Concrete admixtures are used to enhance the properties of concrete for applications in concrete works with special requirements.

Concrete admixtures are used to modify the properties of concrete to achieve desired workability in case of low water cement ratio, and to enhance setting time of concrete for long distance transportation of concrete.

So, it is of much importance for a civil site engineer to know about the properties of admixtures for better selection and application in concrete works.

 

Definition of Concrete Admixtures

As per BIS (IS – 9103: 1999) Page No.1, Concrete Admixture is defined as a material other than water, aggregates and hydraulic cement and additives like Pozzolana or slag and fiber reinforcement, used as on ingredient of concrete or mortar and added to the batch immediately before or during its mixing to modify one or more of the properties of concrete in the plastic or hardened state.

Reasons for Using Admixtures (ACI Committee Report Page – 298)

Admixtures are used to modify the properties of concrete or mortar to make them more suitable for the work at hand or for economy or for such other purposes as saving energy.

Some of the important purposes for which admixtures are used are:

To modify properties of fresh concrete, mortar and grout so as to:

• Increase workability without increasing water content or decrease water content at the same workability.
• Retard or accelerate time of initial setting.
• Reduce or prevent settlement.
• Modify the rate or capacity for bleedings.
• Reduce segregation.
• Improve pumpability.
• Reduce the rate of slump loss.

To modify the properties of hardened concrete, mortar and grout so as to:

• Retard or reduce heat evaluation during early hardening.
• Accelerate the rate of strength development at early ages.
• Increase strength (compressive, tensile or flexural).
• Increase durability or resistance to severe condition of exposure.
• Decrease permeability of concrete.
• Control expansion caused by the reaction of alkalies with certain aggregate constituents.
• Increase bond of concrete to steel reinforcement.
• Increase bond between existing and new concrete.
• Improve impact resistance and abrasion resistance.
• Inhibit corrosion of embedded metal.
• Produce colored concrete or mortar

When to Use Concrete Admixtures

• When properties cannot be made by varying the composition of basic material.
• To produce desired effects more economically.
• Unlikely to make a poor concrete better.
• Not a substitute for good concrete practice.
• Required dose must be carefully determined and administered

How to Use Concrete Admixtures

• Check job specification
• Use the correct admixture
• Never use one from an unmarked container.
• Keep containers closed to avoid accidental contamination.
• Add the correct dosage.
• Avoid adding ‘a little bit extra
• Use a dispenser
• Wash thoroughly at the end the day
• Best if added to the mixing water
• Manufacturer’s recommended dosage is usually adequate
• Trial mixes are important to determine most effective dosage

Types of Concrete Admixtures

Admixtures are classified according to the Indian Standard (IS 9103: 1999) are as follows:

a) Accelerating admixtures

b) Retarding admixtures

c) Water-reducing admixtures

d) Air-entraining admixtures

e) Super plasticizing admixtures

Classification according to American Concrete Institute Committee report.

a) Air-entraining admixtures

b) Accelerating admixtures

c) Water reducing and set controlling admixtures

d) Admixtures for flowing concrete

e) Miscellaneous admixtures

Classification of admixtures according to the book of “Concrete Admixtures: Use and Applications” edited by M. R. Rixom are given in the forward pages.

Air Entraining Agent Concrete Admixtures

These are generally used to improve workability, ease of placing, increased durability, better resistance to frost action and reduction in bleeding.

The common Air-Entraining agents are natural wood resins, neutralized vinsol resins, polyethelene oxide polymers and sulphonated compounds.

Mechanism of Air Entraining Concrete Admixtures

These are anionic, because the hydrocarbon structures contain negatively charged hydrophilic groups, such as COO, SO3 and OSO so that large anions are released in water. Conversely, if the hydrocarbon ion is positively charged, the compound is cation active or cationic.

In other words, anionic surface active agents produce bubbles that are negatively charged, cationic charged cause bubbles to be positively charged, surface active agents of all classes can cause air entrainment in concrete, but their efficiency and characteristics of air-void system vary widely.

Air entraining Admixtures have following properties:

• These are foaming agents, gas producing chemicals. It introduces millions of tiny, stable bubbles of uniform size that are uniformly distributed throughout the mix (usually about 5% of the volume).
• Improves properties of fresh concrete such as workability, cohesion and reduces segregation and bleeding.
• Improves properties of hardened concrete – For every 1% of air there is a 4% loss in strength which is minimized by the reduction in water content. It improves durability of hardened concrete.

Accelerating Admixtures for Concrete

Accelerating admixtures are used for quicker setting times of concrete. It provides higher early strength development in freshly cast concrete.

Main uses of Accelerating Concrete Admixtures

• These admixtures are suitable for concreting in winter conditions
• During any emergency repair work
• In case of early removal of formwork

Disadvantages of Accelerating Concrete Admixtures

• It has increased drying shrinkage
• It offers reduced resistance to sulphate attack
• CaCl2 high risk of corrosion of steel – not permitted in reinforced concrete
• It is more expensive and less effective

Water Reducing Admixtures

Chemical Types for Water Reducing Admixtures

a) Calcium or sodium salt of lignosulfonic acid

b) Poly carboxylic acid

Mode of Action

The principal role on mechanism of water reductions and set retardation of admixtures are usually composed of long-chain organic molecules and that are hydrophobic (not wetting) at one end and hydrophilic (readily wet) at the other.

Such molecules tend to become concentrated and form a film at the interface between two immiscible phases such as cement and water, and alter the physio-chemical forces acting at this interface.

The mechanism by which water reducing admixture operate is to deflocculated or to disperse the cement agglomerates into primary particles or atleast into much smaller fragments.

This deflocculating is believing to be a physio chemical effect whereby the admixture is first of all adsorbed on to the surface of the hydrating cement, forming a hydration “sheath”, reduces the antiparticle separated from one another.

The presence of water reducing admixture in a fresh concrete results in:

(i) a reduction of the interfacial tension.

(ii) an increase in the electro kinetic potentials and

(iii) protection sheath of water dipoles around each particle i.e. mobility of fresh mix becomes greater, partly because of reduction in inter-particle forces and partly because of water freed from the restraining influence of the highly flocculated system which is now available to lubricate the mixture. Hence less water is required to achieve given consistency.

Why Water Reducing Admixtures are used?

a) Concrete having greater workability be made without the need for more water and so strength losses are not encountered

b) By maintaining some workability, but at a lower water content, concrete strengths may be increased without the need for further cement addition

c) While maintaining the same w/c ratio and workability concrete can be made to a given strength as in the reference concrete at lower cement content.

Effect on durability

The straight addition of admixtures of this type does not came any increase in permeability and indeed where the admixture is used to reduce the w/c, then permeability is considerably reduced.

Effect on shrinkage

Admixture of this type when used as workability aids on water reducers do not adversely effect the shrinkage.

Effect on creep

Materials of this type of admixture have no deleterious effect on the creep of concrete.

Detrimental effect

a) While using water reducing agent. Care must be taken in controlling the air content in the mix. Most water-reducing agent entrain air due to their surfactant properties.

b) At high dosages of lignosulphonate material, retardation of the mix occurs.

Applications of Water Reducing Concrete Admixtures

The application of the type of admixtures are as follows —

a) When concrete pours are restricted due to either conge