Different Admixtures Used in Concrete and Their Purposes

Concrete Admixture is any Material added to Concrete other than Sand, Cement, Aggregate & Water to enhance the properties of concrete for applications in concrete works.

Concrete admixtures can improve concrete workability, quality, manageability, acceleration or retardation of setting time among other properties that could be altered to get specific results.

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.

THE MAJOR REASONS FOR USING ADMIXTURES

  • To reduce the cost of concrete construction
  • To achieve certain properties in concrete more effectively than by other means
  • To maintain the quality of concrete during the stages of mixing, transporting, placing, and curing in adverse weather conditions
  • To overcome certain emergencies during concreting operations

Concrete does not always behave as we would like, some of the undesirable behaviour can be seen as disintegration, spalling, cracking, leakage, deflection and settlement. Durable concrete structures can be achieved only through careful attention to many details.

 

TYPES OF ADMIXTURE

The following are Different types of admixture used in Concrete and the Purpose they Serve.

[1] WATER REDUCING ADMIXTURE

Water reducing admixture added to the concrete reduce the water demands of the mix, maintain the workability, increase the strength, reduce the cost, reduce bleeding, reduce segregation, reduce honeycombing, reduce cracking and permeability, increase bond strength of concrete to steel, and reduce drying shrinkage. An average of 12-15 % reduction in the total water content will attain this goal.

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With this type of admixture, concrete properties are improved and help place concrete under difficult conditions. Water reducers have been used primarily in bridge decks, low-slump concrete overlays, and patching concrete. Recent advancements in admixture technology have led to the development of mid-range water reducers.

 

[2] ACCELERATING CONCRETE ADMIXTURE

Accelerating concrete admixture is used to shorten the setting time and increase the early strength of the concrete. These agents are mainly used for concrete needing to be used immediately.

Calcium chloride could be named as the most common accelerator component; however, it could promote corrosion activity of steel reinforcements. Nonetheless, concrete best practices such as proper consolidation, adequate cover and proper concrete mix design could prevent these corrosion issues.

 

[3] AIR ENTRAINING CONCRETE ADMIXTURE

This type of admixture produces a more workable concrete while reducing bleeding and segregation of fresh concrete. Improved resistance of concrete to severe frost action or freeze/thaw cycles. Other benefits from this admixture are:

  • High resistance to cycles of wetting and drying
  • Reduce the amount of fine aggregate needs in the mixture
  • High degree of workability
  • Improve pumpability.
  • High degree of durability

 

[4] SET RETARDING CONCRETE ADMIXTURE

These agents are primarily used for delaying the setting time of concrete by reducing the high temperature of concrete. Set retards are useful, but are not the solution to poorly designed mixes, inferior materials, or low cement factors.

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The set retarder must be uniformly dispersed throughout the whole mix. Failure to do so will result in the admixture remaining in voids, causing some areas to remain plastic. This leads to plastic shrinkage.

Set retarding admixtures are used in concrete pavement construction, allowing more time for finishing ​concrete pavement, reducing additional costs to place a new concrete batch plant on the job site and helps eliminate cold joints in concrete. Retarders can also be used to resist cracking due to form deflection that can occur when horizontal slabs are placed in sections. Most retarders also function as water reducers and may entrain some air in concrete.

 

[5] SHRINKAGE REDUCING CONCRETE ADMIXTURE

This type of admixture could reduce early and long-term drying shrinkage. Shrinkage reducing admixtures can be used in situations where shrinkage Cracking could lead to durability problems or where large numbers of shrinkage joints are undesirable for economic or technical reasons. Shrinkage reducing admixtures can, in some cases, reduce strength development both at early and later ages.

 

[6] SUPERPLASTICIZERS CONCRETE ADMIXTURE

The main purpose of using superplasticizers is to produce flowing concrete with a high slump in the range of seven to nine inches to be used in heavily reinforced structures and in placements where adequate consolidation by vibration cannot be readily achieved.

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It has been found that for most types of cement, superplasticizer improves the workability of concrete. One problem associated with using a high range water reducer in concrete is slump loss. High workability concrete containing superplasticizer can be made with a high freeze-thaw resistance, but air content must be increased relative to concrete without superplasticizer.

 

[7] CORROSION INHIBITING CONCRETE ADMIXTURE

Corrosion inhibiting are used to slow corrosion of reinforcing steel in concrete. Corrosion inhibitors can significantly reduce maintenance costs of reinforced concrete structures throughout a typical service life of 30 – 40 years.

Corrosion-inhibiting admixtures have little effect on strength at later ages but may accelerate early strength development. Corrosion inhibitors do accelerate the setting times of concretes over a range of curing temperatures unless they are formulated with a set retarder to offset the accelerating effect.

 

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