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Embodied Carbon Benefits of Reusing Structural Components in the Built Environment

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This paper provides parametric estimates of embodied carbon reductions when structural components are reused in a typical office building. First, a lower bound of structural material quantities is estimated for a typical steel frame structure in a low-rise office building. The embodied carbon of this conventional design is then compared with values collected from a series of similar existing steel buildings (deQo database) as benchmark. Various scenarios regarding the impact of selective deconstruction, transportation, and cross-section oversizing are modelled and parameterized. The study eventually computes carbon savings over one life cycle of the building project. Results show that reuse remains beneficial for long transport and high oversizing. The discussion calls for more comprehensive studies and refined metrics for quantifying selective deconstruction.
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PLEA 2018 HONG KONG
Smart and Healthy within the 2-degree Limit
Embodied Carbon Benefits of Reusing Structural
Components in the Built Environment:
a Medium-rise Office Building Case Study
ABSTRACT: This paper provides parametric estimates of embodied carbon reduction s when structu ral component s
are reused in a typical office building. First, a lower bound of structural material quantities is estimated for a
typical steel frame structure in a low-rise office building. The embodied carbon of this conventional design is then
compared with values collected from a series of similar existing steel buildings (deQo database) as benchmark.
Various scenarios regarding the impact of selective deconstruction, transportation, and cross-section oversizing
are modelled and parameterized. The study eventually computes carbon savings over one life cycle of the building
project. Results show that reuse remains beneficial for long transport and high oversizing. The discussion calls for
more comprehensive studies and refined metrics for quantifying selective deconstruction.
KEYWORDS: Embodied carbon, Reuse, Circular Economy, Office Building, Steel
1. INTRODUCTION
1.1. Embodied carbon and waste
The Intergovernmental Panel on Climate Change
recommends that the building sector becomes zero
carbon by 2050 in order to meet the Paris Climate
Agreement [1,2] and to avoid extreme climate
catastrophes. The whole life greenhouse gas (GHG)
emissions expressed in carbon dioxide equivalent
(CO2e) and shortened as “carbon” in this paper,
include both, operational and embodied carbon of
buildings.
Operational carbon relates to GHG emissions
during the use phase of the building, which
includes heating, cooling, ventilation, lighting, and
equipment.
Embodied carbon refers to GHG emissions during
all other life cycle phases: material extraction,
component production, transport, construction,
maintenance, and demolition.
Recent technical standards and political initiatives
have successfully reduced the operational carbon of
buildings. However, significant improvements are still
required to lower the embodied carbon of new
buildings.
Besides, up to 50 % of material use in Europe is
related to the built environment [3, 4], which generally
constitutes the most resource intensive sector in many
industrialized countries [5]. In addition, more than
30 % of the waste generated in Europe originates from
the construction sector [6-8]. From these
observations, it follows that the design and
construction of buildings and infrastructures could be
improved by making a more efficient use of materials.
Load bearing systems, because of their high
material mass and energy intensive production, are
currently responsible for the biggest portion of
embodied carbon emissions and waste production in
buildings [9]. Structural engineers have therefore a
responsibility to reduce the environmental impact of
buildings.
1.2. Circular economy and reuse
A potential path to increased sustainability of
building structures is the integration of circular
economy principles in the structural design. Circular
economy, a concept originally introduced by architect
and economist Walter Stahel [10], advocates a closed
loop flow of materials and components in order to
extend their service life [11]. The European
Commission considers that circular economy would
boost competitiveness, innovation, local employment,
business opportunities, and social integration and
cohesion while protecting against shortage of
resources, volatile prices, and air, soil and water
pollution [12]. Circular economy involves five
strategies: reduce, repair, reuse, recycle, and recover
energy. Most sources, including the European Union
[13], prioritize them in the same sequence, i.e. reduce
must take precedence over repair, repair over reuse,
reuse over recycling, and recycling over energy
recovering. Although academic literature evolves to
bring circular economy into the building sector, its
application in building practice remains difficult due to
a number of economic, cultural and technological
reasons, the description of which is out of scope for
this paper. In light of the urgent need to reduce
material waste and embodied carbon in the
Catherine De Wolf*a, Jan Brütting a, Corentin Fivet a
a Structural Xploration Lab, Swiss Federal Institute of Technology (EPFL), Lausanne
*catherine.dewolf@epfl.ch
construction sector, this project explores the
opportunities of redefining materials value chains
through circular economy.
In particular, the reuse of structural elements is a
promising strategy that is still scarcely studied.
Contrary to recycling which requires energy to process
material, e.g. to remelt steel, reuse extends the service
life of components while limiting their physical
transformation and changing their location and/or
function. Reusable structural components may
consequently have a longer service life than the
systems to which they initially belong. Disassembled
buildings become a mine for new constructions, and
functional obsolescence is not a reason for waste
production anymore.
1.3. Problem statement
The industry is currently lacking benchmarks to
assess the beneficial impact of structural reuse. This
paper therefore provides a first answer to the
following question. How would the reuse of structural
components be beneficial for reducing the
environmental impact of office buildings and to what
extents? In particular how impactful are design
parameters that typically arise when considering reuse
strategies, e.g. material transportation, cross-section
oversizing, and selective deconstruction?
2. METHODOLOGY
The load bearing system of a steel frame five-story
high office building is used as a case study. This
building typology is commonly found in urban areas
where land pressures and therefore demolition and
transformation rates are high. The chosen building
typology also fits within the available benchmarks (see
section 2.1) for medium-rise steel office buildings.
First, buildings of similar construction type, i.e. steel
constructions with four to six stories, are selected from
an industry-collected database. The embodied carbon
of those buildings is analysed and defines the
benchmark. This benchmark is then used to relate the
case study to the existing practice. Second, the design
of the case study is analysed and serves as the baseline
of minimally required material quantities and
embodied carbon related to its conventional
construction. Third, embodied savings due to the
reuse of steel structural components in the studied
design are assessed. For various assumptions of cross-
section oversizing, the savings are parametrically
studied as a function of the impact related to selective
deconstruction and transportation.
In total three scenarios are compared:
Benchmark of existing buildings: the lower bound
of the industry-collected office buildings;
Baseline for a conventional office building: the
new construction of a typical steel-framed office;
Reuse design cases: parametric analyses of
buildings reusing steel components from other,
obsolete buildings.
This original methodology can be used to explore
and compare more complex reuse scenarios or other
case studies.
2.1. Benchmark of existing buildings
Benchmarking embodied carbon in structural
systems of buildings has been historically challenging
due to uncertainty and unavailability of data and due
to the difficult comparability of buildings as complex
entities [14]. Leading structural engineering firms have
developed in-house databases to start benchmarking
their own projects [15-17]. The Waste & Resources
Action Programme (WRAP) initiated the collection of
whole building life cycle assessment (LCA) results from
industry, but only the end results of embodied carbon
calculations were collected, leading to a lack of
transparency [18]. In comparison, the database of
embodied Quantity outputs (deQo, available at
http://deqo.mit.edu) collects both embodied carbon
coefficients (ECCs) and structural material quantities
(SMQs) in recent constructions, which offers a greater
degree of transparency to the users [18]. The process
starts by extracting mass and volume of used materials
from the bill of quantities or from building information
models (BIM), shared by global structural design firms
[14,19]. The Carbon Leadership Forum used the deQo
data and other industry-collected databases and case
studies to create the first benchmarks for embodied
carbon in buildings [20-22].
The ECCs (expressed in kgCO2e/kg) of the considered
materials are then used to calculate the total
embodied carbon of existing buildings, as shown in the
following equation:
Embodied Carbonbuilding =
!
""SMQi×ECCi
L
l=1
M
m=1
where:
m is a particular material or component in the
building m = 1, 2, 3,…, M;
l is the number of replacements within the
lifespan of the building for each material
l = 1, 2, 3,, L;
SMQ are Structural Material Quantities (kg);
ECC are the corresponding Embodied Carbon
Coefficients (kgCO2e/kg)
Results from this data collection are evaluated and
presented in boxplots. Figure 1 summarizes structural
material quantities for all stored buildings with four to
six stories and with steel as the main structural
material. The SMQs are normalized by gross floor area.
The diagram is divided into buildings with small gross
floor area (up to 10000 m2) and big gross floor area
(more than 10000 m2). The thick line inside the grey
box of the boxplot reports the median value, whereas
the boundary of the box indicates the inner quartiles.
Whiskers represent the minimum and maximum
values.
Figure 2 similarly indicates the corresponding
embodied carbon, normalized per gross floor area.
What is considered in the material quantities and
embodied carbon results shown in Figures 1 and 2 are
the impacts related to the manufacturing and
construction of the structural steel system, but also to
slabs, connections, load-bearing walls included in the
basement, a base plate, and foundations.
Figure 1: Structural material quantities of 23 existing steel
buildings with four to six stories.
Figure 2: Embodied carbon of 23 existing steel structures
with four to six stories.
To be comparable with the case study building
introduced in the next sub-section, this subset of all
deQo projects results from a query of similar structural
systems, materials, and number of floors. From the
hundreds of buildings in deQo, 23 entries currently
correspond to the criteria aligned with these
constraints.
2.2. Baseline building
To evaluate the environmental benefits of reusing
structural components, the main structure of a
baseline building is designed as a case study. The
building is composed of a steel frame with steel
columns and a grid of primary and secondary steel
beams supporting prefabricated concrete slab
elements. The conventional construction of this
structural system is compared parametrically with
scenarios where steel elements are reused from one
or more dismantled buildings (see next subsection).
The baseline building has a width of 32 m, a length of
60 m and a height of 17.5 m. The building has five
stories, a story height of 3.50 m, ten bays in the length
direction with a column spacing of 6.00 m, and four
bays in the width direction with a column spacing of
8.00 m. A schematic view of the structural skeleton is
shown on Figure 3.
Figure 3: Schematic view of the case study structural
system.
Dead load of the slab elements as well as a
superimposed dead load of 2.0 kN/m2 and a
(conservative) life load of 5.0 kN/m2 are considered.
These assumptions are used to size the baseline
structure from standard I-sections at ultimate limit
state including standard safety factors. The general
strategy for sizing is to utilize cross section capacities
in the best way possible.
A life cycle assessment is performed to quantify the
corresponding embodied carbon of the main
structural elements. For the purpose of this study, an
ECC for the production of new steel, including a typical
recycled content, equal to 1.10 kgCO2e/kg and an ECC of
reinforced concrete equal to 0.15 kgCO2e/kg are used.
These values are averages derived from the Inventory
of Carbon and Energy [23], GaBi [24], Athena [25], and
EcoInvent [26], evaluated in [9]. In addition to
production, impacts related to the transport of
elements over 110 km to the building site are
considered. The transport emissions of
0.36 kgCO2e/(t·km) are obtained from [27] for typical
road freights. The overall embodied carbon for a
conventional construction of the baseline building
(including the new production of steel elements) is
140 kgCO2e/m2 of which 39 kgCO2e/m2 are due to the
steel elements, while 72 kgCO2e/m2 are caused by the
slabs and base plate. The embodied carbon of the
foundations, here assumed as 22.5 kgCO2e/m2, varies
however greatly in practice depending on soil
properties [28].
2.3. Reuse design cases
On the one hand, reuse avoids sourcing raw
materials and requires little energy for reprocessing.
On the other hand, reuse requires energy during the
selective deconstruction of obsolete buildings as well
as for transport, refurbishment and storage. In the
studies of this paper, we only consider the reuse of
load-bearing components.
To design a structure based on an available stock
of reclaimed elements means that a-priori given
geometric and mechanical properties of components
0
200
400
600
800
1000
1200
1400
1600
Small Area Big Area
Structural
Material
Quantities
(kg/m2)
0
200
400
600
800
1000
1200
Small Area Big Area
Embodied
Carbon
(kgCO2e/m2)
might lead to a non-optimal capacity utilization of
available elements that counteracts the potential
savings through reuse [29-30]. Reused structural
elements are ultimately oversized. Few quantifications
of finally achieved benefits exist. Through a parametric
case study, this research evaluates how much
embodied carbon can be saved through the reuse of
structural elements compared to a conventional
construction.
2.4. Embodied carbon comparison
The embodied carbon of the conventional baseline
structure is compared to the case where the same
structure is made from reused steel elements. A
parametric study analyses the sensitivity of
environmental savings through reuse for two key
parameters: selective deconstruction and transport
related carbon emissions.
The total building material quantities of the case
study building include the steel frame, the reinforced
concrete slab elements, a base plate, elevation cores
and foundations. The material quantities and
embodied carbon associated with all non-steel
elements are kept constant in the parametric study
and are equal for both conventional baseline and
reuse scenarios. The parametric study only focuses on
the reuse of the structural steel elements. The
quantities of all concrete elements are here included
in order to allow a comparison of the baseline and
reuse design cases to the buildings extracted from the
deQo database. Connections, bracing systems, and
secondary structure were not considered in this
preliminary design, such that the resulting material
quantities and embodied carbon will be on the lower
bound of the case studies reported in deQo.
Figure 4 summarizes the steps considered for the
LCA of the different reuse scenarios. It is assumed that
the steel elements are reclaimed from obsolete
buildings through selective deconstruction. This
process includes the opening of connections as well as
the hoisting of elements with a crane. A corresponding
impact of 0.267 kgCO2e/kg is reported in [29], which is
based on a review of data provided by Athena in [30].
In the parametric study, this value is varied between
0.0 kgCO2e/kg and 1.0 kgCO2e/kg to account for the
uncertainty of this data.
The transport distances are the second parameter
analysed in the parametric study. Transport distances
between 0 and 500 km from the deconstruction site
over the fabrication site to the building site are
considered.
The last parameter that is analysed is the cross-
section oversizing of the structural steel elements.
Indeed, when structural elements from an obsolete
building are reused in a new configuration, not all
elements can be used at a utilization level as high as in
the original configuration. Among other reasons, this
is due to the unavailability of desired cross sections
[29]. It is therefore assumed that material quantities in
reuse scenarios are ‘oversized’ compared to the
conventional case where cross sections are selected
with optimal size. The extra steel mass is
parametrically varied between additional 0 to 50 % of
the material quantities used in the baseline building.
Figure 4: Diagram representing the impacts of reuse
3. RESULTS
3.1. Influence of transport
Figure 5 illustrates the influence of transportation
distances on the embodied carbon of the reuse design
cases. The considered oversizing of steel element mass
is expressed in 10 % steps by the corresponding grey
lines. In addition, Figure 5 shows the lower bound
benchmark, i.e. the first quartile (Q1) of collected low
area steel buildings (section 3) as well as the embodied
carbon of the conventional baseline building. It is
visible that even with 50 % oversized steel element
sections and a transport distance of 500 km, the
embodied carbon of the reuse design case does not
exceed that of the conventional load bearing system.
These results indicate that longer transport distances
are acceptable in order to facilitate the supply of
reclaimed steel elements. Only when considering
transport distances over 2000 km and an oversize ratio
of 25 % the embodied carbon of the reuse case would
exceed that of the baseline case.
Figure 5: Embodied carbon of benchmark lower bound,
baseline and reuse design cases for varying transport
distances and oversize percentages.
Deconstruction
Fabrication site
transport
transport
Building site
80
90
100
110
120
130
140
150
160
0100 200 300 400 500
Embodied Carbon [kgCO2e/m²]
Transport Distance of reused steel elements [km]
Lower bound (Q1) benchmark (deQo)
Baseline building
Oversize 50 %
Oversize 0 %
3.2. Influence of selective deconstruction
Figure 6 shows the influence of selective
deconstruction related carbon emissions on the total
embodied carbon of the load bearing system made
from reused elements. Again, grey lines indicate the
considered percentage of element oversizing. The
reference ECC of 0.267 kgCO2e/kg for selective
deconstruction obtained from [29] is also indicated.
The results show that embodied carbon of reuse
design cases only exceed the embodied carbon of the
baseline building when elements are oversized and
impacts of the selective deconstruction are
unexpectedly high. As introduced before, the
reference impact of new steel production is
1.1 kgCO2e/kg.
Figure 6: Embodied carbon of benchmarked lower bound,
baseline and reuse design cases for varying selective
deconstruction values and oversize percentages.
In general, the obtained results show that when
oversizing and emissions spent for transport and
selective deconstruction are low, the benefits of
structural reuse are significant. The potential savings
in greenhouse gas emissions through reuse relatively
to the baseline conventional building can be up to
20 % when considering the reference impacts for
selective deconstruction, a transport distance of
300 km and only 25 % oversizing.
4. DISCUSSION AND CONCLUSION
This paper presents the study of a structural
system for an office building realised with new steel
elements and with reused structural elements. The
embodied impact of the building is computed
parametrically and compared to data collected
industry-wide.
Results show that for this case study embodied
carbon savings of 20 % can be obtained by designing
with reused structural elements. It should be noted
that the parametric study is only applied to the steel
structural skeleton. The foundation, core and slabs are
kept at a constant amount of materials. It is assumed
that the same concrete quality was used in all concrete
elements and the same steel quality in all steel
elements for simplicity of the modelling. In addition,
impacts of new connections, bracing system and
secondary structure are not taken into account.
Further research should give separate coefficients for
slabs, foundations, cores, connections, and bracing
elements. However, as these values are kept constant
in this case study, they do not influence the relative
comparison of results.
The embodied carbon savings would be even
higher if the prefabricated concrete slabs could be
equally reused. Indeed, the slabs contributed about
half of the total embodied carbon in the baseline
building. This confirms previous findings [32] that slabs
are the structural elements with the highest
environmental impacts in typical building structures.
Results show that reuse remains beneficial even
when transport distances, selective deconstruction
related impacts, and oversizing are relatively high.
Only when selective deconstruction and oversizing are
both much higher than expected, the impacts exceed
those of a conventional new construction. Impacts due
to selective deconstruction are currently computed as
ratios of structural mass, it therefore depends on the
oversizing. In practice, however, it may be assumed
that selective deconstruction is much more related to
the complexity of the disassembly process than the
weight of the system. Future studies should therefore
include ECCs for selective deconstruction that are not
directly dependent on mass.
In future research, different scenarios will also
include the impacts calculated over multiple life spans,
with the functional unit being one service life. Such
scenarios would account for material degradation
more precisely. The parametric study should also be
extended to concrete elements and should address
serviceability constraints. Further, an optimization of
the utilization of available stock elements would allow
the reduction of oversizing and allow an informed
design processes. In this paper, refurbishment,
storage, new connections, and remaining structural
capacity are neglected. Future work can expand on
including the impacts related to these aspects.
ACKNOWLEDGEMENTS
This project has received funding from the
European Union’s Horizon 2020 research and
innovation programme under the Marie Skłodowska-
Curie grant agreement No. 665667 and from the Swiss
Government Excellence Scholarship.
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... De Wolf et al. [12] highlighted the environmental impact of downsizing and reconditioning activities during the reuse of steel components for the construction of a new building. The assessment was carried out by comparing a reference building composed of reused steel elements with the same building composed of newly produced steel elements. ...
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Whole life cycle emissions of buildings include not only operational carbon due to their use phase, but also embodied carbon due to the rest of their life cycle: material extraction, transport to the site, construction, and demolition. With ongoing population growth and increasing urbanization, decreasing immediate and irreversible embodied carbon emissions is imperative. With feedback from a wide range of stakeholders – architects, structural engineers, policy makers, rating-scheme developers, this research presents an integrated assessment approach to compare embodied life cycle impacts of building structures. Existing literature indicates that there is an urgent need for benchmarking the embodied carbon of building structures. To remediate this, a rigorous and transparent methodology is presented on multiple scales. On the material scale, a comparative analysis defines reliable Embodied Carbon Coefficients (ECC, expressed in kgCO2e/kg) for the structural materials concrete, steel, and timber. On the structural scale, data analysis evaluates the Structural Material Quantities (SMQ, expressed in kg/m2) and the embodied carbon for existing building structures (expressed in kgCO2e/m2). An interactive database of building projects is created in close collaboration with leading structural design firms worldwide. Results show that typical buildings range between 200 and 550 kgCO2e/m2 on average, but these results can vary widely dependent on structural systems, height, size, etc. On the urban scale, an urban modeling method to simulate the embodied carbon of neighborhoods is proposed and applied to a Middle Eastern case study. A series of extreme low carbon case studies are analyzed. Results demonstrate that a novel design approach can lead to buildings with an embodied carbon as low as 30 kgCO2e/m2, which is an order of magnitude lower than conventional building structures today. Two pathways are implemented to lower the embodied carbon of structures: choosing low carbon materials (low ECC) and optimizing the structural efficiency of buildings (low SMQ). This research recommends new pathways for low carbon structural design, crucial for lowering carbon emissions in the built environment.
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To reduce embodied carbon in buildings, two strategies are available. First, material efficiency is improved. Second, the materials are chosen for their low carbon content. The operational carbon of buildings has lowered recently, but for immediate emissions savings innovations in embodied carbon are needed. This research demonstrates that most material mass lies in roofs and floor slabs, rather than in walls and columns. Therefore, the first strategy to reduce impacts would be to lower their material quantities in floor and roof design. For the second strategy, alternative materials are studied. Vaulted masonry structures combine both strategies: vaults span spaces efficiently and masonry has a lower embodied impact than steel and concrete. Results demonstrate that a combination of both strategies effectively lowers the embodied carbon of buildings: typical floor and roof structures range around 440 kgCO2e/m2 whereas vaulted tile masonry can be as low as 60 kgCO2e/m2.
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Sumario: Introduction -- Analysis: Two case studies and their national context -- Syntesis: Feasibility of the substitution of labor for energy -- Appendix I: The European Community and its institutions -- Appendix II: What is the French "Plan"? -- Bibliography
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There are three major areas in which buildings consume energy: (1) energy consumption from operational processes, such as heating and electricity, (2) energy from building material production and supply, and (3) energy from design and construction processes. In recent years, improved operational energy efficiency has shifted the framework for quantifying a building's energy consumption to a total life-cycle approach, which includes energy consumed in the design and construction phases, also known as the embodied energy. Researchers and industry professionals are in the early stages of developing methods and metrics to quantify embodied energy of buildings, particularly focused on building superstructure. To date, no extensive studies have been performed on the material quantities of foundation systems in building structures or their environmental impact. This thesis answers the key question: "How much do foundation systems contribute to the overall material quantities of buildings, and do foundation systems significantly contribute to the overall embodied energy?" Two methods are used to address these questions. First, an analysis was performed on a survey of building materials using a database of embodied energy recently developed at MIT. The database contains information on material quantities of foundation systems from 200 actual buildings. Second, a case study was analyzed in an attempt to evaluate gaps in the database. Ultimately this thesis is intended to provide preliminary benchmarks for material quantities and embodied energy of foundation systems in buildings. The findings in this study show that foundation systems contribute approximately 25% to a building's total weight and contribute nearly the same percent to the building's overall embodied energy. In addition it provides architects, engineers, contractors, and building owners with information related to the sustainability of building structures.
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A study was conducted at the Concrete Center to investigate the embodied CO2 in structural frames for non-residential buildings. The study used the designed and measured schemes produced for the cost model studies. It explored the variations in embodied CO2 predictions, considering two sources of variation, such as the method of the analysis and the specification. The study found that there was little difference between the embodied CO 2 of the different types of structural frames within the uncertainties of the available data. It was found that there was a significant opportunity for the structural engineer to reduce the embodied CO2 of the final structure by careful specification after the frame type had been selected. An assessment was also made of how the structural frame affected the impacts across the whole building for items, such as construction, cladding, substructure, and fit-out.
Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change
IPCC. 2014. Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Synthesis Report, Geneva, Switzerland: IPCC.