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
- Effect of Mix Proportions on Rheological Properties of Concrete
- Consistency of Concrete Affecting Rheology of Concrete
- Effect of Hardening and Stiffening on Rheological Properties of Concrete
- Shape and Texture of Aggregates affecting Rheological Properties of Concrete
- Grading of Aggregates influence on Rheological Properties of Concrete
- Effect of Maximum Aggregate size on Rheological Properties of Concrete
- Effect of Admixtures on Rheological Properties of Concrete
Factors Affecting Rheological Properties of ConcreteFactors 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 ConcreteThe 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 ConcreteThe 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 ConcreteFactors 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 ConcreteThe 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 ConcreteA 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 ConcreteLarger 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 ConcreteThere 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
- 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.
- Increasing the cohesion between the particles
- Reduction in chances of bleeding