The baseline scenario refers to the model conditions associated with the assumptions described in the “Materials and methods” section. The results obtained for this scenario were presented below.
Table 1 depicts characterization and normalization indicators for assumed types of steel sheet piles. Figure 1 illustrates the results for GU 6N piles in the form of process networks with the indicators for human health, ecosystems and resources damage categories. The presented indicators show that the dominant environmental impact (97%) results from the production process of hot-rolled sheets. The remaining part of the environmental burden (2.5–3.3%) is due to transport. These proportions show that, compared to the production processes, the transport distance from the manufacturer to the installation site has an insignificant impact on the final results. If GU 6N steel sheet piles were produced at the location of their application, the overall environmental impact would be reduced by only a few percent. Identical conclusions can be drawn for sheet piles type Larssen 43 (normalization indicators are about twice as high, but the proportions between the environmental impact of sheets production and transport are analogous).
Table 1 Characterization and normalization indicators for steel sheet pilesFull size table
Fig. 1The process network diagrams obtained for steel sheet piles GU 6N
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Table 2 shows characterization and normalization indicators for PVC sheet piles. The same results in terms of endpoint indicators are illustrated in Fig. 2 in the form of process networks. The presented values indicate that the environmental impact is dominated by the sheet piles manufacturing (98%). The process with the highest share is PVC granulate production (84–93% of the total environmental impact), whereas electricity consumption for polymer extrusion is much less significant (the share is below 15%). As in the case of steel sheet piles, the share of transport does not exceed a few percent of the total environmental impact.
Table 2 Characterization and normalization indicators for steel sheet pilesFull size table
Fig. 2The process network diagrams obtained for PVC sheet piles
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The comparison of the indicators obtained for steel sheet piles GU 6N and PVC sheet piles clearly shows that the environmental impact of vinyl products is significantly lower than that of steel piles. Depending on the endpoint, the indicators determined for PVC sheet piles are from 2.2 to 4.4 times lower than values obtained for GU 6N piles and from 5.2 to 10.4 times lower than those for Larssen 43 piles. The largest differences are observed for the category of ecosystems while the smallest differences has been recorded for the category of resources. The individual indicators for the compared types of sheet piles are shown in Fig. 3.
Fig. 3Normalization indicators for the compared types of sheet piles
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The aim of this section is to estimate uncertainties of the analysis outputs resulting from the uncertainties of input parameters of the baseline LCA model. Monte Carlo simulations were applied to generate 95% confidence intervals for the results. The confidence ranges are depicted in Fig. 4. In case of steel sheet piles absolute relative deviations from the mean values ranged 0.6–2.4% and were the largest for the damage category of resources. The results of PVC sheet piles indicated significantly greater absolute deviations which varied within a range of 12–19%. As in the case of steel products, the largest deviations were observed for the resources endpoint. However, despite wider confidence intervals, the comparison of the highest indicators for vinyl piles with lowest indicators for GU 6N piles would not change the conclusion drawn: the environmental impact of PVC products is significantly lower than that of steel sheet piles (depending on the damage category, the indicators obtained for vinyl products are from 1.9 to 3.8 times lower than values for GU 6N piles).
Fig. 4Confidence intervals for the normalization indicators determined
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The baseline scenario model involves a few main assumptions that could affect the outcomes and conclusions drawn: the thickness of steel sheet after 50 years of use, corrosion rate, transport distance, emission standard of transportation vehicles.
According to the assumption 1 (in the “Materials and methods” section), the minimum thickness of steel sheet piles after 50 years of operation is 4.5 mm. As the baseline scenario indicated a substantial advantage of vinyl products over steel products, increasing this value would result in an upscaling of the environmental impact of steel sheet piles and consequently even greater discrepancies in outcomes for both solutions. To test the effect of this assumption on the comparison results, a smaller thickness t50 of steel sheets was assumed: 3 mm (the minimum thickness of cold rolled steel profiles available on the market (InfraRentals 2019; ArcelorMittal 2019a). Including the corrosion loss allowances according to Eq. 1, the initial thicknesses of steel sheet piles, tinit, were:
for the least aggressive environment tinit = 4.2 mm,
for the most aggressive environment tinit = 10.5 mm.
The products with corresponding parameters, taken for further analyses, were steel sheet piles of the following types:
ArcelorMittal PAU 2240 6 mm (42.3 kg/m2),
Larssen 23 11.5/10 mm (155.0 kg/m2) (InfraRentals 2019). Thus, the mass per functional unit is:
21.15 t—for the least aggressive environment,
77.5 t—for the most aggressive environment.
The obtained normalization indicators for the compared types of sheet piles are depicted in Fig. 5. For comparison, the same graph presents the values obtained for baseline scenario (solid bars). The results show that despite the assumed minimum thickness of steel sheet piles, the environmental impact of PVC products is distinctly lower compared to the impact of steel products. In case of steel piles assumed for the least aggressive environment, the differences are not that great as in baseline scenario. For human health and ecosystems damage categories, the indicators determined for PAU 2240 piles are 2.5–2.7 times higher compared to vinyl alternative, but in case of resources endpoint the relative difference equals only 32%. Despite lower differences, the general conclusion remains the same as in the “Baseline scenario” section—steel sheet piles indicate significantly higher environmental impact than PVC sheet piles.
Fig. 5Normalization indicators assuming 3 mm thickness of steel sheet piles after 50 years of use
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According to the discussed outcomes, the rate of corrosion has a significant impact on the environmental impact indicators. The results determined for the most aggressive environment, for which the corrosion rate is the highest, substantially exceed the values obtained for the less aggressive conditions. The indicators concerning sheet piles Larssen 43 and Larssen 23 are 2.4–3.7 higher than those for sheet piles GU 6N and PAU 2240. The assumption of a higher corrosion rate would increase the environmental impact of steel products leading to greater discrepancies between the environmental performance of steel and PVC products. Given this conclusion, the opposite change of the assumption, namely, lower corrosion rate was analyzed in this section. In an extreme case, it could even be assumed that there is no corrosion, but provided the use of the secondary protection technique. In case of not very aggressive environment, like submerged zones, for instance, a corrosion prevention measure such as protective coatings could be sufficiently effective and allow to avoid the application of thickness loss allowances (Ferguson and Trewern 2015). In this regard, the use of 3-mm-thick steel sheet pile (PAL 3030) with coatings was assumed as a material for waterfront construction. As the unit mass of the product is 29.4 kg/m2 (InfraRentals 2019), the total mass related to the functional unit equals 14.7 t. Figure 6 depicts the comparison of normalization indicators for steel sheet piles coated with zinc or powder and PVC sheet piles. The results show that the vinyl product remains the best choice from the environmental perspective; however, the differences between the values obtained for different sheet piles are markedly lower compared to the data presented above. For human health and ecosystems categories, the indicators determined for steel products are 1.9–2.1 times higher compared to vinyl alternative, but in case of resources endpoint the results are almost the same (the relative difference equals only 0.8–2.0%). Excluding the impact of coating processes, the indicator determined for steel piles would be even lower than the value for PVC piles, which is due to a difference within a fossil depletion impact.
Fig. 6Normalization indicators for steel sheet piles with coatings and PVC sheet piles
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The baseline scenario comprised a few assumptions concerning transport processes: transportation distance from sheet piles manufacturer to the place of waterfront construction (500 km) and means of transport (lorries complying with the emission standard of EURO 4 with a total permissible mass exceeding 32 tons).
In the comment to Fig. 1, it was noticed that the assumed transport distance had an insignificant impact on the final results. If steel sheet piles were produced at the place of their application, the overall environmental impact would be reduced by only a few percent. The same conclusion could also be drawn for vinyl sheet piles. The opposite scenario considering longer transportation distance was also analyzed. Figure 7 depicts normalization indicators for the compared types of sheet piles assuming four times greater transport distance than in the baseline scenario (2000 km). The bars indicating steel piles are noticeably higher than the bars denoting PVC products. The relative differences are very similar compared to baseline scenario (compare Fig. 3). Depending on the endpoint, the indicators determined for vinyl sheet piles are from 2.3 to 4.4 times lower than those of GU 6N and from 5.4 to 10.4 times lower than values obtained for Larssen 43. The share of environmental impact resulting from transport does not exceed 10%. The analysis results allow further generalization of conclusions: in the scale of distance corresponding to European region, the environmental impact of transport is insignificant in comparison with the impact of sheet piles production processes. Thus, the assumed transport distances for raw materials, products and waste do not have a crucial impact on the scale of the environmental impact of the entire life cycle.
Fig. 7Normalization indicators assuming transportation distance of 2000 km
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In order to test the assumption concerning emission standard of transportation vehicles, additional analyses were carried out assuming better and worse standards: EURO 5 and EURO 3. The determined endpoint indicators are presented in Table 3.
Table 3 Normalization indicators obtained for various emission standardsFull size table
The obtained results indicate that the total environmental impact of the life cycle of the sheet piles is almost independent of the assumed emission standard. Relative differences of results range between 0.2 and 0.7% depending on the damage category. The greatest differences (obtained for human health endpoint) are illustrated in Fig. 8.
Fig. 8Human health indicators obtained for different emission standards of transport vehicles
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