Investigations often focus their efforts on evaluating effects of strategies for reducing C&D waste but not the final destination of resources,
i.e., in the management of the use of these recycled materials [
12]. Thus, to complete a comprehensive model of management of C&D wastes, this section proposes a simulation model which focuses on the last stage of the process: the final destination of waste. Our work has focused on the recycled aggregates quarry as a case study. We look for economic policy instruments that stimulate the incorporation of these materials in the recycled market [
22]. In Spain, as in other countries with low percentages of recycled C&D wastes, it will be especially necessary to provide incentives for the use of these materials, where raw materials are abundant and very cheap.
4.1. Dynamic Modeling of the Problem
Having analyzed the situation to be modelled through a conceptual understanding, we developed a dynamic simulation model capable of showing the effects of different policies of incentives to the use of C&D waste on the flow of generation, recycling and reuse of C&D waste. Following the Systems Dynamics methodology and using Vensim DSS 6.1. (Ventana Systems UK Ltd.: Wiltshire, UK) software, the purpose of the model was to reproduce the current behaviour of the system [
29,
30] and, thereby, establish a useful support tool for executive decision-making processes [
31] in order to influence the recycling behaviour of the firms.
The causal analysis developed by [
8] pointed to the existence of dynamic relationships between the activity of the generation of waste from C&D and the use of such debris—previously recycled—in works of new construction. The demand for recycled materials depends on the assessment made by the companies as to the relative impact of the use of aggregates made from C&D waste as construction materials on the final cost of the construction work
versus the use of quarried aggregates. Following the strategy considered by the Spanish Government in the PNIR 2008–2015, we have designed a model which reflects the flow between the generation of waste from the C&D of civil works and the use of these waste materials as an alternative to natural quarried aggregates in new constructions, in terms of economic parameters and using system dynamics’ methodology [
29,
30]. The final objective of this model is to provide to the institutions involved in this area a better understanding of the problem of recycling in works of civil engineering. To do this, we analyzed how their decisions regarding the promotion of economic incentives or penalties in the use of C&D waste as building materials may affect the behavior of the construction companies, and, as a result, these decisions also affect the fulfilment of the recycling goal set out in the National Integrated Waste Management Plan (PNIR 2008–2015).
The generation of waste has been triggered by the demand for building materials. Pressure in civil engineering works and new building has led to the demolition of current works. Following this approach, the process of analyzing C&D waste management begins with the accumulation of untreated waste (disposal of untreated C&D waste) (
Figure 3). The accumulation of this waste will depend on the continuous inflow of waste from the demolition of buildings (generation of untreated C&D waste used in building), on the demolition of civil engineering works (generation of untreated C&D waste from civil engineering works), as well as on the outflow of materials treated in the recycling process. This process allows for the separation of materials unsuitable for future use, which will be sent to controlled disposal sites (shipments to disposal sites), and materials suitable for treatment and recovery as aggregates made from C&D waste (recycled C&D waste) (included in the concept of recycling is the percentage of recycled C&D waste as well as the percentage of C&D waste from other recovery operations). The difference between the generation of waste and the recycled volume makes it possible to assess progress towards the established recycling goal in each time period.
Figure 3.
Disposal of untreated C&D waste.
Figure 3.
Disposal of untreated C&D waste.
The accumulation of recycled C&D waste (storage of treated C&D waste) will depend on the inflow of recycled materials and on the outflow of these materials as aggregates, to become a part of the materials used in civil engineering works. The amount of inflow of C&D waste will also depend on the percentage of materials that can be transformed into aggregates versus those that are unsuitable for this purpose and are sent to the disposal facility. This distribution is contingent upon the quality of the recycling process (percentage of recycled material), whose percentage can be changed depending on the technology or infrastructure used. The improved quality of recycled materials is directly related to the sustainable construction goal and promotes the development of new technologies to improve efficiency in the procurement and use of recycled materials.
The storage of aggregates will be the result of the sum of the output of treated C&D waste and the output of natural quarried aggregates in each period of time (
Figure 4). The institutional policies should aim to balance the total aggregates in favor of the C&D waste instead of natural quarried aggregates. However, it requires a better understanding of the system behavior. Because the final behavior of the firms is an economic behavior, the outflow of C&D waste will depend on the internal and external economic incentives available during construction activity. The decision to substitute a specific proportion of natural quarried aggregates for aggregates made from C&D waste will depend on the relative cost incurred.
Figure 4.
Storage of aggregates.
Figure 4.
Storage of aggregates.
The model proposed here presents the analysis of a potential incentive
versus a penalties policy offered by the government. This policy consists of, on the one hand, compensating the companies for the cost difference that may arise between using material made with the mixture of C&D waste/natural aggregate and 100% natural aggregate (external incentives offered for using treated C&D waste) or, as an alternative, punishing the absence of use of C&D waste by increasing the initial cost of quarried aggregates with a specific tax (non-recycling tax) [
32]. The novelty of this approach is that it shows the effects of these policies in the EMS system and how this stakeholder’s behavior is influenced over time.
As the technical specifications of the mix C&D waste aggregate/quarried aggregate, the model presents a maximum distribution percentage of 30% (Weight C&D waste aggregate/Weight quarried aggregate + Weight C&D waste aggregate) × 100, which has been analyzed in previous studies examining the behavior of hot mix asphalts in the construction of roads with low to medium traffic volume [
33,
34]. The increase in this percentage which favours the use of C&D waste will be, in turn, directly linked to the sustainable construction goal of the government.
In order to understand the economic behavior of the firms in the model, if the material made with C&D waste and natural quarried aggregates has technical properties that are similar to the use of 100% natural aggregates, the construction companies will choose the option offering the lowest cost (difference in cost between C&D waste and quarried aggregates). Thus, meeting the goal of reusing C&D waste will be directly related to the differential cost between the two types of aggregates (
Table 3).
The cost of quarried aggregates is not considered to be constant; rather it depends on market demand. Increased demand will cause a rise in cost of this material in the following period (cost ratio quarried aggregates/aggregate demand). Given the lack of statistical data to provide information on the breakdown of the cost components of C&D waste, this aspect was examined using a scenario of T growth in the use of these materials.
Table 3.
Equation description of variables.
Table 3.
Equation description of variables.
Variable | Equation Description |
---|
Disposal of untreated C&D waste | ∫ (generation of untreated C&D waste used in building + generation of untreated C&D waste from civil engineering-shipments to disposal sites-recycled C&D waste) |
Storage of treated C&D waste | ∫ (recycled C&D waste-output of treated C&D waste) |
Storage of natural quarried aggregates | ∫ (generation of natural quarried aggregates-output of natural quarried aggregates) |
Storage of aggregates | ∫ (output of natural quarried aggregates + output of treated C&D waste-consumption of aggregates) |
Generation of untreated C&D waste used in building | (Total C&D waste consumption/ Total C&D waste generation)* (C&D waste consumption building/ Total C&D waste consumption) * demand of aggregates) |
Recycled C&D waste | % of recycled material* Disposal of untreated C&D waste |
Generation of untreated C&D waste from civil engineering | (Total C&D waste consumption/ Total C&D waste generation)* (C&D waste consumption civil engineering/ Total C&D waste consumption) *demand of aggregates) |
Shipments to disposal sites | (1% of recycled material)* Disposal of untreated C&D waste |
Difference between the recycling goal/Actual recycling | recycling goal-(recycled C&D waste/(recycled C&D waste + shipments to disposal sites)) |
Output of natural quarried aggregates | consumption of aggregates-output of treated C&D waste |
Difference in cost between C&D waste and quarried aggregates | medium cost of C&D waste/medium cost of natural quarried aggregates |
Difference between the goal of utilization /Reuse of treated C&D waste | goal of utilization-treated C&D waste used previous year |
Output of treated C&D waste | IF THEN ELSE (Storage of treated C&D waste > 0, IF THEN ELSE ((difference in cost between C&D waste and quarried aggregates\<1: OR: external incentives offered for using treated C&D waste>0),% of mix* consumption of aggregates, 0), 0) |
Consumption of aggregates | IF THEN ELSE (demand of aggregates > 0, demand of aggregates, 0) |
generation of natural quarried aggregates | output of natural quarried aggregates |
In this way, the model designed will allow for the assessment of both the fulfilment of the recycling goal (difference between the recycling goal/actual recycling) and the objective of using C&D waste (difference between the goal of utilization/reuse of treated C&D waste), set out in the National Integrated Waste Management Plan (2008–2015), according to institutional policies (for a detailed overview of the evolution of the C&D waste generated in Spain, see
Section 2).
In the model designed, we tested two policies: an institutional policy based on external incentives and the use of a fixed non-recycling tax as negative incentive for the use of quarried aggregates.
First, the policy based on external incentives relies on economically compensating the firms for the increased cost of using C&D waste. The final goal of this policy will be to change the recycling behavior of the firms until it is economically justifiable. Once the generation and management of C&D waste is high enough to make the investments in recycling and waste treatment plants viable, the relative cost of the waste material will be lower than the cost of natural aggregates. In the future, it will be possible to do away with the external incentive offered by the administration, thus consolidating the autonomous functioning of the model with no need for public intervention.
The second policy consists of requiring the firms to pay a fixed non-recycling tax if they use a 100% quarried aggregate mixture. In this sense, this policy would provide direct incomes to the administration, and also would discourage the use of quarried aggregates in economic terms, because it increases the medium cost of the materials.
4.2. Modeling Results
We considered a scenario of demand for aggregates to be used in construction with an initial growth combined with the subsequent period of stagnation (
Figure 5). This scenario reflects the historical behavior of these materials over the decade from 2001–2010.
Figure 5.
Demand of aggregates.
Figure 5.
Demand of aggregates.
Source: Own elaboration.
4.2.1. First Measure: Policy Based on External Incentives
The scenario of demand of aggregates affects the flow of waste from the time these materials are generated until they are reused through recycling. Considering a C&D waste cost as a constant, the administration will be obliged to apply external incentives while the cost of C&D waste is higher than the cost of natural aggregates, until the increased cost of the latter materials allows the cancellation of this policy due to lack of demand. In this case, the flow diagram will help firms and institutions understand the dynamic behavior of the recycling system. In this model, the application of economic incentives that compensate for the difference in cost between the use of C&D waste and quarried aggregates allows to achieve the proposed goals (
Figure 6). Eventually, firms change their behavior and increase the use of the treated C&D waste in the aggregate mixtures until reaching the rate technically permitted (30% of the total mix of aggregates).
Figure 6.
Output of treated C&D waste.
Figure 6.
Output of treated C&D waste.
Source: Own elaboration.
This policy would be aligned with the economic behavior of the firms, and it would allow exponential growth in consumption of C&D treated waste during the first year. Between year 2 and 12, the growth of the consumption will dampen. Finally, the use of C&D treated waste will remain at the percentage technically allowed (30% of the total mix of aggregates).
4.2.2. Second Measure: Use of a Fixed Non-Recycling Tax
In the absence of any economic incentive for recycling, the administration could try implementing a policy of economic penalties to help to achieve the goal of recycling C&D waste. We consider the same scenario of growing demand for aggregates for 12 years followed by a stagnation of demand, according to historical data (2001–2010).
In this case, instead of proposing economic incentives to reduce the cost differences between C&D waste and quarried aggregates, in what means an added cost for public institutions, the government may impose a fixed non-recycling tax to the use of quarried aggregates used in civil construction. According to the model proposed, this measure will affect companies’ recycling behavior only if the non-recycling tax increases the cost of natural aggregate over 10% (
Figure 7). We also have to take into account that often fiscal measures only have consequences one year later, so there will be a delay in the desired influence on companies.
Figure 7.
Output of treated C&D waste.
Figure 7.
Output of treated C&D waste.
In this case, the policy of penalties (non-recycling tax) will cause a continuous and slight increase in consumption of C&D treated waste from year 1–10. From year 10 onwards, the consumption will stabilize at the maximum percentage technically allowed (30%). Based on this measure, the results are shown with a one year delay since companies are not subjected to this penalty policy until the next period when they are affected by the measure and, therefore, change their behavior. This is the reason why in the graph the levels of tons of waste start from positive values instead of from zero, compared to the scenario of incentives shown in the previous subheading.
The scenarios of simulation we have proposed will serve as a supporting tool for a better understanding of the consequences of different policies that stimulate the use of recycled materials in civil construction. The final goal of this methodology is not to answer the question of which policy should be implemented by the administration, because there is no optimum in this complex reality. In this case, providing economic incentives to construction companies through economic compensation for the difference in cost will increase the percentage of C&D waste materials used by the companies in 12 years. Nevertheless, in the short term, the administration will repay the cost of this policy. On the contrary, the use of economic penalties (non-recycling tax) to help achieve the goal of recycling C&D waste might be considered a most profitable policy for the administration in the short term since the goal is achieved in less time—only 10 years. However, increasing the global cost of quarried aggregates will also boost the total cost of materials of the companies, reducing their competitive advantages in the global market. To sum up, the simulation tool will help us understand the consequences of potential decisions from a dynamic approach, but, ultimately, the public administration will be responsible for selecting the policy most suitable to the complexity of the situation.