improving the sustainability of concrete structures

8/10/2019 IMPROVING THE SUSTAINABILITY OF CONCRETE STRUCTURES

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IMPROVING THE SUSTAINABILITY OF CONCRETE

STRUCTURES

P-C Atcin

1

, S Mindess

2

, A Tagnit-Hamou

1

1. University of Sherbrooke, Canada2. University of British Columbia, Canada

ABSTRACT. It can easily be demonstrated that High Performance Concrete (HPC) is moresustainable than Normal Strength Concrete (NSC) from the points of view of both materialproperties and durability. Therefore, in order to improve the sustainability of concretestructures, it is essential to promote the use of HPCs having the following characteristics: i)robust rheology during the first 90 minutes in the fresh state to facilitate placing ii) limitedautogenous shrinkage iii) very compact skin to avoid the penetration of aggressive agents.In order to improve the robustness of HPC rheology it is necessary to consider both thechemical and phase compositions of the clinkerused to make the binder. It is important thatcement producers optimize the compositionand finenessof their cements rather than theircube strengths. In order to reduce the development of autogenous shrinkage, two approacheshave already been exploited successfully: internal curing and the use of expansive agents;

these technologies should be used routinely. In order to improve the imperviousness of theskin of concrete structures, the specification for external water curing mustbe enforced. Itis relatively easy to do so, as experienced in Montral, Canada: It is simply necessary to paycontractors specifically to water-cure concrete structures. They then become zealous aboutcuring because they can make an easy profit by doing so. If these three points are all properlyaddressed, it is then possible to build sustainable structures that will last a very long time.After all, the Pantheon in Rome has a track record of more than 1800 years!

Keywords: Durability, Sustainability, Carbon Footprint, High Performance Concrete,Highly Sustainable Concrete, Filler, Concrete Curing

Professor Pierre-Claude Atcin is Professor Emeritus, Department of Civil Engineering,Universit de Sherbrooke. His research interests include high performance concrete andconcrete sustainability.

Sidney Mindess is Professor Emeritus, Department of Civil Engineering, University ofBritish Columbia. His primary research interests include concrete sustainability, fibrereinforced concrete, concrete testing, and the behaviour of concrete under impact loading.

Arezki Tagnit-Hamou, FACI, is Professor at the Department of Civil Engineering and Headof an industrial chair at Universit de Sherbrooke. His research interests include physico-

chemistry and microstructure of cement and concrete, supplementary cementitious materialsand sustainable development.

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1. INTRODUCTION

It is imperative for usto improve the sustainability of concrete structures: it is not sensible tocontinue to waste our material and industrial resources in designing and building concretestructures having poor durability and sustainability when using normal strength concrete;

these structures could be built in a very sustainable manner by using low w/c concrete [1].Since the production of 1t of Portland cement generates the emission of almost 1t of CO2, it isnecessary to decrease the amount of Portland cement used to build concrete structures inorder to decrease their carbon footprint.

There are a number of different ways to decrease the carbon footprint of concrete structures.It is possible to: use blended cements in which a certain volume of Portland cement clinker is substituted

by the same volume of cementitious material or filler; decrease the amount of concrete used to sustain the loads acting on the structure;

increase the durability of the concrete.In this presentation, we will focus on the two last points, because they have a much greaterimpact than the first one for decreasing the carbon footprint.

2. INCREASING THE DESIGN STRENGTH TO DECREASE THE

CARBON FOOTPRINT AND INCREASE THE DURABILITY OF

CONCRETE STRUCTURES

The most significant way of decreasing the carbon footprint of a concrete structure is by

increasing its design strength. At the same time, one should substitute some Portland cementby a supplementary cementitious material or by a filler.

In order to illustrate this very simply, let us suppose that in order to sustain a given load L,we decide to build two unreinforced columns, one with a 25 MPa concrete and the secondwith a 75 MPa concrete.

Figure 1 Comparison of 25 MPa and 75 MPa unreinforced concrete columns

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As shown in Figure 1, when building a 25 MPa concrete column:

-first, it is necessary to produce, transport and place 3 times as much concrete as whenbuilding a 75 MPa column, because the cross-section of the 25 MPa column is 3 times largerthan that of the 75 MPa one;

- second, in order to get a 25 MPa strength 9 times out of 10, it is necessary to use about 300kg of Portland cement per cubic meter of concrete (depending on the standard deviation ofthe production of that concrete); however, one would typically use about 450 kg of a blendedcement to produce a 75 MPa concrete. Consequently, using only 1.5 times more binder in the75 MPa concrete, we get 3 times more strength.

Therefore, by using:- 2 times as much cement- 3 times as much aggregate, and

transporting and placing 3 times as much concrete, we end up with a concrete column of

poor quality and durability that has a very high carbon footprint. This is absolutelyunsustainable.

Consequently, if we want to decrease drastically the carbon footprint of concrete elementsworking in compression, we have to increase their design strength. In the case of structuralelements working in flexure, such as beams and floors, it is also better to promote the use of

pre- or post- tensioned low w/c concrete than to design them with normal strength concrete.

Why does the MPa/kg of a binder increase when increasing the design strength?

The answer is very simpleconcrete compressive strength does not depend:-

on the amount of cement used to produce 1 m3of concrete;- on the cube strength of the cement used to produce the concrete;- on the C3S and C3A contents of the cement; or- on the specific surface area of the cement.

Rather, it depends on the water/cement ratio (w/c), as was discovered in about 1892 forcement pastes by Fret [2]and in 1918 by Abrams for concrete [3]. In order to explain this,let us have a close look at the exact meaning of the w/c.

3. THE WATER/ CEMENT RATIO, THE KEY PARAMETER TODECREASE THE CARBON FOOTPRINT OF CONCRETE

For many people, the w/c is only an abstract number having an inverse relationship withconcrete compressive strength. In reality, the w/c is a number in directrelation to the averagedistance between cement particles within a cement paste just before it starts to harden [4]. Inthis presentation, instead of using a sophisticated mathematical model, let us use instead avery simple geometrical model.

Consider a system of circular cement particles having a radius a that are placed at eachcorner of a square having sides equal to 3a as shown below in Figure 2(a).

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It is easy to calculate the w/c of the unit cell shown in Figure 2(b):

- the surface of the cell is 3a3a = 9a2- the surface of the cement particles within the cell is 4 1/4 a2

Let us suppose that the specific gravity of the cement is 3.14 and that 3.142

10

Figure 2 (a and b) Schematic representation of 2 very simple cement pastes

It is easy to show that the w/c of the unit cell is

2 2

2

9 3.14 9 3.14w/c = 0.60

3.14 x 3.14 10

a a

a

= =

Now, let us place a circular particle of cement at the centre of the system whose diagonal is4.24 a long. The w/c of this new unit cell is then 0.14.

Thus, by replacing the water at the centre of the unit cell by a cement particle it is possible todecrease drastically the w/c. Moreover, we can see that in this new system the cement

particles are very close to each other. Consequently the hydrates that will be formed duringcement hydration will only have to be developed over a very short distance in order to bridgethe gaps that separate cement particles from each other, to create strong mechanical links and

produce a very strong concrete. Therefore, it is not necessary to use cements containingcompounds that form crystals having a very rapid growth (C3A and C3S), because very little

glue is necessary to bind the cement particles together and create a very strong matrix.

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4. THE FILLER EFFECT

In order to lower the carbon footprint of concrete, cement producers are currently promotingthe use of blended cements, where a certain percentage of Portland cement clinker is replaced

by a supplementary cementitious material or by a filler. Is this a sustainable solution to

decrease the carbon footprint of concrete structures? The answer is YES if simultaneouslythe w/c of the concrete is decreased, but NOif the w/c is not decreased. This is in spite of thefact that by increasing the C3S, the C3A, the gypsum content and the fineness of the

blended cement, cement producers can succeed in maintaining the same level of short termstrength in the small mortar cubes they use to check the strength and variability of their

production.

Let us go back to Figure 2(a), and let us replace one particle of cement by a particle of fillerof the same diameter, for a 25% rate of substitution. As shown in Figure 3(a), the w/b of thisnew system is still equal to 0.60, but the w/c ratio is increasedto 0.78. The Portland cementhas been diluted so that the average distance between cement particles has been

increased!

The durability of such a concrete will be drastically decreased, in spite of the fact that thestrength of the concrete has been maintained due to the various tricks used by the cement

producers to increase the initial strength of the system.

Now let us replace in Figure 2(b) the central particle of cement by a particle of filler. In sucha system (Figure 3(b)) the ratio of substitution is now 50% and the w/b is decreased to 0.14and the w/c to 0.27. The hydrates formed during the hydration of the cement particlessurrounding the central particle of filler will have to be developed over a very short distance

to bridge the very small gap that separate them from the filler particle. The solid matrixformed will have a very low porosity and a high strength: it will be durable and sustainable atthe same time.

Figure 3 (a and b) Calculation of the w/b and w/c of 2 cement pastes in which 25% and50% of Portland cement clinker has been replaced by filler

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Thereforethe use of cements containing fillers must be promoted only when the w/b issimultaneously decreased.

If the w/b is not simultaneously decreased we will end up with concrete structureshaving a poor durability and, in the long term, we will not decrease the carbon footprint

of concrete structures.

5. CEMENT HYDRATION

There are many different ways to describe cement hydration. In this presentation, thedevelopment of the complex chemical reactions occurring when the cement particles reactwith water will not be considered. Only the physicalaspects of cement hydration which areof interest to civil engineers will be discussed.

When Portland cement hydrates- mechanical links are created-

heat is liberated- the absolute volume of the hydrating cement paste decreases.

5.1 The Le Chatelier Experiment

Back in 1904, Le Chatelier carried out a very simple experiment [5]: he filled with a freshlymixed cement paste 2 glass containers up to the bottom of their long necks. As shown inFigure 4 (a,b,c) the paste was left to harden under water whose level was adjusted to a marksituated at the middle of the neck; in the second case, it was left to harden in air as shown in

Figure 4 (d,e)

Figure 4 Schematic representation of the Le Chatelier experiment

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During the first day, he observed that the level of the water on top of the cement pastedecreased, and then after a few days it stabilized. After a few weeks, he observed that theglass container broke due to the swelling of the hydrated paste.In contrast, when the paste was hydrated in air, he observed that the relative volume of the

paste decreased and that the hardened paste no longer completely filled the glass container.

From this very simple experiment, Le Chatelier concluded:- first, when a Portland cement paste hydrates there is a contraction of the absolute

volume of the paste because some water penetrates into the paste. Le Chatelier wasable to evaluate this absorption as 8% of the initial volume of the paste;

- second, in spite of this reduction in the absolute volume of the cement paste, itsrelative volume increases when the paste is cured under water and results in therupture of the glass container;

- third, when the same paste hardens in air, its relative volume decreases.

Therefore, volumetrically speaking, concrete is not a stable material:- when it is cured in air , it shrinks;- when it is cured under water, it swells;- the absolute volume of a cement paste decreases by 8% during hydration( this

phenomenon is called chemical shrinkage).

5.2 Powers Work on Hydration

During the 1950s, Powers followed the development of cement hydration in a quantitativeway [6]. He found that:-in order to reach full hydration, a cement paste must have a w/c ratio equal to at least 0.42,-water reacts with Portland cement in two ways:

- chemically, to form what he called cement-gel;- physically, to form what he called gel-water (some water molecules are physically

linked to the hydrated cement particles in spite of the fact that they have not reactedchemically).

Powers observations can be illustrated very simply by using the schematic representationproposed by Jensen and Hansen [7], where the degree of hydration is given on the x- axis andthe relative volumes on the y- axis, as shown in Figure 5. Using this representation, let us

now consider a few particular systems.

5.2.1 Hydration of a Cement Paste Having a w/c Equal to 0.42 That Hardens in Air

As may be seen in Figure 6, some water combines chemically with Portland cement to form asolid gel, and some combines physically to form the gel-water. Full hydration also results in areduction of the absolute volume of the hardened cement paste equal to that already measured

by Le Chatelier (8%). Therefore, when concrete hardens, it becomes porous. When a cementpaste hardens under water some water penetrates within the paste to fill the porosity createdby this chemical contraction (Figure 7).

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Figure 5 Jensen and Hansen system of coordinates to represent schematically Powers workon hydration

Figure 6 Schematic representation of the hydration of a 0.42 cement paste in a closed system

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Figure 7 Schematic representation of the hydration of a 0.42 cement paste benefitting froman external source of water

5.2.2 Hydration of a Cement Paste Having a w/c Equal to 0.36 That Hardens Under

Water

The question then arises: Why not use this water that penetrates within the paste to hydrate anadditional amount of cement? Jensen and Hansen (7) have found that when a cement pastehaving a w/c of 0.36 is cured under water it ends up as a non- porous material at the end ofthe hydration process, as seen in Figure 8.

5.2.3 Hydration of a Cement Paste Having a W/C Equal to 0.60

Such a system contains much more water than necessary to fully hydrate the Portland cement.At the end of the hydration, some capillary water remains in the system (Figure 9) and offersan easy way for aggressive ions to penetrate into the concrete. The greater the w/c ratio, thegreater the amount of remaining capillary water and the larger the capillary pores. Such aconcrete exhibits very poor durability when exposed to severe environmental conditions

because aggressive ions can penetrate very easily into the concrete through the large capillarysystem.

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Figure 8 Schematic representation of the hydration of a 0.36 cement benefitting from anexternal source of water

Figure 9 Schematic representation of the hydration of an air-cured 0.60 cement paste___________________________________________________________________________________________________

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5.2.4 Hydration of a Cement Paste Having a w/c Equal to 0.30

Such a system does not contain enough water to fully hydrate all of the cement. Thehydration reaction stops through lack of water as seen in Figure 10. At the end of thehydration process, the unreacted parts of the cement particles act as very hard and rigid solid

inclusions that strengthen the hardened cement.

Figure 10 Schematic representation of the hydration of a 0.30 cement paste benefitting froman external source of water

6.0 CURING LOW W/C CONCRETES

Because they ignore the very simple experiment carried out more than 100 years ago by LeChatelier, the great majority of engineers and scientists that deal every day with concrete

believe that concrete is condemned to shrink in three different ways (excluding carbonationshrinkage).

- plastic shrinkage;-

autogenous shrinkage;- drying shrinkage.

Even the thermal contraction accompanying the cooling of a concrete after it has reached itspeak temperature is called thermal shrinkage in spite of the fact that it is simply a normalthermal contraction.

These 3 forms of shrinkage are due to the development of menisci in the cement paste due tothe:

- evaporation of water from the fresh cement paste (plastic shrinkage);- evaporation of water from the hardened cement paste (drying shrinkage);

-

creation of a porosity due to the chemical shrinkage of the paste (autogenousshrinkage).

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In fact, this not absolutely true; as demonstrated by Le Chatelier, a cement paste swells

or shrinks according to its curing conditions.

To avoid plastic and drying shrinkage, it is only necessary to prevent water evaporation fromfresh and hardened concrete. To avoid autogenous shrinkage, it is only necessary to fill the

very fine porosity created by the chemical contraction as soon as it is created with an externalsource of water (external to the paste). This external source of water can be included inconcrete during its initial mixing by substituting some aggregates by an equivalent volume ofsaturated lightweight aggregates or by introducing Super Absorbent Polymers (SAP)[8]. Thismethod of curing is called internal curing because it is carried out inside the concrete.

From Powers observations, it can be concluded that concrete must be cured according toits w/c (Figure11).

Figure 11 Curing concrete according to its w/c

Concretes having a w/c greater than 0.42 that contain more water than necessary to fullyhydrate the cement must be cured using:

- fogging- external water curing;-

a curing membrane;- permanent sealing.

Concrete having a w/c lower than 0.42 must be cured using some additional water becausethe hydration will otherwise be stopped by a lack of water.Therefore it is necessary either to:

- cover concrete with an evaporation retarder (nota curing membrane) in order to avoidplastic shrinkage and to allow later external water curing when hydration starts inorder to reinforce the concrete skin;

-

provide internal curing to allow hydration down to a w/c ratio of 0.36 within the massof concrete.

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In any case, whatever the w/c ratio is, it is imperative to motivate contractors to watercure the concrete: it is simply necessary to pay them specifically for the water curing. Ifthey can make a profit from water curing they become zealous, though it is always prudent tocontinue to pay inspectors to check on them!

7.0 WHAT SHOULD WE CALL HIGH PERFORMANCE CONCRETETHAT IS ALSO HIGHLY SUSTAINABLE?

It is time now to come back to the acronym of HSC that was initially the abbreviated way inwhich High Strength Concretes were designated, because initially low w/c concretes wereused only to increase the strength of the concretes used to build columns in high-rise

buildings [9]. Later on the acronym HPC (for High Performance Concrete) was created totake into account the fact that these concretes had more than simply a high strength; theywere also much more durable. But, as shown in this presentation, HPCs are more than simplyHSCs having great durability; they are also highly sustainable. Therefore, it is time to identify

them with the acronym, HSC, for Highly Sustainable Concrete.

When using HSC the carbon footprint of concrete structures is decreased drastically,especially when the binder used to make the HSC involves the substitution of part of thePortland cement by a supplementary cementitious material or a filler.

ACKNOWLEDGEMENTS

The authors thank Arame Niang for producing the figures.

REFERENCES

1. ATCIN P.-C., MINDESS S., Sustainability of Concrete, Spon Press, London, U.K.,2011, 301p.

2. FRET R., Sur la compacit des mortiers hydrauliques, Annales des Ponts etChausses, 1892, Vol. 4, 2ndsemestre, pp5-161.

3. NEVILLE A.M., Properties of Concrete, 5th Edition, Prentice Hall, 2011, 846p.

4. BENTZ AND ATCIN P.-C., The Hidden Meaning of the Water-to-Cement Ratio,Concrete International, 2008, Vol. 30, No. 5, pp 51-54.

5. LE CHATELIER H., Recherches exprimentales sur la constitution des mortiershydrauliques, Dunod, Paris, 1904, 196p.

6. POWERS T.C., The Properties of Fresh Concrete, John Wiley and Sons, New York,1968, 664p.

7. JENSEN O. AND HANSEN E., A Model for the Microstructure of Calcium SilicateHydrate in Cement Paste, Cement and Concrete Research,2001, Vol. 30, No. 1, pp 101-116.

8. KOVLER K. AND JENSEN O.M., Novel Technique for Concrete Curing, ConcreteInternational, 2005, Vol. 27, no. 9, pp 39-42.

9. ALBINGER J. AND MORENO J., High Strength Concrete: Chicago Style, ConcreteConstruction, 1991, Vol. 26, No. 3, pp 241-245.

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improving the sustainability of concrete structures

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