VISCOSITY MODIFYING ADMIXTURES IN CONCRETE
VISCOSITY MODIFYING CONCRETE ADMIXTURES
Viscosity modifying concrete admixtures (VMAs) is a family of admixtures designed for specific applications. The European Federation for Specialist Construction Chemicals and Concrete Systems cites the following uses:
- Reduce segregation in highly flowable/self compacting concrete.
- Reduce washout in underwater concrete.
- Reduce friction and pressure in pumped concrete.
- Compensating for poor aggregate grading, especially a lack of fines in the sand.
- Reducing powder content in self compacting concrete.
- Compensating for poor aggregate grading, especially a lack of fines in the sand.
- Reducing powder content in self compacting concrete.
- Reduce bleeding in concrete.
- Improve green strength in semi-dry concrete.
Self-consolidating high-strength concrete mixtures are frequently produced using high range water-reducing admixtures in conjunction with viscosity modifying admixtures, such as cellulose ether, welan or diutan gum. Some VMAs are based on inorganic materials such as colloidal silica, which is amorphous with small insoluble, non-diffusible particles, larger than molecules but small enough to remain suspended in water without settling. By ionic interaction of the silica and calcium from the cement a three dimensional gel is formed which increases the viscosity and / or yield points of the paste. This three dimensional structure/gel contributes to the control of the rheology of the mix, improving the uniform distribution and suspension of the aggregate particles and so reducing any tendency to bleeding, segregation and settlement.
Most Viscosity Modifying Admixtures are supplied as a powder blend or are dispersed in a liquid to make dosing easier and improve dosing accuracy. They have little effect on other concrete properties in either the fresh or hardened state but some, if used at high dosage, can affect setting time and or the content stability of entrained air.
