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Francesco Biasioli, ERMCO – European Ready Mixed Concrete Organization

SOMMARIO

Quando si tratta di sostenibilità è comunemente accettato che se ne debbano considerare non solo gli aspetti relativi all’ambiente, ma anche quelli che coinvolgono l’economia e l’impatto sulla società. Questi tre “pilastri” della sostenibilità sono alla base delle specifiche norme europee e devono essere presi in conto anche per i materiali da costruzione, caso in cui un approccio volutamente banalizzato, usato talvolta per impressionare i non specialisti, può portare a conclusioni fuorvianti se non del tutto errate. Per i tre più diffusi materiali da costruzione – legno, acciaio e calcestruzzo armato – nell’articolo vengono esaminati alcuni aspetti relativi ai tre “pilastri” citati: per il sociale, la sicurezza degli individui, per l’ambientale le emissioni di CO2 e per l’economico i costi. L’intento è di separare i “fatti” dai “miti” intendendo come tali quelle idee che suonano così ragionevoli da essere universalmente accettate anche se basate su ipotesi talvolta errate. Definito il concetto di “unità funzionale” si sviluppa il confronto per una unità semplice, un pilastro dimensionato seguendo le regole di progetto della specifica norma europea di calcolo, il relativo Eurocodice. Per un materiale non omogeneo come il calcestruzzo armato vengono definiti gli opportuni “coefficienti di incremento” delle prestazioni del solo calcestruzzo che tengono conto della presenza dell’armatura. Per finire si esamina cosa cambia passando dai casi ideali ai casi reali e se si considerano altri tipi di unità funzionali. L’articolo è in lingua inglese.

A MYTH: THE STRENGTH/SPECIFIC WEIGHT RATIO

The following is taken from lesson slides about timber structural design given in an Italian University.1

1 http://sparch.unipr.it/didattica/att/7bf4.7390.file.pdf

Sustainability and construction materials:

myths, facts and fallacies

(CALIBRI 24, GRASSETTO)

Sottotitolo articolo (CALIBRI 16, GRASSETO CORSIVO)

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The table gives average values of compressive strength (“resistenza”)2, specific weight (“peso specifico”)3 and elastic modulus (“modulo elastico”) of three materials, timber (“legno”), steel (“acciaio”) and reinforced concrete (“cemento armato”). The sentence above the table says: “Timber is the construction material with the highest strength/specific weight ratio: very efficient!”. The sentence below says “A well-designed timber structure has a similar section to one with (reinforced) concrete, and weight similar to a steel one”. As written, the two sentences address two different questions. The first one based on materials alone states that the “efficiency” of materials should be measured – and therefore materials compared - on the basis of their (compressive) strength/specific weight ratio; the second considers the impact of materials in designing a structure. At first sight the strength/specific weight ratio criterion sounds good – the lighter and stronger the material, the better. But what does this mean when compressive strength is used, as in the table? In this case it may be considered a myth – it sounds convincing, but misleading if we go into detail. The underlying idea is to build an ideal column of constant section A as high as the (compressive) strength of a material allows and to assess materials’ efficiency on the basis of “the greater the height , the better the material”. Let’s define “f” the collapse strength (compressive load per unit area) of a generic material, and “A” the area of its transverse section: the maximum load F the area A can withstand when uniformly loaded (as in a column subject to a theoretically perfectly axial load) is F = (f A). The weight W of a column of height “h” is obtained by multiplying its volume V = (Ah) by w, the specific weight of the material, so W = w V = w (A h). Equating the “resistance” F and the “action” W the height hmax equals the (compressive) strength/specific weight ratio:

F= W f A = w A hmax hmax = (f/w)

Expressing the strength in kg/m2 and hmax in meters, using initials “s” for steel, “c” for concrete and “t” for timber and the data in the table: hmax,s = (4000x104)/7850 = 5100 m hmax,c = (400x104)/2500 = 1600 m hmax,t = (400x104)/500 = 8000 m On the basis of this easily understood, easily calculated “performance” criterion, timber scores best of the three: a really attractive construction material! But something must be wrong or missing in this approach if in the real world the tallest building constructed with concrete, Burj Dubai, is “only” 830 m high, the tallest building made of steel, Taipei 101, is “only” 501 m high and the world’s tallest building made of timber (the most “efficient” construction material according to this criterion) is of 12 timber storeys supported by 3 concrete storeys, built for the EXPO 2015 in Milan (IT). And Hyperion, the tallest tree in the world, a “sequoia sempervirens” in California Redwood National Park is “only” 116 m high! With computers available today why are our engineers (even the best among engineers, Mother Nature) still unable to cope with the intrinsic properties of construction materials? Is it their fault, or is the strength/specific weight ratio, i.e. the maximum theoretical height hmax a misleading indicator, a “myth”? Let’s look at some “facts”. In the structural design of even the simplest structure like an ideal column, correct figures have to be used: the compressive strength of all three materials is in

2 If buckling is not an issue, a specimen of transverse area A fails in compression when loaded with a force F: stress f = (F/A) is the

(ultimate) strength (collapse force per unit area) of the material, usually expressed in N/mm2.

3 The “specific weight” w of a material is its weight W per unit volume V (w = W/V). The symbol used in physics for a material’s

density (mass per unit volume ) is (the lower case Greek letter rho) and w = g, where g is the acceleration of gravity. According to the International System of units, density w is expressed in kg, while specific weight w is expressed in kN/m

3. In common

language metric kilograms (kg) and tons (t) are often used for weight: to convert 1 kN 100 kg, 1 t = 1000 kg 10 kN.

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reality not a single figure as in the table above, but a range of strengths “fk” (the subscript k means “characteristic”, i.e. a single figure evaluated on a statistical basis). These fk values identifying the “strength class” of each material are defined in the relevant product standard and referred to in the relevant design standard (in Europe, in Eurocodes). As a structure designed on the basis of a material characteristic strength would have too high a probability of collapse, engineers use a value lower than the characteristic strength fk of the material - its “design” strength fd = fk/m (d = “design”). This is obtained by dividing “characteristic” values by the material-specific “safety factor”, m, which takes into account both material and design issues - how the material is manufactured, how material properties are influenced by the environment, how accurate are the theoretical models used in design etc. Safety factors, m, may differ from country to country, but max-min values can be easily identified. Dealing with columns in compression, the ranges of characteristic compressive strengths fk, the appropriate safety factors, the compressive design strengths, fd, and other (partly amended) mechanical properties of materials are listed in table 1 for the three materials.

Material Mean density mean

Character. compression strength fk

Safety factor

Design strength fd

hmax = fd /w Slenderness ratio

Slenderness radius h/

kg/m3 N/mm2 - N/mm2 m - m Timber4 350 - 550 19 – 29 2,40 7,9 -12,0 2025 – 2210 20 101 - 111 Concrete 2300-2400 20 – 55 1,4 -1,5 13,3 – 36,7 566 - 1560 50 11 - 31 Steel 7850 390 – 550 1,05 224 - 524 2850 - 6670 150 19 - 45

Table 1. Material properties, height hmax and slenderness radius

If the ranges of maximum heights are re-calculated on the basis of the design strengths, the previous picture is reversed (table 1): steel comes first, then timber and concrete. Though now more realistic, the new maximum height values remain remote from the real world, so something is still missing. The first rule for the sustainable use of any material is to reduce the quantity of material required to an absolute minimum. As engineers say, “structures are designed from top to bottom” (and usually built from bottom to top) because going down, from top to bottom, the total weight W of the structure - and the service loads it supports, if any - obviously increase. Given a maximum material strength, fd, the area of the structure, A, should therefore not be constant but increase from a theoretical minimum at the top to a maximum Amax in the bottom. The geometry of this “uniform resistance column” with (variable) section area A is well known and may be easily recognized in a number of man-made and natural structures (Fig. 1).

Figure 1. Man-made and natural uniform resistance structures

4 Max-min values according to EN338 – Structural timber – for timber classes C20 – C50

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With this variable area A the weight W is reduced, and the height hmax should increase, so the problem remains: with can even a uniform resistance column not reach the theoretical maximum height hmax? Answer: because it would be too “slender” and collapse due to instability - as the Tower of Babel did. This “buckling attitude” is taken into account in design standards by setting limits to the “slenderness ratio”, , of the material. For an ideal column of constant circular shape of radius r, i.e. with no preferred buckling direction, this is:

= K h/r r = K h/

K is a parameter which depends on the effectiveness of the restraints at the extremities of the column and, in a simplified analysis, K = 1,0 may be assumed for all materials, i.e. not to depend on the material. For a given height h the greater the slenderness ratio , the lower the radius and the area A, so the lower the quantity of material required - and vice versa. The h/ parameter therefore takes into account both strength and stability, two aspects to be considered in the case of columns. On the basis of the maximum heights hmax previously calculated and the limit values for given in the relevant Eurocode for timber, (reinforced) concrete and steel, slenderness radii (h/) have been calculated (table 1). On the basis of (h/) concrete scores first and while there is no significant difference between concrete and steel, timber has a (h/) value significantly higher than that of the other two. This is one of the reasons why the highest building in the world is made of concrete and the height of real timber structures is limited : buckling is a problem for this material. Taking into account “facts” it is evident that for construction materials the (compressive) strength to specific weight ratio is misleading when structures have to sustain compression loads. It makes sense for other type of structures – rockets, ships, airplanes, Formula 1 racing cars, bridges – where weight is really THE problem. This is the reason why light alloys like aluminium were used in the past and composite materials such as carbon fibres are used today. The situation will probably change in the future, due to the advance of science and technology which will give us stronger but lighter construction materials, but today facts are as described.

FROM MATERIALS TO FUNCTIONAL UNITS

The above examples demonstrate that for building and civil engineering structures comparison made on the basis of material properties alone is misleading. The question to be asked first should be: what is the function a structural element has to fulfil? A slab has to support a given load (self-weight included); a column has to support a given total surface of slabs, so again a given total load; a foundation has to support a given column supporting a number of slabs so again a given load….and so on. For structural elements, load bearing capacity is the basic criterion, but others may be added – e.g. durability (the maintenance of the load-bearing capacity for the structure’s lifetime assumed in the design), robustness, serviceability, cost, environmental impact etc. In general the “performance” required (structural, thermal etc.) is related to a “functional unit” of the structure. This “functional unit” should be identified first – as small as a single column or as large as a whole building. To properly identify a functional unit, “boundaries” may have to be considered. For example, as structures have to stay firmly on the ground, an optimally designed column would apply to the ground a pressure more or less equal to the design strength, fd, of the material used in building.

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Excluding rock, the load capacity of common soils is a fraction of the design strength fd of any construction material: by consequence columns cannot be directly laid on ground and an intermediate structural element, a foundation, is needed to “spread” the load on the ground. Whatever material our ideal column is made of, if a comparison among structural solutions using alternative materials has to be made, it is mandatory to consider the column together with its foundation – which is usually built with concrete. Other restraints may depend on the way the unit is connected to a larger structure – a column may be differently restrained at its ends so that its slenderness is modified – or on the way in which the functional unit is available on the market – industrially produced components like steel and timber elements usually have fixed finite dimensions described in a catalogue and the nearest currently available section, not the minimum “ideal” one, is assumed by the designer.5 If materials have to be compared, the functional unit should be:

identified first, “boundaries”, if any, included, then optimally designed using each material’s specific design properties and taking into

account what is common practice and/or is available on the market. In the case of structural elements, once the above criteria are satisfied, it is easy to calculate the load, self-weight included, which a functional unit of a given geometry made with a specific material (the reference material) may withstand. This load should then be used to determine the dimensions of the same unit using other structural materials, taking account of their specific properties and structural behaviour. Only when the different solutions, all able to support the same load, i.e. to fulfil the required function, have been identified, a “sustainability” evaluation based on the three “sustainability pillars” can be made. However, to allow rapid comparison and to avoid cumbersome design, a simplified approach may be used, as described in the following.

THE FIRST RULE OF STRUCTURAL DESIGN

Let’s consider a 1 meter long section of an ideal column under a long term compression load theoretically perfectly centred, so that bending has not to be considered. The column is part of a building assumed to have storeys of limited height, again in order not to have to consider buckling, because, as already pointed out, this may be a problem for timber elements. In the absence of buckling, if the column is made of a “homogeneous” material (in this context, timber and structural steel may be considered homogeneous, while reinforced concrete is not) the resisting design axial forces NRd are given by the formula already stated (“t” = timber, “s” = steel):

NRdt = At ftk /t = At ftd NRds = As fsk /s = As fsd [1]

At and As are the timber and steel transverse section areas, ftd and fsd their “design” compressive strengths. In the case of a non-homogeneous material like reinforced concrete (“rc”) the 1 m long column of (external) concrete section area Ac includes an area Asl of longitudinal reinforcing steel bars and an area Aswt of transverse steel used for stirrups; the second is not required in the calculation of the axial resisting load, but it has to be taken into accounts for economic and environmental evaluations. To compare the r.c. non-homogeneous solution with the others it is necessary to ”homogenise”, to transform steel into an ideal concrete using the so called reinforced concrete “enhancement coefficients” which “ideally” multiply a basic concrete characteristic (strength, 5 Steel, timber and precast concrete elements with predefined sections are used in practice. Cast in-situ concrete

sections may be theoretically adjusted to any value, but in practice some modularity is also common.

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cost etc.) to take into account the presence of steel. 6 For axially loaded columns the “strength enhancement coefficient” αc is defined so that a formula with a format similar to that of the steel and timber ones:

NRdrc = Ac (αc fck) /c = Ac αc fcd [2]

Whichever of the three materials is considered, the “function” of a column is to support a given design axial load NEd . To compare the functional units the following equalities apply:

NEd = NRdt = NRds = NRdc NEd = At ftd = As fsd = Ac (c fcd) [3]

Equation [3] states the first rule of sustainable (structural) design: for economic, social and environmental reasons, whatever the material is, use the minimum quantity (area A) technically possible by selecting its technically maximum available performance (the design strength fd), because adequate safety is guaranteed by the specific safety factors . Once the (actual or enhanced) design strength of each material fd has been identified, the equalities above give the theoretical MINIMUM areas of timber At, steel As and concrete Ac, i.e. for each material the section area able to sustain the stated applied load NEd. This approach is far more effective than the use of the strength/specific weight ratio as it is not based on the material properties alone but on the concept of a “functional unit” fulfilling a specific function – in this case to sustain load NEd. However, the reference to the “theoretical minimum” area is partly misleading so it should be used only for a first “rough” evaluation because limitations exist on the commonly available sections for timber and steel and, in the case of reinforced concrete, on the current geometry and the minimum/maximum quantity of reinforcement. As already discussed, comparison should be based on the calculation of the maximum load capacity Rd of the functional unit of a real section made of (any) selected reference material. For columns, once for the reference material has been decided the maximum load capacity NRd NEd is calculated then used to design the real dimensions when other materials are used, in all cases taking into account geometrical material-specific limitations, if any. The resulting solutions can then be compared.

CONSIDERATIONS OF ECONOMY AND OF THE ENVIRONMENT

What is described above mainly deals with safety, the social pillar of sustainability, so it only partly addresses the issue. What about the other two “pillars” - economic and environmental? To address the economic pillar, the cost C of the functional unit made with each material has to be calculated. Once the areas A of the structurally optimized solutions are identified as described above, the volume V of a 1-m long element is obtained simply by multiplying its section area A (in m2) by 1 so V = (A x 1) = A. If the materials’ unit cost c (cost per unit volume)7 in a specific place under the same delivery conditions (e.g. at the construction’s site gate) is known, the costs of 1-m long functional units for timber Ct, steel Cs and reinforced concrete Crc are:8

Ct = ct At = NEd (ct /ftd) Cs = cs As = NEd (cs /fsd) Crc = crc Ac = NEd (crc /cfcd) [4]

Unlike the calculation for strength, the reinforced concrete cost coefficient crc has in this case to include the cost of both the areas of longitudinal steel Asl and transverse steel Aswt used for

6 F. Biasioli - “Measuring reinforced concrete sustainability the “enhancement coefficients” - under publication.

7 While the cost per unit volume (€/m

3) is used for timber and concrete, for steel the cost is usually given in €/kg and has to be

converted into cost per unit volume multiplying it by the steel specific weight w: (€/kg)x(kg/m

3) = (€/m

3).

8 All costs are to the “gate” of the construction site. Costs related to the construction phase have not been considered: for timber

and steel, those related to erection and connections (material and workforce); and for concrete, the costs of scaffolding, casting and curing. Even if they are included, the relative positioning of different materials does not significantly change.

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stirrups. This can be done by applying a reinforced concrete “cost enhancement coefficient” αcc > 1 to the basic cost of concrete cc :

crc = αcccc

What about the environmental pillar? Though the environmental profile of each material may be described using a number of parameters9, for the simplified approach used in this study only the “CO2 footprint” related to the “greenhouse effect” is used. The embodied energy and emission figures, determined as part of the life cycle inventory (LCI) of each building material, may be evaluated using the so called “embodied CO2” (ECO2 – the quantity of CO2 embodied in an unit volume of material10) and a Life Cycle Analysis (LCA) from “cradle-to-(construction) gate”, as for costs. The embodied CO2 of materials is “….still a matter of debate and care should be taken when transferring data collected in one country or region to other countries….”11: as data from different sources give different “embodied CO2” values, standardized information (for example as in Environmental Product Declarations - EPDs) will be, when available, most welcome. Substituting in eq. [4] unit costs (c) with unit embodied CO2 (eCO2) in kg/m3, being as before V = (A x 1) = A: ECO2t = eCo2t At = NEd (eCo2t/ftd) ECO2s = eCo2s As = NEd (eCo2s/fsd) ECO2rc = eCo2rc As = NEd/ (eCo2rc/cfcd) [5] As for costs, to take into account longitudinal and transverse steel an “enhancement

cCO2 >1 has to be applied to the embodied CO2 of concrete eCO2c:

eCO2rc = αcCO2 eCO2c

Given the three units fulfilling the same function (supporting the same load NEd), each using exactly the theoretical minimum of material (the minimum area A), from eq. [4] and [5] a theoretical cost and/or embodied CO2 comparison may be obtained simply by comparing the (real or enhanced) “cost per unit design strength” (c/fd) and “embodied CO2 per unit design strength” (eCO2/fd) ratios of different materials – the lowest they are, the better. We highlight “theoretical” because as stated before, “ideal” solutions may be misleading and it is preferable to compare “real” solutions . Even so, a first classification may be based on comparison of materials’ ratios (c/fd) avoiding to perform any structural design. It has to be recalled that results depend on the costs and embodied CO2 of the materials used, and these are usually different in different countries due to different design habits, materials characteristics and unit prices.

EXAMPLE

In the following, taking as the functional unit an “ideal column” supporting the same axial load NEd three different materials are compared using the cost/strength and embedded CO2/strength theoretical ratios. For strength, reference is made to materials currently used in the EU; costs are those of a country where price data are publicly available;12 embodied CO2 data are taken from

9 Environmental Product Declarations (EPDs) in accordance with EN15804 provide up to 24 different environmental parameters.

10 ECO2 data, usually supplied as dimensionless figures – i.e. kg of CO2 emission per kg of material - are converted into values per 1 m

3 of material by multiplying them for the material specific weight.

11 page 15, Fib Bulletin 67, 2012– Guidelines for green concrete structures - www.fib-international.org,

12 Official prices of construction materials and works are published In Italy either by producers of by the local Chamber of

Commerce, “street” prices are usually lower. The following reference have been used: for timber and steel http://www.elencoprezzi.provincia.tn.it; for concrete: http://www.marx.it/Preislisten/Beton_italienisch.pdf

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literature13. For all materials, a cradle to (construction) gate approach is assumed. The materials’ data used are:

Timber: class C30, ftd 9,6 N/mm2 ct = € 300/m3 eCo2t = 80 kg/m3 Structural steel: class S355, fyd = 338 N/mm2 cs = 12.560 €/m3 eCo2s = 9420 kg/m3 Concrete: class C30/37 XC4, fcd = 20 N/mm2 cc = 95 €/m3 eCO2c= 385 kg/m3 Reinforcing steel: class B500, fyd = 435 N/mm2 cs = 7850 €/m3 eCo2rs = 6700 kg/m3

Assuming a concrete section area Ac reinforced with an area As = 1,5% Ac of B500 reinforcing steel, the enhancement coefficients” of reinforced concrete “are14:

αc = 1,23 αcfcd = 1,23x20 = 24,5 N/mm2

αcc crc =αcccc = 2,86x95 = 272 €/m3 αcCO2 = 1,39 eCO2rc = αcC02 eCO2c= 1,39 x 385 = 535 kg/m3

Steel increases concrete strength by 23%, cost by 186% and embodied CO2 by 39%: optimization is possible by: a) decision about which aspect to focus on; and b) careful selection of cement type, concrete class and quantity of steel: the designer’s choices – conscious or unconscious - make the difference between efficient, cost effective and environmentally friendly solutions and those that perform less well. On the basis of the above data the cost/strength and embodied CO2/strength ratios of the three materials are listed in table 2. Relative ratios are calculated assuming timber as 100% reference: the cost of the steel solution is about 1,2 times higher than the timber one, and about 4 times higher than concrete; the cost of the timber solution is about 2,8 times higher than concrete. CO2 embodied in concrete is about 2,6 times higher than in timber, and CO2 embodied in steel more than 3,3 times higher: timber clearly appears to be the “environmental friendly” material!.

Table 2. Functional unit column - “rough” comparison

But this performance comes at a cost: in the last two rows of table 2 both economic and environmental aspects of sustainability are addressed. The cost of a 1 meter long functional unit is divided by the quantity in kgs of embodied CO2, to give the cost of 1 kg of CO2 in 1 meter of this unit. If both economic and environmental aspects have to be optimized, the

13

Embodied CO2 of concrete, timber and steel: Fib Bulletin 67, 2012– Guidelines for green concrete structures 14

F. Biasioli - “Measuring reinforced concrete sustainability by the “enhancement coefficients” - under publication.

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smaller this number, the better. Considered in this way, timber is actually the least efficient material; steel and reinforced concrete are much better – the figure for steel is about 1/3 of that of timber, and that of reinforced concrete about 1/7. The decision on which to prioritise among economic and environmental indicators is a political and social one, especially in a period of scarcity of financial resources, but if both technical and economic aspects are considered, the optimal solution is clear. If, as described before, a “real” comparison is made by optimally designing a timber unit to identify the maximum load it can support, then using this load to optimally design the structural steel and reinforced concrete units, values in table 2 are confirmed: it can therefore be concluded that the cost/strength and embedded CO2/strength theoretical ratios can be used for preliminary comparison. Though costs vary from country to country and embodied C02 values are far from being universally agreed, this relative positioning is of general validity and explains why reinforced concrete remains the most widely used construction material: simply because, for a required structural and environmental performance, it is by far the most cost effective material. Also, unlike the two competing solutions, the environmental impact of concrete can be further optimized by adjusting the concrete mix design. Timber producers cannot reduce the embodied C02 of their material, but concrete producers can do just that – by choice of cement type (blended cements are both a technical and environmentally friendly solution) and cement content, which also influence the concrete strength class. Using a mechanical term in a non-mechanical way, we may say that concrete producers (and specifiers) have a number of ‘degrees of freedom’ in the design, specification and use of their product. Competitors do not have the same options for technical improvement as concrete producers. If reduction of embodied C02 is the main objective or a functional unit, a concrete “environmentally-friendly” solution can easily be identified using a different cement type and/or a higher concrete class and/or quantity of reinforcement. Even these conclusions are far than final. The study above is based on the optimal design (i.e. the minimum area) of the timber solution: as such it may be misleading, since there are severe limitations to the maximum load timber can support. As an example, a (300x300) mm C30 timber section can provide a maximum 860 kN (86 t) design axial load. This load capacity may be adequate for buildings with a limited number of storeys – where timber is actually used - but if more storeys (and by consequence a design load above 860 KN), are required, the

timber solution cannot be applied at all, as technically it does not work. In this case, steel or reinforced concrete are the only technically available solutions for designers. Furthermore, steel and timber section areas which perfectly match a required load capacity may not be commercially available. As column sizes increase in finite steps, so do their load capacities. Steel columns are usually made using so-called HEA profiles (fig. 4) whose height and base increase in 20 mm steps: for a given design strength, the load capacity of such columns approximately

Figure 4. HEA section increases in proportion to their area, in steps of about 20%, so in a number of cases some “waste” of material is unavoidable. This is unlikely for concrete, because of the flexibility given by the many combinations of areas of reinforcement and concrete and performances.

CONCLUSIONS

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The approach presented in the paper based on the use of “enhancement coefficients” for strength, cost and embodied CO2 of reinforced concrete allows the structural optimization and the evaluation of cost and embodied CO2 of concrete functional units. The comparisons made with other materials demonstrate that reinforced concrete is an intrinsically cost-effective and environmentally friendly construction material. Though developed for a simple “ideal” functional unit – a no-buckling column under a centred load – this approach may be used for more complex reinforced concrete functional units – eccentrically loaded columns, slabs, beams etc. For these elements not only the load capacity, but other considerations (e.g. deflection limits under service loads, the geometry of the section etc.) may govern the quantities of material required and by consequence cost and environmental impact, so different values of reinforced concrete “enhancement coefficients” have to be calculated but comparisons and/or optimization may be done as outlined above. Due to its flexibility in design and on site (supply of the exact quantity required, casting in even complex forms etc.), reinforced concrete is likely to score extremely well when considering all sustainability issues for these functional units also. In making the case for concrete solutions, we must appreciate and understand that in the case of a two-component composite material like reinforced concrete, design alternatives are always possible to match a required load and/or environmental performance – we can:

change the section area of concrete and/or of reinforcing steel, and/or optimise the concrete mix design to meet any specific sustainability or engineering

requirements. These “degrees of freedom” make reinforced concrete a very flexible and attractive construction material as the required quantities and properties can be provided exactly – and this means less material consumption, reducing both cost and environmental burden. In considering the sustainability of concrete, its other intrinsic concrete properties (available at no extra cost!) should also be taken into account - resistance to fire; thermal mass; and in the case of ready-mixed concrete and a number of precast concrete products a local almost “km 0” production. Furthermore, sustainability cannot be considered without also taking into account the impact of the use phase of a building and subsequent recyclability. Concrete, has a design life of at least 50 years, and can be entirely recycled, back into concrete, or into road bases. So, the conclusion is obvious: Ignore the myths, concentrate on the facts and, for really sustainable construction, use concrete!