The European Green Deal is an integral part of the European Commission’s strategy to implement the goals of the 2030 Agenda (i.e. reducing greenhouse gas emissions by 55 percent compared to the 1990 values and achieving climate neutrality by 2050).
To analyse the energy and environmental impacts of the building sector throughout its life cycle, it is necessary to refer to indicators of the sustainability of materials, construction systems, and buildings such as embodied energy (EE) and operational energy (OE).
The embodied energy is the energy required for the extraction and processing of raw materials, the transport and assembly of finished products on site, and for maintenance, renovation, demolition, and waste disposal at the end-of-life.
Operational energy, on the other hand, is the amount of energy required to operate the building during its lifetime. The embodied and operational energy rates correspond to consequent CO2e emission factors: embodied carbon (EC) and operational carbon (OC).
Therefore, designing an energy-efficient building in the operational phase is a necessary but not sufficient condition, considering the additional emission rates resulting from the use of resources in other phases of the life cycle.
The construction sector accounts for 37 percent of global carbon dioxide (CO2) emissions into the atmosphere; it accounts for 34 percent of all global energy consumption, 50 percent of raw material extraction, and consumes one third of drinking water.
It uses a considerable amount of raw materials and energy during the life cycle phases of the constructing process: production, construction, use, maintenance until demolition (i.e., a linear process ‘from Cradle to Grave’).
The desirable process to reduce resource consumption and mitigate environmental impact should be circular. The closing of the circle is realised with the re-introduction into the system through the recycling operations of products that would otherwise reach the end of their life, reducing waste as much as possible, in accordance with the ‘C2C—from Cradle to Cradle’ principle.
State of the Art
The life cycle assessment (LCA) method is one of the most widely used tools for identifying the most suitable materials in sustainable construction. It compares the environmental impact of different building materials and directs the design toward the use of materials with low energy consumption. This method considers the whole life cycle, from the production of materials to their end-of-life.
Initial studies on the embodied energy of buildings found that over a 50-year lifespan, embodied energy accounted for 45 percent of the total energy needs. For this reason, the life cycle analysis mainly focused on reducing the OE and neglected the energy consumption of the other life cycle phases.
This reduction has been made possible in the last two decades by stricter energy regulations and standards in accordance with the evolution of various European legislations (CEN).
The European standard EN 15978:2011 specifies the calculation method, based on life cycle assessment (LCA) and other quantified environmental information, to evaluate the environmental performance of a building.
It also defines the system boundary that is applied to the building level: the production phase (including extraction of raw materials, A1; transport of raw materials, A2; production of building materials, A3; their transport, A4; and their installation, A5); the use phase (including use, B1; maintenance, B2; repair, B3; replacement, B4; renovation, B5); the end-of-life phase of the building (including deconstruction, C1; transport of materials, C2; waste treatment, C3; and the final disposal phase, C4); and the beyond-life phase of the building, module D (scenarios for reuse, recycling, and energy recovery from obsolete materials and associated environ- mental impacts).
Wu et al. highlighted, by means of an LCA approach, the prevalent consumption of operational energy in an office building in China.
They also pointed out that the treatment of building materials at the end of their life is an important aspect affecting the environmental performance of the building.
Some environmental improvements aimed at reducing energy consumption and CO2 emissions throughout the building’s life cycle are also provided.
Bontempi highlighted that the development of reliable and often open-access databases containing embodied carbon (EC) and EE (embodied energy) associated with many raw materials has contributed to the development of sustainability analysis tools based on energy use and emissions.
Thanks to the use of general parameters (EC and EE), in principle, these approaches can be applied to all sectors and can be considered as pre-screening methods, preliminary to a complete LCA.
A further evolution of the LCA methodology can be found in studies that investigate A to C modules (i.e., including end-of-life considerations, ‘C2G—from Cradle to Grave’).
In this sense, evaluating both the embodied and operational emissions of different office building retrofit scenarios (in Norway), Rabani et al. extended the scope of the investigation to module C.
Di Ruocco et al. investigated the criteria and approaches adopted in a series of case studies to develop a useful tool to identify the ‘sustainable trade-off’ of opaque envelopes.
This study proposes, through the comparison of the application of two methods (TOPSIS and COMPROMISE), the analysis of the materials used in the de- sign by considering a series of performance and environmental characteristics in relation to the content of recycled materials, the end-of-life destination of these materials: embodied energy and embodied carbon.
Therefore, to protect non-renewable resources and reduce land consumption, Di Ruocco et al. developed a methodology aimed at the sustainability of rehabilitation and conservation interventions of protected buildings through a life cycle building (LCB) approach, minimising CO2e emissions.
The model was developed based on modules A, C, and D (A1–A3: production, C1–C4: end-of-life, and D: benefits beyond the system boundaries). The results show that the most relevant contributions, in terms of CO2e reduction, come from the use of dry-assembled wood technology units.
Gomes et al. evaluated the environmental, economic, and energy performance of different flat roof solutions, C2C, to support the selection of the best alternative to be used in each building. The results showed that inverted flat roof solutions have, in general, a worse environmental performance than traditional solutions.
In light of the state-of-the-art analysis, the following considerations have emerged:
· The circular C2C (form cradle to cradle) approach has only been partially investigated;
· Among the case studies analysed, very few refer to timber construction systems;
· No study has evaluated, within the C2C approach, the potential of wood systems for carbon storage.
Therefore, with reference to the criticalities and gaps found in the state-of-the-art, this study means to elaborate a methodology applicable to timber construction systems that allows one to evaluate the reduction in embodied energy and carbon in the transition phases between the cradle to grave.
Specifically, this study aimed to investigate the potential of wood building systems, in terms of reducing CO2e emissions related to the selective deconstruction process, at the end-of-life, by evaluating the different performance of two wood building systems:
· Frame structure;
· XLAM panel structure.
Tools and Methods
The tools used for the purpose of methodological development are:
To estimate the CO2e,1 component, the emissions from the machinery and equipment required for selective deconstruction operations were considered.
To estimate the CO2e,2 component, the integration of virgin raw material of the amount of scrap as loss of damaged material, as a result of deconstruction operations, was considered. Emissions result from the production of the amount of mate- rial to be integrated and the EC factor from an open-access database, Inventory Carbon and Energy (ICE), corresponding to the A1–A3 phases (cradle to gate).
To estimate the CO2e,3 component, the emissions derived from transporting the deconstructed materials from the demolition site to the processing/storage company were calculated.
To estimate the CO2e,4 component, the emissions of the machinery and equipment required for restoring the wooden components, in order to put them back into the market, were calculated.
To estimate the CO2e,5 component, the negative factor indicated by ICE for the cradle-to-gate phase was considered, multiplied by the amount of material recovered by selective deconstruction (therefore net of the amount of XLAM of scrap).
Therefore, this value expresses the potential of the circular approach of the methodology: it corresponds to the gain, in terms of emissions, connected to the reuse of the components in the next life cycle, thus avoiding (for this quantity) the impact related to the extraction of virgin raw material and its transformation (corresponding to the A1–A3 phases).
The methodology also takes into account an Italian sustainability protocol, CAM—Minimum Environmental Criteria, which defines the environmental requirements useful for identifying the best design solution, product, or service with regard to the life cycle of the work.
The methodology developed was divided into the following phases:
Phase I: Definition of the scope of investigation
Phase II: Technological characterisation of the building
Phase III: Estimation of CO2e emissions
Phase IV: Assessment of the level of disassembly
Phase V: Estimation of waste streams and quantities and the verification of the Italian CAM parameters
Specifics of Five Methodology Developed Phases
Phase I: Definition of the Scope of Investigation
According to the European Standard EN 15978:2001, the aim of the proposed methodology is to estimate the environmental impact, in terms of CO2e emissions, of the end-of-life phase of wood building systems. Specifically, the scope of the investigation was constituted by modules C and D that concern, respectively, the end-of-life phase and the benefits/charges derived from reuse, recovery, and potential recycling activities.
Phase II: Technological Characterisation of the Building
The technological characterisation of the building was based on the UNI 8290-1:1981 standard, thanks to which it is possible to break down the building into classes of technological units (1st level), each class of technological unit is in turn broken down into technological units (2nd level), and each technological unit is in turn broken down into technical elements (3rd level).
Air conditioning, plumbing, fire protection, and burglar alarm systems as well as and exterior furniture are not included in the classification as they are considered invariant.
In addition, the type of connections of the different materials that make-up the technical elements were evaluated; to do this, the UNI 11277:2008 standard was taken into consideration by associating each construction system with an installation technology.
The identification of the type of construction system, the prevailing materials that make-up the technical elements, and the type of connection has a fundamental importance to subsequently define the work necessary for disassembly and selective demolition.
Phase III: Estimation of CO2e Emissions
In the third step, the CO2e emissions generated by a selective disassembly and demolition process during the whole end-of-life phase of a building with a timber construction system were estimated.
The CO2e emissions were obtained from the sum of five rates:
Positive rate of CO2e,1 from the demolition activities.
Positive rate of CO2e,2 from scrap resulting from demolition activities.
Positive rate of CO2e,3 from off-site transport.
Positive rate of CO2e,4 from transformation/treatment activity for further reuse.
Negative rate of CO2e,5 as emissions credit for storage in the material.
Phase IV: Assessment of the Level of Disassembly
The potential for the re-use of building bodies, building components, and building materials is closely related to their level of disassembly, which represents the ability of a building, building component, or building material to be disassembled and re-introduced into the production cycle, therefore establishing a sustainable continuity between the end-of-life phase (decommissioning of the building) and the production phase of the individual building components.
The current approach to assessing the level of disassembly (LID) of a technological unit, technical element, or prevailing material is dictated by UNI 11277:2008.
The method proposed by the UNI standard assigns a score, from 0 to 5, exclusively according to the laying technology and the construction system of the prevailing material. Once the score of each prevailing material is known, the level of disassembly of the technological unit is considered equal to the average of the scores of the prevailing materials composing it.
This method, being devoid of indicators or information on the phases following the disassembly phase and preceding the reintroduction of the single prevailing materials within the production cycle, allows for a partial evaluation of the level of disassembly (LID) of a building or a single technological unit. To define an integrated method for the evaluation of the level of disassembly, the research proposes an implementation of the classification made by the UNI 11277:2008 standard.
The integrated experimental method implements the classification made by the UNI 11277:2008 standard. Two other parameters are added to the construction system and laying technology parameter: damage generated by handling and transport and transformation activities. The score (LID) to be assigned to each prevailing material, belonging to a given technological unit, is obtained from the sum of the products between the recovery potential, chosen for each parameter according to the sub-parameters, and the weights assigned to each parameter. The score to be assigned to the technical element is obtained as the average of the scores of the prevailing elements composing it.
Phase V: Estimation of Waste Streams and Quantities and Verification of CAM Parameters
The last step of the methodology is to estimate the kinds and quantities of waste generated during the whole disassembly and selective demolition process. The estimation of waste streams is conducted by assigning to each material the EWC (European Waste Code) provided by the List of Wastes.
The estimation of the quantities of waste for reuse, recycling, and disposal is carried out by assigning a future destination to each prevailing material, depending on the nature of the material (EWC):
Processing centres: By-products with a future destination in processing centres are intended for re-use.
Recycling centres: By-products with a future destination in recycling centres are in- tended for recycling activities. This quantity also includes the percentage of scrap.
Recycling centres: Dangerous and non-dangerous waste with a future destination in disposal centres are intended for disposal activities.
With reference to the end-of-life phase, it is particularly interesting to verify criterion 220.127.116.11 of the Italian Ministerial Decree 11/10/2017, according to which at least 50 percent of the weight/weight of building components and prefabricated elements excluding plants, must be subject to selective demolition at the end of their life and be recyclable or reusable. Of this percentage, at least 15 percent must be made up of non-structural materials.
Application to the Case Studies
The selection of case studies took into account the following requirements:
· Buildings made with predominantly dry technology system.
· No.1 building made with a wood framed construction system.
· No.1 building made with a XLAM panel construction system.
The selected buildings must present the characteristics of contemporary architectural works, in terms of the quantity of publications, citations, and web presence.
The first case study selected was ‘Villa GP, a single-family villa located in Valdagno (VI), built in 2018 by IRODA Studio with a timber frame structure.
The second case study was ‘Cenni di cambiamento’, a social housing complex built in Via Cenni, Milan, by RPA Rossi Prodi Associati s.r.l. in 2009. The intervention, characterised by an XLAM load-bearing structure, is the largest in Europe; in fact, due to the vastness of the complex, the methodology was applied to only one of the four buildings.
Pre-demolition verification, through the drawing up of an inventory of materials and building elements of the building system, is meant to provide a clear picture of the building structures to be demolished including an estimate of the waste materials that will be released, in order to implement a correct deconstruction.
With the help of the guidelines on the correct management of demolition waste, provided in recent years by the EU, through the protocol for the management of construction and demolition waste and the protocol ‘Guidelines for waste verifications prior to demolition and building renovation works', an inventory of the materials and building elements of the building organism was drawn up for each selected case study.
Once the technological units and the technical elements characterising each case study were determined, the CO2e emissions generated by a selective disassembly and demolition process during the whole end-of-life phase of the two selected case studies were estimated.
An analysis of the results showed that in both case studies (i.e., for both the frame and the XLAM construction system), the final CO2e balance was negative, so the disassembly and selective demolition process therefore took place with zero emissions, thanks to the compensatory emission credit for storage in the wood.
Results from the Case Studies
In summary, with reference to the goals set by the study, the application of the methodology to the investigated buildings allowed us to extrapolate two families of results:
(1)CO2e Emissions of Module C and Consequent Benefits in Module D (UNI 15978)
a.Villa GP: Discussion of Results, with Reference to the Application of Modules C and D
Module C of the Villa GP case study resulted in a unit emission of +2.916 kg CO2e/m3, from the perspective of obtaining a benefit (module D), in terms of emissions reduction, of ?4.470 kg CO2e/m3, allowing for a negative balance in the transition from the cradle to the grave, estimated at ?1.554 kg CO2e/m3.
b.Via Cenni Complex: Discussion of Results, with Reference to the Application of Modules C and D
Module C of the Via Cenni complex case study resulted in a unit emission of +1.080 kg CO2e/m3 from the perspective of achieving a benefit (module D), in terms of emissions reductions, of ?10.670 kg CO2e/m3, allowing for a negative balance in the transition from the cradle to the grave, estimated at ?9.590 kg CO2e/m3.
(2)Compliance with the Disassembly Threshold (Italian CAM)
a.Villa GP: Discussion of Results, with Reference to Italian CAM Compliance
The Villa GP case study, exceeds the minimum thresholds required by CAM, both in terms of the percentage of disassembly to total components (100% > 50%) and the inci- dence of non-structural components only (55.73% > 15%).
b.Via Cenni Complex: Discussion of results, with Reference to Italian CAM Compliance
The Via Cenni complex case study exceeded the minimum thresholds required by CAM, both in terms of the percentage of disassembly potentiality out of the total com- ponents (75.25% > 50%), and the incidence of non-structural components only (15.41% > 15%). This second case also had a disposal rate, which was 24.75 percent.
The results show that the case studies present different values in terms of the goal to be pursued:
In terms of CO2e emissions, in the end-of-life phase, the Via Cenni complex presents a more virtuous construction system compared to Villa GP;
In terms of the disassembly potentiality, the Via Cenni complex, while exceeding both thresholds imposed by the Italian CAM, presented lower values compared to Villa GP as well as a residual amount of materials to be taken for disposal.
Therefore, the final emerging considerations are:
With regard to the level of CO2e emissions at the end-of-life, the case study of greater volume (Via Cenni complex) allowed us to optimise the use of machinery in the deconstruction and transport phases.
Regarding the level of disassembly potentiality, the XLAM construction system has more complex connections, and is therefore more difficult to be disassembled than a ‘frame’ construction system.
Reducing Carbon Emissions
The construction sector, being responsible for 39 percent of carbon dioxide emissions, has a key role to play in achieving this goal. To change this trend, the construction sector needs to transform the life cycle assessment (LCA) pathway from linear to circular to ensure a more efficient use of natural resources.
The studies investigated in the state-of-the-art have not taken into account the potential of going beyond module C of BS EN 15978:2011, and should be considered as partial works still linked to a linear vision.
In contrast, closing the circle of processes is essential, and this goal is only possible by providing a series of strategies downstream of the demolition of a building system, preferably of a selective type, and the reintroduction of components/materials into a new ‘C2C—from Cradle to Cradle’ production cycle.
In order to pursue the transition from a linear to circular economy and to promote sustainable economic growth, a very important aspect to be considered is the management of construction and demolition waste (CDW).
With a view to maximising the potential for the reuse of building components and thus reducing C and DW, both the design phase and the end-of-life phase of a building organism play a fundamental role.
In the design phase, a sustainable strategy oriented toward the reintroduction of materials into the production cycle is represented by the choice of dry technological systems rather than wet ones, as they are characterised by greater disassembly potentiality, and therefore favour the reuse of building components with the same function; in contrast, wet building systems allow for the reintroduction of components into the production cycle with different functions from the original ones, most of the time with lower performance, only following treatments that produce energy consumption.
In the end-of-life phase, a sustainable strategy oriented toward the reintroduction of materials into the production cycle foresees the preparation of a disassembly and selective demolition plan that allows for the reuse or recycling of materials, building components, and prefabricated elements used.
The Italian CAM for Buildings (Minimum Environmental Criteria) requires that at least 50 percent by weight of the building components must be recyclable or reusable, and of this percentage, at least 15 percent must be non-structural materials.
The study developed a method to assess the environmental impact of the disassembly and selective demolition process by identifying the process with the lowest CO2e emissions to minimise waste at the end-of-life phase. The methodology was applied to two wooden architectures, whose materials are able to store CO2 due to their organic nature, which will be returned to the environment after incinerating (i.e., when no further life cycle is possible for it).
As shown above, in both case studies (i.e., for both the frame and XLAM system), the final CO2e balance was negative due to the offsetting emission credit from the storage property of the wood components.
This research has shown that buildings designed with a predominantly wood construction system can make a significant contribution in terms of reducing the CO2e emissions throughout their life cycle. This is due to the following main aspects:
· Easy selective demolition process, resulting in waste reduction and a high reuse rate of components at the end-of-life.
· Consequent reduction in virgin raw material as a result of the system’s potential use at the end-of-life.
· Possibility of reducing the overall emissions of the wooden building (near zero) due to the carbon storage properties of the wooden components.