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Disclaimer:

This document is a part of my M.Sc. thesis on Use of Superplasticizers for High Strength Concrete, published on 1999, Tribhuwan University, Institute of Engineering, Nepal. Readers' discretion is advised. The author bears no responsibility to any loss or damage by use of this document.

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Cement Concrete and Superplasticizers

Er. Saroj Bhattarai, M.Sc. Structures,

Senior Divisional Engineer, Department of Roads, Nepal

What is Cement Concrete?

Structure of concrete

Concrete in its basic form is a conglomerate of fine and coarse aggregate bound together

by hydrated cement paste. At macroscopic level concrete can be considered as a two-

phase material, consisting of aggregate particles dispersed in an incoherent mass of the

hydrated cement paste (hcp). Large part of concrete is covered by coarse aggregate. The

voids between the coarse aggregates are filled with mortar from cement paste and fine

aggregates. Further at microscopic level, a third phase – the transition zone, can be

distinguished as an interface between the aggregate particles and bulk hydrated cement

paste (Fig. 2-1-1).17

Strength of concrete

Among the two main phases of concrete, the aggregate phase mostly remains

unchanged during the hardening process. But the cement undergoes through several

changes once it comes into contact with water. This changing process is called

hydration and the paste is known as hydrated cement paste (hcp). The hcp has a very

complex structure which is a combination of solids such as calcium silicate hydrate (C-

S-H), calcium hydroxide (CH), calcium sulfoaluminates (C-A-S-H), unhydrated clinker

grains; different types of voids such as interlayer space in C-S-H, capillary voids, air

voids and water in different states such as capillary water, absorbed water, interlayer

water and chemically combined water.

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The strength of concrete is mainly due to the van der Walls force of attraction between

the products of hydration themselves and between these products and aggregate

particles. This force of attraction is dependent upon the volume and size of voids

present in the bulk hcp and the transition zone. In the initial stage of hardening water

films form around aggregate particles which means a higher water/cement ratio closer to

the aggregates. Due to the high w/c ratio the crystalline products of hydration near the

aggregates (i.e. at the transition zone) are of relatively larger size and therefore form a

more porous structure. That is why the transition zone is considered the weakest part at

least at an early age of concrete. However, later the voids in the transition zone start to

be filled with products of slow chemical reaction between the cement paste constituents

and the aggregates, which increases the strength of the transition zone. Usually at an age

of three months or more the strength of the transition zone tends to be equal or

sometimes even greater than that of the bulk cement paste 17. But, since the structure is

loaded much earlier the microcracks present in the transition zone, which are there even

Figure 2-1-1 Simplified diagrammatic representation of the three phases of concrete C-S-H = Calcium silicate hydrate; CH = Calcium hydroxide; C-A-S-H = Calcium sulfoaluminates

C-S-H

Aggregate Transition zone Bulk Cement Paste

CH C-A-S-H (Ettringite)

3

before load is applied, tend to propagate further, leading ultimately to failure of the

concrete.

Relationships between water/cement ratio, porosity and strength of

concrete

There are several factors that influence on strength of concrete. As stated by Gilkey*,

"For a given cement and acceptable aggregates, the strength that may be developed by a

workable, properly placed mixture of cement, aggregate and water (under the same

mixing, curing and testing conditions) is influenced by the:

(a) ratio of cement to mixing water

(b) ratio of cement to aggregate

(c) grading, surface texture, shape, strength and stiffness of aggregate particles

(d) maximum size of the aggregate"

Generally the factors (b) through (d) are of lesser importance than factor (a) when usual

aggregates up to 40 mm maximum size are employed 19. That means the ratio of cement

to mixing water or water/cement (w/c) ratio plays a major role in strength of concrete.

Based on works of T.C. Powers it has been found that there is a certain relationship

between w/c ratio and total volume of capillary porosity. A study with constant volume

of cement (100 cm³) but varying water-cement ratios (0.7, 0.6, 0.5 and 0.4) showed that

after 100 percent degree of hydration showed that the paste with w/c ratio of 0.7 had 37

percent of capillary voids while the same with w/c ratio of 0.4 had only 11 percent 17.

Under the assumption made in this study with a 0.32 w/c ratio paste there would be no

capillary porosity after a 100 percent hydration. (Fig. 2-1-2).

* From Discussion paper by H.J. Gilkey: Water/cement ratio versus strength – another look, J. Amer. Concr. Inst., Part 2, 58, pp. 1851 – 78 (Dec. 1961). Quoted from [19].

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For normally hydrated portland cement mortars, Powers* showed that there is an

exponential relationship of the type S = kx³, where S is the compressive strength, x is

the solid-to-space ratio (which is inversely proportional to porosity) and k is a constant

equal to 34,000 psi (234 MPa). Fig. 2-1-3 explains this relationship graphically.

Although this relationship can not be directly applied to concrete because presence of

microcracks in the transition zone makes it a relatively non-homogeneous complex

material, but since the strength of concrete is governed both by strength of cement paste

and strength of transition zone, the validity of this relationship is retained.

The effect of porosity is seen not only on strength but on durability also. P.C. Aitcin et

al 2 observed that when concrete is subjected to external aggression the most effective

way to decrease the intensity of this aggression is to reduce its porosity and

permeability. This is why w/c ratio has always been the controlling factor of concrete

durability.

* T.C. Powers, J. Am. Ceram. Soc., Vol. 41, No. 1, 1958. Quoted from [19]

0 0.1 0.2 0.3 0.4 0.5 0.6

0.3

0.4

0.5

0.6

0.7

100% Hydration

75% 50%

25%

Capillary Porosity, Vol. Fraction P

W / C Ratio

Figure 2-1-2 Influence of water/cement ratio and degree of hydration on capillary porosity (figure partially extracted from [17], p.33)

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From the discussion it is clear that water content in a unit volume of concrete plays a

major role in development of strength of concrete. Obviously, greater water/cement

ratio causes more porosity and hence lesser strength, provided that other parameters are

kept constant. Earlier in 1918 after extensive testing at the Lewis Institute, University of

Illinois, Duff Abrams found that a relation existed between water/cement ratio and

strength of concrete, which is represented by:

where, fc is the compressive strength of concrete, k1 and k2 are empirical constants and

w/c is the water/cement ratio. Many mix design methods are based on this relationship.

As an example, the Indian Standard method suggests to use the same relation (Fig. 2-1-

4) for selection of w/c ratio for required target mean strength 18. It should be noted here

however that for w/c ratio lower than 0.3, disproportionately high increase in strength

can be achieved for very small reduction in w/c ratio. This may be due to significant

increase in the strength of the transition zone at very low w/c ratio.17 Therefore the

above equation may not hold good for very high strength concrete. But nevertheless,

effect of w/c ratio is not found to be inverse in any case.

0 0.2 0.4 0.6 0.8 1.0

100

75

50

25

125

150

175

200

fc=234 x³

Gel / Space ratio (x)

Str

engt

h of

5 c

m c

ube

mor

tar

(fc)

, MP

a

Figure 2-1-3 Porosity – Strength relationship of cement mortar as established by T.C. Powers in J. Am. Ceram. Soc., Vol. 41, No. 1, pp. 1-6, 1958. (figure partially extracted and modified from [17], p.45)

cwc k

kf

/2

1

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Workability of concrete

From the previous sections it is clear that for higher strength of concrete the

water/cement ratio should be decreased. But for a given amount of cement, and given

type and amount of aggregate, reduction of water beyond a certain volume will result in

stiff consistency and poor workability. A concrete mix with poor workability is difficult

to place and compact by conventional methods. As a result, the desired strength and

durability characteristics are very difficult to achieve.

Workability of concrete is defined in ASTM C 125 as the property determining the

effort required to manipulate a freshly mixed quantity of concrete with minimum

loss of homogeneity. There is another term – consistency, which describes the state of

fresh concrete in terms of its wetness 19. But wetness alone does not make the mix

workable since mixes with same consistency may vary in workability. Properties of

aggregate, and presence of admixtures are other factors that control workability. There

is yet another term – cohesiveness, which describes the tendency to bleed or

segregate17. Thus workability is a composite property which can not be determined by a

single factor.

Figure 2-1-4 Generalized relationship between free w/c ratio and compressive strength of concrete suggested in Indian Standard, IS : 10262-1982. (Figure from [18])

0.3 0.35 0.4 0.45 0.5 0.55

40

30

20

10

0

Water / Cement ratio

0.6

50

60

28-d

ay c

ompr

essi

ve s

tren

gth

of c

oncr

ete,

N/m

m²

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Workability is measured by several methods. But no single test can measure all the

properties that describe complete workability of the mix. Some popular methods are:

slump test; Vebe test; compacting factor test; remoulding test; flow test etc. The slump

test is the easiest and widely used method for measuring workability. Although slump

test, as other tests, does not describe the workability as a whole, but due to its simplicity

and portability of the equipment it is the most popular method of controlling

consistency of the mix in field conditions. Many codes including the Indian Standards

allow slump test as measurement of workability.

Workability and high strength concrete

Effect of water on strength of concrete was already discussed in sections 2.1.2 and

2.1.3. From the discussions it is clear that workability (particularly in terms of

consistency) and strength are two opposing properties of concrete. With the decreasing

water content, fresh concrete becomes more and more difficult to mix, place and

consolidate. For the production of high strength concrete, the opposing effects of water

cement ratio on consistency and strength of concrete cannot be harmonized without the

use of water reducing admixtures 17. There are examples of using zero slump concrete,

but such concrete requires special arrangement for placing and compacting, which may

not always be possible, particularly in heavily reinforced parts of the structure. As

mentioned elsewhere in this text (Section 1.2) workability, especially for high strength

concrete, is the basic necessity in the context of this country since most of the concrete

works are carried out manually.

Superplasticizer and its role

Superplasticizers are high range water reducing admixtures, which are able to reduce

the amount of water by up to 1/3 of that required for normal concrete mix.

Superplasticizers were first introduced in around 1964/65 in Japan 24 and have already

found a wide acceptance in the concrete industry. As classified by ASTM C 494-92,

superplasticizers fall in to Types F and G, High-range Water Reducing and High-range

Water Reducing with retarding, respectively. These are water-soluble organic polymers

which have to be synthesized, using a complex polymerization process, to produce long

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molecules of high molecular mass. 19 The main types of these admixtures are based

on15:

Lignosulphonates

Hydroxycarboxylic acid salts

Melamine sulphonates

Naphthalene sulphonates

The last two, sulfonated melamine-formaldehyde condensates (SMF) and sulfonated

naphthelene-formaldehyde condensates (SNF) are most commonly used. The molecular

weight of superplasticizers is in the range of 20,000 to 30,000 17; the larger molecular

mass, within limits, improves its efficiency 19 from the view point of excessive

retardation or air entrainment at higher dose 15.

Superplasticizers do not alter fundamentally the structure of hydrated cement paste, the

main effect being a better distruibution of cement particles and consequently, their

better hydration19. The main action of the long molecules of superplasticizers is to wrap

themselves around the cement particles and give them a highly negative charge so that

they repel each other. This results in deflocculation, and better dispersion of the cement

particles19. (Fig. 2.1.6). 15 Thus, superplasticizers themselves do not enhance the

property of cement or other ingredients of concrete. It is the temporary deflocculation of

cement particles that has to be exploited.

Figure 2.1.6 Action of superplasticizer on cement particles.

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Some of the main features of the superplasticizers and their use are as follows:

Capable of reducing water content up to 30 percent for a given workability

Inert organic chemical, action on the cement particles – mostly physical, does not

alter fundamental structure of the hydrated cement paste. Although some recent

experiments have shown that naphthalene based SP can react particularly with C3A

and substantially reduce the initial surface hydration rate 11.

Problems of segregation and bleeding are minimum even with slight overdose.

Suitable for concrete produced at site and placed immediately due to rapid slump

loss (within 30 to 60 minutes). But now-a-days there are superplasticizers with

retarders available in the market, so this problem is somehow negotiated.

It is necessary to find out its compatibility with the cement being used and optimum

dose before using it in mass production. Optimum dose depends upon type of

cement, desired strength and w/c ratio, desired workability and type of aggregate.

The Figure 2.1.7 shows the effect of superplasticizer on workability (slump) and

compressive strength 14.

Figure 2.1.7. Effect of superplasticizer on workability and strength.

For a constant slump 65 mm, average

increase in 28 days strength = 30%

Time, days

Compressive strength

w/c=0.38, No admixture

w/c=0.30, admixture 1.2%

Mix ratio 1:0.65:1.95

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Why superplasticizer?

There are other methods also available for increasing the strength of concrete. Some of

them are:

Using higher grade cement

Lowering the slump value (making zero slump concrete)

Increasing cement content to meet the normal w/c ratio

Use of normal water reducing agents

Before the introduction of superplasticizers these methods were used for production of

high strength concrete. But there were problems associated with these methods.

Higher grade of cement means more fine particles. Although the fineness is mainly

responsible for early development of strength but there is better hydration of all the

particles, which leads to some increment in final strength. But cost of grinding to higher

fineness is considerable and the finer the cement the more rapid deterioration on

exposure to the atmosphere.19

Lowering the slump or making zero slump concrete makes it very difficult to mix, cast

and compact which is a serious problem when these procedures are done manually. The

problem is more pronounced in reinforced concrete. (Please refer to Section 2.1.4 also)

Increasing the cement content to meet the normal w/c ratio is always associated with the

cost. Furthermore the increased heat of hydration may cause cracks and increment in

volume particularly in heavy sections.

Normal water reducing admixtures (ASTM Type A, D & E) are relatively cheaper but

these admixture are capable to reduce water content only up to 10 – 15 percent.

Overdose causes considerable retardation and segregation.

Superplasticizers can reduce the required amount of water by as much as 30 percent

without lowering the workability. Superplasticizers are costlier than normal water

reducer, but in overall scenario, the total cost can be decreased by taking advantage of

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better workability, early strength development and reduced quantity of concrete work.

Although some types of superplasticizers can cause retardation, other adverse effects

due to accidental over-dosage are minimum. Since the action of superplasticizer on

cement is primarily physical and subsides in due time, there are no major harmful

chemical reactions. These are the main reasons for choosing superplasticizer for the

study.

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8. Collection of Abstracts of Articles (Collected by S. Bhattarai), Collection of abstracts of articles on high strength concrete and superplasticizers published in the Internet web sites, U.R.Ls.- http://www.tfhrc.gov, http://www.pubs-asce.org, http://www.aci-int.org, 1999

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11. GAGNE, R., BOISVERT, A & PIGEON, M., Effect of Superplasticizer Dosage on Mechanical Properties, Permeability, and Freeze-Thaw Durability of High-Strength Concretes With and Without Silica Fume, ACI Material Journal, vol 93, No.2, March-April, 1996, pp. 111-120.

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14. KUMAR, V., ROY, B.N, & SAI, A.S.R., Effect of Superplasticizer on Concrete, Indian Concrete Journal, vol 63, No. 1, January, 1989, pp. 31-33, 42.

15. LIMAYE, R.G., Admixtures for High Strength Concrete, Indian Concrete Journal, vol 70, No. 12, December, 1996, pp. 689-691.

16. MANJREKAR, S.K., Use of Superplasticizers: Myths and Reality, Indian Concrete Journal, vol 68, No. 6, June, 1994, pp. 317-320.

17. MEHTA P.K & MONTEIRO, P.J.M, Concrete - Microstructure, Properties, and Materials (Indian edition), Indian Concrete Institute, Chennai, India, 1997

18. N. Krishna Raju, Design of Concrete Mixes, CBS Publishers & Distributors, New Delhi, India, 1997

19. NEVILLE, A.M., Properties of Concrete (4th edition), ELBS/Addison Wesley Longman, England, 1996

20. PENTTALA, V., Possibilities of Increasing the Workability Time of High Strength Concrete (proceedings of the RILEM colloquium, Hanover Oct 3-5, 1990), Properties of Fresh Concrete, Chapman & Hall, 1990, pp. 67-76.

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21. PLANTE, P., PIGEON, M., & SAUCIER, F., Air-Void Stability, Part II: Influence of Superplasticizers and Cement, ACI Material Journal, vol 86, No. 6, November-December, 1989, pp. 581-589.

22. PLANTE, P., PIGEON, M., & SAUCIER, F., Air-Void Stability, Part III: Field Test of Superplasticized Concretes, ACI Material Journal, vol 87, No. 1, January-February, 1990, pp. 3-11.

23. PUNKKI, J., GOLASZEWSKI, J., & GJORV, O.E., Workability Loss of High-Strength Concrete, ACI Material Journal, vol 93, No. 5, September-October, 1996, pp. 427-431.

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39. Interviews with the suppliers of chemical admixtures and technicians involved in the use of such admixtures. (Mr. B.R. Tater, Mr. Prashanta Mathema from H.G. Enterprises; Mr. Rajesh Thapaliya from Indco Trading Co.; Mr. Ramesh Sapkota from Himalayan Engineering Associates).