*1.1. The Needs of Current Pavement Maintenance Systems and Practices*

In today's global landscape, there are significant challenges for development being faced by countries of all sizes and types. One of the primary concerns is the condition of the roadway network as this is a key determinant for development. This is because it is considered the gateway to mobility and access for citizens, which in turn leads to economic and social benefits for the nation and its people [1]. This concern is further worsened by the continuing budget reductions for road authorities for pavement maintenance and rehabilitation programs [2]. These reductions result in authorities not having sufficient financial resources to maintain their networks in an optimal state.

There are attempts to utilize optimization systems such as the pavement management system (PMS), which is based on utilizing the available financial resources in the most efficient and valued manner based on the needs of the road network [3]. However, while the use of the PMS helps authorities to make optimized decisions, it is also highly dependent on the availability of data on the condition of the roads in the network. The acquisition of this road condition data can be quite costly, as the most accurate technologies to date largely involve expensive equipment and vehicles featuring elements such as laser profilers [4]. These systems in many cases require significant time and substantial training for the authorized personnel working in the road authorities. As a result of this, agencies quite often rely on the use of manual surveys to obtain this data [5] and, as a result, these data can be considered subjective and, in some cases, inaccurate. This leads to the development of poor maintenance strategies or, in many cases, a pre-set strategy with no capacity to adapt to real-time circumstances and challenges. To this end, it has been the goal of many authorities and agencies to find lower-cost solutions for monitoring road conditions for the purpose of building accurate and robust asset databases. *Infrastructures* **2020**, *5*, x FOR PEER REVIEW 2 of 20 condition of the roads in the network. The acquisition of this road condition data can be quite costly, as the most accurate technologies to date largely involve expensive equipment and vehicles featuring elements such as laser profilers [4]. These systems in many cases require significant time and substantial training for the authorized personnel working in the road authorities. As a result of this, agencies quite often rely on the use of manual surveys to obtain this data [5] and, as a result, these data can be considered subjective and, in some cases, inaccurate. This leads to the development of poor maintenance strategies or, in many cases, a pre-set strategy with no capacity to adapt to realtime circumstances and challenges. To this end, it has been the goal of many authorities and agencies

There are several different areas of research in this domain [6,7]. Generally, the two most researched areas of study of automated pavement distress collection systems are the ones based on systems utilizing lasers and imaging technologies [8]. There are advantages and disadvantages of both of these systems, with the laser-based systems generally being more accurate, but the imaged-based ones carrying a lower cost. However, with both systems, there is still a need for continuous physical surveys to be carried out on the road to inspect conditions. to find lower-cost solutions for monitoring road conditions for the purpose of building accurate and robust asset databases. There are several different areas of research in this domain [6,7]. Generally, the two most researched areas of study of automated pavement distress collection systems are the ones based on systems utilizing lasers and imaging technologies [8]. There are advantages and disadvantages of both of these systems, with the laser-based systems generally being more accurate, but the imaged-

Apart from the aforementioned technologies, there is significant research built around the use of in situ monitoring systems for the acquisition of accurate information concerning the conditions of the pavement. Such systems allow documenting the level of service of the road asset via the use of embedded sensors and technologies, which can allow for remote and continuous monitoring over the life cycle of the pavement for fatigue [9–13]. Such systems do not require frequent surveys and the state of the pavements can be monitored without any disruption to the traffic or road network, which is an advantage over the systems mentioned before. These embedded systems typically work by monitoring strains of the asphaltic layers, which can then be interpreted to help road agencies to discern information on the condition of the pavement. Accurate post-processing and analysis of data coming from the sensors are fundamental in order to define an adapted and cost-effective management plan. The information can be commonly utilized within a PMS for particular needs, as shown in Figure 1 below [14]. The full extent to which the information can be utilized within the PMS is not covered within this study, but the ways in which the data collected can be used for determining needs of the pavement and for planning interventions are the main focus of the work. Furthermore, the data obtained through the sensors can be considered under information quality level 4, which considers the structure and condition of the pavement for planning and performance evaluation. based ones carrying a lower cost. However, with both systems, there is still a need for continuous physical surveys to be carried out on the road to inspect conditions. Apart from the aforementioned technologies, there is significant research built around the use of in situ monitoring systems for the acquisition of accurate information concerning the conditions of the pavement. Such systems allow documenting the level of service of the road asset via the use of embedded sensors and technologies, which can allow for remote and continuous monitoring over the life cycle of the pavement for fatigue [9–13]. Such systems do not require frequent surveys and the state of the pavements can be monitored without any disruption to the traffic or road network, which is an advantage over the systems mentioned before. These embedded systems typically work by monitoring strains of the asphaltic layers, which can then be interpreted to help road agencies to discern information on the condition of the pavement. Accurate post-processing and analysis of data coming from the sensors are fundamental in order to define an adapted and cost-effective management plan. The information can be commonly utilized within a PMS for particular needs, as shown in Figure 1 below [14]. The full extent to which the information can be utilized within the PMS is not covered within this study, but the ways in which the data collected can be used for determining needs of the pavement and for planning interventions are the main focus of the work. Furthermore, the data obtained through the sensors can be considered under information quality level 4, which considers the structure and condition of the pavement for planning and performance evaluation.

**Figure 1.** Typical uses of pavement management information in a pavement management system (PMS). **Figure 1.** Typical uses of pavement management information in a pavement management system (PMS).

The use of the data can, therefore, extend the road service life and improve its safety. For the purpose of this paper, conventional strain gauges and piezoelectric sensors are considered for monitoring road conditions, and consequently triggering maintenance activities. An insight into their use is explored and the possibilities of their use in road condition monitoring are identified through an experimental case study.

### *1.2. Environmental Concerns about Employing New Detection Systems*

With the use of these embedded technologies, it is possible for early detection of pavement distresses and for preventative maintenance to be employed instead of the costlier corrective maintenance practices that would be needed once the pavement would have already failed [15–18]. In scenarios typical in small authorities, there is usually a pre-set maintenance plan for the life cycle of the pavement based on the experience of the area and the available funds. With the use of these types of maintenance plans, there is no customization based on the real conditions of the roads. Therefore, limited preventative maintenance is done to help lengthen the pavement life cycle and save money for the authority. They essentially operate on the 'worst-first' approach, wherein the pavements are allowed to reach their failure point without any preventative measures deployed [1]. As a result, the use of embedded sensors would be a welcome addition for the authority.

However, the use of these proactive sensors can result in more frequent maintenance interventions as more preventative interventions would be utilized based on the triggers of the sensors to delay the use of the more costly corrective maintenance interventions when the pavement has suffered both functional and structural failure. This result brings into question the environmental friendliness of using these approaches, as more frequent interventions can have more severe environmental impacts.

Transportation can be quite energy-intensive, and thus the associated environmental impacts can be adverse. In many cases, however, the construction, operation, and maintenance of the road pavements or road networks have been considered less significant in terms of environmental impacts, when compared with the potential environmental impacts by the vehicles utilizing the specific road or road network during its life cycle [19,20]. Given the need for more sustainable transportation infrastructures and asphalt pavements, it has lately become apparent that aspects such as the road construction and maintenance could lead to significantly increased amounts of energy consumed, and hence to higher amounts of emissions [19,21]. To further investigate, the aforementioned environmental implications the life cycle assessment methodology can be utilized as described in international standards [22,23].

This study, however, has as a main objective to compare the environmental impacts of three different alternatives, namely, three different maintenance pipelines, focusing only on the use phase of the asphalt road and specifically on its maintenance. Numerous studies have been conducted so far that assess the environmental impacts of asphalt pavements over their life cycle. For instance, Häkkinen & Mäkele assessed the environmental impacts of the pavement construction, maintenance, and traffic, followed by Chappat and Bilal, who also focused on the same aspects of the environmental assessment of a road [24,25]. Other researchers also included the environmental impacts arising owing to the construction of the necessary earthworks, surrounding a road pavement [19,26], while Hoang et al. only assessed the environmental impacts of the road construction and maintenance [27].

It thus becomes evident that the use of life cycle assessment for roads is strongly correlated with the objective of the study and can be implemented. In the specific investigation, as it has a comparative nature, the comparison is undertaken with the same pavement structure. However, with alternative maintenance strategies each time, the stage of pavement construction, the impacts of the earthworks' construction, the traffic impacts, and the end of life were omitted from the study. This is because of the fact that a comparative study would not benefit from the inclusion of identical aspects in all the alternatives. In other words, the omitted aspects would have no influence on the outcomes of the study.

### *1.3. Aim of the Study 1.3. Aim of the Study*

This paper carried out a life cycle assessment (LCA) case study to quantify the environmental impacts of the maintenance pipelines based on three different scenarios (technologies). The baseline scenario is where no sensors are embedded in the pavement structure, and thus a preset maintenance plan is followed; the second scenario utilizes piezo-electric sensors and the third utilizes conventional strain gauges, in order for optimized maintenance pipelines to be achieved. The LCA was carried out using results from the experimental test section, where the sensors were deployed in an accelerated pavement testing setup. Environmental impacts of devising maintenance plans based on intervention triggers from the sensors as opposed to a typical pre-set plan were compared and analyzed. This paper carried out a life cycle assessment (LCA) case study to quantify the environmental impacts of the maintenance pipelines based on three different scenarios (technologies). The baseline scenario is where no sensors are embedded in the pavement structure, and thus a preset maintenance plan is followed; the second scenario utilizes piezo-electric sensors and the third utilizes conventional strain gauges, in order for optimized maintenance pipelines to be achieved. The LCA was carried out using results from the experimental test section, where the sensors were deployed in an accelerated pavement testing setup. Environmental impacts of devising maintenance plans based on intervention triggers from the sensors as opposed to a typical pre-set plan were compared and analyzed.

*Infrastructures* **2020**, *5*, x FOR PEER REVIEW 4 of 20

### *1.4. Structure of the Study 1.4. Structure of the Study*

Before the LCA case study could be done, it was also very important to understand how the sensor results are read and interpreted, as this will establish the practicality of using them in real-world conditions. To this end, Section 2 describes the gauges and sensors utilized in the study and Section 3 explains the experimental setup of the study. The results of the sensors in the case study are then provided in Section 4, detailing how results are read and analyzed, whereas Section 5 deals with the formulation of the maintenance strategies. Finally, the results of the LCA are provided in Section 6, with further discussions being made on the results of the LCA and the use of the embedded sensors. Before the LCA case study could be done, it was also very important to understand how the sensor results are read and interpreted, as this will establish the practicality of using them in realworld conditions. To this end, Section 2 describes the gauges and sensors utilized in the study and Section 3 explains the experimental setup of the study. The results of the sensors in the case study are then provided in Section 4, detailing how results are read and analyzed, whereas Section 5 deals with the formulation of the maintenance strategies. Finally, the results of the LCA are provided in Section 6, with further discussions being made on the results of the LCA and the use of the embedded sensors.
