Originated from a Greek word “rheo” which means “flow” and “logia” which means “study” Rheology is the study of flow of matter in liquid, soft solid or solid state under conditions in which they respond to plastic flow other deforming elastically when a external force is applied.
Rheological properties of a cement mix means deformation of hardened concrete and placing and mixing of freshly mixed concrete.The mechanical behaviour of hardened cement paste,which exhibits both elastic and inelastic deformations, can be expressed in rheological terms.
Factors Affecting Rheological Properties:-
1. If the amount of coarse aggregate in a concrete mix is more the desirable amount the voids cannot be filled with the available mortar, which will lead to loss of cohesion and mobility. Such a mix is termed harsh and requires a great amount of effort to place and compact. On the other hand, an excessive amount of fine aggregate or entrained air in a concrete mixture will greatly increase the cohesion and render the concrete difficult to move.
2. Slump test is the measure of consistency of concrete mix or it is an indicator of relative water content of the mix. An increase in the water content or slump above that required to achieve a workable mix produces greater fluidity and decreased internal friction. Thus, a water content more than that needed will not improve the rheological properties of concrete. But very low slump with decrease the workability of a concrete mix making it impossible to place is some areas.
3. Elevated temperature, use of rapid hardening cement, cement deficient in gypsum and use of accelerating admixtures, increase the rate of hardening which reduce the mobility of concrete.
4. The rough and highly angular aggregate particles will result in higher percentage of voids being filled by mortar, requiring higher fine aggregate contents and correspondingly higher water content. Similarly an angular fine aggregate will increase internal friction in the concrete mixture and require higher water contents than well rounded natural sands.
5. A well graded aggregate gives good workability. Gap graded aggregate affects void system and workability. These effects are greater in fine aggregate.
6. An increase in the maximum size of aggregate will reduce the fine aggregate requirement to maintain a given workability and will thereby reduce the surface area to be wetted and hence the cement content necessary for a constant water/cement ratio .
7. The admixtures which have significant effect on the rheology of concrete are plasticizers and super-plasticizers, air-entraining agents, accelerators and retarders.Lignosulphate salt based plasticizers (0.15%) reduce the water content by 10% without any detrimental effect. Super-plasticizers and plasticizers prevent the formation of flocculated structure by changing the inter-particle attraction/repulsion.
With proper attention to the rheological properties can increase work efficiency and reduce the material cost as well as the cost of construction.
