Building Materials, Third Edition

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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.

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1. w 2€X A concrete mix having an axcess 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.


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   ™X 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.


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   X 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.


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   X 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.


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   2—X A well graded aggregate gives good workability. Gap graded
   aggregate affects void system and workability. These effects are greater in fine aggregate.


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   X 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.


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   X 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.