The rheological properties of concrete are dependent on the factors like mix proportions, consistency, the properties of each ingredient present in the mix, the mixing amount and the admixture amount, the time elapsed after mixing.
Factors Affecting Rheological Properties of Concrete
Factors affecting rheological properties of concrete based on flow characteristics of concrete are:
- Mix proportions of concrete
- Consistency of concrete
- Hardening and stiffening of concrete
- Textures of aggregates
- Use of concrete admixtures
Effect of Mix Proportions on Rheological Properties of Concrete
The proportioning of concrete mixture is done so that adequate workability is obtained during the construction. This would give full assurance about the performance and the characteristics of the hardened concrete.
A case where the concrete mix is having excess aggregate will have scarcity in mortar to fill up the voids caused. This result in decreasing cohesion ,hence decrease the mobility of the whole mix. This mix is regarded as harsh and would demand larger effort for placing as well as transporting.
The harshness is also caused by a lower value of air content. Increasing the entrained air would lessen the use of fine aggregates. Or in other words, the use of fine aggregates or entrained air will result in increasing the cohesion making the mix difficult to move.
The use of fine aggregate means the surface area for contact is increasing, thus asking for more mortar, to cover the surfaces with the paste. This is to maintain the mobility. But this, in turn, results in problems like drying shrinkage thus cracking.
Under practical consideration, the mix is designed for excess fine aggregate and excess cement, than what would be necessary for an optimum measurement.
The concrete mix becomes sticky, with high cement content. This would result in a mix which is sluggish in nature. This is mainly seen for cast in-situ concrete for normal slumps. The workability of rich mixes reduces with a lower water-cement ratio and a higher cement content.
Consistency of Concrete Affecting Rheology of Concrete
The measure of relative water content in the concrete is given by the consistency of the concrete mix. The concrete fluidity increases with an increase of slump or the water content, with an intention to make the mix workable. This disadvantage of fluidity will decrease the internal friction of the concrete particles.
Segregation and bleeding in concrete have higher chance to occur with increasing fluidity. It must be kept in mind that the water content above an allowable value won’t help in increasing the rheological properties of concrete. Too less water content won’t have proper mobility as well as good compactibility.
When taking rheological properties into consideration, one percent increase in air entrainment is equivalent to one percent increase in the amount of fine aggregate in the mix. This, in turn, is equivalent to 3% increase in water content.
Effect of Hardening and Stiffening on Rheological Properties of Concrete
Factors that promotes the rate of hydration and hence hardening, will reduce the mobility of the concrete. Reasons of higher temperature, rapid hardening cement, using accelerated admixtures will reduce the concrete flowability capacity.
Using dry aggregates or aggregates that are highly porous would absorb a larger amount of water or they have larger surface areas that are supposed to be wetted.
Shape and Texture of Aggregates affecting Rheological Properties of Concrete
The aggregate shape and texture do affect the rheological properties of concrete in a large extend. The incorporation of highly angular aggregates that are highly rough in texture, will ask for larger mortar to fill up the voids. This would demand higher fine aggregate as well as water content.
The fine aggregates that are angular in nature have property to increase the internal friction within the concrete mix. But due to this, they demand higher water than natural sand commonly used that is well rounded in shape.
Grading of Aggregates influence on Rheological Properties of Concrete
A good quality workability is guaranteed by a well-graded set of aggregates. This well grading system helps in filling the voids in a systematic manner.
The absence of any grade will remain void to be filled by either the cement or fine aggregates. This brings a change in the quantity. This effect is more pronounced in fine aggregates than in coarse aggregates.
The water demand increases with the increase in fineness of the fine aggregates. This will lead to a concrete mix which is sticky.
When the fine aggregates become coarser in size, there is less cohesion. This mix is said to be harsh. As there is no bonding between each other, there are chances of bleeding. Hence proper adjustment must be done in the grading of aggregates, to ensure quality in workability and above mentioned factors.
Effect of Maximum Aggregate size on Rheological Properties of Concrete
Larger the size of aggregates, lesser is the demand for fine aggregate. A lesser fine aggregate content will show a reduction in total surface area. This, in turn, will reduce the surface area that must be wetted. Hence there is a reduction in the cement content which is necessary to maintain a constant water cement ratio for the mix.
Effect of Admixtures on Rheological Properties of Concrete
There are many varieties of admixtures that are incorporated into concrete mix designs. These admixtures are chosen based on the requirement and the type of characteristics for which the concrete should perform.
Among different admixtures, those that have a significant influence on the rheology of concrete are:
- Air entraining agents
The above-mentioned admixtures are used for three different characteristics:
- To gain workability without compromising the strength and durability
- To give workability, with small amount of water and providing higher strength
- To give workability as well as strength with a lesser amount of cement content.
When considering normal water cement system. There are particles that are suspended in an electrolyte system, which make the particles to stick to each other. This phenomenon can be named as flocculation.
The addition of admixture will help in the prevention of this flocculation, and hence the flocculated structure, which will bring a change in the attraction of inter-particle to repulsion.
Now this repulsion is created by introducing a membrane around the cement particles, by the plasticizers or the Superplasticizers. The particles with this membrane lose its capability to attract and hence the flocculation is prevented.
This absorbed membrane can vary its thickness based on a number of Superplasticizers. And hence the rate of flocculation can be made to zero. In addition to above phenomena, there are repulsive electrical forces that are generated between the particles. These are created by the absorption of ionized compounds. This will result in an increase of plastic viscosity.
The air entraining agents in the concrete mix will introduce air bubbles, that modifies the surface tension of the aqueous phase of the concrete mix. The air bubbles that are entrained will have a size ranging from 10 to 250 micrometers. This is made happen with the compounds that have a head group which is hydrophilic in nature with a negative charge and a tail which is non-polar and hydrophobic in nature.
The entrained air bubbles will create a bridge between the cement particles, with a negative charge. These air entraining agents significantly affects the rheological properties of the concrete by
- Increasing the cohesion between the particles
- Reduction in chances of bleeding
The accelerators or retarders are incorporated in concrete to vary the setting time depending upon the time required or available for transportation, placing and any other related factors. These bring the change in stiffening properties of the concrete mix. A retarder used to bring a reduction in the workability loss. This brings an increase of setting time.