Similar to workability, the rheological properties of fresh concrete are affected by the mix proportions, namely the amount of each constituent, properties of the ingredients, the amount of mixing, the presence of admixtures, and the time elapsed after mixing. Though these factors have properties of fresh concrete, but here these factors are discussed in relation to their influence on the flow properties of concrete.
Factor # 1. Mix Proportion:
The concrete mix is proportioned to provide workability needed during placement and compaction to assure the stipulated performance characteristics of the hardened concrete.
In case there is an excess amount of coarse aggregate in a concrete mix, it will be deficient in mortar to fill the void system, resulting in a loss of mobility and cohesion. Such a mix is known as a harsh mix and requires more efforts to place and compact it. The harshness in the mix may also be developed by low air content. An increased air entrainment may reduce the excessive use of fine aggregate.
On the other hand an excessive amount of entrained air or fine aggregate in concrete will increase the cohesion to such an extent that will make the concrete difficult to move i.e. its mobility will decrease. The more amount of fine aggregate in a concrete mix increases the surface area of the particles which requires more amount of paste to coat the surface of the particles to have the same mobility. This high amount of cement paste results in higher drying shrinkage and cracking.
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In practice usually concrete mixes are proportioned with an excess amount of fine aggregate and higher cement content than needed for a concrete of optimum fine aggregate content.
The higher cement content makes the concrete slow moving and sticky, especially in the normal range of slump for cast in-situ concrete. Further the higher cement content and lower water-cement ratio reduces the workability of rich mixes from that measured immediately after mixing.
Factor # 2. Consistency:
The consistency of concrete is an indicator of the relative water content in the concrete mix. It is measured with the help of the’ slump test. The increase in slump or water content above that required to get a workable mix produces greater fluidity and decreased internal friction. The decrease in cohesion of the mix increases the potential for segregation and increased bleeding. Thus more amount of water content, than that needed to produce a workable mix will not improve the rheological properties of concrete.
Contrary to this, too low of water content or slump will reduce the mobility and compatibility of concrete which will cause difficulties in placing and compacting the concrete. It has been observed that as far as rheological properties of concrete are concerned an increase of one percent of air entrainment is equivalent to an increase of water content by 3% or 1% of fine aggregate. On the other hand an excessively dry mix may result loss of cohesion and may cause an increased dry segregation.
Factor # 3. Hardening and Stiffening:
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The rate of hardening of concrete increases with the use of rapid hardening cement, cement deficient in gypsum and the accelerating admixtures, the increased rate of hardening of concrete reduces its mobility. The porous and dry aggregate reduce the workability of the concrete by absorbing water from the concrete mix. The workability also reduces if the surface area of aggregate particles to be wetted is increased.
Factor # 4. Aggregate Shape and Texture:
The rough and highly angular aggregate particles produce higher percentage of voids which need more paste to fill these voids. Thus there is need for more fine aggregate and water content in the mix. Similarly angular fine aggregate will increase the internal friction in the concrete mix requiring more water content than needed when well-rounded natural sand is used. Thus the shape and texture of aggregate influences the rheology of the concrete to a great extent.
Factor # 5. Aggregate Grading:
A well graded aggregate gives a good workability. Gap grading or a change in the size distribution of aggregate may cause appreciable effect on the workability and void system of the concrete mix. These effects have been found more in the fine aggregate than coarse aggregate. The water requirement in the mix increases with the decrease in the size of fine aggregate particles, resulting in increasingly sticky concrete.
On the other hand as the size of fine aggregate particles increases i.e. fine aggregate becomes coarser, the cohesion of the concrete reduces resulting in harsh concrete mix and the tendency of bleeding increases. Thus to maintain the workability of the concrete mix adjustments in the grading of fine aggregate becomes necessary due to the above noted changes.
Factor # 6. Maximum Size of Aggregate:
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An increase in the maximum size of aggregate will reduce the surface area of the aggregate particles to be wetted resulting in the reduction in the cement content necessary for a constant water-cement ratio.
Factor # 7. Admixtures:
The rheology of concrete has been found to be effected by the following admixtures:
(a) Plasticizers and super plasticizers.
(b) Air entraining agents.
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(c) Accelerators and retarders.
These admixtures are used in the following three ways:
(i) To give the same workability with less water content, resulting in higher strength.
(ii) To give the same workability and strength with reduced cement content. However the cement content should be sufficient from workability considerations.
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(iii) To give increased workability with the same long term strength and durability.
Plasticizers based on lingosulphonate salt (0.15%) have been found to reduce water content by 10% without any adverse effect on the properties of concrete. Super plasticizers have made it possible to produce flowing or self levelling concrete which can be compacted with minimum effort into the corners of the form work and around the congested reinforcement. For a self levelling concrete the maximum yield value may be kept as 350 Pa. Such a concrete can be obtained by the use of synthetic naphthalene or metamine resins or ultra-pure lignosulphate. However concrete with very high workabilities are likely to segregate.
The water-cement system constitutes a system of particles suspended in a high electrolyte concentration bringing the particles close enough to stick together. This phenomenon is known as flocculation. The presence of plasticizers modify the condition in the water-cement suspension to prevent the formation of flocculated structure by altering the inter particle repulsion/attraction.
The plasticizers or super plasticizers interact with cement particles forming a membrane of absorbed charged molecules around each particle. These charged molecules physically prevent the particles to come so closer to each other to stick and the flocculation of particles is prevented. Thus the membrane forms an electrostatic or steric barrier for the particles to come closer to have contact.
The attraction between the particles can be reduced to zero by increasing the thickness of this membrane. The mechanism of deflocculating or dispersion of cement particles reduces the yield. In addition of attractive forces, repulsive electrical forces are also developed between particles due to the absorption of ionized compounds which increase the plastic viscosity through the operation of secondary electro-viscous effect.
Thus the admixtures prevent close contact between the cement particles by a combination of electrostatic or steric repulsion, weakening the structure which can be formed at rest and reducing the yield value. Beyond an optimum value, the increase in the concentration of plasticizer does not decrease the yield rate appreciably. However with the increase in cement content of the mix, reduction of yield value increases. The value of plastic viscosity increases if the sand content in the mix is low and decreases with higher sand content. It is due to the fact that when concrete with low sand content flows, it is the flouted cement in the mix that separates the coarse particles.
In case the cement is deflocculated then these coarse particles come closer and touch each other developing a greater resistance to the flow of concrete, resulting an increase in the plastic viscosity of the concrete even though cement paste is reduced. On the other hand concrete with more sand content needs less cement to fill the voids in the coarse aggregate as sand present in the concrete will fill these voids.
Factor # 8. Air Entraining Agents:
The air entraining agents produce spherical bubbles of 10 to 250 micron in diameter by modifying the surface tension of the aqueous phase in the mix. Most commonly materials used as air entraining agents are sodium olerate, sodium caprate, alkyle sulphate and pemric acid salts etc. The air bubbles are produced by these compounds which have negatively charged head group as (-COO–, SO2–3, SO2–4) which is hydrophilic (water attracting) and a non-polar tail which is hydrophobic (water repelling).
Thus the air bubbles generated have an apparent negative charge which forms bridges between cement particles resulting in increased yield value. These air bubbles act as ball bearings allowing large particles to slide over each other more easily, decreasing the plastic viscosity. The rheological properties of the concrete are changed very significantly by the air entrained agents.
The air entrainment increases the cohesion and reduces the tendency of bleeding in the mix. It has been observed that 4 to 6% air entrainment by volume increases the cohesion of the mix and sand content may be reduced by about 5%.
Factor # 9. Accelerators and Retarders:
The accelerators will reduce and the retarders will extend the workability time for a given mix. The time of retention of improved workability is critical for the concrete where it is to be transported before placement. The reduction in workability is similar to the stiffening of concrete due to slow chemical reaction during the induction period.
The addition of admixtures change the nature of membrane formed around the cement particles and the composition of aqueous solution resulting the change in the rate of stiffening. This effect especially has been observed when super plasticizers are used.
A retarder plasticizer reduces the workability loss and increases the retention time due to slowing down the setting of cement. Super Plasticizers have been found to effect much improvement in workability without using retarders which are essential with conventional plasticizers.