ADMIXTURES OF CONCRETE PRESENTED BY K.LAVANYA@LAKKA BAI M.TECH(ACT)

INTRODUCTION: Admixtures is defined as a material, other than, Cement Water Aggregate That is used as an ingredient of concrete and is added to the batch immediately before or during mixing. Admixtures are used to modify the properties of ordinary concrete so as to make it more suitable for any situation.

HISTORY: History of admixtures is as old as the history of concrete. It embraces a very vast field, but a few type of admixtures called Water Reducers or High Range Water Reducers, generally referred as plasticizers and superplasticizers, are of recent interest. Specially developed in Japan & Germany around 1970.

Later on they were made popular in USA and Europe even in middle East and Far East.Unfortunately, the use of Plasticizers and Superplasticizers have not become popular in India till recently (1985).

Why Are They Used? To modify properties of fresh & hardened concrete To ensure the quality of concrete during mixing, transporting, placing & curing To overcome certain unexpected emergencies during concrete operations (ie, set retarders)

Why Are They Used? Increase slump and workability Retard or accelerate initial setting Reduce or prevent shrinkage Modify the rate or capacity for bleeding Reduce segregation Retard or reduce heat evolution during early hardening Increase strength (compressive, tensile, or flexural)

Decrease permeability of concrete Increase bond of concrete to steel reinforcement Increase bond between existing and new concrete Improve impact and abrasion resistance (hardness) Inhibit corrosion of embedded metal; Anti-washout & Produce colored concrete.

How Applied? Most admixtures are supplied in a ready-to-use form and added at plant or jobsite. Pigments and pumping aids are batched by hand in VERY small amounts

CLASSIFICATION OF ADMIXTURES Chemical admixtures Mineral admixtures

tyPES of chemical admixtures: According to characteristics effects produced by them chemical admixtures used are as follows. Plasticizers Super Plasticizers Retarders and retarding admixtures Accelerators and Accelerating Plasticizers Air-entraining admixtures

Grouting admixtures Air – detraining admixtures Gas – forming admixtures Corrosion inhibiting admixtures Shrinkage reducing admixtures Water or damp proofing and permeability reducing admixtures

Minerial admixtures: Cementitious Pozzolanic Ground Granulated Blast Furance Slag (GGBS) Fly ash Silica Fume Rice Husk

Plasticizers (Water-reducing ): These admixtures are used for following purposes: To achieve a higher strength by decreasing the water cement ratio at the same workability as an admixture free mix. To achieve the same workability by decreasing the cement content so as to reduce the heat of hydration in mass concrete.

To increase the workability so as to ease placing in accessible locations Water reduction more than 5% but less than 12% The commonly used admixtures are Ligno-sulphonates and hydrocarbolic acid salts. Plasticizers are usually based on lignosulphonate, which is a natural polymer, derived from wood processing in the paper industry.

ACTION OF PLASTICIZERS: Dispersion: Portland cement being in fine state will have a tendency to flocculate in wet concrete, these flocculation entraps certain amount of water used in the mix. When its used, they get adsorbed on the cement particles,creates particle to particle repulsive forces which overcome the attractive forces.

This repulsive force is called Zeta Potential, which depends on the base, solid content, quantity of plasticizer used. When cement particles are deflocculated, the water trapped inside the flocs gets released & now available to fluidify the mix. Lubricating: The agents are organic by nature, thus they lubricate the mix reducing the friction and increasing the workability.

Retarding Effect: Plasticizers get adsorbed on the surface of the cement particles and form a thin sheath. This sheath inhibits the surface hydration reaction between water and cement as long as sufficient plasticizers molecules are available. Quantity of plasticizers will decrease as the polymers become entrapped in hydration products.

The following mechanisms may take place simultaneously: Reduction in the surface tension of water. Induced electrostatic repulsion between particles of cement. Lubricating film between cement particles. Dispersion of cement grains, releasing water trapped within cement flocs. Inhibition of the surface hydration reaction of the cement particles, leaving more water to fluidify the mix.

Superplasticizers(Hrwr): Its developed in Japan & Germany during 1960 & 1970. They are chemically different from normal plasticizers. Reduce water requirement by 12-30% The mechanism is more or less same as the plasticizers

No plasticizers  With 0.3 per cent superplasticizer

They flowing concrete with very high slump in the range of 7-9 inches (175-225 mm) to be used in heavily reinforced structures

ADVANTAGES OF superplasticizers: Significant water reduction Reduced cement contents Increased workability Reduced effort required for placement More effective use of cement More rapid rate of early strength development; Increased long-term strength; & Reduced permeability.

High Passing ability:

DISADVANTAGES OF superplasticizers: Additional admixture cost (the concrete in-place cost may be reduced) Slump loss greater than conventional concrete Modification of air-entraining admixture dosage Less responsive with some cement Mild discoloration of light-colored concrete

Classification of superplasticizers Sulphonated malaine-formaldehyde condensates (SMF) Sulphonated naphthalene-formaldehyde condensates (SNF) Modified lignosulphonates (MLS) Polycarboxylate derivatives

Copolymer of carboxylic acrylic acid with acrylic ester (CAE) Cross linked acrylic polymer (CLAP) Polycarboxylate ester (PC) Multicarboxylate ethers (MCE) Combination of above

Retarders: Retarding admixtures delay hydration of cement. To delay or extend the setting time of cement paste in concrete. These are helpful for concrete that has to be transported to long distance, and helpful in placing the concrete at high temperatures. Used in casting and consolidating large number of pours without the formation of cold joints

Used in grouting oil wells. Commonly known retarder is Calcium sulphate. Appropriate amount of gypsum to be used must be determined carefully. Use of gypsum is recommended when adequate inspection and control is available. Otherwise addition of excess amount may cause undesirable expansion and indefinite delay in setting of concrete.

Retarding plasticizers: Many a time this extent of retardation of setting time offered by natures will not be sufficient. Instead of adding retarders separately, retarders are mixed with plasticizers or super plasticizers at the time of commercial production. Such commercial brand is known as retarding plasticizers or retarding superplastizers.

Retarding plasticizers or superplastizers are important category of admixtures. Used in Ready mixed concrete Industry for the purposes of retaining the slump loss, during high temperature, long transportation, to avoid construction or cold joints, slip form construction regulation of heat of hydration.

Accelerators: Accelerating admixtures are added to concrete to shorten the setting time & increase the rate of hardening & strength development of concrete. Its increase in the rate of hydration of the hydraulic cement. These are used in cold weather conditions (below 5˚C or 41˚F)

Some widely used and effective chemicals that accelerate the rate of hardening of concrete mixtures, including calcium chloride, other chlorides, triethanolamine, silicates, fluorides, alkali hydroxide, nitrites, nitrates,& formates.

benefits of accelerators: Reduced bleeding, Earlier finishing, Improved protection against early exposure to freezing and thawing, Earlier use of structure, Reduction of protection time to achieve a given quality, Early removal of form, and In some case, early load application.

Accelerating plasticizers: Certain Ingredients are added to accelerate the strength development of concrete to plasticizers or super plasticizers. Such accelerating super plasticizers, when added to concrete result in faster development of strength The accelerating materials added to plasticizer or superplasticizers are triethenolamine chlorides, calcium nutrite, nitrates and flousillcates etc., The accelerating plasticizers or accelerating super plasticizers manufactured by well known companies are chloride free

Air-entraining admixture: Microscopic air bubbles intentionally incorporated in mortar or concrete during mixing, usually by use of a surface-active agent; typically between 10 &1000 μm in diameter and spherical or nearly so. It is the process whereby many small air bubbles are incorporated into concrete and become part of the matrix that binds the aggregate together in the hardened concrete. These air bubbles are dispersed throughout the hardened cement paste

The air voids present in concrete can be brought under two groups: Entrained air Entrapped air

Air entraining agents: Natural wood resins Animal and vegetable fats and oils such as tallow, olive oil and their fatty acids such as stearic and oleic acids Various wetting agents such as alkali salts or sulphonated organic compounds Water soluble soaps of resins acid Miscellaneous materials such as sodium salts of petroleum sulphonic acids, hydrogen peroxide and aluminium powder, etc

Factors affecting air antrainment: Type and quantity of air entraining agents used Water cement ratio of mix Type and grading of aggregates Mixing time Temperature Type of cement Influence of compaction Admixtures other than air entraining agents used

Effect of air entrainment on the properties of concrete: Air entrainment will effect directly the following three properties of concrete Increased resistance to freezing and thawing Improvement in workability Reduction in strength

Incidentally Air entrainment will affect the properties in following ways: Reduces the tendencies of segregation Reduces the bleeding and laitance Decreases the permeability Increases the resistance to chemical attack Permits reduction in sand content, water content, cost, & heat of hydration Reduces unit weight, alkali aggregate reaction, the modulus of elasticity

Conclusion: Plasticizers, superplasticizers, & Air-entraining admixtures is mostly used. Superplasticizers is widely used all over the wold. India is catching up with the use of superplasticizer in the construction of high rise buildings, long span brides Because It reduced cement content, increased workability, & increased strength.

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