Viscosity Modifying Admixture (VMA)

Admixtures known as viscosity-modifying admixtures, or VMAs, are used to alter the viscosity, cohesiveness, workability, and other characteristics of freshly mixed concrete. The next section discusses the function of VMA and its applications in various types of concrete.

The purpose of the VMA (viscosity-modifying additive) in concrete

The primary purpose of a viscosity-modifying additive in a concrete mix is to change the rheological characteristics of the material, particularly the fresh concrete’s plastic viscosity. When VMA is added to the concrete mix, the concrete’s plastic viscosity rises and its yield point somewhat rises as well. For ideal concrete rheology, the yield point—an additional rheological property—should be maximized in addition to the plastic viscosity. However, VMA has no effect on concrete’s yield point. Therefore, to change the concrete mix’s yield point, plasticizers or super plasticizers are used in addition to VMAs.

Advantages of Admixtures that Modify Viscosity (VMAs)

The following is a list and discussion of many concrete kinds that exhibit varying behaviors due to the inclusion of viscosity-modifying admixtures to their design mixes:

1. Self-leveling Pavement
2. Concrete Pumped

3. Concrete Beneath Water
4. Concrete that is lightweight
5. Concealed Concrete
6. Concrete that has been sprayed
7. Concrete with Pores
8. Concrete with Aggregates That Aren’t Well Graded

1. Self-leveling Pavement

• Segregation may happen if a self-compacting concrete mix with a lower powder content is left exposed to fluctuations in moisture.
• Adding a viscosity-modifying additive to self-compacting concrete improves its robustness to moisture changes and boosts its resistance to segregation.
• VMAs lessen concrete mix bleeding and improve the stability of self-compacting concrete mixtures.

2. Concrete Pumped

• The most frequent issue with pumped concrete is blockage of aggregates at pipe bends.
• Adding a viscosity-modifying additive makes pumped concrete more cohesive and reduces segregation during pumping.
• The lubricating action produced by VMA lowers the friction between the pump-wall and the concrete mix during pumping.

3. Concrete Beneath Water

• When putting and setting underwater concrete, it is essential that the concrete be extremely cohesive to avoid washout.
• Underwater concrete is treated with a viscosity-modifying additive to improve the cohesiveness of the mix and minimize washout. Thus, another name for it would be anti-wash out admixture.Additionally, it makes concrete constructions submerged in water more durable.
• Generally speaking, when concrete is submerged in water, cement dust from the concrete causes the water to get murky. However, the use of VMAs prevents dust generation and maintains water’s transparency, making the placement of concrete easier.

4. Concrete that is lightweight

• Lightweight aggregates, which typically have good permeability, are used in light weight concrete. These aggregates may take up water from the mixture, which makes it less workable.
• Another issue is that the aggregate’s small weight may cause it to float over the extremely workable concrete mix. Viscosity modifying admixtures reduce the water absorption of aggregates, but the aggregate must be at least partially saturated before added to the mix. By adding VMA, you can increase the mix’s viscosity and avoid this issue.

5. Concealed Concrete

• A high water content is necessary for semi-dry concrete to reach its maximum strength. However, adding more water changes its characteristics.
• Adding a viscosity-modifying additive to a semi-dry concrete mix allows for easier attainment of final strength by extending the range of the ideal water content.
• Smooth surface finish is also provided by VMA for semi-dry concrete.

6. Concrete that has been sprayed

• By enhancing the cohesiveness of the concrete mixture, the use of a viscosity-modifying additive reduces the rebound of particles from sprayed concrete.

7. Concrete with Pores

• Admixtures that change viscosity stop cement paste from washing out of porous concrete mixes.
• By strengthening the link between cement paste and aggregate, VMA makes concrete stronger.

8. Concrete with Aggregates That Aren’t Well Graded

• The use of manufactured or poorly graded aggregates in concrete mixes can cause segregation, which can be lessened by adding a viscosity-modifying additive (VMA). Adding VMA can also stop finishing issues with this kind of concrete mix from occurring.