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Chemical Admixtures

Admixtures defined as material other than water, aggregates, and hydraulic cements used as an ingredient of concrete or mortar and added to the batch immediately before or during mixing.

Most of the time Mineral admixtures are used to improve workability and durability and to harden concrete.

Chemical Admixture is defined as a chemical other than cement, water; aggregate that is used as ingredient of concrete to improve properties Such as workability, curing temperature range, setting time or color.

Some chemical admixtures have been in use for a very long time such as calcium chloride to provide a cold-weather setting concrete. Others are more recent and represent an area of expanding possibilities for increased performance. Although many new compounds of are being developed for use in concrete, chemical admixtures and mineral admixtures are derived from waste products from other industries, and as such may be complex mixtures of compounds. Thus the chemistry of concrete chemical admixtures is complex topics required in –depth knowledge and experience.

The effectiveness of an chemical admixture depends upon factors such as type, brand, and amount of cementing materials; water content; aggregate shape, gradation, and proportions; mixing time; slump; and temperature of the concrete.

Generally the cost effectiveness of a chemical admixture should address the additional cost required to handle an additional ingredient, as well as the economic effect the chemical admixture may have on the cost of transporting, placing and finishing the concrete.

1.      Types of Admixture

Generally Admixtures divided in two major categories; Chemical admixture and Mineral admixture of concrete.

Mineral admixtures are minerals added Portland cement mix approximately > 15% by weight of cement like raw or calcined pozzolans; Fly ash produced from burning bituminous coal; Fly ash normally produced from burning lignite (subbituminous) coal both pozzolanic and cementatious.

Chemical Admixtures usually added to Portland cement mix about1-4% by weight of cement.

There are different means to classify admixture type that are available in the construction industry among these the two common ways of classification is based on the state of admixture and based on the function they give.

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1.1.            Classification Based on the State of Chemical Admixture

There are different means to classify admixture type that are available in the construction industry among these the two common ways of classification is based on the state of admixture and based on the function they give.

Two classes of admixtures are available based on the form of admixture these are

  1. 1)Admixtures in liquid form

Usually Chemical admixtures which are manufactured in industries aligned with research and frequent experiments made for specific use to assist or to create particular behavior of concrete.

  1. 2)Admixtures in powder form

Most of the time mineral admixtures found in surface of the earth or manufactured in industries to make concrete more helpful.

Some of such admixtures pre-mixed with Portland cement like; Pozolana and colored cements which are added in cement manufacturing plants in specified proportion.

 1.2.            Classification Based on the functions

Admixtures can be classified by function as follows:

1. Air-entraining admixtures

2. Water-reducing admixtures

3. Plasticizers

4. Accelerating admixtures

5. Retarding admixtures

6. Hydration-control admixtures

7. Corrosion inhibitors

8. Shrinkage reducers

9. Alkali-silica reactivity inhibitors

10. Coloring admixtures

11. Miscellaneous admixtures such as workability, bonding, damp proofing, permeability reducing, grouting, and gas-forming, ant washout, foaming, and pumping admixtures

1.3.            Classification Based on ASTM

ASTM C 494, “Standard Specification for Chemical Admixtures for Concrete,” classifies admixtures into seven types. ASTM classifies Chemical admixtures based on their purpose; stated as follows:

Type A: water-reducing

Water-reducing, set-controlling admixtures are added to concrete during mixing to increase workability, improve durability, provide easier placement, control the setting time, and produce easier finishing with less segregation of the ingredients.

This is accomplished while allowing a reduction of the total water content and providing the ability to control the time of setting to meet changing jobsite and climatic conditions.

The strength improvement resulting from water-reducing admixtures is primarily a result of reducing the watercementitious materials ratio and increasing cement efficiency. For a given air content, concrete strength is inversely proportional to the water-cementitious materials ratio and, therefore, the reduction in water needed to achieve the desired slump and workability when a water-reducing agent is used will effect an increase in strength. The result of waterreducing admixtures in improving strength, however, often exceeds the results of simply reducing the water content.

Type B: retarding

Type C: accelerating

Type D: water-reducing and retarding

Type E: water-reducing and accelerating

Type F: high-range water-reducing

Type G: high-range and retarding

1.4.            Commonly Used Chemical Admixture in Ethiopia

Chemical admixtures are designed to enhance the properties of concrete or mortar in the plastic and hardened states, increase efficiency of cementitious material, and improve the economy of the concrete mixture. The use of a chemical admixture or combination of chemical admixtures may be the only viable strategy to achieve the desired results. In certain instances, the desired objectives may be best achieved by changes to the mixture proportions in addition to using the proper chemical admixture.

The most frequently used chemical admixtures in Ethiopia can be categorized in to three classes of chemical admixtures based of their functional used in the site. Each of the categories are discussed in detail below.

  1. 1)Accelerators these are used to increase the rate of setting or the rate of hardening at early age or both effects simultaneously. These type of chemical admixture most of the time used by general contractors and real estate developers for the following reasons

ü  To increase the progress of concrete works which helps in reduction of completion time of the project and effective use of form works.

ü  For the productions of blocks and precast elements

`2) Water-reducing and set retarding chemical admixtures to reduce the water requirement of concrete or to retard the setting time of the concrete or both. Most of the time these types of chemical admixtures are applied for long hauling of ready mix concrete in hot temperature and for the placement of massive concrete.

3) Damp proofing acts to make the surface of pores no wetting that means they increase the contact angle of water with pores surface. This will increase the external force required to over comes the surface forces and penetrate the pore. These types of chemical admixtures are suitable for masonry or concrete structures subjected to hydrostatic water pressure such as culverts, canals, manholes water reservoirs and basements.

1.5.            How to select chemical admixtures

Most of the time there are many basic reasons to consider while using chemical admixtures; but first one have to see what concrete have by its own. Then try to list out special conditions encountered and list what are special properties to encroach. There are many misleading promotions one has to be aware of.

Some of points to considerer while selection types of admixture    

  1. 1.Workability
  2. 2.Bleeding and Settlement
  3. 3.Consolidation
  4. 4.Hydration, Setting Time, and Hardening
  5. 5.Curing
  6. 6.Drying Rate of Concrete
  7. 7.Strength
  8. 8.Density
  9. 9.Permeability and Water-tightness
  10. 10.Abrasion Resistance
  11. 11.Volume Stability and Crack Control
  12. 12.Resistance to Freezing and Thawing
  13. 13.Alkali-Aggregate Reactivity
  14. 14.Carbonation
  15. 15.Chloride Resistance and Steel Corrosion
  16. 16.Chemical Resistance
  17. 17.Sulfate Attack
  18. 18.Seawater Exposures
    1. a.Role of Admixture in cement proportion aspect

2.      Merit and demerit of Using Chemical Admixture

    b.Role of Admixture in construction time aspect

 c.Role of Admixture in construction methodology aspect

 d.Role of Admixture in construction cost aspect

 e.Role of Admixture in special concrete production aspect

 f.Role of Admixture in environment protection

3.      Demand and Supply of Chemical Admixtures in Ethiopia

In this time the use and practice of admixtures in Ethiopia is beginning stages, it only accounts two decades. The market availability and utilization of admixture in the country is very small when we compare it with developed countries. This may be due to the recent introduction of the material into the market and lack of awareness toward the importance and use of admixtures.

In these days there are a few companies that distribute admixture by importing from different countries of the world such as South Africa, Saudi Arabia, Germany and Great Britain. Most of the suppliers are located in the capital city, Addis Ababa.

These companies only import those admixtures that are commonly used in the construction industries.

3.1.            Chemical Admixture Practice in Ethiopia

Chemical admixture are applied for the following site works

  1. For ready mix concrete
  2. For production of precast elements
  3. For production of blocks
  4. For concrete structures subjected to hydrostatic water pressure
  5. Other construction works

 3.2.            Admixture suppliers Distributor of Chemical

1. AB-HAM P.L.C Fosroc Admixture

2. Afro Chemicals & Steel P.L.C BASF Admixture

3. Con Mix Addis Ababa Branch Con Mix Admixture

4. Lica P.L.C Con Mix Admixture

5. Sika Addis Ababa Branch Sika Admixture

Those companies has supplied concrete admixtures for various nationally and internationally funded projects under taken by both local and foreign construction firms. All suppliers give the specification, technical support and sent skilled man power to site in order to support the consumers.

The major consumers of the admixtures in the country are national and international contractors, real-estate developer and government enterprises for varies applications starting from the production of blocks to application of admixture for varies concrete works. The table below shows the major consumers and their main purposes.

3.3.            Major Consumers Chemical Admixture

Examples Main Purpose

  1. 1.General Contractors MIDROC GC
  3. 3.T/Brehan Anebay GC

-For ready mix concrete

-For early strength of concrete

-For congested steel reinforcement

Real Estate Developers Different Companies -For production blocks

  1. 1.Road sector contractors DMC Damp proofing
  2. 2.Government Enterprises WWCE Damp proofing

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Special conditions to use Chemical concrete admixture

Storage of Chemical Admixtures

Dosage of Chemical Admixtures

The following points should be considered before using concrete chemical admixture:

  • Follow the manufacturer’s instructions regarding dosage, application details and instructions and product description
  • Make relevant tests to check that the desired effects are being obtained. It is particularly important because the precise effect of a chemical admixture will depend on many factors, cement composition, aggregate characteristics and impurities, mix proportions, presence of other chemical admixture type and length of mixing, time of addition, concrete temperature, and environmental condition.
  • The presence of any potentially detrimental substance such as chlorides, sulfates, sulfides, nitrates or ammonia.
  • Potential occupational hazard for works should be considered
  • Conditions for storage & recommended shelf life

Preparation of chemical admixture and procedure for procedures for introducing it into the concrete mix.

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