Responsible management of construction resources.

Responsible management of construction resources.
Amortization of the concrete’s embodied environmental impact as
a sustainable strategy.
Speakers:
Vilches Such, A.1; Lizana Moral, F.J.1; Barrios Padura, Á.1; Heredia Díaz, I. 2; Serrano
Jiménez, A.J. 1
1
2
Higher Technical School of Architecture, Building Department, Seville, Spain
Quality and Sustainable Development Manager, Holcim (Spain), Madrid, Spain
Abstract: Cementitious derivate products are one of the most used in building sector. Despite having a
low unitary environmental impact relating to other materials, its intensive use means a high impact to
environment. This paper introduces the research carried out in Seville´s University with the main
target to evaluate strategies in order to improve and optimize the use and management of structural
concrete, and analyse its environmental and service life performance. Besides locating the main
energetic and emissions impacts in concrete production, it is determined the dosages with lower
amortization time period (CO2e/years) regarding to its durability properties. Depending on the case of
study it can get a better environmental performance using a product with a higher embodied impact
but with better durable properties that will allow amortize the resource till the end of its service life.
Concrete, life cycle assessment, serviceability, sustainability
1. Introduction
Reducing greenhouses gases emissions and diminishing energetic dependency of countries are
world priority objectives. The Building Sector is responsible of the 30% of CO2e emissions
and the 40% of primary energy consumed. Those impacts to the environment occur mainly in
the life cycle stages of materials production, transport and the energy needed for building
operation.
This research is focused on the analysis of CO2e emissions at the stage of materials
production in order to establish efficient and sustainable design strategies.
The incessant information about the sustainability of products makes it complicated to discern
which are the most reliable aspects and criteria to evaluate this concept. We should call into
question the established issues and theories regarding the sustainable management of
resources in Building Sector, underlining that most of the material used in construction works
(ceramic, cement and aggregates), around the 80%, are not included under labelling programs
neither environmental certification programs[1].
The significant concern about the reduction of the concrete environmental impact is reflected
in the large amount of research projects on this topic: outlining those which propose clínker
substitution by additions, the use of recycled aggregates, the optimization of heat exchange in
cement plant, and the waste valorisation as alternative fuel.
1
All this questions are identified by the cement industry that states how the European Cement
Sector is close to the limit that could be achieved due to the best available technologies and
rationalization. Recent reports published by the Cement Sustainable Initiative (CSI) confirm
that the current production technologies of clinker do not allow significant improvement
potential regarding to energy efficiency. They highlight how Cement Sector has the
possibility to reduce CO2e emissions using waste as fuel and alternative raw materials
(additions and secondary raw material from ceramic industry to clinker).
1.1 Environmental impact of the cementitious derivate products.
Diverse research carried out with the aim of analysing concrete life cycle lead to clinker
production as the main source of energy consumption and emissions to the atmosphere [2-7],
estimating that regarding to the type of cement used (due to the amount of clinker), CEM I or
CEM III/B, may be a variation between 60-70% of CO2e emissions.
These emissions occur because a huge amount of energy (often provided by fossil fuels) is
required in clinker production. Firstly, greenhouse gases are released due to the combustion
process of fuel. Secondly, calcium carbonate is decomposed at 900ºC releasing significant
amount of carbon dioxide, around a 59% (CaCO3 + 1450ºC CaO + CO2) [4].
This is the reason why many researches propose to reduce clinker contents. However, others
show that reducing it lowers its durability [8].
In this line, the present paper focuses on the evaluation and assessment of the importance of
concrete durability in the definition of its environmental performance.
1.2 Building materials’ durability as a sustainable key factor.
The principal differentiation with other researches is the inclusion of durability as essential
evaluation criteria. Regarding sustainability, life service and resource optimization should be
taken into account as well as the environmental aspects. This holistic approach pretends to
reveal the effect of concrete damages in relation to its initial dosage and its environmental
impacts associated, that will affect directly to the concrete’s life service and to the
environmental assessment along time.
2. Methodology for environmental impacts and lifetime calculations.
The methodological processes include the following three aspects: (i) quantification of
environmental impact, (ii) concrete service life calculation, and (iii) correlation between
variables through the structural calculation of a column.
2.1 Environmental impact quantification.
Nowadays, there is lack of common criteria to define the boundaries of the Life Cycle
Assessment (LCA). The control of those limits is still under development to certain products
through the EN 15804:2012, which establish the Category Product Rules (RCP) regulations
2
regarding to building materials. Because of that, and due to the high weight in the impact of
the concrete that the type of conglomerate has [2-7], firstly it was made a comparative work
of 21 different samples of cement in order to get representative means values of each type.
The data was obtained from different Database [4,9-11] and from Environmental Product
Declaration [12]. The means values used for this research are presented below:
Energy consumption
CO2e emissions
Cement type
Samples
[MJ / kg cem.]
Samples
[kg de CO2e / kg cem.]
CEM I
10
4,46
9
0,843
CEM II
9
3,73
8
0,742
CEM IIIa
2
3,05
2
0,420
Table 1. Average values used for each type of cement. Self elaboration.
To determinate the impact of the components (aggregates, water and additives) it was used
the BEDEC Database [9], from the Instituto Tecnológico de la Construcción. The values used
in the research are the followings:
Energy consumption
CO2e emissions
Component
[MJ / kg]
[kg CO2e / kg]
AGGREGATE
0,150
0,00790
WATER
0,006
0,00029
ADDITIVES
93,000
13,73000
Table 2. Values used for each concrete component. Source: BEDEC database.
2.2 Estimation of the service life of concrete.
Concrete durability is conditioned by diverse aspects: physical, chemical, mechanical,
biological and environmental [13]. Because of these limitations and due to geographical and
environmental adaptation, the procedure carried out to obtain the life service of concrete was
through failure process of corrosion by concrete carbonation model shown in the Annexe 9 of
the Spanish structural code EHE-08 [14]. It was considered 30mm of steel covering and an
environmental aggression of IIa (XC2 or XC4 according to Eurocode 2): outdoor environment
without chlorides, exposed to rain with an annual precipitation higher than 600mm or
foundations. The total lifetime estimated is the sum of the initial period (ti) and the
propagation period (tp):
Equation
(1)
Unit
80 Ø Factors:
tL Service life (year)
ti Initiation period by carbonation model (year)
tp Propagation period by corrosion (year)
d Cover depth (mm)
Kc Carbonation rate
ϕ Diameter of steel (mm).
Vcorr Corrosion rate (µm/year).
Table 3. Estimated service life of concrete by carbonation corrosion model. Source: Annexe 9 EHE-08.
This model does not consider the influence of water/cement ratio either the maximum amount
of cement [8].
3
2.3. Calculation criteria
9 dosages of different concrete from a unique plant for C25/30, C30/37 and C35/45 were
implemented, as different aggregates require different dosages of cement. As high strength
category is not commonly used, for C40/50 and C45/55 were used a different plant dosages,
these environmental impact results, although are slightly lower, can be used for the research
having this into consideration. As well, another criteria used was to include different type of
cement for every characteristic strength. This allowed us to relate all factors in a specific
model: a three meters column under an axial stress.
From the structural concrete element, for each dosage it was calculated the environmental
impact per cubic meter. This impact is equal to the summation of the unitary multiplied by the
amount of material of every component. It was calculated the service lifetime (years) for
every dosage regarding to the previous equation (Eq. 1).
2.4 Indicators used
To estimate the amortization along time of the concrete embodied impact kg of CO2e/year and
MJ/year indicators were used. It was obtained through Eq.2.
Equation
(2)
Unit
!" #$%&'(/*#+&,
Factors:
IT , Impact amortization factor
An, Resistant section area
h, Support height (3m)
icategory, Unitary material impact for each category: (CO2
e/m3) o (MJ/m3)
tL, Estimated service life years of concrete
Table 4. Calculation of impact amortization factor. Self elaboration.
3. Results
The emissions (kg de CO2e) and embodied energy (MJ) results are showed in the following
tables for each dosage. It can be deduced, from the embodied carbon results of table.5, that
cement is the major component in the concrete impact with a weight around 75-85%.
Aggregates and water have a lower percentage, between 3-8%, and additives represents
around 10-18%. As well, dosages with CEM III/A accounted a 36,5% less emissions than
CEMII/A to C25/30 concretes, and 46,1% less emissions regarding to CEM I to C30/37.
According to the table.6, CEM III/A accounted a 13,2% less embodied energy than a CEM
II/A to C25/30, and a 28,4% less embodied energy than a CEM I to C30/37.
Cement
Strength
Class
(EN 1992)
(N/mm2)
C25/30
C30/37
Designation
according to
EHE-08
HA-25-B-20-IIa
HA-25-B-20-IIa
HA-30-F-20-IIa
HA-30-B-20-IIa
HA-30-B-20-IIa
Cement type
CEM II/A-S 42,5 N/SRC
CEM III/A 42,5 N
CEM I 52,5 SR
CEM II/A-S 42,5 N/SRC
CEM III/A 42,5 N
Embodied Carbon (Kg CO2e)
%
Total
Cement Aggregate Water Additives
kg CO2e/m3
additions
6-20
36-65
0-5
6-20
36-65
84,31%
75,26%
81,90%
85,40%
76,80%
4,68%
7,39%
3,68%
4,35%
6,91%
0,01%
0,02%
0,01%
0,02%
0,02%
10,99%
17,33%
14,41%
10,24%
16,27%
303,6
192,5
380,9
325,8
205,1
4
C35/45
C40/50
C45/55
HA-35-B-20-IIa
HA-35-B-20-IIa
HA-40-B-20-IIa
HA-45-B-20-IIa
CEM II/A-S 42,5 N/SRC
CEM III/A 42,5 N
CEM II/A-V 42,5 R
CEM II/A-M (P-V) 42,5 R
6-20
36-65
6-20
12-20
85,20%
76,52%
81,18%
81,52%
3,57%
5,66%
3,78%
3,39%
0,01%
0,02%
0,01%
0,01%
11,22%
17,80%
15,02%
15,08%
383,2
241,5
365,6
414,1
Table 5. Calculation result of Kg CO2 e/m3 impact associated of different type of concretes. Self-elaboration.
Strength
Class
(EN 1992)
(N/mm2)
C25/30
C30/37
C35/45
C40/50
C45/55
Cement
Designation
according to
EHE-08
HA-25-B-20-IIa
HA-25-B-20-IIa
HA-30-F-20-IIa
HA-30-B-20-IIa
HA-30-B-20-IIa
HA-35-B-20-IIa
HA-35-B-20-IIa
HA-40-B-20-IIa
HA-45-B-20-IIa
Embodied Energy (MJ)
%
Cement Aggregate
additions
Cement type
CEM II/A-S 42,5 N/SRC
CEM III/A 42,5 N
CEM I 52,5 SR
CEM II/A-S 42,5 N/SRC
CEM III/A 42,5 N
CEM II/A-S 42,5 N/SRC
CEM III/A 42,5 N
CEM II/A-V 42,5 R
CEM II/A-M (P-V) 42,5 R
6-20
36-65
0-5
6-20
36-65
6-20
36-65
6-20
12-20
72,16%
67,92%
72,12%
73,83%
69,75%
74,85%
70,86%
70,15%
71,10%
15,13%
17,43%
11,62%
14,19%
16,41%
11,83%
13,70%
12,33%
11,15%
Water
0,05%
0,06%
0,04%
0,05%
0,06%
0,05%
0,05%
0,04%
0,04%
Additives
Total
MJ/m3
12,66%
14,59%
16,22%
11,92%
13,78%
13,27%
15,38%
17,48%
17,71%
1784,8
1549,2
2291,8
1896,0
1639,9
2194,3
1893,8
2128,6
2388,8
Table 6. Calculation result of MJ/m3 impact associated of different type of concretes. Self-elaboration.
Depending on the different strength, the section needed for each column would be different.
The most strength used the smaller section size is got; therefore the amount of material
needed for the whole column would be lower. Taking into account this structural behaviour
was important to analyse the results and its further discussion in conclusions.
Geometry support
Strength
Class
Cement type
(EN 1992)
(UNE-EN 197-1)
(N/mm2)
C25/30
C30/37
C35/45
C40/50
C45/55
CEM II/A-S
CEM III/A
CEM I
CEM II/A-S
CEM III/A
CEM II/A-S
CEM III/A
CEM II/A-V
CEM II/A-M
Section1
(cm2)
1220,4
1220,4
1017,0
1017,0
1017,0
871,7
871,7
762,8
678,0
Impact of structural element
Total
%
Concrete
kg CO2e/m3 emissions CO2e
3
(m )
kg CO2 e relative
0,366
0,366
0,305
0,305
0,305
0,262
0,262
0,229
0,203
303,6
192,5
380,9
325,8
205,1
383,2
241,5
365,6
414,1
111,2
70,5
116,2
99,4
62,6
100,2
63,2
83,7
84,2
100,0%
63,4%
104,5%
89,4%
56,3%
90,2%
56,8%
75,3%
75,8%
Durability
Amortization
Estimated
service
live2
%
CO2e/year CO2e/year
relative
397
317
600
600
417
878
539
1246
1719
0,280
0,223
0,194
0,166
0,150
0,114
0,117
0,067
0,049
100,0%
79,4%
69,1%
59,1%
53,5%
40,7%
41,9%
24,0%
17,5%
1. Section necessary for an axial force of 3051kN, obtained for a concrete support on the ground floor of a five-storey residential building.
2. Estimated service life for a covering of 30mm and Ø12mm of diameter steel, by carbonation corrosion model.
Table 7. Amortization kg CO2e/year of a concrete structural element exposed to an environment IIa (high
humidity). Self-elaboration.
Strength
Class
Cement type
(EN 1992)
(UNE-EN 197-1)
(N/mm2)
C25/30
C30/37
C35/45
C40/50
C45/55
CEM II/A-S
CEM III/A
CEM I
CEM II/A-S
CEM III/A
CEM II/A-S
CEM III/A
CEM II/A-V
CEM II/A-M
Geometry support
Impact of structural element
Section1
(cm2)
Concrete
(m3)
MJ/m3
1220,4
1220,4
1017,0
1017,0
1017,0
871,7
871,7
762,8
678,0
0,366
0,366
0,305
0,305
0,305
0,262
0,262
0,229
0,203
1784,8
1549,2
2291,8
1896,0
1639,9
2194,3
1893,8
2128,6
2388,8
Total
% MJ
emissions
relative
MJ
653,4
567,2
699,2
578,5
500,3
573,8
495,3
487,1
485,9
100,0%
86,8%
107,0%
88,5%
76,6%
87,8%
75,8%
74,5%
74,4%
Durability
Amortization
Estimated
service
live2
MJ/year
% MJ/year
relative
397
317
600
600
417
878
539
1246
1719
1,647
1,791
1,166
0,964
1,199
0,653
0,920
0,391
0,283
100,0%
108,7%
70,8%
58,5%
72,8%
39,7%
55,8%
23,7%
17,2%
1. Section necessary for an axial force of 3051kN, obtained for a concrete support on the ground floor of a five-storey residential building.
5
2. Estimated service life for a covering of 30mm and Ø12mm of diameter steel, by carbonation corrosion model.
Table 8. Amortization MJ/year of a concrete structural element exposed to an environment IIa (high humidity).
Self-elaboration.
4. Discussion
Reinforced concrete (composed of aggregates, cement, water and additions) use traditionally
dosages according to the aggregate type from every cement plant. This is done with the aim of
getting a certain mechanical strength, compactness and service lifetime capable to endure
weather conditions on site where the building is located. The Spanish structural code (EHE08) regulates the water/cement ratio decreasing this relation for high environmental
aggressive conditions that could deteriorate concrete or rust the steel.
High proportions of cement with high levels of clinker produce high strength and compact
concrete, however it has a high embodied environmental impact in terms of energy needed
and CO2e emissions.
The durability studied through the EHE analysis method considered the covering of steel and
the sheltering from rain as determining factors. In the current research, the used values are
30mm (d) as cover thickness and 0,5 as rain exposure factor in order to determinate the
carbonation rate (kc). According to the carbonation model, a difference from 2 to 3 times
more of service lifetime for rain-exposed concretes it is obtained (which are shown in the
results) than for non rain-exposed concretes. Regarding the calculations, the lowest value of
estimated service lifetime is 317 years (C25/30; CEM IIIa) and the higher is 1700 years for
high strength concretes (C45/55; CEM II), with dosages of high level of cement. When
concrete section is bearing a stress close to the maximum allowable, the lowest impact of the
material is obtained with high strength concretes because of the lower amount of material
needed. It is obtained in the results that for the same type of cement, a high strength concrete
(C45/55) will have embodied emissions of the 75,8% regarding to a low strength concrete
(C25/30). As well, the amortization of the impact of the structural element would be of a
17,5% at the end of its service life, according to the degradation model used. Therefore, the
use of high strength concrete is recommendable even though they are composed of cements
with higher unitary impact.
In the Building Sector, the most used calculation strengths are 25, 30 and 35 N/mm2. Within
this strength the lower impact option regarding to a carbonation model is the 35N/mm2
concrete using a CEM IIIa. Nevertheless, if possible using higher strength concretes would be
less impacting despite the fact that the type of conglomerate is CEM II or CEM I.
However, if the column is sized due to a minimum section (the Spanish building code
determinates the smallest section as 25x25cm for columns), it is better to use lower embodied
carbon concrete with CEM IIIa.
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Acknowledgements
The authors acknowledge the information and support given by Herena Amaya Ruiz Fuentes,
Technic Assistance and Sustainability Applications responsible of Holcim Spain. This paper
has been developed from the results obtained in the framework of the Holcim Grant to
Sustainable Development 2013. This grant is conceded to a recent graduate from the Higher
Technical School of Architecture of the University of Seville, Spain.
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