Chapter IV Concrete Making Materials · Advanced Concrete Technology - Zongjun Li 24 • finely...

CIV415CONCRETE TECHNOLOGY

Chapter IVConcrete Making Materials

“Admixtures”

Assist.Prof.Dr. Mert Yücel YARDIMCISpring, 2014/2015

Advanced Concrete Technology - Zongjun Li 1

Admixtures


Historically, an admixture is almost as old as concrete itself.

The Romans used animal fat, milk, and blood to improve theirconcrete properties.Although these were added to improve workability, blood was avery effective air-entraining agent and might well have improvedRoman concrete durability. In more recent times, calcium chloridewas often used to accelerate the hydration of cement.

The systematic study of admixtures began with the introduction of air-entrainingagents in the 1930s, when it was accidentally found that cement ground withbeef tallow (grinding aid) had more resistance to freezing and thawing than acement ground without beef tallow. Nowadays, as we mentioned earlier,admixtures are important and necessarycomponents for modern concrete technology.

Admixtures – definition & classification


An admixture is defined as a material other than water, aggregates,cement, and reinforcing fibers that is used in concrete as aningredient, and added to the batch immediately before or duringmixing.

• Air-entraining agents• Chemical admixtures• Mineral admixtures


Air-entraining agents (ASTM C260): to improve the frost resistance of concrete.

Kimyasal katkılar (ASTM C494, BS 5075 and EN 934-2):Any chemical additive to the concrete mixture that enhances the properties ofconcrete in the fresh or hardened state.• Reducing the water demand of mixture (water reducers)• Controlling the setting time and strength gain rate (accelerators and retarders)• Viscosity modifying agent (adjusting the viscosity of paste)• Shrinkage reducing chemicals• Alkali-silica mitigating admixtures

Mineral katkılar (ASTM C494 and BS 5075):consists of finely divided solids added to concrete to improve its workability, durability, and strength. Fly ash, slag and pozzolans are important categories of mineral admixtures.

Miscellaneous admixturessuch as latexes, corrosion inhibitors, and expansive admixtures.


Water-reducing admixtureThey are used to reduce the water content of a concrete mixture while maintaininga given constant workability.The resultant effect of reduced water content is increased strength and durabilityof concrete.

Water reducers may also be employed to “plasticize” the concrete, i.e., to makeconcrete flowable. In this case, the water content (or water-to-cement ratio) is heldconstant, and the addition of the admixtures makes the concrete flow better, whilethe compressive strength (which is a function of the water-to-cement ratio) is notaffected.

Another use of water reducers is to lower the amount of cement (since water isproportionately reduced) without affecting strength and workability. This makesthe concrete cheaper and more environmentally friendly, as less cement isconsumed.

Water reducers are classified broadly into two categories: (1) normal and (2) high range.

Water-reducing admixture


The normal water reducers are also called plasticizers, while the high-range water reducers

are called superplasticizers.

Normal water reducer (Plasticizer)

• Reducing water demand by 5 – 10%• Lignosulfonate salts of sodium and

calcium

High range water reducer (Superplasticizer)

• Reducing water demand by 15 – 40%• Sulfonated naphthalene formaldehyde (SNF)• polycarboxylic ether (PCE)

Lignosulfonates (normal and sugar-refined), andSNF-based water reducers work on the mechanismof lowering zeta potential, which leads toelectrostatic repulsion to separate cement particlesfrom flocculation, thus releasing the entrappedwater by cement particle clusters



Polycarboxylic ether-based water reducers are polymers withbackbone and side chains. The backbone gets adsorbed on thesurface of the cement grains, and the side chains cause dispersionof cement grains by steric hindrance.This phenomenon relates to the separation of the admixturemolecules from each other due to the bulky side chains,demonstrated in Figure 2-30.Steric hindrance is a more effective mechanism than electrostaticrepulsion.

The dosages of water reducer for the liquid form can refer to either the solid content or liquid mass and is expressed as weight percentage of the binder.



Superplasticizers (SPs) are used mainly for two main purposes:• to produce high-strength concrete at a w/c ratio in a range of 0.23–0.3;• to create “flowing” concrete with high slump flow in the range of 500

to 600 mm, the self-compacting concrete.

Another benefit is that a lower w/c ratio would lead to better durabilityand lower creep and shrinkage.

The major drawbacks of superplasticizers• retarding setting (especially in large amounts)• causing more bleeding• entraining too much air.

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The most common problem in the application of water reducers inconcrete is incompatibility with cement and aggregate, which refers tothe abnormal behavior of a concrete due to the superplasticizer used.

Common problems:• Shorten setting time,• delayed setting time,• rapid slump loss,• improper early-age strength development.

These issues in turn affect the hardened properties of concrete,primarily strength and durability.

Compatibility between cements and superplasticizers is affected bymany factors, including cement composition, admixture type anddosage, and concrete mixture proportions.




The C3A content or, more specifically, the C3A to SO3 ratio has aprofound effect on the compatibility between cement and SPs.

When the C3A content of cement is high and the sulfate availability islow, superplasticized concretes experience high rates of slump loss.

When there is less C3A available, higher amounts of SPs tend toadsorb on C3S and C2S, resulting in a reduction in the rate of strengthdevelopment.



Most of the chemical admixtures are in liquid form.The solid content varies between 30% - 50%. The remaining is water.The additional water added coming from the superplasticizer should be deducted from the mixing water amount.

Kg/m3Cement 460Water 1610-3 mm sand 8935-12 mm crushed stone 53612-22 mm crushed stone 357Superplasticizer 4.60TOTAL 2412

If the solid content of the superplasticizer is 30%Mixing water should be (161-4.60*0.70)

Shrinkage-reducing admixture


Shrinkage-reducing admixtures (SRA) provide a significant technicalapproach to reduce the drying shrinkage of concrete.

The main mechanism of SRA in reducing drying shrinkage of concreteis that the SRA lowers the surface tension of the pore solution andsubsequently reduces the stresses in the pore solution that aredirectly proportional to the surface tension.

With the reduction of the driving stress, the drying shrinkage can bereduced.

Setting-control admixture


Setting-control admixtures are used to either extend or shorten theplastic stage of concrete to meet the special requirements of theconstruction of concrete structures.

If the admixture is used toextend the plastic period, it iscalled a retarder.If the admixture is used toshorten the plastic period, it iscalled an accelerator.

Some of the admixtures are designed to retard (I and IV) or accelerate (V) the setting. On the other hand, there are some admixtures presenting dual-role depending on the dosage.

Setting-control admixture


Retarding admixtures:• Mainly used to offset fast setting caused by ambient temperature, particularly in hot weather;• As a Setting control of large structural units to keep concrete workable throughout the entire

placing period;• to meet the requirement of long transportation time from the concrete plant to the

construction site.For example, for the construction of building built up in a very far place from the concrete plant,the concrete has to be transported by trucks (mobile mixers). In this case, 5 to 6 h or even longermay be required.

Accelerators:• It is mainly used when the more quick setting is required.(plugging leaks in swimming pools, water tanks, and pipelines; emergency repairs for highways,bridges, airport runways, and tunnels, etc.)• Winter construction in cold regions.Soluble inorganic salts, such as calcium chloride, are by far the best known and most widely usedaccelerators.

A side effect of using chloride, however, is that it induces corrosion of thereinforcement in concrete structures. Nowadays, calcium nitrate is used instead ofcalcium chloride.


Air-entraining admixtureAir-entraining admixtures entrain air inthe concrete.

Surface-active agents having two poles

The agents are concentrated at theair–water interface, the hydrophilicside with water and the hydrophobicside with air.The surface tension is lowered sothat bubbles can form more readilyand then stabilize once they areformed.

Commonly, carboxylic acid or sulfonic acidgroups are used as to achieve thehydrophilicity, while aliphatic or aromatichydrocarbons are used for hydrophobility.

The entrained air is produced by admixturesthat cause the mixing water to foam, andthe foam is locked into the paste duringhardening.

Entrappedair void


Entrained air voids are different from entrapped air voids.Entrained air voids are formed on purpose, while an entrapped air void isformed by chance when the air gets into the fresh concrete during mixing.

Entrapped air voids may be as large as 3 mm; Entrained air voids usually range from 50 to 200 μm.


Advantages of air-entraining admixtures• The workability of concrete can be improved because air bubbles

act as lubricants in a fresh concrete. Subsequently, the wateramount can be reduced for a targeted workability.

• The ductility of concrete can be improved, since the air bubblesgenerated by the air-entraining agent provide more room fordeformation to occur.

• The permeability of concrete can be improved due to the effectof air-entraining agent in enclosing the air bubbles.

• The impact resistance of concrete, can be improved as the airbubbles provide more deformation.

• Finally and most importantly, the durability, especially theability to resist freezing and thawing of concrete, can besignificantly improved by adding an air-entraining agent.

Spacing factor in air-entrainment


The smaller the spacing factor(which is defined as the averagedistance from any point in thepaste to the edge of a nearestentrained air bubble), the moredurable the concrete.

For the case of a spacing factorgreater than 0.3 mm, the airentrained has a small ornegligible effect on durability.


The volume of air required to give optimum durability has been found to be about 4–8% by volume of concrete.

The actual fraction depends on the maximum size of aggregate: the larger the aggregate size, the lower requirement for air content.

Mineral Admixtures


Mineral Admixtures


• finely divided siliceous materials.• added to concrete during mixing in relatively large

amounts.• Industrial by-products are the primary source of

mineral admixtures.• silica fume, slag, fly ash, and metakaoline.

Silica fume


• a by-product of the silicon metal and ferrosilicon alloy industries.

• very fine spherical particles consisting of noncrystallinesilica.

• silica fume is also called condensed silica fume or microsilica.

Size Smaller than

0.50 µm 99%

0.20 µm 95%

0.10 µm 70%

0.05 µm 20%

Silica fume


• The surface area around 200,000 cm2/g. Thisvalue is typically around 3,700 cm2/g for normalPortland Cement.

• Presents very high puzzolanic activity.

• The normal percentage of silica fume used toreplace Portland cement is from 5 to 15%.

• However, it creates handling problems and increases the water requirement in concreteappreciably, unless superplasticizer is used.

Size distribution of silica fume


Puzzolanic reaction of silica fume


A denser concrete microstructure can be achieved anda high compressive strength can be reached.

Pozzolanic reaction is defined as

Cement + water = C – S – H (Tobermorite gel) + Ca(OH)2

pozzolan + calcium hydroxide + water = Calcium Silicate Hydrate (secondary)

Silica fume is a very active pozzolanic material and readily reacts with CH and water to form secondary C–S–H.

The process can further reduce the porosity and permeability in concrete as well as the possibility of chemical reaction of CH with other ions to form harmful products.

Metakaolin


High-reactivity metakaolin (MK) is one of the recently developedsupplementary cementing materials for high-performance concrete.

It is produced by calcining purified kaolinite clay in a specifictemperature range (650 to 800◦C) to drive off the chemically boundwater in the interstices of kaolin and destroy the crystalline structure,which effectively converts the material to the MK phase, anamorphous aluminosilicate.

Unlike industrial by-products, such as silica fume (SF), fly ash, and blast-furnace slag, MK is carefully refined to lighten its color, removeinert impurities, and control its particle size.

Metakaolin


The particle size of MK is generally less than 2 μm, which issignificantly smaller than cement particles, though not as fine as SF.

It is typically incorporated into concrete to replace 5–20 % ofcement by mass.

Silicon dioxide and aluminum oxide are the two main components in MK and little CaO exists in MK.

Similar to silica fume, metakaolin improves concrete performance by the packing effect and by reacting with calcium hydroxide to form secondary C–S–H.

Advantages of metakaolin


MK was found to improve concrete properties while offering goodworkability.

Concrete modified by MK requires less water-reducing admixturethan that modified by SF to achieve a comparable fluidity.

MK is particularly effective in reducing the rate of diffusion ofsodium and chloride ions. The diffusion coefficient can be reducedby about 50% when 8% MK is added into the concrete mixture.

MK and SF had similar functions in improving strength of concrete, reducing free drying shrinkage, and chloride diffusion rate.

Fly ash


Fly ash (pulverized fuel ash) is a by-product of an electricity-generating plant using coal as fuel.

Fly ash can be divided into two categories according to type of coal burned (ASTM C618)

Bituminous coal

burning ligniteor sub-bituminous coal.• class F is called low-

calcium fly ash• class C is called high-

calcium

Fly ash


The size distributions of fly ash are slightly smaller than those of Portland cement with more than 50% under 20 μm.

Fly ash


• Fly ash can be used to produce a so-called blended cement.

• In this case, fly ash is milled with clinker and gypsum in the last procedure in cement production.

• Fly ash can also be used to produce modified concrete by adding it to concrete during the mixing process.

Fly ash

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The incorporation of fly ash into concrete has certainadvantages and some disadvantages.

• Incorporation of fly ash into concrete can improve the workabilitydue to the spherical shape and glassy surface of fly ash particles.

• Incorporating fly ash into concrete can reduce the hydration heatof fresh concrete and is good for mass concrete structures.

• By replacing cement with fly ash, the cost of concrete can bereduced, since fly ash costs low than cement.

• Because fly ash is an industrial by product, it lowers the energydemand in producing concrete.

Fly ash


• Fly ash concrete are low early age strength and longer initialsetting time due to the low reactivity of fly ash (it is not favorablein cold-weather concreting).

• The normal replacement of cement by fly ash is around 25 to30% by weight.

Slag


Slag is a by-product of iron or steel production.

For iron production, blast furnaces are used and for steelproduction, either a basic oxygen furnace or an electrical furnace.

The slag produced in iron production is different from that fromsteel production. Currently, the slag used in the concrete industryis mainly the slag from iron production, and its full name isground, granulated blast furnace slag or GGBS.

GGBFS


CaO and SiO2 are the two main components, and the CaO content inslag is quite close to that of high-calcium fly ash.

Ground granulated blast-furnace slag and high-calcium fly ashes are similar inmineralogical character and reactivity. Besides a similar content of CaO, both areessentially noncrystalline, and their high-calcium glassy phases have a similarorder of reactivity.

GGBFS


Compared to low-calcium fly ash, which usually does not make anysignificant contribution to the strength of Portland cement concreteuntil after about 4 weeks of hydration, the strength contribution byhigh-calcium fly ash or granulated blast-furnace slag may becomeapparent as early as 7 days after hydration.

Similar to fly ash, slag can also be used in both cement and concrete production.

In cement production, slag can be milled with clinker and gypsum to produce Portland–slag blended cement.

In concrete production, up to 50% of cement can be replaced by slag.Slag-modified concrete shows an improved slump retention as well as both earlyand long-term strength

Benefits of using mineral admixtures in concrete


Chapter IV Concrete Making Materials · Advanced Concrete Technology - Zongjun Li 24 • finely...

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