Viscosity Modifying Admixture for Enhancing the Concrete Property
Chemical admixtures have made a significant contribution to the development of modem concrete technology. Today's construction practices need concrete of such high consistencies to enable it to be placed by tremie, conveyed through pipeline by pumping, self- leveling and compacting, all at a very low w/c ratio to achieve high strength and durability. This has become possible only by the use of chemical admixtures. There are hardly any important concrete in the world without the use of admixtures. They have become an integral part of modem concrete, a back bone material of modem construction.
One of the innovations in chemical admixture is the development of improved Viscosity Modifying Admixtures (VMAs). VMAs are essential to control the stability and cohesion of concrete with very specific rheological requirements, such as self-compacting concrete, underwater concrete, or shotcrete.
Viscosity Modifying Admixture and its Functions:
Viscosity Modifying Admixtures (VMA) also known as Viscosity Enhancing Admixtures, are water-soluble polymers that increases the viscosity and cohesion of cement-based materials. Such enhancement of the liquid-phase viscosity is essential in flowable systems in order to reduce the rate of separation of material constituents and improve the homogeneity and performance of the hardened product. The key function of a VMA is to modify the rheological properties of the cement paste.
VMA's are a family of admixtures designed for specific applications. They are used to:
• 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
• reduce bleeding in concrete
• improve green strength in semi-dry concrete
Most VMA's are based on high molecular weight polymers with a high affinity to water. By interaction of the functional groups of the molecules with the water and the surfaces of the fines, VMA's build up a three dimensional structure in the liquid phase of the mix to increase the viscosity and/or yield point of the paste. The strength of the three dimensional structure affects the extent to which the yield point is increased.
Some VMA's 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 point 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 VMA's are supplied as a powder blend or are dispersed in a liquid to make dosing easier and improve dosing accuracy. The dosage depends on the application but typically ranges from 0.1 to 1.5% by weight of cement but can be varied for specific applications.
Viscosity Modifying Admixtures for Self-Compacting Concrete
Self-consolidating concrete (SCC) describes a specialized, high-slump concrete mixture able to flow and consolidate under its own weight with little or no vibration and without segregation or excessive bleeding. Its resistance to segregation can be enhanced by reducing the water-to-cementations materials ratio (W/C) or by incorporating a Viscosity Modifying Admixture.
During the transportation and placement of SCC the increased flow ability may cause segregation and bleeding, which can be overcome by providing the necessary viscosity, which is usually supplied by increasing the fine aggregate content; by limiting the maximum aggregate size; by increasing the powder content; or by utilizing VMA.VMAs can be used to enhance the resistance to segregation and bleeding. Viscosity Modifying Admixtures make the concrete more tolerant to variations in the water content of the mix so that plastic viscosity is maintained and segregation prevented. The use of a VMA can reduce the risk of separation of material constituents during transport and placement of SCC, and until the onset of hardening.
Viscosity Modifying Admixtures for Underwater Concrete
Placing concrete underwater can be particularly challenging because of the potential for washout of the cement and fines from the mixture, which can reduce the strength and integrity of the in-place concrete. Although placement techniques, such as tremies, have been used successfully to place concrete underwater, there are situations where enhanced cohesiveness of the concrete mixture is required, necessitating the use of an anti washout or viscosity modifying admixture (VMA). Some of these admixtures are formulated from either cellulose ether or whelan gum, and they work simply by binding excess water in the concrete mixture, thereby increasing the cohesiveness and viscosity of the concrete. The overall benefit is a reduction in washout of cement and fines, resistance to dilution with water as the mixture is placed, and preservation of the integrity of the in-place concrete.
The major effect of a VMA for underwater concrete is to increase the cohesiveness of the fresh concrete to such a level that during and after placing the fresh concrete is not washed out. In this way it gives the concrete structure the required strength and durability.
Because concrete when placed underwater is very difficult or impossible to compact it requires a high workability. VMA's used for underwater concrete usually decrease the workability and for this reason are normally used in combination with a superplasticizing admixture to recover the self-placing properties.
Viscosity Modifying Admixtures for Pumping Concrete
Pumped concrete has gained increasing importance over recent years, for both economic and technical reasons, but as a result of developments in construction practice, the requirements on pumped concrete have become more demanding and have approached the limits of normal concrete technology. VMA is used to meet these demands and to reduce fluctuations in concrete performance.
The most common problem with pumping concrete occurs when the coarse aggregate particles start to lock together, usually at a bend or other slight constriction. The pump pressure forces the lubricating mortar fraction to separate from the mix, leaving a plug of coarse aggregate which eventually blocks the line. Traditionally this has been solved by increasing the fines content of the mix but is not always technically and economically acceptable and may not be effective in the most demanding applications. The VMA is a more effective solution, preventing this segregating effect by making the concrete more cohesive without the need to change the mix design.
Viscosity Modifying Admixtures for Lightweight Concrete
Most lightweight aggregates are porous with water absorption much higher than normal aggregates. They can readily absorb water causing the mix to dry out and lose workability. Pumping lightweight aggregate concretes presents additional problems if the aggregates are not pre-soaked and fully saturated. The pump pressure drives water into the aggregate causing the mix to dry out and block the lines. VMA's can reduce the amount of water which gets absorbed, allowing partially saturated aggregate to be used.
The low density of lightweight aggregates can also causes flotation problems in high workability concretes, VMA will increase the paste viscosity helping to prevent flotation and keeping the mix homogeneous.