CHEMICAL ADMIXTURES IN CONCRETE

 

CHEMICAL ADMIXTURES IN CONCRETE

 A material other than water, aggregates, or cement that is used as an ingredient of concrete or mortar to control setting and early hardening, workability, or to provide additional cementing properties.

Mostly of people are having some contradiction on considering additives as admixtures although both alter some properties, but they are put on different substances since additives are put during cement production while admixtures are put during concrete mixing.

Reasons for using admixtures

• To suitable concrete of certain properties for complex structures.
• For economic purposes
• For maintaining quality of concrete during all stages of concrete work

Admixtures can be divided into two categories as follows

• Mineral admixtures
• Chemical admixtures

In this post will talk more about chemical admixtures as used in modifying properties of fresh and hardened concrete.

Chemical admixtures these are water soluble compounds that are added to concrete during mixing for altering concrete properties such as setting time, workability, mostly they are added in concrete in small desirable quantity to a make full chemical process per requirement.

 

The following are the examples of chemical admixtures, their processes, effects on mixed concrete.

Air entraining admixtures

These admixtures when added to the concrete mix, they tend to entrap air (microscopic bubbles) into a concrete, entrapped air allow expansion and contraction of concrete, so it increases durability of concrete exposed to freezing and thawing.

The entrapped air as surfactants tends alter the surface tensions properties between particles in mix this affects the workability and cohesion between particles that reduce tendency of bleeding and segregation.

Also the entrapped air bubbles acts as bearings to reduce friction between particles that allow compaction in low workable concrete (semi solid concrete).

Water reducers (Plasticizers)

These admixtures tends to reduce amount of water requirement in archiving certain slump of concrete, this means with the reduced amount of water the recommended workability is attained.

Typical water reducers reduce water requirement to about 5 percent to 12 percent.

For example,

If the design requirement is 180 liters, if we are going to use water reducer of 10 percent the water required will be.

                                    10% x 180 = 18 liters

                        Now water required will be

                                    180 – 18 = 172 liters

This reduction of water requirement is reduction in water cement ratio, with the reduction of water cement ratio (water requirement) and workability high strength concrete is attained.

The recommended amount of water reducer has to be applied, because more water reducers cause more workable concrete that produces weak concrete.

Retarder admixtures

These are admixtures that delay setting and hydration process of the concrete, they are useful in extending setting time of concrete, but they are also used in attempt decrease slump loss and extend workability especially prior to placement at elevated temperatures.

Accelerators

These are admixtures which increases the rate of hydration of hydraulic cement, due to this the setting time of cement is reduced, this causes early hardening and strength development of cement concrete, mortar or grout.

Accelerators can be either that reduce the time for the mix to change from the plastic to the hardened state (setting time) or that develop early strength of about 120%,130% in 24hrs,48hrs respectively.