GUIDE FOR ADMIXTURES IN CONCRETE

 ADMIXTURES USAGE & SPECIFICATIONS

Concrete admixtures are natural or manufactured chemicals or additives added during concrete mixing to enhance specific properties of the fresh or hardened concrete, such as workability, durability, or early and final strength.

Concrete admixtures are of different types and they are as follows:

1. Water Reducing Admixtures
   
2. Retarding Admixtures
3. Accelerating Admixtures
4. Air entraining concrete admixture
5. Pozzolanic Admixtures
6. Damp-proofing Admixtures
7. Gas forming Admixtures
8. Air detraining Admixtures
9. Alkali Aggregate Expansion Inhibiting Admixtures
10. Anti-washout Admixtures
11. Grouting Admixtures
    
12. Corrosion Inhibiting Admixtures
13. Bonding Admixtures
14. Fungicidal, Germicidal, Insecticidal Admixtures
15. Coloring Admixtures

Here we classify as  - 1. Mineral Admixtures  2. Chemical Admixtures

MINERAL ADMIXTURES

Mineral Admixtures are finely divided siliceous materials which are added to concrete in relatively amounts. They can be broadly divided into two groups, namely,

1. Reactive mineral admixtures, which could be either pozzolonic(for example, low calcium fly ash, silica fume), or cementitious (for example, ground granulated blast furnace slag), or both cementitious and pozzolonic (for example, high calcium fly ash)

2. Inert mineral admixtures, which have no cementitious or pozzolonic value and are generally added as a filler material (for example, silica flour, limestone powder, etc.).

When the materials from the first group comprising of reactive mineral admixtures are used to partially replacement, they react with the calcium hydroxide in the hydrated cement paste to form complex compounds which result in a reduction in permeability, improvement in the ultimate strength, water tightness and durability, besides imparting economy to the mix. However, these admixtures need to be uniformly blended while mixing the concrete.

Incidentally, blended cements such as PPC and PBSC contain mineral admixtures as per relevant Indian standards. They are manufactured under controlled conditions in a factory and contain these admixtures uniformly. These cements are most suitable for site as well as ready mixed concrete.

The IS 456:2000 permits the use of the following mineral admixtures, provided uniform blending with cement is ensured:

1. Fly Ash

2. Ground Granulated Blast-furnace Slag (GGBS)

3. Silica Fume

4. Rice Husk Ash

5. Metakaolin.

The use of mineral admixtures directly at site in concrete is still in its infancy in India and is mainly restricted to the ready mixed concrete. Excepting silica fume, none of these admixtures are readily available commercially in the market. While the specifications of silica fume, rice husk ash and metakaolin are yet to be formulated by the Bureau of Indian Standards, fly ash conforming to Grade I of IS 3812 and GGBS conforming to IS 12089 may be used as part replacement of ordinary Portland cement provided uniform blending with cement is ensured.

1. Limits of Mineral Admixtures to be used with cement

S.No.	MINERAL ADMIXTURE	% TO BE USED	REFERENCE
1	Fly Ash (PFA)	15 - 35	IS 1489 (Part-1) 1991 Amendment No.3, July 2000
2	Slag (GGBS)	25 - 70	IS 455 - 1989 Amendment No.3, July 2000
3	Silica Fumes	05 - 10	IS 456 - 2000

2.Requirements of Fly Ash for use as pozzolana and Admixture 

 

Characteristic	Requirement of  Fly Ash Grade I
Physical Requirements
Fineness, minimum specific surface, m2/kg	320
Lime reactivity, average compressive strength, N/mm2, Minimum	4.0
Minimum compressive strength, at 28days, N/mm2,  minimum	Not less than 80 percent of the   strength of corresponding plain  cement mortar cubes
Drying shrinkage, percent, maximum	0.15
Soundness expansion, Autoclave test, percent, maximum	0.8
Chemical Requirements
Silicon dioxide(Sio2) plus aluminium oxide(Al2O3)plus iron oxide(Fe3O2), percent by mass, minimum	70.0
Silicon dioxide(SiO2), percent by mass, minimum	35.0
Magnesium oxide(Mgo), percent by mass, maximum	5.0
Total sulphur as sulphur trioxide(SO3), percent by mass, maximum	2.75
Available alkalis as sodium oxide (Na2O), percent by mass, maximum	1.5
Loss on ignition, percent by mass, maximum	12.0

CHEMICAL ADMIXTURES

Chemical admixtures are sometimes called the fifth ingredient of concrete, other than cement, coarse and fine aggregates and water. They are inorganic or organic materials—solid or liquid—which when added to the normal components of a mix (either concrete, mortar or paste), interact with the cementitious system through chemical, physical or physico-chemical means, modifying one or more properties of the mix in the fresh, setting, hardening or hardened state.

A number of advantages can be derived with the use of admixtures. For example, in the fresh state of concrete, depending on the type of admixture used, they can increase the workability without increasing the water content, reduce or prevent settlement, modify the rate and /or capacity of bleeding, reduce segregation and reduce slump loss, retard or accelerate the time of initial/final setting.

Aside from altering the properties of the fresh mix, they can retard or reduce heat evolution during early hardening, accelerating the rate of strength development at early ages, increase the compressive strength of concrete, improve durability, control alkali-aggregate reactivity, produce aerated concrete, improve bond between old and new concrete, inhibit corrosion of reinforcement, produce coloured concrete/mortar, etc.

Chemical admixtures can be classified according to the purpose they are used, or according to the type of materials constituting them.

Commonly-used admixtures are:

1. Water reducing /plasticizing admixtures

2. Set controlling admixtures, or retarders

3. Air entraining admixtures

4. Accelerating admixtures

5. High range water reducing, or super plasticizing admixtures.

Besides the above, other types of admixture are also used. These include Grouting admixtures, pumping aids, bonding admixtures, expansion-producing admixtures, fungicidal, germicidal and insecticidal admixtures, etc.

Commercially available admixtures may contain materials that separately belong to one or more groups. For example, a water-reducing admixture may be combined with a retarding admixture, and so on. The effectiveness of an admixture depends upon such factors as type, brand and amount of cement; water content; aggregate shape, grading and proportions; mixing time; slump; and temperatures of concrete and air.

Trial mixes should be made with the admixture and the job materials at temperatures and humidity anticipated on the job. In this way the compatibility of the admixture with other job materials, as well as the effects of admixtures on the properties of the fresh and hardened concrete, can be observed. The amount of admixture recommended by the manufacturer, or the optimum amount determined by laboratory tests should be used.

Physical requirement for the main types of admixtures as given in IS 9103:1999 are given in the table

Requirement	AA	RA	WRA	AEA	NSA	RSA
Water content, percent of control sample, maximum	-	-	95	-	80	80
Time of setting, allowable deviation from control sample, hours:
Initial              Maximum	-3	+3	+/-1	-	-	+4
Minimum	-1	+1	-	-	+1.5	+1
Final                Maximum	-2	+3	+/-1	-	+/-1.5	+/-3
Minimum	-1	+1	-	-	-	-
Compressive strength, percent of control sample, minimum:
1-day	-	-	-	-	140	-
3-day	125	90	110	90	125	125
7-day	100	90	110	90	125	125
28-day	100	90	110	90	115	115
6-month	90	90	100	90	100	100
1-year	90	90	100	90	100	100
Bleeding, percent increase over control      sample, maximum	5	5	5	5	5	5
Loss of workability	-	-	-	-	*	**
Air content, percent, maximum, over control	-	-	-	-	1.5	1.5

Notes: AA: Accelerating admixture; RA: Retarding Admixture;

WRA: Water-reducing admixture; AEA: Air entraining admixture;

NSA: Super plasticizing admixture (normal);

RSA; Super plasticizing admixture (retarding)

* At 45 min the slump shall not be less than that of control mix concrete at 15 min

** At 2 hour, the slump shall not be less than that of control mix concrete at 15 min.