There are many factors that influence the performance of admixtures in concrete such as type of admixture, dosage, compatibility of the admixture with cement, mix design, ambient temperature, and the presence of other admixtures.
These factors influence the degree of achievement of desired properties for which admixtures are introduced. Admixtures are added to concrete mixture to obtain certain properties such as accelerate setting, early strength development, improve durability, enhance concrete cohesiveness for underwater placement, decrease permeability, increase pumpability, reduce water content and many more desire properties.
Factors Affecting Performance of Admixtures in Concrete
1. Type of Admixtures
The admixture would be more effective its molecular weight is high. For instance, high molecular weight of softwood sodium lignosulfonate leads to better dispersing efficiency, slump retention, less retardation, and maximum water reduction can be increased by a range of 10-20%.
2. Dosage
The quantity of admixture should be optimum. Excess of admixture may cause segregation or bleeding. It may also result in the extension of set time and strength. The optimum does should be estimated by trials.
3. Compatibility with Cement
All admixtures may not produce same results with different cements. Therefore, before using any admixture, its compatibility with cement has to be established. Properties of cement like fineness, chemical-composition, C3A content etc. affect the performance of admixture.
Incompatibility between cement and admixtures lead to several problems such as rapid loss of workability, segregation of concrete, acceleration/ retardation of setting, and low rates of strength gain. Therefore, trials have to be made before finalizing an optimum does of admixture.
4. Mix Design
All constituents of mix affect the performance of the super-plasticizer as given below:
- Water: more water in the mix improves the physical interaction and dispersion of admixtures.
- Coarse aggregate: proportioning and grading of coarse aggregates influence the performance of concrete admixture.
- Fine aggregate: proportioning, grading and silt content also influence the performance of concrete admixture.
- Cement: its fineness, C3A content influence the performance of admixture. Higher C3A reduces efficiency of admixture.
- Other admixture: presence of other admixtures also influences the performance of concrete admixtures.
Therefore, proper trials before actual use are very vital for effectiveness of admixture.
5. Ambient Temperature
Performance of concrete admixture such as water reducing admixture is influenced by ambient temperature and humidity to which concrete is exposed. The increase of temperature would increase saturation dosage of water reducing admixture such as lignosulfonate.