Admixture
According to ACI 212.R-10, admixture is a material that is added to a batch of concrete either right before or right during the mixing process. Admixtures are blended to enhance the qualities of either freshly-poured or hardened concrete, or their combination. Admixtures can be classified as either normal or retarding, water-reducing, air-entraining, accelerating, retarding, or hyper plasticizing (retarding).
Admixtures must fulfil specific requirements in order for them to fulfil their intended function. For example, ACI 212.3 R-10 stipulates that chemical admixtures must meet ASTM C494 standards. This would guarantee the creation of concrete with the appropriate qualities
1. Admixtures’ Physical Requirements
The physical requirements of various additives used in conjunction with concrete are shown in the below Tables
Water Content
| Description | Accelerating Admixture | Retarding Admixture | Water-reducing Admixture | Air-entraining adjuvant | Admixture of super plasticizing (Normal) | Admixture of super plasticizing (retarding) |
| Water Content (max) in % | – | – | 95.0 | – | 80.0 | 80.0 |
Allowable Setting Time
| Description | Accelerating Admixture | Retarding Admixture | Water-reducing Admixture | Air-entraining adjuvant | Admixture of super plasticizing (Normal) | Admixture of super plasticizing (retarding) |
| Min, Hr | -1.0 | +1.0 | – | – | +1.5 | +1.0 |
| Max, Hr. | -3.0 | +3.0 | +/- 1 | – | – | +4.0 |
Compressive Strength , % of Control Sample (minimum)
| Description | Accelerating Admixture | Retarding Admixture | Water-reducing Admixture | Air-entraining adjuvant | Admixture of super plasticizing (Normal) | Admixture of super plasticizing (retarding) |
| 1 Day % | – | – | – | – | 140 | +1.0 |
| 3 Days in % | 125 | 90 | 110 | 90 | 125 | 125 |
| 7 Days in % | 100 | 90 | 110 | 90 | 125 | 125 |
| 28 Days in % | 100 | 90 | 110 | 90 | 115 | 115 |
| 6 Months in % | 90 | 90 | 100 | 90 | 100 | 100 |
| 1 Years in % | 90 | 90 | 100 | 90 | 100 | 100 |
Flexural Strength , % of Control Sample (Minimum)
| Description | Accelerating Admixture | Retarding Admixture | Water-reducing Admixture | Air-entraining adjuvant | Admixture of super plasticizing (Normal) | Admixture of super plasticizing (retarding) |
| 3 Days in % | 110 | 90 | 100 | 90 | 110 | 110 |
| 7 Days in % | 100 | 90 | 100 | 90 | 100 | 100 |
| 28 Days in % | 90 | 90 | 100 | 90 | 100 | 100 |
Length Change , % increased over control sample (Maximum)
| Description | Accelerating Admixture | Retarding Admixture | Water-reducing Admixture | Air-entraining adjuvant | Admixture of super plasticizing (Normal) | Admixture of super plasticizing (retarding) |
| 28 Days in % | 0.010 | 0.010 | 0.010 | 0.010 | 0.010 | 0.010 |
| 6 Months in % | 0.010 | 0.010 | 0.010 | 0.010 | 0.010 | 0.010 |
| 1 years in % | 0.010 | 0.010 | 0.010 | 0.010 | 0.010 | 0.010 |
Bleeding & Aire Content , % increased over control sample (Maximum)
| Description | Accelerating Admixture | Retarding Admixture | Water-reducing Admixture | Air-entraining adjuvant | Admixture of super plasticizing (Normal) | Admixture of super plasticizing (retarding) |
| Bleeding in % | 5.0 | 5.0 | 5.0 | 5.0 | 5.0 | 5.0 |
| Air Content in % | – | – | – | – | 1.50 | 1.50 |
• Not more than 15 mm less than concrete with a control mix.
** The slump at 45 minutes cannot be less than the control mix concrete’s at 15 minutes.
*** After two hours, the slump must not be less than the control mix concrete’s after fifteen minutes.
2. Admixture Quality Requirement
The admixtures must satisfy the following standards of quality in order to be considered compatible with concrete:
The attributes of combination
The data sheet must include characteristic information about the type of admixtures, forms that are available, basic material utilized, dosage information and administration techniques, precautions to be followed, and standard compliance.
3. Make-Up of Chemicals
This is a crucial factor, and the maker must provide information on how the admixture fits in with the cement that is on the market as well as other ingredients in various environmental conditions.
Admixture’s chemical composition, including its amount of soluble sulphate and chloride, must adhere to specifications provided by BIS and other organizations.
4. Firm Content
The concentration and efficiency of the admixture are determined by the solid content; the higher the solid content, the higher the efficiency and the less additive would be needed to change the properties of the concrete.
5. Particular Gravity
Admixture consistency is mostly determined by its specific gravity and solid content; variations in specific gravity will affect the consistency of the concrete mix.
6. Mass Per Molecule
When it comes to concrete, the larger the molecular weight, the higher the efficiency.
7. Amount
For every combination, the manufacturer must provide information on dose and application techniques.
8. Specifics of Compatibility
Basic compatibility information, potential issues with compatibility resulting from various cement types and their impacts on water absorption or demand, etc., will be provided.
9. The Process of Hydration
The admixture shouldn’t alter the cement paste’s hydration process, causing it to solidify, nor should it have a high chloride content that could corrode steel
10. Acclimatization to Air
Unless otherwise noted, the amount of air entrained in the concrete shouldn’t be more than 1%.