Chemical Admixtures for ConcreteINTRODUCTION

Özge Andiç Çakır, PhD

Why Concrete is the most widely used Construction Material?

• Concrete making materials are easily available almost all over the world;

• It can be easily shaped when it is freshly mixed,

• It becomes a stone-like material of desired characteristics when it hardens;

• It has minimal maintenance requirements during service;

• It has various uses in building structures in different climates.

Concrete: Definition

• Concrete is a composite material that consists essentially of a binding medium within which particles or fragments of coarse and fineaggregate are embedded.

• For Portland cement concrete, the binder is a mixture of cement and water.

Evolution of Concrete

Mixing cement &

water

Hydration starts

Cement paste stiffens

Setting

Concrete gains strength

Hardening

Fresh (Plastic) Concrete

Hardened Concrete

Fresh State: Workability?

Workability is the ease of

• mixing,

• transporting,

• placing and,

• compacting the fresh concrete without any segregation.

Thus, a workable concrete should be:

• Easily mixed and transported,

• Uniform throughout a given batch and between batches,

• Flowable, such that it is capable of filling completely the forms,

• Compactible with no segregation of materials,

• Finishable.

What else do we expect from convenient fresh concrete?

• Desired workability,

• Desired setting time,

• Desired rate of hydration heat

• Uniformity with minimum bleeding

What about Hardened Concrete?

• Required strength at specified ages,

• Volume stability, (no excessive shrinkage or expansion),

• Sufficient durability, (sufficient resistance to environmental attacks such as freezing and thawing, sulphate attack, acid attack, chemical reactions of aggregates with alkalies, wear, and corrosion of steel embedded in concrete.)

How can we obtain desired hardened and fresh concrete properties?

Desired quality can economically be achieved by;

• intelligent selection of basic concrete making materials (aggregate, cement, water),

• proper proportioning of the ingredients,

• proper application of mixing, placing, compacting and curing operations.

Is this always enough? Engineers always ask for more!

What about a special concrete for a special application?

• Extended or reduced setting time?

– Hot weather concreting, cold weather concreting, faster applications..

• Lower rate of hydration heat ?

- Less cracking in massive structures..

• Acceleration of strength gain?

- Faster demoulding..

• Increased resistance to some durability problems?

- Freeze-thaw resistance, steel corrosion inhibition, etc.

Lets think about a solution?

• Adding one or more extra materials (admixtures!) to the basic concrete making materials during the process of concrete mixing might help.

However, do not forget!

An admixture is not a panacea for poor mix design or sloppy concrete practice, and the user should first ensure that the desired result cannot be attained by improvements in these areas.

Admixtures: Definition

An admixture for concrete is defined as “a material other than water, aggregates and hydraulic cement that is an ingredient of concrete and is added immediately before or during mixing”.

Classification

• There are a number of admixtures for concrete production.

• There are also many classifications.

• There is a basic classification in the literature regarding their nature and utilization procedure:

- Mineral admixtures,

- Chemical admixtures

Mineral Admixtures

These are finely divided solids added to concrete to improve its workability, strength, durability and economy or to control the rate of hydration of concrete.

- Natural pozzolans,

- Artificial pozzolans (fly ash, silica fume, ground granulated blast furnace slag).

Chemical Admixtures

Literally all concrete admixtures are chemicals,

However, the term chemical admixture is restricted to water-soluble compounds other than air-entraining agents.

They are added primarily to control setting and early hardening of fresh concrete or to reduce its water requirements.

Air-Entraining Admixtures are not chemicals, but...

• “Air-entraining admixtures” are organic materials, usually in solution form.

• They introduce a quantity of air into fresh concrete to improve their frost resistance.

• In fact, they are used in the same way the other chemical admixtures are used, added into the mix water.

Chemical Admixtures providing more solutions: Miscellaneous admixtures

More solutions instead of controlling setting time or fluidity of concrete,

- gas forming, - expansion producing, - bonding, - pumping aid, - colouring, - damp-proofing, - permeability reducing,- corrosion inhibiting etc.

In Conclusion; chemical admixtures are used because individually they may;

• Increase workability without increasing water content,

• Decrease water content at the same workability,

• Decrease both water and cement (keeping constant w/c ratio) to provide economy,

• Retard or prevent settlement or create slight expansion,

• Modify the rate and/or capacity of bleeding,

• Reduce segregation,

• Improve pumpability,

In Conclusion; chemical admixtures are used because individually they may;

• Reduce rate of slump loss,• Retard or reduce heat evaluation during early

hardening,• Accelerate the rate of strength development at

early ages,• Increase durability,• Decrease permeability to liquids,• Increase bond between concrete and reinforcing

steel,• Improve impact and abrasion resistance,• Inhibit corrosion of embedded steel,• Provide coloured concrete or mortar

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