Admixtures in Concrete

CONCRETE TECHNOLOGY

5/26/2014 Prepared by Dr SK Lim 1

TOPIC 1 ADMIXTURES IN CONCRETE

Admixtures

A material added to plastic concrete or mortar, other than water, aggregates, cement, and fibre to change one or more properties of fresh or hardened concrete


properties of fresh or hardened concrete (ASTM C494).

They are added in concrete by either weighed or measured by volume as recommended by the manufacturer.

When to Use Admixture?

When the desired modification of properties of fresh or hardened concrete cannot be achieved by changes in the composition of the mix proportion.


the mix proportion.

Classification

Classified by chemical and functional physical characteristics (Hewlett, 1978):

Air entrainers Water reducers


Water reducers Retarders Hydration controller admixtures Accelerators Supplementary cementitious admixtures Specialty admixtures

Chemical Admixtures

Chemical added in very small amount or quantity to the mixture to develop special properties in fresh or hardened concrete.

1. Accelerating admixture


2. Retarding admixture3. Water-reducing/ high-range water-reducing

admixture4. Air-entraining admixture5. Others

Reasons for Using Admixtures

Portland Cement Association (PCA) in USA identifies 4 major reasons for using admixtures (Kosmatka et al., 2002)

1. Reduce cost of concrete construction 2. Achieve certain properties in concrete more


2. Achieve certain properties in concrete more effectively than by other means.

3. Ensure quality of concrete during the stages of mixing, transporting, placing, and curing in adverse weather conditions.

Overcome certain emergencies during concrete operations.

Accelerating Admixture To accelerate the setting and early strength development of concrete,

particularly in cold weather applications. Meet the requirement of ASTM D 98

E.g. Calcium Chloride (CaCl2) The initial set time of 3 hours for a typical concrete reduced to 1.5 hours by

1% calcium chloride of the cement weight. Adding 2% calcium chloride reduces the initial setting time to 1 hour. Typical final setting times are 6 hours, 3 hours, and 2 hours for 0%, 1%,

and 2% calcium chloride.


and 2% calcium chloride. Compressive strength at 3 days at least 25% higher than concrete without

admixture. The amount of CaCl2 should be limited to 2% by weight of cement. Excessive amounts of calcium chloride in concrete mix may result in rapid

stiffening, increase in drying shrinkage and creep of concrete and corrosion of reinforcement.

CaCl2 is not recommended in hot weather concreting and for pre-stressed concrete and may be ineffective at temperature below freeze before it reaches the initial set.

Reasons to use Accelerators

Reduce the amount of time before finishing operations begin.

Reduce curing time. Increase rate of strength gain


Increase rate of strength gain Plug leaks under hydraulic pressure

efficiently.

The PCA recommends against using

calcium chloride under the following

conditions: Concrete is pre-stressed. Concrete contains embedded aluminum such as

conduits, especially if the aluminum is in conduct with steel.


with steel. Concrete is subjected to alkali-aggregate reaction. Concrete is in contact with water or soils containing

sulfates Concrete is placed during hot weather. Mass applications of concrete.

ACI recommends the following limits to

water-soluble chloride ion content based

on percentage of cementMember Type Chloride Ion Limit, %Pre-stressed concrete 0.06


Reinforced concrete subjected to chloride in service

0.15

Reinforced concrete protected from moisture

1.00

Other reinforced concrete 0.30

Retarding Admixture

Some construction conditions require that the time between mixing and placing or finishing the concrete be increased.

Added to concrete to delay its setting and hardening, especially in hot weather applications (temperature greater than 320C).

Hot weather increases the rate of hardening which makes it difficult to place and finish the plastic concrete.

Recommended in large structures and difficult situations such as large piers and foundations, to delay the initial set as well as to


large piers and foundations, to delay the initial set as well as to keep concrete workable throughout the entire placing period (eliminate cold joint).

Sugar in any form is a retarder. The use of retarder may reduce the strength at early age. Because most retarders also act as water reducers, they are

frequently called water-reducing retarders. Based on ASTM C 494, type B is simply a retarding admixture,

while type D is both retarding and water reducing, resulting in concrete with greater compressive strength because of the lower water-cement ratio.

Retarding Admixture Cont.

Retarding admixtures consists of both organic and inorganic agents.

Organic retardants include unrefined calcium, sodium, NH4, salts of lignosulfonic acids, hydrocarboxylic acids, and carbohydrates. Inorganic retardants include oxides of lead and zinc,


Inorganic retardants include oxides of lead and zinc, phosphates, magnesium salts, fluorates and borates.

As an example of a retardant's effects on concrete properties, lignosulfate acids and hydroxylated carboxylic acids slow the initial setting time by at least an hour and no more than three hours when used at 65 to 100 degrees Fahrenheit.

Reasons using Retarders

Offsetting the effect of hot weather Allowing for unusual placement or long haul

distances. Providing time for special finishes (e.g.,


Providing time for special finishes (e.g., exposed aggregate)

Water-Reducing Admixture

One of Surfactants, also known as surface-active chemical

To gain workability and maintain quality. Used to reduce the quantity of mixing water required,

which increase the compressive strength, and to produce concrete of desired consistency or high slump.


which increase the compressive strength, and to produce concrete of desired consistency or high slump.

Can reduce water required by 12% or more. Its effect is due to increased dispersion of cement

particles, causing a reduction in the viscosity of the concrete

Increase mobility of the cement particles in the plastic mix.

Levels of Effectiveness

Conventional (low-range) Mid-range High range (Superplaticizer)


Water Reducers Mechanism Cement grains develop static electric charge on their surface

as a result of the cement grinding process. Different charges attract to each other, causing the cement

grains to cluster or flocculate which in turn limits the workability. The chemicals in the water-reducing admixtures reduce the static

attraction among cement particles. The molecules of water-reducing admixtures have both positive

and negative charges at one end, and a single charge (usually negative) on the other end.


negative) on the other end. These molecules are attracted by the charged surface of the

cement grains. The water reducers neutralize the static attraction on the cement

surfaces. As a result, the clusters of cement grains are broken apart.

Mutual repulsion of like charges pushes the cement grains apart, achieving a better distribution of particles, more uniform hydration, and a less-viscous paste.

High-Range Water Reducer

Also called Superplasticizer (ASTM C494). Superplasticizers are linear polymers containing sulfonic acid

groups (water-soluble organic polymers) attached to the polymer backbone at regular intervals (Verbeck, 1968).

The sulfonic acid groups are responsible for neutralizing the surface charges on the cement particles and causing dispersion,


surface charges on the cement particles and causing dispersion, thus releasing the water tied up in the cement particle agglomerations and thereafter reducing the viscosity of the paste and concrete (Mindess and Young, 1981).

Superplasticizer is added in small dosage with mixing water (typically 0.2% by weight of cement) to increase slump.

Flowing concrete- slump greater than 180mm Dosage rate may vary from brand to brand for different types of

cement.

High-Range Water Reducer Cont.

By using superplasticizer, it is possible to have w/c ratio in the range of 0.3 to 0.45which would be impossible otherwise.

Addition of superplasticizer increases cost by


Addition of superplasticizer increases cost by 5% BUT savings in labour can be as high as 33%.

Families of Superplasticizers

Most of the commercial formulations belong to one of four families:

1. Sulfonated melamine-formaldehyde condensates (SMF)


condensates (SMF) 2. Sulfonated naphthalene-formaldehyde

condensates (SNF) common used3. Modified lignosulfonates (MLS) 4. Polycarboxylate derivatives

Effect of Superplasticizer

Increase in workability Increase in compressive strength Eliminates segregation

Allow good dispersion of cement particles in


Allow good dispersion of cement particles in water

Concrete With Superplasticizer

Has smooth surface Much less likely to chip and spall Has fewer plastic shrinkage cracks Higher compressive strength (up to 70-80 MPa) Less shrinkage


Less shrinkage Low permeability Less time to place and finish the concrete (stays

workable for 30 to 60 minutes) An accelerating curing period because of rapid gain

in strength Early removal of formwork

Air-Entraining Admixture

Added to entrain air/ produce tiny air bubbles in concrete to provide space for water to expand upon freezing.

Air entrainment is a process by which a large


Air entrainment is a process by which a large number of microscopic air bubbles are dispersed throughout the concrete.

Diameter of bubbles in the range of 20 to 200m

Air-Entraining Admixture

Air entrainers are usually liquid (ASTM C 260) The agents enhance air entrainment by lowering the surface

tension of the mixing water. Anionic (-) air entrainers are hydrophobic (water hating/ repelled

water). The negative charge of the agent is attracted to the positive


The negative charge of the agent is attracted to the positive charge of the cement particle.

The hydrophobic agent forms tough, elastic, air-filled bubbles. Mixing disperses the air bubbles throughout the paste, and the

sand particles form a grid that holds the air bubbles in place. Other types of air entrainers have differerent mechanisms but

produce similar results.

Advantages of Air-Entraining Admixtures

It causes the disruption of the continuity of capillary pores result in reduction in permeability and reduces internal stresses caused by expansion of the pore water on


caused by expansion of the pore water on freezing.

Foaming agents are added in small amount (typically about 0.05% by weight of cement) during mixing and can be used to entrain 3 to 10% air.

Advantages of Air-Entraining Admixtures

Cont.

The use of air-entraining admixture can:- Improved workability- Reduce bleeding

Reduce segregation


- Reduce segregation- Shrinkage-compensated (e.g. aluminium

powders)However:- Lower compressive strength

Variety of Air-Entraining Materials

Salts of wood resins (Vinsol resin) Synthetic detergents Salts of sulfonated lignin (by-product of paper

production).


production). Salts of petroleum acids Salts of proteinaceous material Fatty and resinous acids Alkylbenzene sulfonates Salts of sulfonated hydrocarbons.

Chemical Admixture

Description Applications

Water-reducing Admixture To get dense concrete, to improve workability


Retarding admixture To delay setting and hardening, hot weather concreting, large structures

Accelerating admixture To accelerate setting and early strength development, cold weather concreting

Air-entraining admixture To improve durability and workability

Mineral Admixtures

Are natural pozzolanic materials or industrial by-products that are used in concrete to replace part of cement

It is known as supplementary cementing material


material Are generally added in relatively large

quantities in comparison with chemical admixtures

Contains reactive silica (SiO2) /amorphous silica

Reactive Silica Rocks and MineralsROCKS Reactive Minerals

Arenite TridymiteArkose Cristobalite

Shale Volcanic glass Sandstone Cryptocrysalline (or

microcrystalline) quartz Limestone Strained quartz

Chert


ChertFlint

QuartziteQuartz-arenite

GneissArgilliteGranite

GreywackeSiltstone

Silica CrystalsTetrahedral coordination of silica (SiO2), the basic building block of the most ideal glass former.

Crystal structure-Tetrahedral coordination of silica (SiO2), the basic The amorphous structure of


of silica (SiO2), the basic building block of the most

ideal glass former.glassy silica (SiO2) in two-dimensions. No long-range order is present; however there is local ordering with respect to the tetrahedral

arrangement of oxygen (O) atoms around the silicon (Si) atoms. Note that a fourth oxygen atom is bonded to each silicon atom, either behind the plane of the screen or in

front of it; these atoms are omitted for clarity.

The Effect of pH on Silica

Solubility.


The pore solution pH influences the solubility of silica. The higher the pH (the more OH-), the more likely silica will dissolve.

Blended Cement- Green Materials

(Environmental-friendly)

Portland cement consists of 95% cement clinker (carbon dioxide emissions through manufacturing process) and 5% gypsum.

To reduce CO2 emissions, produce blended


To reduce CO2 emissions, produce blended cement = OPC + pozzolans/ mineral admixtures (industrial wastes or by- products)

Green technology- reduce, reuse and recycle the wastes (industrial ecology), so reduce the environmental impact.

Pozzolans

Are siliceous or siliceous and aluminous materials which in themselves possess little or no cementitious property

BUT in finely divided form and in the presence of moisture, these materials can


presence of moisture, these materials can react with calcium hydroxide of cement at ordinary temperature to form compounds of cementitious properties

This reaction is called POZZOLANIC REACTION

Pozzolanic Activity The Pozzolanic reaction primarily occurs between

amorphous siliceous materials (namely, pozzolanand pozzolana, a finely divided volcanic ash, rich in Obsidian/ igneous rock, a mineral glass commonly found in lava (molten rock expelled by a


found in lava (molten rock expelled by a volcano) ) and slaked lime (calcium hydroxide) to form calcium silicate hydrates

At the basis of the Pozzolanic reaction stands a simple acid-base reaction between calcium hydroxide, also known as Portlandite, or (Ca(OH)2), and silicic acid (H4SiO4, or Si(OH)4).

Pozzolanic Reaction

Hydration process:cement + H2O C-S-H gel + Ca(OH)2

Pozzolanic reaction: Ca(OH)2 + H4SiO4 > Ca2+ + H2SiO42- + 2


Ca(OH)2 + H4SiO4 > Ca + H2SiO4 + 2 H2O > CaH2SiO4 2 H2O (Calcium Silicate Hydrate gel)

or summarized in abbreviated notation of cement chemists:

SiO2 + Ca(OH)2 C-S-H gel

Pozzolanic Reaction

The ratio Ca/Si, or C/S, and the number of water molecules can vary and the here above-mentioned stoichiometry (the calculation of quantitative (measurable) relationships of the reactants and products in a balanced chemical reaction) may differ.

As the density of CSH is lower than that of Portlandite and pure silica, a consequence of this reaction is a swelling of the reaction products.


products. This reaction may also occur with time in concrete between alkaline

cement pore water and poorly-crystalline silica aggregates (ASR). This delayed process is also known as alkali-silica reaction (ASR),

or alkali-aggregate reaction (AAR), and may be very damageable to concrete structures because the resulting volumetric expansion is also responsible for spalling and decrease of the concrete strength.

Example of Pozzolans

Ground Granulated Blast Furnace slag (GGBFS)

Pulverised Fuel Ash (PFA) Silica Fume (SF)


Silica Fume (SF) Rice Husk Ash (RHA) Palm Oil Fuel Ash (POFA)

Method of Replacement

Replacing part of the cement content (by weight of cement)

Can be added during mixing process or grind together with cement (pozzolan cement)


together with cement (pozzolan cement)

Effect of Pozzolanic Admixtures

Lower early strength Lower heat of hydration Higher ultimate strength

Less permeable


Less permeable Reduce cost Increase workability

Slag (ASTM C 989) Is a non metallic product, consisting essentially of

silicates and aluminates of calcium bases Not only pozzolanic but also cementitious It is develop in a molten condition simultaneously

with iron in a blast furnace Is the glassy (amorphous, non crystalline solid)

granular material formed when molten blast furnace


granular material formed when molten blast furnace slag in rapidly chilled as by immersion in water.

The granular material is then ground to less than 45 microns.

Specific gravity in the range of 2.85 to 2.95 Amount replacement: From 20% up to 80% Level of reactivity- Grade 80, 100, or 120


Ground granulated blast furnace slag

Fly Ash Is a by-product of burning powdered coal in electric generating

power plant Generally finer than Portland cement and consists mainly of small

spheres of glass involving silica, alumina, and ferric oxide Particles diameters range from 1m to more than 0.1mm, with an

average of 0.015mm to 0.02mm, and are 70% to 90% smaller than 0.045mm/ 45m.

Primarily a silica glass composed of silica (SiO2), alumina (Al2O3), iron oxide (Fe O ), and lime (CaO)


2 2 3iron oxide (Fe2O3), and lime (CaO)

Amount of replacement: 20 to 70% Spherical shape of fly ash increases workability. Fly ash extends the hydration process, allowing a greater strength

development and reduced porosity. 20% replacement- smaller pore size distribution, lower heat of

hydration decelerate early strength, but extended reaction permits a continuous gaining of strength.

Classification of Fly Ash (ASTM C 618)

Three types: Class N Fly Ash- Raw or calcined natural pozzolans,

including diatomaceous earths, opaline cherts and shales, ruffs and volcanic ashes or pumicites, and some calcined clays and shales.


some calcined clays and shales.- Class F Fly Ash- low calcium fly ash (5-10% CaO).

Has no cementitious properties but can react with calcium hydroxide (pozzolan properties)

- Class C Fly ash high-calcium fly ash (15 to 30% CaO). Has cementitious properties in addition to pozzolanic properties


Pulverised Fuel Ash (PFA) powder

Silica Fume (ASTM C 1240)

Is a by-product of the electric arc furnaces in the silicon metal and ferrosilicon alloy industries

Consist of amorphous/ non-crystalline silica (85 to 90% silicon oxide)

Very fine particles less than 0.1m (100 times finer than cement)


finer than cement) Highly pozzolanic and very reactive Is available in wet or dry forms. Amount replacement: 10 to 20% Produce high strength concrete (15,000 psi), reduce

concrete corrosion induced by deicing or marine salts, highly resistant to penetration by chloride ions.


Silica Fume

Rice Husk Ash

Is an agricultural fly ash which is obtained from burning and grinding of rice husk

Contains reactive silica Amount replacement: 10 to 30%


Amount replacement: 10 to 30%


Bottom RHA from fix grate boiler

RHA from circulating fluidised bed boiler

Palm Oil Fuel Ash

Obtained from the burning of palm oil clinker Amount replacement: 10 to 30%


UTILIZATION OF PALM OIL

FUEL ASH (POFA) IN CONCRETE


Natural Pozzolans

A pozzolan is a siliceous and aluminous materials, possesses little or no cementitious value.

In finely divided form and in the presence of moisture, react chemically with calcium hydroxide at ordinary temperature to form compounds possessing cementitious properties.

Natural occurring pozzolan, e.g. volcanic ash,


Natural occurring pozzolan, e.g. volcanic ash, combined with burned lime, were used about 2000 years ago.

Up to 15% of the weight of Portland cement is hydrated lime.

Adding a pozzolan to Portland cement generates an opportunity to convert this free lime to a cementitious material.

Fiber for Concrete

To control plastic shrinkage cracking and drying shrinkage cracking.

They also lower the permeability of concrete and thus reduce bleeding of water.

Some types of fibers produce greater impact, abrasion and shatter resistance in concrete.


abrasion and shatter resistance in concrete. Generally fibers do not increase the flexural strength

of concrete, so it can not replace moment resisting or structural steel reinforcement. Some fibers reduce the strength of concrete.

E.g. of fibers: steel, plastic, glass and natural materials

Fiber for Concrete Cont.

The shape of fibers described by the aspect ratio, length/diameter. E.g. steel fiber, diameters from 0.25mm to 0.9mm with aspect ratio 30 to 150mm.Addition of fibers reduced workability of


Addition of fibers reduced workability of concrete, so, limited to maximum 2% by volume of the mix.

Fiber reinforced concrete successfully used for floor slabs, pavements, slope stabilization, and tunnel linings.

Benefits of Polyproplene [CH3]n Fiber

Improve mix cohesion, improving pumpability over long distances


long distances Improve freeze-thaw resistance Improve resistance to explosive spalling in case of a

severe fire Improve impact resistance Increase resistance to plastic

Factors Affecting the Suitability and

Performance of Admixture in Concrete

Type of cement Mix proportions and method of mixing Temperature and time when the admixture is

added into the mix


added into the mix Chemical composition of the admixture

Monitoring and Precaution

Selection of material Trial mix Short-term and long-term effect

Eliminate over-dosing on site


Eliminate over-dosing on site Manufacturers advice

Special Admixtures- Water Proofing

Admixture (WPA)

Concrete absorbs water because surface tension in capillary pores in the hydrated cement paste pulls in water by capillary suction.WPA aim at preventing this penetration of


WPA aim at preventing this penetration of water into concrete (make concrete hydrophobic)

WPA react with calcium hydroxide, e.g. stearic acid, vegetable or animal fats to make concrete hydrophobic.

Special Admixtures- Water Proofing

Admixture (WPA)

Other WPA, e.g. very fine material containing calcium stearate or hydrocarbon resins or coal tar pitches which produce hydrophobic surfaces.

Side effect: improve workability due to finely divided wax or bituminous emulsions, which entrain some


wax or bituminous emulsions, which entrain some air. Also improve cohesion of concrete but may result in sticky mix.

Vital to ascertain WPA contains no chlorides. Water proof membranes are emulsion-based

bitumen coatings, possibly with rubber latex, which produce a tough film with some degree of elasticity.

Special Admixtures- Anti Bacterial Agent

(ACI 212.3R-91) Bacteria/ fungi, or insects mechanisms are: releasing corrosive

chemicals through metabolic action, and creation of an environment which promotes corrosion of steel. Staining of the surface can also result.

Mineral or organic acid (from bacteria) + cement paste (alkaline) = neutralize, extra acids destroy concrete.


= neutralize, extra acids destroy concrete. Agent toxic apply on concrete surface to the attacking

organisms: anti-bacterial, fungicidal, or intersecticidal. Copper sulfate and pentachlorophenol- control the growth of

algae or lichen on hardened concrete BUT their effectiveness is lost with time.

Admixtures which may prove toxic should not be used.

References

Malhotra, V. M. 1989. Superplasticizers: A global review with emphasis on durability and innovative concrete. In ACI SP-119: Superplasticizers and other chemical admixtures in concrete, ed. V. M. Malhotra, 1-19. Detroit: American Concrete Institute.

Malhotra, V. M., ed. 1989. ACI SP-119:


Malhotra, V. M., ed. 1989. ACI SP-119: Superplasticizers and other chemical admixtures in concrete. Detroit: American Concrete Institute.

Mielenz, R. 1984. History of chemical admixtures for concrete. Concrete International: Design and Construction 6 (4):40-54 (April).

Mindess, S., and J. F. Young. 1981. Concrete. Englewood Cliffs, N.J.: Prentice-Hall, Inc.

Documents

Admixtures in Concrete