearly understand where the data points lie (better or worse trend). On a logarithmic scale, the center of gravity (where the eye is drawn) lies at the geometric mean, where the line starts at 1 and not 0. Hence, applying the logarithmic scale plot (see Figure 4), all the impact categories below the 1 pts line are referred to as ZONE 1 (better performance zone). Hence, it can be said NMAM performs better in five categories: global warming (0.3102 pts/FU), \begin{table}
\begin{tabular}{c c c} \hline \hline
**Impact Category** & **Reference Unit** & **Impact Result** \\ \hline Environmental impact \(|\) global warming & kg CO\({}_{2}\)-Eq & 7.41900 \(\times\) 10\({}^{3}\) \\ Human health \(|\) respiratory effects, average & kg PM2.5-Eq & 8.79600 \(\times\) 10\({}^{2}\) \\ Environmental impact \(|\) ozone depletion & kg CFC-11-Eq & 3.68100 \(\times\) 10\({}^{-2}\) \\ Environmental impact \(|\) eutrophication & kg N-Eq & 1.47700 \(\times\) 10\({}^{1}\) \\ Human health \(|\) carcinogenic & kg benzene-Eq & 2.16300 \(\times\) 10\({}^{3}\) \\ Environmental impact \(|\) photochemical oxidation & kg NOx-Eq & 3.01270 \(\times\) 10\({}^{1}\) \\ Human health \(|\) non-carcinogenics & kg toluene-Eq & 6.01233 \(\times\) 10\({}^{6}\) \\ Environmental impact \(|\) ecotoxicity & kg 2.4-D-Eq & 1.08133 \(\times\) 10\({}^{4}\) \\ Environmental impact \(|\) acidification & moles of H+Eq & 1.86282 \(\times\) 10\({}^{5}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: LCIA results of unmodified (conventional) asphalt materials per FU. ozone depletion (0.2323 pts/FU), eutrophication (0.6779 pts/FU), photochemical oxidation (0.0217 pts/FU), and ecotoxicity (0.0634 pts/FU). All the impact categories above the 1 pts line are referred to as ZONE 2 (worse performance zone). NMAM performs worse in this zone in 4 categories: respiratory effects (36.9556 pts/FU), human health carcinogenic (5.8258 pts/FU), human health non-carcinogenic (182 pts/FU), and acidification (41.0101 pts/FU). The worst performance in acidification, which is the increase in hydrogen ions (H+) concentration within the environment as a result of the presence of acids, can be attributed to the sulfuric acid used in the production of nano-silica and the cause of sulphur dioxide and nitrogen oxides released during transportation of the materials and including asphalt materials. As mentioned previously, the modification of asphalt materials with nanomaterial causes only 0.4% per unit increase in global warming. This is because carbon dioxide (the main cause of global warming) is released during the production of bitumen, aggregates, asphalt mixing, and also during transportation. In short, the fact that the modification of asphalt materials with nanomaterial causes just less than or equal to 1% increase in impact score across all impact categories suggests that modifying asphalt materials with nanomaterials does not cause an unreasonable risk to the environment.