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CHEMICAL ADMIXTURES FOR CONCRETE (contd.,)

Types of Chemical admixtures are: Superplasticizers

Air-entraining agents Accelerators

Retarders

AIR ENTRAINING ADMIXTURESAir entraining admixtures have a large use in Europe, North America, Japan and such

countries in the higher latitudes.

This is because of ensuring durability against repeated freezing and thawing Air entraining admixtures are made from:

(i) Natural wood resins

(ii) Synthetic detergents

(iii) Salts of petroleum acids

What is Air Entrainment? The production within the concrete mix of a large number of small bubbles of air.

Normally these bubbles are less than 1mm in diameter.

The bubbles must be stable and remain in the concrete as it is transported andplaced so that they are still present in the hardened mass of concrete.

To provide freeze thaw resistance the bubbles must be evenly spread throughout the

concrete

Chemistry of air entrainment

Air entrainers are almost all anionic surfactants Natural wood resins Vinsol, Wood resins

Animal and vegetable fats and oils

oleic acid, coconut oil derivatives Synthetic materials

alkyl/aryl sulphates and sulphonates

Non ionic and cationic materials can be used but are usually not as stable. They may be

useful in blends with particular objects.

Mechanism of Entrainment

Anionic surfactants have a hydrophobic and a hydrophilic end. This makes themcollect at the air water interface with the hydrophobic end in the air. This reduces

surface tension and allows the formation of a stable bubble

The hydrophilic end is polar and is attracted by charges on the surface of cementand aggregate particles. This attaches the bubble to the surface and helps to

produce a stable structure in the mix.

Air is not added to the mix. The air that is already in the mix during the mixingprocess is made stable.

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Combinations of materials are used to provide stability under different conditionsand to produce a desired range of bubble sizes.

Air stabilization in concrete

Why air entrain concrete?Frequently, most consideration is given to the effects of air entrainment in hardened

concrete, primarily freeze-thaw protection, but it must be remembered that there areconsiderable benefits to be gained in the fresh concrete.

Freeze - Thaw resistance

Increasing mix cohesion

Improved mix workability Improved surface finish

Increased mix volume - yield

Many of these benefits can be obtained with only small increases in air content and donot necessarily require the addition of a specific air entraining admixture.

Mix cohesion problems Cohesion problems occur when the internal structure in a concrete mix is

insufficient to hold the mix together. This internal structure is partly due to

physical support of the particles by each other and partly due to electrostatic

interactions between the particles. If there is a lack of materials of a particular size in the grading of the aggregate

then the physical support may not be complete. This can cause a separation of the

mix Constituents. If there is insufficient attraction between the particles then there can be a

separation

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How air helps cohesion

Typically 1 or 2 % additional air over that normally present in the mix will besufficient to give improvements in cohesion. Air bulks the volume of the paste,

allowing the other materials in the mix to be better covered.

Air tends to form at the right size to replace any gaps in the sand grading. This

means that the structure of the concrete mix is maintained properly, reducing thetendency to segregate.

Electrostatic action between air bubbles and aggregate acts to hold mixcomponents together, Air bubbles act as elastic buffers in the concrete mix,

allowing mix components to move past each other more easily. This reduces the

need to add more water to make the mix mobile and therefore reduces thetendency to bleed.

Effect of air on bleed

Effects of improved Cohesion

Segregation reduces, minimizing problems of settlement of aggregates. Water is held within the concrete, reducing bleed and cutting down surface finish

problems Lack of excessive bleed water means that the surface is not weakened through

having an increased water:cement ratio and does not have the same tendency to

show shrinkage cracks.

Reduction in strength

1% air will usually reduce compressive strength by 5 to 6% requiring anallowance to be made when designing the concrete mix.

However, the increased workability and placeability of an air entrained mix willreduce the water content needed, which will have some counter effect on the drop

in strength.

Low cohesion or harshness in a mix may increase water demand as more water isadded in an attempt to improve the compaction of the mix. Using air to improve

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cohesion reduces the need for extra water and therefore may not actually reduce

compressive strength as much as would otherwise be expected

Admixtures and setting and hydration control (Accelerators / Retarders)

Reasons for retardation To overcome the faster set of concrete at higher temperatures

To allow for longer delivery and placing times To allow later deliveries of concrete to be added without causing cold joints

To meet the needs of particular construction methods, Such as slipforming.

Reasons for acceleration To overcome the slower setting of concrete at low temperatures

To meet the needs of particular construction methods, such as floor finishing

To produce rapid early strength gain To allow rapid turnover of moulds

NOTE : If the strength gain is the only criterion then often the use of asuperplasticiser to obtain a high level of water reduction, combined with the use ofsteam curing if necessary, will produce better strength gain than the the use of an

accelerator.

Chemicals usedRetarders - mostly organic chemicals

Carbohydrates

Hydroxycarboxylic acids and salts Phosphates

Accelerators - mostly inorganic chemicals

Calcium chloride, Formates, Nitrates Thiocyanates

Silicates

AluminatesCalcium chloride is the most effective accelerator. However it will

cause corrosion of embedded steel and must not be used where

reinforcement is present

What is Affected?

Cement hydration

Setting time Working life

Strength gain

Although retardation and acceleration are primarily affecting the chemistry of thehydration reaction, it is usually the physical results of this that are required.

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The effect of retarders and accelerators on the hydration curve

The effect of retarders and accelerators on temperature rise

Four hours retardation The concrete is to be placed after four hours

Possible four hour journey to site

Four hours more workability than a normal mix is required

Flowing concrete is needed for four hours The concrete is not to set for four hours

Setting delayed by four hours from control mix is required

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Only one of these (the last) relates to a definition of retardation which will befound in admixture standards, but all of them might be the need of the customer.It is important that the exact needs are determined before and attempt at a solution

made.

Other effectsThe exact level of retardation or acceleration obtained in a particular mix will depend on

a number of factors, including: Level of admixture

Increased dosages will have increased effects but this is usually not a straight line

effect. Temperature

Cement chemistry

Starting workability

EFFECTS OF CHEMICAL ADMIXTURES ON FRESH CONCRETE

Water reducers/ plasticizers. Improvement in workability when used as plasticizer

Better appearance, and uniformity

Better finishing characteristics Less bleeding; greater pumpability

Some retardation of setting times.

High range water reducers/ super plasticizers Whatever is true of plasticizers more of that with superplasticizers, generally

Higher dosages; greater benefits in properties of fresh concrete

Air entraining agents Workability increases

Mixes are more cohesive, Finishability better. Freeze- Thaw resistance.

EFFECTS OF CHEMICAL ADMIXTURES ON HARDENED CONCRETE

Water reducer / Plasticizer

when used as plasticizer no significant effect on 28 day strength, modulus of

elasticity or permeability When used as water reducer- reduced creep and shrinkage. Other properties are

unaffected.

When W/C is reduced enhanced strength, higher modulus of elasticity and allround improvement in all properties including permeability.

High range water reducer/ superplasticizer whatever is true for water reducer it is true for high range water reducer as well

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On a statistical basis the concrete is always more uniform, better compacted and

variability of strength is loss Long term gain in strength and other properties often noted.

Air entraining agents

Lead to lower unit weight of concrete, inversely proportional to air entrained At the same W/C ratio, compressive strength, flexural and tensile strengths are

reduced Approximately, for every 1% air entrained (by volume), 5% loss of strength

Loss of strength overcome by a small reduction in W/C ratio made possible by

reducing water content to a small extent Durability is better since capillary absorption of water is blocked by air bubbles

Freeze-Thaw resistance, Resistance to aggressive solutions

Can chemical admixtures be trusted? An usual question. Especially when the marketing pressures are high. The answer

is yes, but.., The but arises from a need of the user to ensure compatibility of a commercialproduct with the cement he has at hand.

This is done not only by going through chemical manufacturers technical

information but also testing. Testing is done on cement, mortar and concrete with and without chemical

admixture. IS 9103 prescribes physical requirements.

The bigger the job the more elaborate can be the tests, including chemical tests on

the cement itself.

Whose responsibility?

This is that of the Site/Project Engineer, the same person who is responsible forconcrete quality.

Contractor should assist in carrying out the necessary tests

Who pays?

It is in the interest of contractor to use admixtures in order to produce better andmore uniform concrete

He can ensure that the cost is recouped either in labour saving, or in placement

costs or in cement saving.

He is the ultimate beneficiary as well as the owner

Reference:

1. Properties of Concrete, A.M. Neville, Fourth Edition, Pearson Education Asia pvt.,Ltd., 2000.

2. Concrete- Microstructure, Properties and Materials, P.K. Mehta and Palulo J.M.

Monteiro, Tata Mcgraw Hill., 2006.

3. Concrete Technolgy Theory and Practice, M.S. Shetty, S. Chand and Company

Ltd.,, 2005.4. Current Literature..,