CONCRETE TECHNOLOGY

Air entraining admixtures (AEA)

1.Increase the workability

2.Increase the resistance to frost attack

Factors affecting the Air-Entrainment

1. Increase in the fineness of cement decreases the air content.

2. Increases in the alkali content of cement increases the air content.

3. Well rounded particles are conductive to air entrainment.

4. Increase in the percentage of fine fraction decreases the air entrainment

5.Dust on the aggregate decrease the air content.

6. Gravel concrete entrain more air than crushed stone concrete.

7. Increase in the slump increases the air content.

8. An increase in the concrete temperature decreases the air content.

9.Excess vibration reduces air content.

10. Amount of air-entrained decreases with the increase in the fineness of pozzolans.

11.Increases in the carbon content of pozzolanas reduces the amount of entrained air.

Compounds that are used for air-entrainment are

Natural wood resins Sulphonated compounds Fats and oils.

Effect of Entrained air on the properties of concrete:

1.Increases the workability

2. Increases the resistance to weathering

3. Reduces the bleeding

4. Reduces the water sand content, there by increases the strength.

Water reducing Admixture (WRA)

Water reducing admixture reduces the use of water To obtain required Slump

Use of less cement to get the same compressive strength. To achieve a higher slump at a given water cement ratio.

WRA

Calcium sulphate is used as water reducing admixture.

Retarding Admixtures

Admixtures which are lengthen the setting time and workability time are known as retarding admixture.

Retarding admixtures are helpful in condition where temperature condition is very high.

Ammonium chloride, Ferrous and Ferric chlorides, Calcium borates and oxy-chlorides, Alkali bicarbonates.

Mineral Admixtures.

Commonly used mineral admixture in concrete is pozzolan.

Pozzolans can be classified as natural and artificial.

Natural Pozzolans

1. Clays2. Trass, Pumicities and Perlite3. Opaline cherts4. Santorin earth5. Shacks6. Diatomaccous earths7. Volcanic tuffs

Artificial Pozzolans

1. Blast furnace slag2. Fly ash3. Silica fume

Effects of Pozzolons on the Properties of Fresh concrete.

1. Effect on Workability.

2.Effect on Bleeding.

3. Effect on Strength

4. Effect on Sulphate Resistance

5. Effect on Temperature Rise.

6. Influence on Alkali-Aggregate Reaction

Effect on Workability

Addition of a mineral admixture to concrete mixes not deficient in fines, particularly rich mixes, generally decreases the workability for a given water content. Because of this, addition of mineral admixture to such mixes with out a reduction in cement generally entails an increase in the total water content, and may result in an increase in the drying shrinkage and absorptivity, and decrease in the strength.

Effect on Bleeding

The ratio of surface area of solids to volume of water may be increased by increasing the amount of cement or by the addition of a suitable mineral admixture.

Effect on strength

The effect of a mineral admixture on the strength of concrete varies with the type of admixture and characteristic of concrete mix. Generally the strength of lean mixes is increased and strength of rich mixes is decreased. Contribution to the development of strength of rich mixes is decreased.

Effect on Sulfate Resistance.

Pozzolanic and cementatious materials have been used in structures exposed to sea water or other sulfate bearing water. Pozzolonic materials are normally used in the proportion of 1 part of pozzolan to 5 parts of portland cement to 1 part of pozzolan to 2 parts of portland cement, either by weight or by absolute volume. Use of pozzolanic material with other than sulphate-resisting portland cements increases the resistance of the concrete to aggressive attack of sea water, and natural acid waters. The relative improvement is greater for concrete with low cement content. The use of pozzolan with sulfate-resisting portland cements does not increases the sulfate resistance. Presence of chemically active aluminium compounds in pozzolan causes reduction in the sulfate resistance of the concrete.

Effect on Temperature Rise

Pozzolanic and cementatious admixture have been used in large hydraulic structures where it is necessary to reduce the portland cement content in order to decrease the temperature-rise resulting from heat generated on the hydration of the cement. The proportions used are generally similar to those used for improving sulfate resistance of concrete.

Influence on Alkali-Aggregate Reaction

The use of pozzolan for specific purpose of preventing excessive expansion caused by alkali-aggregate reaction was recommended in 1947.

Three types of alkali-aggregate reactions are generally recognized. Alkali-Silica Reaction Alkali- Carbonate Reaction Alkali-Silicate Reaction. Alkali-Silica Reaction

It involves the intraction between the alkali in the concrete with aggregates containing active silicon dioxide. Unreactive silica such bas quartz, has an orderly arrangements of its silicon oxygen tetrahydral within its crystal structure, whereas the reactive forms consists of randomly arranged tetrahydral networks. Such aggregates as opal and chert react with alkali in the concrete to form a gel on the surface of the aggregates which imbibes moisture causing gel to swell. The swelling develops pressure on its surroundings and results in disruptive cracking.

Alkali-Carbonate Reaction

It is due to the presence of certain carbonate containing aggregate such as argillaceous dolomite limestones that contain metastable calcium carbonate. These carbonates reacts chemically with alkali hydroxides creating products whose volume is larger than that of the reactants as given by the equation below;

CaMg(Co3 ) +2 NaoH→ Mg(oH)2 +CaCo3 +Na2Co3

Alkali-Silicate Reaction

It is the result of aggregates interacting with the alkali and producing expansive stresses within the mass of the concrete.

The rate and extent of the alkali aggregate reaction depands on various factors:

1.Particle size of the alkali-active ingradients.

2. Temperature

3. Total quantity of rectants

4. Availability of the alkali and hydroxl ions to the reaction sites which involves the characteristics of the pores and capillaries in the concrete paste.

5.Availability of pore liquids as an ion transport medium

Superplasticizers

These are the modern type of water reducing admixtures, basically a chemical or a mixture of chemicals that imparts higher workability to concrete. Superplasticizers can be used in concrete for three different purposes or a combination of these,

To increase workability without changing the mix composition.

To reduce the mixing water and w/c ratio in order to increase the strength and improve the durability.

To reduce both water and cement in order to reduce creep shrinkage and thermal strains caused by heat of cement hydration.

Most commonly used superplasticizers are water-reducing admixtures containing of either naphthalene or melamine sulphonate condenced in the presence of formaldehyde

Presently available superplastiziers are broadly clasified in the following categories. Category I-Sulphonated melamine formaldehyde condensate (SMF) Category II-Sulphonated napthalene formaldehyde condensate(SNF) Category III- Modified ligno-sulphonates(MLF) Category IV- other organic compounds such as sulphuric acid ester and carbohydrate esterts.

Commercial ligno sulphates used in admixture formulations are predominantly calcium or sodium based chemicals with sugar contents of 1 to 30 percent. The density of superplastizers is in the range of 10 to 11kN/cubic meter with solid content ranging from 22 to 46 percent by weight of the commercial product.