Chemical Admixtures in Concrete A state-of- the-art-report

Introduction

Concrete, nowadays is not just mixing of cement, aggregate & water but it also comprises of chemical and mineral admixtures. It is becoming a more and more effective construction material as a result of addition of admixtures and improvements in production techniques. The chemical admixtures, in particular are frequently used to enhance the characteristics of both fresh and hardened concrete such as to extend or accelerate setting characteristics, entrain air, reduce water content, increase cohesiveness, enhance flow, introduce self leveling properties, improve durability and enhance strength parameters. Therefore, the role of chemical admixture in concrete is becoming important each year. It is often emphasized that the new admixtures play a more important role in concrete than new cement [1]. The recent development and advancement in chemical admixtures particularly organic based advanced super plasticizers are playing important role in production of a variety of innovative concretes such as ultra high strength concrete, high performance and self leveling concrete [2-8]. Chemical admixtures are usually used to reduce the cost of concrete construction by modifying plastic & hardened properties to ensure the quality of concrete during mixing, transporting, placing, and curing. However, sometimes, the cost of admixtures is comparable to that of the cement in high-performance concrete due to its high dosage. Admixtures are, however, not almighty. There are also problems associated with it including the production of stiffer concrete, variation in initial slump and slump loss using some types of cement, large variations in the flow characteristics using combination of different admixtures etc. Therefore, it is important to ensure the compatibility of admixtures with cement and additives before its application in concrete.

Chemical admixtures are inorganic or organic materials other than Portland cement, water, and aggregate and added to the mix immediately before or during mixing. These are added into the mix not normally exceeding 5% by mass of cement or cementitious materials. Admixtures interact with hydrating cement by physical, chemical or physic-chemical actions. The history of admixtures is as old as the history of concrete. First reported chapter on chemical admixture is published in ASTM STP 169 B in 1966 [9]. However, specifically developed water reducers or high range water reducers in Japan and Germany in late 1960s created a great change in the development of admixtures & attracted the concrete producers due to its great efficiency. This paper presents a state of the art report on chemical admixtures by addressing to limited depth due to length restrictions of paper.
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Mechanism of Chemical Admixtures

The idea of adding admixtures to concrete is not new. It is found in roman literatures that their masons use to add a egg whites or blood in concrete [10], which can be now explained that hemoglobin is also an excellent dispersant of cement particles. It is observed by Aitcin [11] that the chemical admixtures are not much complex products; their action is dictated by the complex laws of physics, chemistry and thermodynamics, whereas for Dodson [12], there are only four types of admixtures:

1. Those that disperse cement particles;
2. Those that modify the kinetics of hydration;
3. Those that react with one of the sub products of hydration reaction;
4. Those that have only a "physical" action.

The chemical, physical or physico-chemical actions of admixtures in cement concrete are quite complex. In fact, cement itself has extremely complex compounds including calcium silicates, calcium aluminates, gypsum. It also contains many alkalis and other calcium salts. It is observed by several researchers that one or more of the following mechanisms may take place simultaneously [13-18]:

• Reduction in surface tension of water.
• Modification in the rate or capacity of bleeding.
• Reduction in expansion caused by the reaction of alkalies.
• Increase bond of concrete to steel reinforcement.
• Electrostatic repulsion between particles of cement.
• Lubrication between cement particles.
• Dispersion of cement grains and releasing of water trapped within cement flocks.
• Delay in the hydration reaction of the cement thus leaving more water to fluidify the mix.
• Modification in the morphology of the hydrated products.
• Steric hindrance prevention in the particle-to-particle contact

The mechanism of advanced superplasticizers, which enhances the flowabilty and segregation resistance may be defined based on the following four effects such as depletion effect, depletion coagulating effect, Tom's effect, and Tribology effect in addition to DLVO & steric effect [18].

Depletion Effect: According to DLVO theory or steric effect theory, a polymer dispersing agent adsorbs to cement particles and disperses them, there must be a depletion effect that occurs when a non-adsorbing polymer with a molecular weight in tens of thousands ingresses between particles and disperses them due to volume repulsion (Fig. 1a).

Depletion coagulating effect: With the depletion effect, a polymer with a molecular weight from hundreds of thousands to millions cannot ingress between the cement particles and so the particles become coagulated, which provides segregation resistance (Fig. 1b).

Tom's effect: Since linear polymers line up along the direction of concrete flow, friction resistance decreases which enhances the flowability (Fig. 1c).

Tribology effect: Low molecular weight compounds having lubrication properties reduce the friction resistance between particles, which also increases flowability (Fig. 1d).

Classification of Chemical Admixtures

ASTM C 494/C494M-13, "Standard Specification for Chemical Admixtures for Concrete," classifies admixtures into eight types as follows [19]:


Whereas Indian standard IS 9103 [20] classifies chemical admixtures into five categories:

1. Accelerating admixtures,
2. Retarding admixtures,
3. Water-reducing admixtures,
4. Air-entraining admixtures, and
5. Superplasticizing admixtures.

Air-entraining admixtures

The air-entraining admixtures (AEAs) produces a system of microscopic air bubbles dispersed throughout in the concrete to provide resistance to freezing and thawing. Because of the tiny bubble's size (10-100 µm in diameter) there are literally billions of bubbles in each cubic meter of air-entrained concrete. In plastic concrete, entrained air also improves workability and may reduce bleeding and segregation of concrete mixtures. The air entraining agent is typically either surfactant that helps in bubble stabilization by reducing the surface tension of water, or substances that produce a water-repellant precipitate when mixed with cement concrete. It is known that formed air bubbles in concrete are unstable & have a limited lifetime. The interfaces between the dispersed air and the surrounding matrix contain free surface energy, and the thermodynamic tendency is to reduce the interfacial surface areas [21]. The traditional air-entraining agents are: vinsol resin; fatty acid salt which are replaced by synthetic surfactants such as; alkyl sulphates; olefin sulphonates; diethanol amines; alcohol ethoxylates; betaines.


All air-entraining agents are almost anionic surfactants. Anionic surfactants have a hydrophobic and a hydrophilic end. This makes them collect the air at the air water interface with the hydrophobic end in the air. The hydrophilic end is polar and is attracted by charges on the surface of cement and aggregate particles. This attaches the air bubble to the surface and helps to produce a stable microscopic air bubbles structure in the mix as shown in Fig. 2. Air is not added to the mix. The air that is already in the mix during the mixing process is made stable.

The air bubbles can move more freely in concrete when it is highly fluid; therefore, there is increased occurrence of bubble coalescence and rupturing. Therefore, increase in slump flow increases the demand for AEAs to entrain a given volume of air [22-23]. Air entraining admixtures for use in concrete must meet requirements of ASTM C 260 [24].

Accelerating admixtures

An accelerating admixture is used to accelerate the rate of hydration of the hydraulic cement and thus decreasing the time of setting and increasing the rate of strength development of concrete. Accelerating admixtures are used to decrease setting time and increase early strength gain, particularly in cold weather conditions to expedite the start of finishing of concreting, reduce finishing time, reduce the time required for curing, early removal of formwork and reducing overall construction period. The most widely used accelerating admixture is calcium chloride [25]. The recommended dose of calcium chloride are typically vary from 1-2% by the weight of cement. Fig. 3 shows the effect of calcium chloride on setting time of Portland cement. It is observed that addition of calcium chloride reduces water requirement by a small amount over the required water to produce same slump without calcium chloride. Besides accelerating strength gain, calcium chloride causes an increase in drying shrinkage, potential reinforcement corrosion, discoloration and an increase in the potential for scaling. The effect on setting time depends on its doses, types of cement used, temperature of concreting and other factors. The calcium chloride does not entrain air but it enhances the effectiveness of air-entraining admixtures in concrete.

Calcium chloride has a potential cause for promoting corrosion in reinforced concrete structure. Therefore other non-chloride accelerator are used in RC structures where corrosion is a measure concern. Common other accelerating admixtures are: inorganic sodium or calcium salts of nitrites, nitrates, thiocyanates, and thiosulphates [26], of which thiocyanates are the only class with the potential for promoting corrosion. The most common organic accelerating admixtures are triethanolamine and calcium formate. Alkali metal salts, sodium fluoride, aluminium chloride, sodium aluminate and potassium carbonates are often used as quick setting admixtures particularly in shotcreting applications [7]. Lithium-based salts are commonly used to accelerate calcium aluminate cements. Nonchloride accelerators such as nitrites, nitrates & thiocyanates are most effective at a temperature below 21oC and performing much better in the range of 5-10oC [7]. Accelerators are designated as Type C admixtures under ASTM C 494 [19].

Retarding admixtures

Retarders are those chemicals that delay the initial setting of concrete by an hour or more by affecting the rate of hydration of cement products and/or reducing the rate of water penetration to the cement particles. Retarders are often used in hot weather concrete to mitigate the rapid setting caused by high temperatures of fresh concrete (>27oC). In addition, retarders are also used in large construction projects to allow more time for transporting, placing and finishing of concrete. Retarders do not decrease the initial temperature of concrete. High temperatures often causes an increased rate of hardening that makes placing and finishing of concrete very difficult. One of the most practical methods of offsetting this effect is to reduce the temperature of the concrete by cooling the mixing water and/or the aggregates. The retarders can be formed by organic and inorganic materials. The organic material consists of unrefined Ca, Na, NH4, salts of lignosulphonic acids, hydroxycarboxylic acids, and carbohydrates. The inorganic material consists of oxides of Pb and Zn, phosphates, magnesium salts, fluorates, and borates. The retarders are typically based on solutions of phosphates, phosphonates, sucrose, gluconate, polysaccharides. The bleeding rate and bleeding capacity of concrete increases by adding retarders. Retarding admixtures are useful in extending the setting time of concrete, but they are often also used to decrease slump loss and extend workability, especially prior to placement at elevated temperatures [27]. Retarders should meet the requirements for Type B in ASTM C 494 [19]. Most commercially available retarders also function as water reducers conforming to Type D admixtures are lignosulphonates acids and hydroxylated carboxylic acids. The use of lignosulphonates acids and hydroxylated carboxylic acids retard the initial setting time for at least an hour and no more than three hours when used at 18.3 to 37.8oC.

Water-reducing admixtures

Water-reducing admixtures are generally used in three different ways: (1) to produce concrete with a lower water to cementitious materials ratio (w/cm); without altering cement content or slump; (2) to produce a higher slump, without altering cement content or w/cm; or (3) to produce concrete with reduced cement content, without altering cement w/cm or slump. In the first case, usually greater strength is obtained. In the second case, easier placing of concrete is obtained. In the third case, a reduction in cost of concrete is obtained. Typical water reducers reduce the water content by approximately 5% to 10%. Normal water reducers are usually based on lignosulphonate (Fig. 4), this is a natural water soluble organic polymer derived from neutralization, precipitation and fermentation processes of the waste liquor obtained during production of the paper making pulp from wood. When lignosulphonate is added to concrete it disperses the cement particles and reducing the amount of water needed to achieve a given consistency of concrete. The other water reducers are hydroxycarboxylic acid and hydroxylated polymers. In terms of cost, lignosulphonate is cheapest of all the water-reducers but a high dosage of this is required to produce significant water reduction.


The development in water-reducers has undergone for different range of water reduction as shown below:


Fig. 5 shows the compressive strength development of concrete containing various types of water-reducing admixtures, whereas Fig.6 represents effect of water-reducing admixtures on viscosity of cement concrete.

Water-Reducing and Retarding Admixtures

Set-retarding admixtures are used in at lowering of the setting time and enhancement in compressive strength development. The conventionally used water-reducing and set-retarding admixtures are some of the following: (1) lignosulphonic acids and their derivatives (2) hydroxylated carboxylic acids and their derivatives (3) sulphonated napthalene/melamine condensates (4) carbohydrates, ploysaccharides and sugar acids (5) inorganic salts such as borates and phosphates. In each of these, the primary component has both water-reducing and set-retarding properties. The water reduction resulting from the use of these conventional admixtures is typically from 5–12% [17]. A part of the water reduction found with lignosulphonate water-reducers is because of the additional air entrained by these materials. Water-reducing admixtures are effective with all types of cements, whereas their action depends on the cement characteristics, the temperature, the admixture dosage etc. Lignosulphonate water-reducing retarders usually entrain 2–3% of air when used in normal dosages, whereas hydroxylated carboxylic admixtures do not entrain air. However, both materials increase the effectiveness of air-entraining admixtures from the standpoint of volume of air produced, so that less air-entraining admixture may be required when added to concrete containing one of these other admixtures. It is important to note that while the air-void spacing obtained with water-reducing retarders is slightly greater than that for an equivalent amount of entrained air produced by typical air-entraining admixtures, the performance of concrete containing water-reducing retarders, as measured by freezing and thawing tests, often has been found to be better than concrete of the same air content, but without the water-reducing retarders. This increase might be the result of the reduction in w/cm. Contrary to expectations, water-reducing retarders usually have not been found effective in reducing slump loss resulting.

High-range water reducing admixtures

High-range water reducing admixtures (HRWRA) also known as super-plasticizers are a relatively new and improved version of water reducing admixtures. These admixtures are chemically different from normal water reducers. These are capable in reducing the water to the extent up to 30% or even more without reducing the workability of concrete in contrast to normal water reducers where possible reduction is up to 15%. HRWRA are added to ASTM C 494 as type F in 1980 and its use in producing flowing concrete in 1985 which is covered by ASTM C 1017. These admixtures are used to produce high strength concrete by taking advantage of their ability to reduce water cementitious material ratio substantially and producing flowable concrete as shown in Fig.7. The first generation superplasticizers are derivatives of lignosulphonates and its modified salts. However, these types of admixtures are not capable of producing high slump concrete. In addition, they often inadequate in workability retention of the concrete.


The second generation superplasticizers are sulphonated melamine formaldehyde (SMF) and sulphonated naphthalene formaldehyde (SNF). SMF is produced by normal resinification of melamine formaldehyde whereas SNF is produced from naphthalene by oleum or SO3 sulphonification; subsequent reaction with formaldehyde leads to polymerization and the sulphonic acid is neutralized with sodium hydroxide or lime. The HRWRA based on melamine or some version of napthalene generally has very poor slump retention, often requires addition of HRWRA at the job site. Due to this it becomes unsuitable for ready mix (RMC) plants. These are excellent for precast concrete construction and are also suitable for cold climatic construction. Whereas, SNF possess good slump retention characteristics, enabling their use in ready mixed concrete plants. Moreover, the cost of SNF is half the cost of SMF approximately which makes its wide applications all over the world. However, the incompatibility issues often arises with the use of SNF.

The most significant new development in HRWRA in the last decade is polycarboxylate ethers (PCE) [28-30], which is known as third generation superplasticizers, others are polyacrylates and monovinyl alcohols. These polymers consist of an ionic backbone grafted with pendant nonionic side chains. These polymers provide dispersion by means of both electrostatic and steric repulsion, the latter of the two believed to be the dominant mechanism [30]. As a result, these polymers are highly efficient dispersants, typically 2 to 3 times more effective than second generation superplasticizers. These types of chemical admixtures exhibit fewer compatibility problems as compared to SNF. Moreover, these can cause a reduction in water content upto 40% and are highly preferred to make high and ultra-high strength concrete, where the w/cm may be as low as 0.20. Water reduction is essentially linear with increasing dosages of polymers including lesser retardation effect because of less surface of cement grains covered with adsorbed polymer. Another benefit of PCE is its ability to use need based specific applications. They exhibit excellent slump retention characteristics and does not affect gain in strength of concrete. Their cost is twice as much as SNF but they can work at comparatively lower dosages than SNF and lignosulphonates, thus the overall cost of concreting is not much affected.

The primary disadvantage of PCEs is a propensity to entrain some amount of air in the absence of an air-entraining agent. The amount of air-entrained increases with increasing dosage, similar to a lignosulphonate. This is the result of the surfactant-like structure of the polymer, namely a hydrophobic backbone with hydrophilic side chains. To offset this deficiency, many commercial products are formulated with defoamers or other components to minimize air entrainment [7].

There is a need for development of flowable and segregation resistant high performance and self-leveling concrete which has led to development of a polycarboxylate based advanced superplasticizer (ADSP) containing cross-linked polymer. The first ADSP to enable the attainment of excellent flowability & segregation resistance was introduced in Japan in year 1987 & was known as 4th generation superplasticizers [3,18]. ADSPs exhibit high water reduction and good slump retention. ADSPs can be classified into four types based on their main components:

Napthalene based: Beta-napthaline sulphonate formaldehyde condensate (BNS) is commonly called naphthalene sulphonate, polyalkylaryl sulphonate, or alkutnapthalene sulphonate. Action derivative polymer, reactive polymer, or modified lignin (secondary components exhibiting slump retention) are added to naphthalene-based ADSP.

Melamine based: Melamine sulphonate formaldehyde condensate (MS) is called melamine modified methylol melamine, or sulphonated melamine. In addition, slump retentive components as well as MS are added to melamine-based ADSP.

Polycarboxylate base: Various polycarboxylate-based water reducing agents are developed. Polycarboxylate-based water-reducing agents are classified into two types: acrylate-acrylic ester (PCA) and olefin-maleate based. Polycarboxylate-based ADSPs with new functions can be developed because unlike BNS and MS, the chemical structure of polycarboxylate-based water-reducing agents can accept side chains exhibiting new functions by grafting and copolymerization. New functions can be added by controlling the chemical structures and their molecular weights as shown in Fig. 8.

Aminosulphonate based: Aromatic amino sulphonate-based polymer compounds are three dimensional condensates, similar to BNS- and MS-based agents.

For normal Portland cement paste prepared by either simultaneous or delayed addition of the above admixtures, the variation of setting times are shown in Fig. 9. The effect of the addition depends on the type of admixture. The effect of delayed addition of the naphthalene sulphonic and amino sulphonic acid-based admixtures is larger, while that of the lignin sulphonic and polycarboxylic acid-based admixtures is smaller [31].

Mid-range water reducing admixtures

Mid-range water reducing admixtures (MRWRA) were first introduced in 1984 [30,32]. These admixtures provide significant water reduction (between 6 and 12%) for concretes with slumps of 125 to 200 mm without the retardation associated with high dosages of conventional (normal) water reducers. While there is no specific ASTM C494 M classification for MRWRA, they are certified to a Type A and some are able to meet the requirements of a Type F (HRWRA). In Australian code AS 1478.1:2000, MRWRA is classified. These are mainly lignosulphonate (LS), beta-napthalene sulphonates (BNS) and polycaroxylate ethers (PCE) based admixtures are used in concrete as MRWRA. Mid-range water reducers can be used to reduce stickiness and improve finishability, pumpability, and placeability of concretes containing silica fume and other supplementary cementing materials. Some can also entrain air and be used in low slump concretes.

Viscosity Modifying Admixtures (VMA)

Many applications of concrete require that the concrete mix remain highly cohesive during transportation, pumping and placing prior to setting. Sufficient cohesion may not always be achieved by mix proportioning alone. Therefore, in these application, viscosity modifying admixture plays an important role. VMA is also frequently used to produce self compacting concrete (SCC) having high flowability, resistance to segregation in both dynamic and static state until hardened. Early approaches to maintain stability and minimizing bleeding were achieved by increasing the proportion of fine materials such as, cement, sand, filler etc. in the mix. However, these concretes are more susceptible to creep and shrinkage [33]. In contrary to above addition of VMA makes it possible to use a more conventional concrete mix by maintaining desired stability. This class of admixtures is used to modify the rheological characteristics of cementitious mixtures. VMAs are long chain water-soluble polysaccharides such as gums or starches, oxygenerated polymers, or dispersion of fine particles such as colloidal silica that enhance the water retention capacity of the concrete mix. While many of the natural polymers are practically available only as a powders, there are some products in liquid form at concentration practical for addition in the concrete. Early use of VMA was in mortars and grouts to minimize the bleeding and improve the adhesion. More recently the interest in adding them to self-consolidating concrete (SCC) has significantly expanded their use [34].

According to Khayat [34], these admixtures can act in the following ways:

Adsorption: Long chain polymer molecules adhere to periphery of water molecules, thus adsorbing and fixing part of the mix water and thereby expanding: this causes an increase in the viscosity

Association: Molecules in adjacent polymer chains develop attractive forces, thus further blocking the motion of water by forming a viscous gel

Intertwining: At low shear rates, polymer chains intertwine and entangle, causing an increse in the viscosity

Cold Weather Admixtures

This class of admixtures is used to facilitate placement of concrete under conditions where the concrete temperature is below the freezing point of water and would normally freeze without some form of protection. Freeze resistance admixtures have been used in former Soviet Union since 1950 to suppress the freezing point of concrete and allow placing and curing of concrete below freezing point. Typically non-chloride set accelerator are basic ingredients of freeze resistant admixtures. These admixtures work on both levels of freezing point depression and an acceleration of the hydration process. They generally contain inorganic salts and/or organic compounds and must be applied at relatively higher dosages (2 to 6%), depending upon the conditions. Generally a water reducing component is included to further lower the water content and increase the salt concentration of the pore solution to minimize the chance of freezing before the concrete has set. Quite acceptable time of setting and early strength development can be achieved with the use of these admixtures [7]. Chemicals for anti-freeze admixtures include sodium and calcium chloride, potash, sodium nitrite, calcium nitrate, urea and binary system such as calcium-nitrite-nitrate and calcium chloride-nitrite-nitrate [15]. These type of admixtures are covered under ASTM C 1622-2005.

Corrosion Inhibitors

The reinforcing steel in concrete is normally protected from corrosion by the high pH of the concrete environment which forms a stable passive oxide film created by hydroxide ion on the steel surface. This oxide film can be destroyed by a sufficiently high concentration of chloride ions or because of decrease pH from carbonation resulting to corrosion. The product of corrosion reactions create a volume expansion causing cracking and spalling of the concrete structure including reduction in the cross-sectional area of the reinforcing bars. Corrosion inhibitor are a means for delaying or eliminating the onset of corrosion in concrete structures. The corrosion inhibiter can be defined according to ISO 80441989 as "A chemical substance that decreases the corrosion rate when present in the corrosion system at suitable concentration, without significantly changing the concentration of any other corrosion agent". Corrosion inhibitors are usually used as a second line of defence to prevent corrosion that may occur due to cracking. The other preventive measures are decrease in permeability and reduction in chlorides & water ingress by addition of mineral admixtures, sealing the surface by external membrane etc. The corrosion inhibitors can be classified based on the base material such as inorganic organic or vapour phase. The more common method is to classify them by electro-chemical reaction that they predominantly affect which are the anodic or cathodic reaction or both. Commercially available corrosion inhibitors include: calcium nitrite, sodium nitrite, dimethyl ethanolamine, amines, phosphates, and ester amines. Most widely used anodic inhibitors, such as calcium nitrite, block the corrosion reaction of the chloride-ions by chemically reinforcing and stabilizing the passive protective film on the steel. This ferric oxide film is created by the high pH environment in concrete. The nitrite-ions cause the ferric oxide to become more stable. A certain amount of nitrite can stop corrosion up to some level of chloride-ion. Therefore, increased chloride levels require increased levels of nitrite to stop corrosion. A typical dosages vary from 5-30 lit./m3 for a 30% solutions. These inhibitors also accelerate the hydration process therefore it must be used in combination with retarding admixture to achieve acceptable time of setting particularly in summer season.

Cathodic inhibitors act on the oxygen reaction on the steel surface and they reduce the corrosion rate by a decrease in corrosion potential. The reduction of oxygen is the principal cathodic reaction in alkaline environments ([35].The most commonly used cathodic inhibitors are sodium hydroxide and sodium carbonate, which are supposed to increase the pH near the steel, and reduce the oxygen transport by covering the steel surface. Phosphates, silicate and polyphosphates are also used [36].

Inhibitors when compared to the other corrosion protection methods have some advantages such as versatility and cost. Their use in concrete can help to delay the initiation of corrosion of the embedded steel exposed to chloride attack and carbonation. However, after the initiation of corrosion, their effectiveness was reported to be less significant despite some contradictory results.

Shrinkage-Reducing Admixtures

Shrinkage-reducing admixtures, introduced in early 1980s in Japan, are used in bridge decks, critical floor slabs, machine foundation grouting and buildings where cracks and curling must be minimized for durability or aesthetic reasons. The admixture primarily consists of low molecular weight polyoxyalkylene alkyl ethers. They are liquids at room temperatures and soluble in aqueous systems and some hydrocarbon solvents. The typical dosages of admixtures varies from 0.75-2% by the weight of cement which reduces unrestrained drying shrinkage as much as 80%. The primary mechanism of action of shrinkage reducing admixture is to interfere with the surface chemistry of the air/water interface within the capillary pores, reducing surface tension effects and consequently reducing the shrinkage as water evaporates from within the concrete. Other type of shrinkage reducing admixtures are glycol derivatives. Drying shrinkage reductions of 25% and 50% have been demonstrated in laboratory tests. These admixtures have negligible effects on slump and air loss, but can delay setting. They are generally compatible with other admixtures [32, 37].

The addition of shrinkage reducing admixture has shown a reduction in compressive strength at 28 days by upto 15% [38]. This can be offset by using water reducing admixture in the concrete. Depending on the type of SRA a slight increase in air content in the concrete has been observed.

Hydration Controlling Admixtures

Hydration controlling admixtures (HCA) introduced in the late 1980s, are used in ready-mixed concrete resulting as a waste from wash water and returned plastic concrete disposal of these materials is an economic burden for the concrete producer, as well as an sustainability concern. Reuse of the material into freshly batched concrete generally accelerates the hydration of cement and consequently loss of slump, makes its use impractical at many instances. Making these concrete as reusable materials, uses of hydration controlling admixtures are beneficial. transportation of concrete. They consist of a two-part chemical systems:

(a) A stabilizer or retarder that essentially stops the hydration of cementing materials and

(b) An activator that reestablishes normal hydration when added to stabilized concrete

The stabilizer can suspend hydration for 72 hours and the activator is added to the mixture just before the stabilized concrete is used. These admixtures make it possible to reuse concrete returned in a ready-mix truck by suspending setting overnight. The admixture is also useful in maintaining concrete in a stabilized non-hardened state during long hauls. The concrete is reactivated when it arrives at the project. This admixture presently does not have a standard specification [39]. Doses of these admixtures and the corresponding amounts of accelerators to reactivate, are highly variable and depends on a number of factors [40]. These include age of concrete, length of stabilization time required, temperature of the concrete, and the time of setting required after reactivation, etc. HCA are formulated using carboxylic acids and or phosphorus containing organic acids and their respective salts.

Other Chemical Admixtures

Several other types of admixtures are developed and used for specific purposes [41]. Color admixtures are used to color concrete for aesthetic and safety reasons. Red concrete is used around buried electrical or gas lines as a warning to anyone near these facilities. Yellow concrete safety curbs are used in paving applications. Generally, the amount of pigments used in concrete should not exceed 10% by weight of the cement. Pigments used in amounts less than 6% generally do not affect concrete properties. Pigments should confirm to ASTM C 979 [42].

Fungicidal, germicidal, and insecticidal admixtures are used to partially control bacterial and fungal growth on or in hardened concrete. The most effective materials are polyhalogenated phenols, dieldrin emulsions, and copper compounds. The effectiveness of these materials is generally temporary, and in high dosages they may reduce the compressive strength of concrete [42].

The admixture for suppressing alkali silica reactions are used to minimize the deleterious effects of alkali silica reactions. Two types of chemical admixtures are used to mitigate the ASR effects are, air-entraining admixtures and lithium salts [7]. The lithium salts such as LiOH, LiCl, Li2CO3, LiNO3 are reported to be very effective in eliminating expansion due to ASR over long period of time. The preferred salts of lithium is LiNO3.

Bonding Admixtures are used to enhance bonding properties of hydraulic-cement-based mixtures and it generally consist of an organic polymer dispersed in water (latex). In general, latex forms a film throughout the mixture. Polymer latex, as a concrete admixture is formulated to be compatible with the alkaline nature of the Portland cement and the various ions present.

Flocculating admixtures are basically synthetic polyelectrolytes, such as vinyl acetate-maleic anhydride copolymer. These admixtures increase the bleeding rate, decrease the bleeding capacity, reduce flow, increase cohesiveness, and increases the early strength.

Air-detraining admixtures also known as defoamers or deaerators, are used to reduce the air content in a variety of applications. Air-detraining chemicals are tributyl phosphate, dibutyl phosphate, dibutylphthalate, polydimethylsiloxane, dodecyl (lauryl) alcohol, octyl alcohol, polypropylene glycols, water-soluble esters of carbonic and boric acids, and lower sulphonate oils. These admixtures lower the surface tension and need to insoluble in water to be effective.

Effects of Admixtures on Properties of Concrete

The influence of important admixtures on the properties of fresh and hardened concrete as reported in several literatures [1,4,7,8,12,14,15,17,19,20,26,41,43] is presented in Table 1. Lignosulphonate-based air-entraining (AE) water-reducing agents are used in various concrete structures for over 50 years. Polycarboxylate-based superplasticizers, which are the main superplasticizers in use today, are in the market for last 20 years [44]. The concept of polymer modification of cement mortar and concrete is not new, since considerable research and development of polymer modification have been performed for the past 70 years or more. As a result, various polymer-based admixtures have been developed, and polymer-modified mortar and concrete using them are currently popular construction materials because of their good cost-performance balance.

New Inventions and Discussions

The recent developments of the beneficial effects of some organic molecules on very specific properties of concrete has been often reported fortuitous, but it can now be explained scientifically. The very recent science of chemical admixtures has progressed quite rapidly, but still alot researches have to be done, because in some cases we do not yet understand well enough the interaction mechanisms governing the mixture of Portland cement and advanced chemical admixtures.


New generations of high-range & mid – range water reducing admixtures continue to make concrete easier to place & finish-saving time, increasing productivity, and reducing labor requirements. Several other significant achievements are made in the field of chemical admixtures such as: polycarboxylate-based superplasticizers, PCS for self-compacting concrete, shrinkage reducing admixtures, ASR controlling agents, admixtures for CLSM (controlled low strength material), hydration stabilizing agents for returned concrete, antifreeze admixtures (non-corrosive, alkali-free), viscosity modifying admixtures, slump extending admixtures, nano-admixtures for high early strength, admixtures for pervious concrete, surface enhancing admixtures. Polycarboxylates have been developed that have a dual mode of action by providing dispersion by electrostatic repulsion and steric hindrance [45-50].

Conclusion

Chemical admixtures are used singularly or in combination to improve the desired properties of concrete or mortar in the plastic and hardened states. Types and dosages are selected in accordance with climatic conditions for maintaining workability and pumpability, strength, w/cm, air content, setting time, and early and final strengths. Proposed mixtures and admixture choices are often confirmed with successful test placements on site. Chemical admixtures can be used to develop concrete of a required workability and strength characteristic at lower cement contents than the comparative plain concrete with no adverse effect on the durability of the concrete or total structure. It is more profitable to use an admixture rather than to increase the amount of cement in order to achieve a given compressive strength. Chemical admixtures have become a very useful and integral component of concrete. Admixtures are not the only solution for every ill the concrete producer, architect, engineer, owner, or contractor faces when dealing with the many variables of concrete, but they do offer significant improvements in both the fresh and hardened state to all concrete. Continued research and development will provide additional reliability, soundness economy, and performance for the next generation of quality concrete.

Regardless of what new and exciting chemical admixtures are introduced into the concrete construction, researches at a routine basis are needed to be made for the production of chemical admixtures that can produce cost-effective, high quality and sustainable concrete.

Acknowledgement

The authors thank to the Director, CSIR-Central Building Research Institute, Roorkee for granting permission to publish this paper. The contributions of Mr Anmol Anand and Mr Sandeep Rana, Project Assistant, CSIR-CBRI, Roorkee are acknowledged.

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NBMCW December 2013