Rheology of Concrete:
Rheology is the science of flow and deformation of matter and describes the interrelation between force, deformation and time.
The term comes from Greek word rheos meaning to flow.
Rheology is applicable to all materials, from gases to solids.
The rheological principles and techniques as applied to concrete include the deformation of hardened concrete, handling and placing of freshly mixed concrete and the behaviour of its constituent parts, namely, cement slurries and pastes.
The rheology of fresh concrete like workability includes the parameters of stability, mobility and compactability.
The mechanical behaviour of hardened cement paste, which exhibits both elastic and inelastic deformations, can be expressed in rheological terms.
Factors Affecting Rheological Properties
Mix Proportions:
A concrete mix having an excess amount of coarse aggregate will lack sufficient mortar to fill the void system, resulting in a 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.
Consistency:
The consistency of concrete measured by the slump test is an indicator of the relative water content in the concrete 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. On the other hand, too low a slump or water content will reduce the mobility and compactibility which may pose difficulties in placement and consolidation.
Hardening and Stiffening:
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.
Aggregate Shape and Texture:
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.
Aggregate Grading:
A well graded aggregate gives good workability.
Gap graded aggregate affects void system and workability.
These effects are greater in fine aggregate.
Maximum Size of Aggregate:
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.
Admixtures:
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.
The air-entraining agents introduce spherical air bubbles 10 to 25 mm in diameter by modifying the surface tension of the aqueous phase in the mix. The bubbles act like ball bearings to allow larger particles to flow past each other more easily thus decreasing plastic viscosity.
The air-entrainment changes the rheology of concrete very significantly by increasing cohesion and reducing tendency for bleeding.
Mixture Adjustment:
Proper attention to the rheological properties of a mixture can effectively reduce construction and material costs. The properties of the materials used and field conditions have a great influence.
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