Admixture is a material that is used as an ingredient for the preparation of concrete, and is added to the batch immediately prior to or during mixing materials (ACI 212.R-10). The purpose of blending admixtures is to improve fresh or hardened concrete properties or the combination thereof. Examples of admixtures include accelerating, retarding, water-reducing, air-entraining, super plasticizing admixture (normal), and super plasticizing admixture (retarding).
There are certain requirements that admixtures should meet otherwise it would fail to complete its mission. For instance, ACI 212.3 R-10 specifies that chemical admixtures shall conform to the specifications of ASTM C494. This would ensure the production of concrete with desired properties.
1. Physical Requirements of Admixtures
Table 1 provides physical requirements of admixtures.
Table 1 - Physical Requirements of Various Admixtures used in combination with Concrete
|Water contents, percent of control sample, maximum||-||-||95||-||80||80|
|Time of setting, allowable deviation from control sample, hours|
|Compressive strength, percent of control sample, minimum|
|Flexural strength, percent of control sample, minimum:|
|Length change, percent increase over control sample, maximum|
|Bleeding, percentage increase over control sample, maximum||5||5||5||5||5||5|
|Loss of workability||-||-||-||-||**||***|
|Air content, percent, maximum, over control||-||-||-||-||1.5||1.5|
AA – Accelerating Admixture
RA – Retarding Admixture
WRA – Water-reducing Admixture
AEA – Air-entraining admixture
NSA- Super plasticizing admixture (Normal)
RSA- Super plasticizing admixture (retarding)
- Not more than 15 mm below that of control mix concrete
** At 45 min, the slump shall not be less than that of the control mix concrete at 15 min
*** At 2 hours, the slump shall not be less than that of the control mix concrete at 15 min
2. Quality Requirement of Admixture
When the admixtures are said to be compatible with concrete, it should meet the following quality requirements :
1. Characteristics of Admixture
The data sheet shall give the characteristic details such as type of admixtures, available forms, basic material used, details on dosage and its application methods, precaution to be taken and compliance with specified standards.
2. Chemical Composition
This is an important parameter and manufacturer shall furnish details of suitability of admixture with cement available in market and other ingredients in different environment condition.
The chemical composition such as chloride and soluble sulphate content of admixture shall comply with specification given by BIS and other organisation.
2. Solid Content
The solid content will determine the concentration and efficiency of admixture. Higher the solid contents, higher the efficiency and would require less amount of admixture to alter the properties of concrete.
3. Specific Gravity
The consistency of admixture mainly depends on its specific gravity and depends on solid content. If the specific gravity changes, the consistency of concrete mix will vary.
4. Molecular Weight
Higher the molecular weight higher the efficiency when it is used in concrete.
Details of dosage and application methods shall be furnished by manufacturer for all admixtures.
6. Compatibility Details
Basic compatibility details, possible compatibility problems due to different type of cement and their effects on water demand or water absorption etc., shall be furnished.
7. Hydration Process
The admixture should not change the hydration process of cement paste to convert into solid mass and it should not contain high chloride which would corrode steel.
8. Air Entrainment
Air entrained in the concrete should not exceed 1% unless otherwise specified.