Life cycle assessment of SBS modified bitumen waterproofing membrane

Building waterproofing membranes constitute essential components of construction materials. This study utilizes the life cycle assessment (LCA) methodology to develop a comprehensive life cycle inventory and conduct an environmental impact assessment of typical styrene-butadiene-styrene (SBS) modified bitumen waterproofing membrane products. The main research objective of this paper is to identify the key factors influencing the environmental impact of products and to provide data for the selection of green materials in the construction industry. The results reveal that the production of raw materials for SBS-modified bitumen waterproofing membranes contributes the most to all environmental impact categories, followed by the transportation and energy production stages, while the production stage of the membranes exhibits a relatively smaller contribution. Within the raw material production stage, the bitumen production process exhibits the highest contribution to fossil fuel depletion (FFP) environmental impact, surpassing 50%. The production process of polyester tire cloth demonstrates the greatest influence on human toxicity potential (HTPc), global warming potential (GWP), freshwater eutrophication (FEP), marine eutrophication potential (MEP), and mineral resource scarcity potential (SOP), accounting for 54.63%, 32.86%, 68.55%, 72.41%, and 61.69%, respectively. Additionally, the production process of base oil exhibits the most significant contribution to fine particulate matter formation (PMFP) and terrestrial acidification potential (AP) among the environmental impact indicators. Variations in the quantities of polyester tire cloth and base oil exhibit the most notable influence on the environmental indicators of the product. Specifically, the sensitivity analysis reveals that polyester tire cloth has the greatest impact on mineral resource scarcity potential (SOP); base oil exhibits the highest sensitivity to fossil fuel depletion (FFP).


1
Introduction Waterproofing materials could waterproof or enhance resistance to leakage in building construction by forming an overall waterproof layer on the surface of a building, and the quality and application of it directly relate to the structural effect and lifespan of the building project.The building waterproofing industry has developed rapidly with the constant development of the construction industry in China [1] .In recent years, driven by the goals of carbon peaking and carbon neutrality in China, green and low-carbon construction has become a prevailing trend for future development.Statistics indicate that in 2020, the nationwide total carbon emissions from the entire lifecycle of buildings amounted to 5.08 billion tons of CO2, accounting for 50.9% of the country's total carbon emissions.Within these emissions, the production phase of building materials contributed 2.82 billion tons of CO2, representing 28.2% of the total carbon emissions [2] .Consequently, green building materials play a pivotal role in supporting the transformation towards sustainable construction practices in China.In the waterproofing materials industry, further development is required, focusing on the exploration of novel materials or technologies that prioritize green and low-carbon characteristics.This endeavor will provide essential support for the development of green buildings, the improvement of construction quality, and the achievement of low-carbon transformation.
Within the realm of waterproofing materials in construction, styrene-butadiene-styrene (SBS) modified bitumen waterproofing membranes exhibit excellent properties such as high tensile strength, aging resistance, water repellency, and tolerance to extreme temperatures.As a result, they are extensively used in waterproofing applications for industrial, residential, and municipal construction projects in China [3] .Currently, the product structure of waterproofing membranes in China is dominated by three types: SBS/APP modified bitumen waterproofing membranes, synthetic polymer waterproofing membranes, and self-adhesive polymer-modified bitumen waterproofing membranes.Among them, SBS/APP modified bitumen waterproofing membranes hold the largest market share, accounting for 37.09% [3] .In recent years, environmental impact assessment of building materials has become a research focus, with a predominant use of life cycle assessment (LCA) methods.LCA is a systematic compilation and evaluation of inputs, outputs, and potential environmental impacts throughout the life cycle of a product system.Conducting LCA for materials can assist companies in adopting a holistic approach to design green materials, going beyond individual processes.It enables quantitative analysis of the overall environmental performance of material production processes and supports technological improvements and green material selection [4] .However, the current research on environmental load assessment of building materials based on LCA is primarily concentrated on bulk construction materials such as ready-mixed concrete, cement, and steel reinforcement.Ignacia Zabalza and other researchers [5] primarily focused on assessing the environmental performance of building materials through a comprehensive evaluation using the life cycle assessment (LCA) methodology.Their evaluations took into consideration various factors, including environmental impacts, energy consumption, improvements in ecological benefits, and the potential for sustainability.Gong et al. [6] developed an LCA-based model to conduct a comprehensive evaluation of the life cycle environmental impacts of cement, steel, and glass in the construction industry.Alessandro et al. [7] and Som S et al. [8] separately employed the LCA methodology to assess the environmental impacts of non-load-bearing concrete block walls and building insulation products.Bojana et al. [9] utilized the LCA methodology to analyze the environmental impacts of building materials throughout their entire life cycle in a specific construction project in Sweden.The findings of Bojana's study revealed that concrete floor slabs had the greatest contribution to CO2 emissions, while wood structures and cellulose insulation materials exhibited comparatively lower environmental impacts.Sungwoo Lee et al. [10] developed a green template as a specific environmental impact assessment tool using the life cycle assessment (LCA) methodology and conducted a case study to test its applicability.Proske T [11] and Flower et al. [12] have employed environmental performance evaluations to identify the ecological advantages of fly ash and slag concrete.These findings have also been validated by Zhang Yurong et al. [13][14][15] .Regarding waterproofing materials, there have been limited but noteworthy investigations conducted by both domestic and international scholars in this domain.Miriana Gonçalves [16] et al. have comparatively assessed the lifecycle environmental and economic impacts of various waterproofing solutions for flat roofs.The outcomes indicate that bitumen membranes emerge as the optimal choice, while synthetic EPDM membranes exhibit the highest value among different roof types.Levent [17] et al. have conducted an environmental impact analysis of waterproofing applications in buildings, specifically examining the influences on the environment resulting from energy and resource consumption during material production and construction processes.Utilizing a lifecycle assessment methodology, they have investigated the environmental impacts of commonly employed waterproofing systems in Turkey, encompassing traditional and inverted flat roof systems.The results demonstrate that mechanically fixed PVC waterproofing systems in traditional flat roofs exert the highest cumulative environmental impact during both production and construction processes, whereas bitumen-based liquid waterproofing systems employed in inverted flat roofs exhibit relatively lower environmental impacts.
This research endeavors to investigate the environmental implications associated with typical waterproofing membrane products within the waterproofing materials industry in China.It involves the collection of environmental burden data pertaining to the production of SBS-modified bitumen waterproofing membranes and their upstream supply chains.Subsequently, a comprehensive life cycle inventory is established, followed by an in-depth assessment of the environmental impacts.This study lies in its ability to quantitatively analyze the holistic environmental benefits throughout the lifecycle of waterproofing materials, employing a multi-dimensional approach.This analysis provides a foundation for informing both technical enhancements in waterproofing material production and the adoption of eco-friendly material selection strategies.

2
Method and Materials

Functional unit and system boundary
The functional unit was selected as the production of 1 m 2 SBS modified bitumen waterproofing membrane in this study.The system boundary was delineated to encompass the "cradle-to-gate" life cycle stages, including raw material production stage， energy production stage and materialization stage.Raw material production stage includes the production stage of raw materials such as bitumen, SBS, and base oil, as well as their transportation stage; energy production stage includes electricity production, natural gas production and diesel production; materialization stage includes the manufacturing process of SBS-modified bitumen waterproofing membrane and in-plant transport.However, the use and end-of-life stages of the product were excluded from the system boundary.A schematic representation of the system boundary is illustrated in figure 1.

Data source
The data for the manufacturing process of SBS-modified bitumen waterproofing membrane were sourced from a comprehensive field survey conducted at a representative enterprise in the waterproofing materials industry.This survey encompassed the collection of consumption data for raw materials (including bitumen, base oil, polyester reinforcement fabric, SBS modifiers, talc, etc.), energy consumption data (such as natural gas, diesel, and electricity), direct emission data for pollutants, and transportation data (including transportation modes and distances).Notably, the emissions of NOx, SO2, and PM were directly measured using the company's online monitoring system, while other emission data were estimated through the application of emission factor methodologies [18] .The data related to the transportation of raw materials (including transportation modes and distances) were collected from the enterprises.Rigorous validation and consolidation of the survey data facilitated the development of a comprehensive input-output inventory for the production of 1 m 2 of SBS-modified bitumen waterproofing membrane, which is presented in Table 1.Background data refers to the resource consumption, energy production, and pollutant emissions data associated with the processes of raw material acquisition, transportation, and energy production that are beyond the operational boundaries of the enterprise but have direct relevance to the production of the studied product.In this study, background data for energy production, such as electricity and natural gas, as well as fuels, were obtained from the Sino-center database.Additionally, background data for raw material production were sourced from reputable databases such as the European Ecoinvent database and ELCD database.These databases provide comprehensive and scientifically reliable information, enabling a comprehensive understanding of the environmental impacts associated with the background processes of resource extraction, energy generation, and material production.

LCIA Methodology
This study constructed environmental impact assessment based on the ReCiPe 2016 Midpoint (H) methodological system.The selection of environmental impact categories includes human toxicity potential (HTPc), fossil fuel depletion (FFP), freshwater eutrophication (FEP), marine eutrophication potential (MEP), terrestrial acidification potential (AP), global warming potential (GWP), fine particulate matter formation (PMFP) and mineral resource scarcity potential (SOP), and the correspondence between the environmental impact categories and the substances in inventory was illustrated in Table 2.
Table 2. Environmental impact indicators of SBS modified bitumen waterproofing membrane.

Sensitivity analysis method
Sensitivity analysis is a quantitative method employed to assess the extent to which model input variables influence the output results [19] .Adopting a sensitivity analysis perspective, this study explores the impact of variable variations on the output results, by manipulating the input quantities of various sources of uncertainty, thus observing the fluctuations in the target variable and identifying the sensitive factors.Furthermore, this analysis enables the identification of the most effective improvement areas within the life cycle of SBS-modified bitumen waterproofing membranes, in terms of environmental impact.The outcomes of sensitivity analysis aid in the identification of crucial factors, offering guidance for environmental management and decision-making, and facilitating optimal management and mitigation of environmental impacts.
The hypothetical model used to analyze Sensitivity in this study was shown in follow.
Formulas ： S --Sensitivity coefficient, which indicates the degree of variation of the environmental impact indicator with uncertain sensitivity factor x； --Environmental impact indicators , such as global warming potential (GWP)； --Uncertain sensitivity factor, such as the dosage of the bitumen; ∆ --Indicates changes in environmental impact indicators; ∆ --Denotes the change in the uncertainty sensitivity factor x parameter.

3
Results and discussion

Contribution analysis of processes
Based on the collected data and employing the SimaPro software, a life cycle assessment was conducted to evaluate the environmental impacts of SBS-modified bitumen waterproofing membrane.
The assessment encompassed the determination of the contribution potentials of the various life cycle stages to eight environmental impact categories, as depicted in figure 2.
The findings reveal that the raw material production stage exerts the greatest influence across all environmental impact categories.It demonstrates the highest contribution potential among all impact types, followed by the transportation stage and the energy production stage, while the production stage of the waterproofing membrane exhibits the lowest contribution potential.This outcome can be attributed to the extensive utilization of petroleum-based chemical constituents, such as bitumen, base oil, and SBS modifiers, which possess significant implications for diverse environmental impact categories.Consequently, the raw material production stage holds the highest potential for environmental impact contributions.In contrast, the production process of SBS-modified bitumen waterproofing membrane involves distinct operations including material selection, formulation, solubilization, impregnation, composite molding, and rolling.It lacks intricate chemical transformations or high-temperature procedures, thereby resulting in a relatively lower contribution potential during the production stage in terms of environmental impacts.
Furthermore, the significance of primary fuel transportation in environmental impact contributions should not be overlooked.The combustion of fuels during transportation imparts substantial effects on environmental impact categories such as fossil fuel depletion (FFP), terrestrial acidification potential (AP) and global warming potential (GWP).Hence, companies should comprehensively consider the transportation distances associated with primary raw material procurement to mitigate costs and the corresponding environmental impacts.

Contribution analysis of raw materials
Due to the significant contribution potential of the raw material production stage across all environmental impact categories, a detailed analysis was conducted to assess the contribution potentials of individual materials within this stage to various environmental impact indicators.The results, presented in figure 3, demonstrate that bitumen production has the highest contribution to the environmental impact category of fossil fuel depletion (FFP), accounting for 51.6% of its life cycle.This can be attributed to the fact that bitumen is predominantly derived from petroleum-based chemical products, resulting in the consumption of crude oil and other petroleum-related raw materials during its production process.
In the case of polyester reinforcement fabric production, it exhibits the highest contribution potentials to multiple environmental impact categories, including human toxicity potential (HTPc), global warming potential (GWP), freshwater eutrophication (FEP), marine eutrophication potential (MEP), and mineral resource scarcity potential (SOP).The contributions are significant, accounting for 54.63%, 32.86%, 68.55%, 72.41%, and 61.69% of their respective life cycles.These high contributions primarily arise from the production processes involved in manufacturing long and short-fiber polyester reinforcement fabric.
Moreover, base oil production demonstrates the highest contribution potentials to the environmental impact categories of fine particulate matter formation (PMFP) and terrestrial acidification potential (AP), accounting for 30.04% and 27.03%, respectively.These impacts stem from the emissions of particulate matter and nitrogen and sulfur compounds during the base oil production process.

Sensitivity analysis
Based on the analysis of the life cycle results mentioned above, it is evident that the environmental impacts of SBS-modified bitumen waterproofing membrane primarily stem from the raw material production stage.Therefore, a sensitivity analysis was conducted on the raw material production stage to investigate the effects of variations in energy and material inputs on the environmental burden during the production stage of the modified bitumen waterproofing membrane.Five raw materials with significant usage, namely bitumen, base oil, polyester reinforcement fabric, SBS modifier, and talc, were selected as uncertain sensitivity factors for the raw material production stage.Electricity and natural gas were chosen as uncertain sensitivity factors for the energy production process.By varying these uncertain sensitivity factors by ±5%, the sensitivity of the environmental burden in the production stage of SBS-modified bitumen waterproofing membrane to these factors was calculated.The results are presented in Table 3.
Table 3. Sensitivity analysis of key parameters.Based on the findings presented in Table 3, it is evident that the consumption of raw materials and energy in SBS-modified bitumen waterproofing membranes exhibits a certain level of sensitivity to various environmental impact indicators.Notably, with regards to the impact categories of global warming potential (GWP), fine particulate matter formation (PMFP), and terrestrial acidification potential (AP), the usage of polyester tire cloth and base oil demonstrates the highest sensitivity.Specifically, the sensitivity analysis reveals that polyester tire cloth and talc have the greatest impact on mineral resource scarcity potential (SOP), accounting for 62.29% and 30.44%, respectively.Moreover, bitumen and base oil exhibit the highest sensitivity to fossil fuel depletion (FFP), with sensitivity values of 47.59% and 23.44%, respectively.In light of these findings, it can be concluded that variations in the quantities of polyester tire cloth and base oil have the most significant influence on various environmental impact indicators.Therefore, for companies operating in this domain, optimizing the composition of raw materials, reducing the usage of polyester tire cloth and base oil, and adopting environmentally friendly alternatives with lower environmental impacts based on resource availability represent effective strategies for reducing the overall environmental footprint throughout the production process of SBS-modified bitumen waterproofing membranes.

Conclusion
This study aims to examine the environmental impact of typical SBS-modified bitumen waterproofing membranes.It involves the development of a comprehensive product life cycle inventory and the execution of an environmental impact assessment.Additionally, key factors influencing the environmental burden of the product are explored through contribution analysis and sensitivity analysis.The findings of this study can be summarized as follows: (1) The raw material production process exhibits a dominant role across various environmental impact indicators, contributing the most among all environmental impact categories.Subsequently, the transportation stage and energy production stage make substantial contributions, whereas the production stage of the waterproofing membranes exhibits the smallest impact.
(2) Among the various raw materials utilized in SBS-modified bitumen waterproofing membranes, the production of bitumen accounts for more than 50% of the environmental impact associated with fossil fuel depletion (FFP).The production of polyester tire cloth demonstrates the highest contribution to human toxicity potential (HTPc), global warming potential (GWP), freshwater eutrophication (FEP), marine eutrophication potential (MEP), and mineral resource scarcity potential (SOP).Furthermore, the production of base oil has the most significant impact on fine particulate matter formation (PMFP) and terrestrial acidification potential (AP).
(3) Within realm of SBS-modified bitumen waterproofing membranes, variations in the quantities of polyester tire cloth and base oil exhibit the most notable influence on the environmental indicators of the product.
Consequently, these conclusions underscore the significance of adjusting the composition of raw materials and reducing the consumption of polyester tire cloth and base oil.Alternatively, selecting environmentally friendly alternatives with reduced environmental impacts can be pursued to alleviate the environmental burden associated with SBS-modified bitumen waterproofing membranes.Furthermore, other measures such as employing renewable energy sources, augmenting the proportion of renewable energy within the energy mix, adopting thermal energy cascade utilization techniques, and implementing waste heat recovery technologies can also contribute to mitigating the environmental burden throughout the entire life cycle of the product.

Figure 2 .
Figure 2. Environmental impacts of each unit process.

Figure 3 .
Figure 3. Environmental impacts from raw materials production.

Table 1 .
Inventory of SBS modified bitumen waterproofing membrane production.