According to the data provided by the European Environment Agency (EEA), noise pollution is a major environmental health problem in Europe [1
] that is causing issues like sleep disorders leading to period of unwanted awakening [2
], learning impairments [3
], hypertension ischemic heart disease [7
], and especially annoyance [10
]. Road traffic is the dominant noise source in urban and suburban areas. According to the Environmental Noise Directive (END) 2002/49/EC, EU Member States are required to determine exposure to environmental noise from major transport and industry sources by means of strategic noise maps [12
]. Strategic noise maps are the basis for the preparation, adoption, and publication of action plans for the prevention and reduction of harmful noise exposure, and the specific measures included in the action plans are decided at the Member State level [12
In the last few years, noise action plans have aimed at raising awareness of noise as an environmental problem and promoting the use of more environmentally friendly modes of transport (electric vehicles, vehicles with quieter engines, and low-noise tires [14
]). Moreover, new mitigation systems for road traffic (the main source of noise) were proposed, and several projects have been devoted to producing extended real-time noise measurements to obtain a realistic picture of the noise distribution over urban areas [15
]. Still, road traffic noise remains a significant environmental problem: Around 100 million people in EU Member States are exposed to road traffic noise levels above 55 dB(A) Lden, while 32 million are exposed to very high noise levels above 65 dB(A) Lden [1
]. Lden is defined as “a descriptor of noise level based on energy equivalent noise level (Leq) over a whole day with a penalty of 10 dB(A) for night-time noise (23.00–7.00) and an additional penalty of 5 dB(A) for evening noise (19.00–23.00)” [16
Road traffic noise is mainly produced by road–tire interactions [17
], and the most important parameters affecting noise emission are the tire model [18
], pavement age [19
], and pavement texture [21
] and mixture [24
]. Because of the abovementioned factors, a common solution for road traffic noise abatement used both in urban and suburban environments is targeting noise sources through road traffic management, e.g., by replacing road surfaces and introducing low-noise road surfaces, improving the traffic flow, and lowering speed limits. In urban agglomerations, this measure is followed by measures related to land use and urban planning. The second most commonly used measure applied to major roads (“a regional, national or international road, designated by the Member State, which has more than three million vehicle passages a year” [12
]) located outside residential areas is changing the noise propagation paths through the construction of noise barriers between the noise source and the receiver [14
]. The term “noise barrier” can be used to encompass every type of structure used to reduce noise, including earth mounds, noise walls, and their combinations [26
]. New types of noise barriers—sonic crystal noise barriers—were also developed recently [27
]. All these structures differ in terms of their construction elements. An earth mound features a berm at the top and sloping sides, noise walls are constructed from horizontally stacked panels [26
], while sonic crystal noise barriers are non-homogeneous structures created from the arrangement of scatterers in a periodic configuration with square, rectangular, or triangular patterns [27
The application of road traffic noise barriers began more than 50 years ago in both the USA and Europe [30
]. The noise wall exploitation behavior, repair, and/or replacement frequency of aged or deteriorated wall panels became an important issue in the last decade. However, despite the long-term experience in the application of noise walls (and research on the sustainability of noise barriers as well as other noise abatement measures [32
]), when deciding on the panel material to be used in the design phase, designers still encounter numerous uncertainties associated with the exploitation behavior of noise walls constructed with panels made from different materials [37
], including their stability, durability, and resistance to fire, impacts, and atmospheric influences. The main question is how the imminent degradation of panels will affect the efficiency of the wall structure, its life-cycle costs, and its long-term sustainability in specific locations and conditions. There is a wide range of materials available for the construction of panels (wood, woodcrete, concrete, glass/glasscrete, stone/brick, aluminum/steel, acrylic, etc.), and all panels can be systematized into four basic types: concrete, metal, wood, and transparent. The uncertainty in panel service life quality is almost equal between the panels built from established materials and the panels built from new materials, which are now developing at an ever-faster pace due to the desire to increase the sustainability of noise walls [37
The choice of panel material is influenced by several factors, including the noise wall dimensions, location and local environmental conditions, aesthetic requirements (including local architectural considerations, public perception, and acceptance of the structure), and price [38
]. According to [39
], different approaches are taken when choosing the panel material within various countries in the EU. In northern EU countries, the landscape approach is the most prominent, in central EU countries, the technical approach (functionality and durability of the wall) is employed first, followed by the architectural approach, and, in southern EU countries, a cost-wise approach (lowest price criterion) dominates when choosing a panel material [39
The abovementioned distribution of the approaches among EU Member States is not surprising since new southern Member States have yet to fully develop and implement their road traffic noise pollution abatement measures, including the construction of noise barriers. For instance, strategic noise mapping conducted on the Croatian highway network showed that more than 520 km2
of sensitive areas are exposed to road traffic noise levels above 55 dB(A) Lden [40
] along 1300 km of the network [41
]. Developed action plans consider addressing the issue of high levels of road traffic noise primarily by constructing noise walls [42
]. The key issue here is that noise walls can use as many resources and have as much of an impact on the built environment as other large structures, even though they are still broadly considered road equipment. The price, i.e., the expenditure of public funds, is usually a major deciding factor for the scale of a typical noise wall project. The average cost of approximately 120 €/m2
for the cheapest option—a wall with timber panels—installed on both sides of the carriageway with a total length of 4 km and an average height of 4 m amounts to a total resource cost of nearly 2 million € [43
]. Furthermore, the choice of panel material plays an important role in the overall sustainability of noise walls. The sustainability of a noise wall is broadly defined as the optimal consideration of technical, environmental, economic, and social factors during the design and construction, maintenance and repair, and removal/demolition stages of noise wall projects [43
]. Therefore, in the design phase, to control the costs of the construction, maintenance, and removal of noise walls, it is necessary to obtain all relevant information on the characteristics of the panels based on which the satisfactory and rational selection of their materials can be made [37
The aim of the research presented in this paper was to reduce uncertainty in the selection of panel materials during the noise wall design phase and to support the process of road traffic noise protection management in Southern European countries by shifting the emphasis in decision making from the panel’s initial price (price of acquisition and installation) to the long-term sustainability and safety of the entire road traffic noise protection project. Section 2
provides a systematic overview of the concepts and EU regulations related to noise reducing devices and the required panel characteristics. By reviewing the publicly available literature and databases, the characteristics of concrete, metal, and wood panels were identified and systematized as follows: the share of panels used on infrastructure, common panel composition, acoustic performance, mechanical resistance and stability, safety requirements, procedures and installation costs, service life expectancy and durability, lifecycle costs, cradle-to-gate sustainability, and recyclability. The observed trends in the choice of panel materials during the last 50 years facilitated a more detailed review of lightweight concrete panels made with expanded clay, plant biomass, and recycled tire rubber aggregates. Section 3
presents the results of a meta-analysis on concrete, metal, and wood panels conducted by comparing the reported panels’ acoustic and non-acoustic characteristics and economic and environmental sustainability features. The scores used in the multi-criteria analysis and the results of the performed evaluation are then presented. To fill a knowledge gap observed in the literature, we also provide the input data and results of the cradle-to-grave approach in a comparative lifecycle assessment of lightweight concrete panels with expanded clay and rubber granules. Section 4
discusses the results and interprets them from the perspectives of previous studies and the working hypotheses. Future research directions are also highlighted. Section 5
concludes the paper.
3. Data Analysis and Evaluation
The review of the available research results and technical information on the acoustic and non-acoustic characteristics, long-term performance, and technical and economic sustainability of noise walls (which is described in Section 2
) resulted in a collection of data with highly dispersed values. This data were then systematized for further meta-analysis. The minimal reported values of the acoustic and non-acoustic characteristics, long-term performance, and technical and economic sustainability of the panels were identified.
The minimal values of the acoustic characteristics for the analyzed types of panels are reported in [51
]. It should be noted that the acoustic performance of the panels strongly depends not only on their materials but also on the thickness of their absorbing layer, their texture, and the cross-sections of their surfaces [89
] (flat, trapezoid, or undulating (low or high waves)).
The different mechanical and stability characteristics of concrete, metal, and wood noise walls limit the possibilities of their application, affect their construction procedures, and, consequently, impact their costs. The minimal values of non-acoustic characteristics, technical characteristics, and reported average procurement and overall average construction costs in €/m2
for each considered panel type constructed in Europe are reported in [44
The minimal expected service lives of panels made of different materials, their maintenance and replacement, and their lifecycle costs are reported in [44
]. Reported cost data correspond well with previous research performed in [37
], where it was concluded that the two most important variables in determining lifecycle costs for noise walls are the initial construction cost and service life and the costs of maintenance activities remaining small compared to the cost of noise wall construction and replacement.
Metal aluminum and steel panels, as well as wood timber and willow panels, were considered separately in a further analysis because the review showed that there were significant differences between their minimal service life and (cradle-to-gate) carbon and water footprints reported in [44
The identified minimal values of the specific noise wall performance measurements for each analyzed panel type are presented in Table 2
For the multicriterial evaluation of concrete, metal, and wood panels, the limit values of each performance criterion that define the score from 1 to 5 were chosen (Table 3
). The limit value choice was based on the prescribed and/or desired performance identified during the literature review. Each panel type was assigned a score based on the collected data for minimally achievable performance given in Table 2
and the defined ranges given in Table 3
. The defined scores from 1 to 5 are presented in Table 4
. Figure 2
shows the calculated average score for each performance group and the overall average score for each panel type.
The conducted analysis showed that concrete panels have the highest average scores for every performance group besides cradle-to-gate sustainability, as well as the highest overall average score. The cradle-to-gate sustainability score of concrete panels is worse due to the large carbon footprint of their production. This result initiated a comprehensive cradle-to-grave assessment of concrete panels made with lightweight concrete using expanded clay and a recycled tire rubber aggregate. This analysis was performed to identify opportunities for improvement in lightweight concrete sustainability by quantifying the impacts that each product has on the environment throughout its full lifecycle, from production and manufacturing to the disposal phase. Since the CO2-eq emission factor for wood chips is considerably lower than that for the expanded clay and rubber aggregate, the lightweight concrete panel solution with a wood cement absorbing layer containing wood chips of various sizes from debarked wood was excluded from further comparisons.
Previous research conducted in [59
] showed that the carbon footprint of concrete blocks with expanded clay aggregate Liapor in a cradle-to-grave assessment amounts to 1000 kg CO2
-eq. At the same time, the carbon footprint of concrete blocks with tire rubber aggregates amounts to 200 kg CO2
-eq. The greenhouse gas emissions caused by the incineration of tire rubber were added for the concrete blocks using an expanded clay aggregate, which increased the aggregate’s carbon footprint by more than a factor of three.
A cradle-to-grave comparative lifecycle assessment of noise protection wall panels with expanded clay (Liadur) and rubber granules (RUCONBAR) was conducted on the RUCONBAR prototype optimization process. Panels that were analyzed had an average absorption layer thickness of 7 cm and identical structural layers. The performed analysis did not include fuel consumption and emissions for the heat production in manufacturing facility, nor did it include sanitary water consumption, fuel consumption for the inner transportation of lift trucks, or the production of machines involved in the production process. No allocation procedure or partitioning of energy supply between the other product systems at the location was considered, and the only considered impact from 20-year usage involved the wall’s transport to the location of installation (the chosen distance was 100 km, and the transport was conducted by a heavy truck of 24 t).
The analysis showed that the main contributions to the carbon footprint from the panels were due to their production processes (Table 5
), which included the acquisition and production of raw materials and their transport to the relevant location, while the maintenance and disposal processes had a significantly smaller impact. In this analysis, recycled rubber was not included in the inventory as an input material. Instead, it was considered as the final product for recycling end-of-life tires. Thus, only the energy consumption of the mechanical grinding process and transport to the production site of the noise protection wall was included. For the disposal of walls, the most probable scenario was assumed, where 90% of the construction waste produced in Croatia was still landfilled. For RUCONBAR, we used a disposal scenario where 90% of the absorption layer is recycled and reused for a new absorption layer.
Due to the significant proportion of cement in the supporting layer, for both wall types, a sensitivity analysis was conducted using different types of cement, with constant parameters used for additional materials. By using cement types such as blast furnace slag cement, containing 4% cement, 50% iron slag from blast furnaces, and 46% clinker, a significant reduction in the overall environmental impact was observed. For LIADUR, global warming potential (GWP) was reduced from 120 to 99 kg CO2-eq/m2, while for the RUCONBAR, the GWP decreased from 117 to 94 kg CO2-eq/m2.
The results of this environmental assessment showed a lower environmental load for the noise protection wall with recycled rubber. As both types of noise walls use the same structural layer, the absorption layer determined this outcome. The comparative analysis of this element as an independent unit is outlined in Table 6
. In this analysis, the environmental benefits from using recycled rubber, such as avoiding impacts from the disposal processes of old tires, were not considered.
Among the observed panels, concrete panels have the highest mechanical resistance, longest service life, lowest lifecycle costs, and lowest water footprint. They also address all the required safety characteristics very well, i.e., they provide good fire resistance (with practically no toxic gases or wind-borne ember emissions in the case of a fire), have no light reflection (glare that can disturb road users is a common problem for metal panels), and possess only a small risk of falling debris in the case of a vehicle impact (they do not shatter like metal or wood panels). Furthermore, analysis of the historical data on the application of different types of noise walls showed that the choice of materials used in the production of panels shifted completely from wood to concrete in recent decades. Consequently, further investigations were focused on the concrete panel characteristics, specifically for panels with sound absorptive layers made with commonly used lightweight aggregates (expanded clay and plant biomass) and recycled tire rubber aggregates.
Based on the available data, the carbon footprints of concrete blocks with expanded clay aggregate are five times higher than the carbon footprints of concrete blocks with tire rubber aggregates in the cradle-to-grave approach. At the same time, the cradle-to-grave approach in a comparative lifecycle assessment of panels with an average absorption layer thickness of 7 cm and an identical structural layer resulted in significantly smaller differences in carbon footprints. This was caused by the exclusion of the environmental benefits from using recycled rubber (avoiding impacts from the disposal processes of old tires) from the analysis.
The results of the performed analysis are based mainly on publicly available data on noise walls constructed in Northern and Central Europe and the United States and, as such, may not be specific enough to give detailed insights into South European practices and experiences. To provide better insight into the service life efficiency of noise walls and panels and to improve the process of noise wall design and management in South European countries, further data analysis of panel production, application, and disposal will be conducted while focusing on those countries. Moreover, to further investigate the potential benefits and limitations of lightweight concrete panel application, the influence of the shape and thickness of the sound absorptive layer on the acoustic properties of these panels, as well as the impact of climate change on their durability, should be addressed in detail.
Noise wall panels can be systematized based on the primary material used in their production considering four basic types: concrete, metal, wood, and transparent. The choice of panel material plays an important role in the overall sustainability of the noise wall, so it is necessary to have all relevant information on the characteristics of these materials and panels (as the final element of the noise wall) in the noise wall design phase. The research presented in this paper aimed to reduce uncertainty in the selection of panel materials and types and support the process of road noise management in southern European countries by shifting the emphasis in decision making from the price to the long-term sustainability of the entire road traffic noise protection project.
An overview and systematization of the data and a multicriterial evaluation of the characteristics of concrete, metal, and wood panels showed that concrete panels have the highest overall average score. They also have the highest average score for their acoustic properties, non-acoustic properties, and long-term performance. These panels have a lower score for cradle-to-gate sustainability because of their high carbon footprint. At the same time, metal panels have the lowest transportation embodied energy and are highly recyclable at end-of-life, while wood panels’ average construction costs and primary energy use are the lowest. Further analysis showed that the main contributions to the carbon footprint of the concrete panels are due to their production processes, while the maintenance and disposal processes have a significantly smaller impact. For foreign experiences in noise wall construction and maintenance, the current market, and the infrastructure share, it is safe to assume that when choosing the materials for panels, preference should be given to concrete. The best way to indirectly reduce concrete panels’ life-time carbon footprints in the design process is to use panels made with aggregates from secondary raw materials, such as recycled tire aggregates.