Rheology of Concrete – Rheological Parameters of Fresh Concrete

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The study of rheology of concrete provides information on properties of fresh concrete such as deformation, behavior of mix, and placement of mixed concrete. Rheology is a term that is mainly used for fluids whose flow properties are complicated in nature, other than fluids like liquids or gases. The term rheology may be defined as the study of the science of the flow and deformation of materials. In the concrete study, the concept of rheology may be applied to analyze the hardened concrete deformation, the behavior of cement paste and slurries, handling and placing of mixed concrete in its fresh state. Hence rheology is applied in all states of concrete (fresh to hardened).

Rheology of Fresh Concrete

When we deal with the rheology of fresh concrete, parameters that are to be considered are stability, mobility and compactability. These are the main factors which measure the suitability of a concrete mix. Now when dealing with rheology, the measurement is more based on stress, strain or rate of strain and the time factors. The stability, mobility and the compactability factors are expressed in terms of the forces or stresses dealing with the concrete mix. These are caused due to the transmission of mechanical stresses within it. The rheology of fresh concrete can be expressed by means of following flow chart in fig.1.

Fig.1. The Rheological Parameters of Fresh Concrete

Stability Parameter in Fresh Concrete Rheology

Stability is the property of a concrete mix when the aggregate particles within the mix possess a homogeneous dispersion and resemble a sampling in a random manner. This property is shown during its conveyance, placement as well as during compaction. Now there are two factors that measure the stability of the mixture. They are

• Segregation
• Bleeding

The segregation can be defined as the phenomenon of the settlement of aggregate mixture in the homogeneous dispersion due to the weak concrete mix. The weak concrete mix is termed to be an unstable mix. Now how to extend the mix can resist the failure through segregation depends upon the cohesion between the individual particles. Segregation is not limited to wet consistencies alone. It can appear in the dry mix too. Now in wet mix, the segregation mainly occurs when the water content level in the mix is such a way that throughout the course of transportation, placing and compaction, the paste cannot hold the aggregate in a well-distributed manner. A lower water-cement ratio results in a crumbly mix that results in dry segregation. This mainly appears during its handling. Now the dry segregation cope up slowly during compaction. During compaction with vibration, they gain fluidity and cohesion with time as well as resistance to shear. Bleeding is a phenomenon of water release when the concrete mix has an unstable mortar. The bleeding has to be under controlled or its chances of occurrence have to reduced.

Mobility Parameter in Rheology of Concrete

The ability of the concrete fluid mix to flow is defined as its mobility. This mobility is happening by the action of mechanical stresses, which is also called as moment transfer. There are many factors that restrict the concrete flow like:

• Cohesive forces
• Frictional forces
• Viscous forces

The adhesive forces between the aggregates and whole matrix result in increasing cohesiveness, which in turn restricts the flow of concrete mix. The cohesiveness is a factor that results in increasing tensile strength of the concrete mix. This parameter as discussed above is a factor that avoids the possibilities of segregation. The viscosity is defined as the resistance to flow. Now this parameter would measure how to extend the concrete mix are movable. This also shows the capability of the concrete mix to arrange them within the matrix (Mostly during the placement of mix in the mould). There won't be any flow under lower stresses with the mixing behavior like a solid. This means it will be having a higher viscosity. With the increase in stress, the strength that binds the matrix together would decrease. This decrease will be insufficient to resist the flow, thereby decreasing the viscous forces within the fluid. Hence the solid behavior is converted into liquid form. The frictional force internally within the mix occurs when the mixture gets displaced. This situation will make the aggregates to translate and rotate. The main governing factors that would help the concrete mix to have resistance against deformation are:

• Shape and Texture of the aggregates used in the concrete mix
• The extent to which the mixture is rich
• Water - cement ratio
• The type of cement

Another factor that governs the mobility of concrete mix is the angle of friction. The triaxial compression test is performed to find the mobility of the concrete mix. Vee - Bee test along with compacting factor test to find the relative mobility of the mix at the site.

Compactability Parameter in Rheology of Concrete

The ease with which the concrete is compacted can be represented by the parameter compactability. The compaction process involves the expulsion of air bubbles within the concrete mix and repositioning the aggregates so that a dense mass is obtained. Care is taken to avoid segregation. Compactability is measured by means of compacting factor test. The method has certain limitations as it sticks to the hopper test apparatus. This test show variation in the result when the mix is high or low workable in nature. The proper measurement of the compaction can be determined by two stages. The first stage of measurement determines the density of the mix in its loose or un-compacted state. Here the mix is simply placed on the hopper without any kind of compaction. The next stage involves the measurement of compacted mix. The mix is placed in three layers, each compacted with a 25mm diameter internal vibrator. The above two values compared with the standard compaction factor test, would give us transmission of the mixture from its loose stage to compacted state. This gives the measure of void content present tin the concrete. The extent of void content will help to give an indication of durability, permeability and the strength of concrete. The concrete mixture is best chosen by knowing the rheological properties of concrete. The test like workability, Vee - Bee and compaction factor tests are found to have limited scope as they measure only a single parameter and are called single point tests.