Concrete living walls: addressing questions of concrete sustainability and
urban density
Benjamin Riley
MAP ARIA ENSAL
3 Rue Maurice Audin
69120 Vaulx-en-Velin
Téléphone: 04 78 79 50 50
Fax: 04 78 80 40 68
Email: [email protected]
Key words :
air-entraining, annual global anthropogenic CO2 production, biophilia, calcination, CBSA,
chemical admixtures, China, city, concrete, concrete mix design, Core Based Statistical,
densification, densification, density, embodied carbon, embodied energy, environmental
impact, environmental protection, expansion, fine aggregate, fine aggregate, fly ash, fossil
fuels, fractal, green wall, green walls, high-density, biophilic, living wall, living walls,
material, megacities, megacity, mix, mobility, organic residues, overpopulation, plasticizer,
population density, population-weighted density, pure sands, recycled aggregate, recycled
aggregate, sand theft, sea sand, Shenzhen, social connectivity, sustainable development,
urban, urban density, urban scaling, US census, vertical, verticalization, waste glass,
workability, world population, Yangtze
Nombre de mots : 4,721
Introduction
Building better cities in the face of overpopulation amid shifting climates will require
practicable solutions to meet the biophilic[1] needs of city dwellers, mitigate air pollution, and
reduce the urban heat island effect. Living walls could play a prominent role in returning
nature to urban canyons while helping to cleanse and cool the environment. However, to be
successful on a large scale living walls would have to overcome the pitfalls of existing
systems: primarily ensuring that the system will have the durability to last as long as the
lifespan of the building. One solution could be material-based: what if the components of a
living wall system could be simplified to be constructed of one very durable material? What if
that material could allow living walls to be integrated into a building’s structure? In this
regard, considering all the most common and accessible building materials, perhaps concrete
is the material with the most flexibility and durability. Conceivably concrete can be
economically engineered into a durable growing medium for plants. Concrete's ubiquity - it is
the most widely used construction material in the world – and ability to be produced locally
increase its worldwide potential as a living wall substrate.
Growing plants on a vertical surface gives the appearance of natural simplicity, but
achieving a successful living wall is a complex problem with many factors including ensuring
the appropriate support structure, maintaining the proper amount of water, oxygen, nutrients
and pH levels, choosing plants that can survive seasonal climatic changes, and establishing
the appropriate lighting conditions.
These technical challenges are numerous and perhaps solvable, but the first question
that must be answered is not how can we meet these challenges but whether or not it is
justifiable to do so, i.e, the larger and holistic questions must be answered first. To that end,
this paper will be the first in a series that will determine if proposing this new living wall
system is a valid response to the challenges facing cities by answering the following
questions, of which the first two will be addressed herein: Can concrete be a sustainable
material?; What urban density population will reach us?; Can buildings be conceived that
adapt to shifting climates?; Is verticality a relevant response to urban densification?; Can
high-rises be a sustainable building typology?; Can plants grow on a material with cement as
its base?; What is the proportion and availability of concrete on facades?; Can green walls
satisfy the Biophilic needs of city dwellers?; Can green walls increase the biodiversity of
cities?; Can living walls be sustainable?; Can living walls be sustainable even with irrigation?;
Who are the beneficiaries of green walls?; How can living walls become affordable?
1. Can concrete be a sustainable material?
•
Defining how to judge concrete’s sustainability
“Humanity has the ability to make development sustainable to ensure that it meets the needs
of the present without compromising the ability of future generations to meet their own
needs[2].” This is how sustainable development was first summarized by the Brundtland
report in 1987. And most agree that the industrial production of a product such as concrete
should strive to mimic the closed looped cycles of natural systems wherein all waste is
eventually reused within the same system, i.e., incorporating what is known as a cradle to
cradle design methodology[3] [5]. And in terms of the environmental impact of concrete,
sustainability would mean that: air, water, and soil pollution is avoided along with the
emissions of greenhouse gases; natural resources are not used in a rate greater than that which
they can be regenerated; and that the development improves “the living standard and quality
of life without affecting the environment[6].” In terms of measuring a product’s impact it is
necessary to look at all four stages of a building product’s life-cycle; material manufacturing
(including the extraction of raw materials), construction, use and maintenance, and end of life
(which includes the energy consumed and environmental waste produced to demolish and
dispose or recycle the building product)[3].
•
Evaluating concrete
Evaluating concrete by the above criteria will paint an unfavorable picture of concrete’s
global environmental impact. The reason for this has more to do with the astounding quantity
of concrete that is employed around the world than to its impact when compared with other
building materials. More concrete is used than any other man-made building material. In fact,
of all of the building materials only water is used in greater quantity than concrete. Some
have identified concrete as the single most important man-made material in the world[7].
According to the Cement Association of Canada, the annual global production of concrete is
approximately 5 billion cubic yards (3.8 cubic meters)[8]. To understand what all that means
in terms of material usage and environmental impact, one must first identify the materials that
form concrete.
Looking at the components of concrete one finds that the actual mix designs of
concrete vary greatly, but an example of a standard concrete mix design is one with 11
percent Portland cement, 41 percent coarse aggregate (gravel, crushed stone or recycled
aggregate), 26 percent fine aggregate (sand), 16 percent water, and 6 percent air[9]. So what is
the impact on the environment of each of these components, and what can be done to reduce
that impact? Each of these components will be individually addressed, beginning with
cement.
Much of the focus on making concrete more sustainable has been on lessening the
environmental impact of cement, which is the ‘glue’ of concrete. This is because most
concrete uses Portland cement, which uses an enormous amount of energy in terms of
electricity, process heat, and transport[8]. It is estimated that concrete production contributes
5% of the annual global anthropogenic CO2 production. The general rule is that for every
metric ton of Portland cement produced approximately one metric ton of CO2 is created[10].
The two sources of CO2 generated during the manufacturing of cement are from the burning
of fossil fuels and calcination (calcium carbonate is heated and broken down into calcium
oxide)[11]. Calcination alone accounts for two-thirds of the CO2 produced. One method of
reducing the carbon footprint of Portland cement is to replace a portion of it with alternative
cements, such as with the waste materials from blast furnaces (slag) and coal-fired power
stations (fly ash). Another method is to simply reduce the amount of Portland cement by
optimizing the mix design. This can have the effect of increasing the durability of the material
because of all the ingredients in concrete it is the cement that is the most porous. It is through
its pores that concrete is vulnerable to penetration by sulfates, chlorides, and other corrosive
materials[10]. Other approaches to reducing the carbon footprint are to reduce the specific
heat consumption of the cement kilns (less heat requires less energy) and using other energy
sources in lieu of fossil fuels[12].
It is interesting to note that in 2014 cement production rose to 4,180,000,000 tons.
That is 4.2 billion tons, approximately. China alone accounted for 60 percent of that total,
followed by India with nearly 6.7 percent of the world’s cement production[13].
Course aggregates in concrete take up the lion share of the standard concrete mix
design. Crushing stone and collecting gravel creates a depletion of non-renewable natural
resources. There is only one viable way to reduce the amount of virgin course aggregates in a
standard concrete mix design and that is to replace the virgin material with either waste or
recycled material. Waste material can be iron slag[14] or other industrial solid waste[15].
Recycled aggregates that come from recycled concrete could also play a large role in reducing
the consumption of raw materials for course aggregate.
Fine aggregates in concrete are almost always made from sand. Getting sand for
concrete that has the appropriate characteristics can prove challenging in many parts of the
world. The building boom that has accompanied the rise of China, India, and Brazil in recent
years has put unprecedented pressure on what people are realizing is a material with a finite
supply. Sand takes thousands of years to form naturally and, like oil, is being consumed faster
than it can be replenished. However, there is plenty of sand. More than enough to meet our
needs, but it is not always located in close proximity to where it is needed[16]. Competition
for sand resources is having drastic effects on the environment. Dredging for sand in seas and
oceans is severely damaging to marine ecosystems (marine ecosystems take a very long time
to re-stabilize). Sand thievery, i.e., the illegal removal of sand in natural deposits, is
decimating natural beaches creating erosion and inland saltwater infiltration[16].
We use sand for many things, including making microchips, but about three-fourths of
all sand used by man is put into concrete. Like the pure quartz sand that is needed to make
computer chips and glass, sand for concrete must have certain properties of purity or else
there can be negative consequences. For example, sand from the sea must be cleansed of
residual salt and sand from deserts are mixed with large amounts of minerals that have a
negative impact on concrete performance. The best sand is river-washed sand collected in
rivers and fresh-water lakes. These pure sands can be twice as expensive as other sands, e.g.,
sea sands. An extreme example of the economic impact of sand consumption took place
recently in the Chinese construction industry, where raw and unprocessed sea sand was
illegally used in the construction of a development in Shenzhen, China, causing a scandal that
halted construction once the substitution had been discovered. It was feared that the buildings
constructed of concrete made with this sand would face collapse in the distant future because
the residual chlorine and salt can become corrosive to the steel reinforcement in the
concrete[17] [18]. Moreover, oftentimes sand is illegally obtained, stolen from poorer country
lacking regulations or the means to protect their resources, otherwise sand is stolen from
designated protected areas that are vulnerable due to their remote locations or the inability to
be safeguarded. Stolen sand is usually cheaper than sand purchased legally, but the physical
properties of the sand can be less easy to guarantee, as the Shenzhen example illustrates. Sand
can be replaced by other materials, but the cost of grinding, pulverizing and cleaning recycled
materials is usually cost prohibitive. Recycled glass, for example, can be ground into a fine
sand-like consistency but has to be carefully cleaned to have sugars and other organic residues
removed[19] (pulverized waste glass can also be used as a substitute for Portland
cement)[20].
It’s difficult to comprehend the enormity of 800 million cubic yards (611,643,886
cubic meters) of water, but that is the annual amount of water used in the production of
concrete if it were to be used in proportion to the above figures. Even with examples it is
difficult to put this figure into perspective, but that volume of water is roughly 1,000th of the
minimum annual flow of the Yangtze’s river[21], or approximately half of the freshwater
needs of the country of Israel[22]. To lower the water consumed in concrete production, some
researchers have tried analyzing various alterations of the mix design. For example, by
substituting Portland cement with large amounts of fly ash and not air-entraining the mix,
researchers were able to reduce the amount of water to between 8.8% and 19.4%[23]. Another
way of reducing water in the mix, and reducing the volume of concrete in general, is to use
ultra-strong varieties of concrete so that less material provides the necessary performance.
Chemical admixtures can be added to the mix to maximize water reduction. Reducing the
amount of water is necessary to achieve ultra-high strengths, but plasticizers and other
chemical admixtures are needed to improve the workability of the mix[10].
•
Comparing materials
An analysis of the environmental impact of concrete in terms of its individual components
must be tempered by comparing concrete to its alternatives. According to the Inventory of
Carbon & Energy (ICE) Summary, concrete (as used in floor slabs, columns, and load-bearing
structures) has a much lower embodied energy (1.11 MJ/kg) and embodied carbon (0.043
kgC/kg) than the traditional construction materials of brick (3.00 MJ/kg; 0.060 kgC/kg), steel
(24.4 MJ/kg; 0.060 kgC/kg), and wood (8.5 MJ/kg; 0.125 kgC/kg) [24], the latter being the
only renewable resource (but wood is also the resource that is most limited in construction
typology due to the life safety concerns related to its fire resistance). Regardless of material
type, it is the construction systems that must be analyzed and compared. For example, a solid
load-bearing wall made of concrete will have a greater volume of material than a cold-rolled
steel-framed wall, and there are many applications for concrete that cannot be replaced with
steel or wood, e.g., in road construction, wherein material alternatives are so limited that
concrete is virtually the only economically viable option.
2. What urban density population will reach us?
As of 2014 the world population reached 7.2 billion and, as opposed to earlier predictions, the
world population will continue to grow through the end of this century[25]. As of June 2014,
54% of the world’s population lives in urban areas, a number that is expected to rise to 66%
by the year 20501. As of 2014, 82% of North Americans live in urban areas, and that
percentage is 80% in Latin America and the Caribbean, 73% in Europe, 48% in Asia, and
40% in Africa[27]. These latter two regions are urbanizing the fastest, faster than all of the
other regions, and by 2050 it is expected that Asia will become 64% urban and Africa 56%
urban. Worldwide, cities are gaining nearly 60 million residents each year[28].
•
Expanding populations take (and make) shape
Cities adapt to increasing populations by changing their scale and these adaptations will have
a predictable effect on a city’s form. The formal changes in terms of growth patterns are
typically fractal in nature[29], and the effects of form in terms of density are a densification in
the city core and horizontal sprawl at its edges. There is disagreement on how high-density
relates to the three pillars of sustainable development[25]: social development; economic
development[30],[31]; and environmental protection[32]; but the latest scientific theory using
the hypothesis of urban scaling [“namely that certain properties of all cities change, on
average, with their size in predictable scale-invariant ways”[33]], makes it probable that for
the foreseeable future the pattern of city growth that includes high-density cores is likely to
proliferate. For example, taking the enormous variability of private wealth distribution (and
its effect the on exclusivity of highly-dense areas) away from the debate about the
densification of cities, and it becomes clear that the economical sustainability of highly dense
areas of cities will continue to be a powerful driver of densification2. Many new case studies
«
1
Concise Report on the World Population Situation in 2014 ».
[26]
2
« Baldwin et al. - 2003 - The core-periphery model Key features and effects.pdf », consulté le 9 février 2015,
http://econ.sciences-po.fr/sites/default/files/martinp/CPmodel.pdf.
have proven that an optimal city has a self-assembled balance between density, mobility, and
social connectivity[34].
•
The difference between population density and urban density
Human population density, or simply population density, is the mid-year population divided
by the land area[35]. In other words, population density is the population of a given political
boundary divided by the land area of that political boundary. Population density statistics are
useful for governmental bodies to monitor urbanization trends, e.g. rural vs urban statistics,
and settlement conditions on a continental, national, and city-wide level[36]. The problem
with population density statistics is that a city’s average density does not provide useful
information to aid in local decision-making, e.g. in designing a public transit system[37]. For
example, Los Angeles’s low population density could be used as an argument against
augmenting its public transit system, whereas if one were to look at the diffusion of density
within LA’s metro area, one would find sufficiently numerous high-density areas that indicate
a need for greater public services.
Urban density differs from population density and is defined as the population within
the political boundary of an urban area divided by the land area of the political boundary of
the developed urban area, i.e., parks, lakes, and rural areas are removed[32]. The benefit of
the urban density calculation is that it is experiential. Urban density figures can give us a
better idea of how density is experienced in a city[32]. The concept of urban density
sometimes finds its way into density statistics and is referred to as net density. When added to
population density, the term net is used to indicate that an area has been subtracted from the
boundary of the density calculation, e.g. undeveloped or inaccessible parcels of land, so as a
result the population density will increase.[38] In this way, the term net population density
could be used synonymously with urban density.
Unfortunately, comparing urban densities between cities, or even between areas within
the same city (as would prove advantageous within the context of studying mega-cities) is
difficult due to a lack of data and the difficulty of finding data recorded using identical
parameters. Typically, governing bodies use the simpler population density to provide a
general idea of comparative densities. This is useful for a rough picture of a city’s density, but
urban density figures could aid in identifying city areas that are the most in need of studies to
determine the impacts of overpopulation, nature deprivation, and climate change, and in
predicting the global increase of highly-dense urban areas. This would also aid in analyzing
the link between density and verticality, for the purposes of predicting the scale of the
challenges city dwellers will face.
•
Population-weighted density and population distribution
In response to this need, the United States Census bureau developed alternative methods for
calculating population density. The most recent US census – 2010 – includes figures for what
is called population-weighted density. This can be understood as the concentration of density
within a metro area[39]. According to the US Census bureau, population-weighted density
“…can be thought of as the average of every inhabitant’s census tract density.”[40] The
numbers are derived from the different densities reported in all the census tracts within a
metro or micro boundary, or CBSA (Core Based Statistical Area).[40] The difference between
population density and population-weighted statistics are profound, as the data from the
Census special report indicates. For example, Honolulu, Hawaii has a population density of
1,586.7 per square mile, but has a population-weighted density of 11,548.2 persons per sq. mi,
the latter being more illustrative of the density inhabitants of Honolulu experience in and
around the central city.
Additionally, the recent US census special report includes data on the distribution of
density within cities. These figures provide a useful reading of where density is concentrated
in a city, and typically reflect the prototypical distribution of people within cities: drastically
denser at the city core, then a precipitous drop in population outside of the core and which
decreases steadily toward the periphery. This is not the rule, but it is the standard.
There is one lesson that should be kept in mind when using population density
statistics and that is that the parameters are variable and can be misleading. Density statistics
are subjective because the words that are used to define them can have different meanings.
Take the term the net density of Honolulu, for example. Density in the context of urban
planning will always measure some degree of human contact within a defined boundary, but
that can be in terms of residential density, e.g., the number of residents or number of homes in
a given area, economic density, e.g., the number of jobs in an area, or development density,
e.g., the number and typology of buildings in a given area[38]. The term net can mean that
undeveloped lots such as lakes are subtracted from the boundary area, or not at all[38]. And
Honolulu can mean the City of Honolulu or the greater aggregate of the Honolulu
metropolitan area[38]. All this means that density statistics will be challenging to employ for
constructive purposes and relatively easy to exploit for political purposes.
•
Megacities
The most dramatic aggregation of population is occurring in megacities. A megacity is
typically defined as a city with more than 10,000,000 inhabitants[41]. As of 2014 there are 28
megacities in the world, a number that is expected to grow to 41 urban agglomerations by the
year 2030[27]. According to the UN’s 201 urbanization prospectus, Tokyo, with a population
of 38 million inhabitants is the largest urban agglomeration, followed by Delhi with 25
million inhabitants, Shanghai with 23 million, and Mumbai, Mexico City and Sao Paolo each
with 21 million inhabitants[27]. Note that these are southern cities, cities located in the global
south, which is where the majority of mega-cities are located. Looking ahead, the world is
expected to gain 2.6 billion people by 2050 with 90% of them in Asia and Africa[27].
•
Verticalization and its link to urban density
For cities, densification is typically followed by verticalization in the model first established
in North American cities3. This model predicts that the growth of an urban population leads
to vertical expansion in city cores. The architectural impacts of verticalization [“the rapid
increase of inner city apartment high-rise buildings, resulting in the development of “vertical”
city parts and urban densification »][43] are many. New Urbanists and some architectural
3
[42] p.126
theorists, such as Nikos A. Salingaros and Léon Krier, abhor skyscrapers. Krier has referred
to them as typological aberrations, typological hypertrophies[44]. Equally passionate, others
claim that residential high-rises are the sustainable solution to limited space and escalating
populations.
Conclusions
Discovering whether or not concrete can be a sustainable material is an important step in
meeting the challenges facing sustainable development. But, realistically, the question has
only one answer. Whether or not one can be an advocate for using concrete or accept its use is
not a justifiable question in light of the overwhelming global need and demand for what is a
relatively inexpensive construction material. Concrete is made of ubiquitous materials that
can be mixed and employed by people with an extremely wide-range of competencies ranging
from the technologically primitive and rising to the most advanced echelons of emerging
technology. One must accept its use and, if in a position to do so, deploy it in the most
judicious manner conceivable. So the only relevant answer to the question of whether or not
concrete can be a sustainable material is that it must be made to be as sustainable as humanly
possible. If not, the global reach of concrete is so broad and profound that the result will be
disastrous. Turning a blind eye to the question of how to increase its sustainability would be
tantamount to actively and consciously participating in the destruction of our environment.
Therefore, it is necessary to use an ecological lens to look at economizing the ingredients of
concrete, beginning by seeing its components as part of a larger closed-loop and natural
economy. Furthermore, the technological advancements of concrete can be used to optimize
the mix design which in turn can be used to influence the design process at the earliest stage
possible, the stage of conception.
Understanding urban density and how it will influence cities will aid city planners, managers,
architects, engineers, urban and landscape designers to choose the appropriate development
solutions to make cities as livable as possible. The latest statistical modeling theories make it
clear that the control over the successful development of a city is much more complex and
independent than traditionally understood, and mostly governed by complex and diffuse
social and economic networks that thrive and intensify with urban densification. Still, the
more we understand about where and how urban densification is unfolding the more
influential the designers, planners and managers of urban landscapes can be.
In summary, if green walls are to play a positive role in confronting the three most pressing
challenges facing contemporary cities – overpopulation, access to nature, and climate change
– they will need to be designed and built to satisfy the requirements of a sustainable city, i.e.,
they must be able to satisfy the social, environmental, and economic needs of their urban
environments. However, the impact of green walls on a city can be multi-faceted and multiscalar, which results in their benefits overlapping two or more of these three realms. Green
walls will have an impact on their local environment and can have a broader contextual
impact on the larger urban network. Moreover, if green walls are to be constructed with
concrete then it will also be necessary to measure the environmental impacts of concrete and
predict the building typologies that would benefit from their installation. This article, the first
in the series, explored the intricacies and challenges of determining concrete’s sustainability
and how population growth will influence what cities will become. Exploring the problems
confronting cities while analyzing the materials that will build them is critical if we are to
keep an eye towards how the built environment can positively contribute to sustainable urban
growth.
GLOSSARY
Biophilic design aims to re-establish the instinctive bond between all living things.
Population density formula: D=Σ(Pidi)/ΣPi, where D is the population-weighted density of a metro or
micro area, and Pi and di are the population and density of the ith census tract, respectively.[40]
The term core based statistical area (CBSA) is a collective term for both metro and micro areas.[45]
A metropolitan area contains a core urban area of 50,000 or more population. [45]
A micropolitan area contains an urban core of at least 10,000, but less than 50,000, population.[45]
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