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Rheology of Concrete- Factors Affecting Rheological Properties of Concrete

WHAT IS RHEOLOGY OF CONCRETE?


The Rheology may be defined as the science of the deformation and flow of materials and is concerned with the relationships between the stress, strain, rate of strain and Time.
The rheological principles and the techniques as applied to concrete include the deformation of hardened concrete, handling and placing of the freshly mixed concrete and the behavior of its ingredients.

The rheology of fresh concrete like workability include the parameters of stability, mobility, and compatibility, which are necessary to determine the suitability of any concrete mix. For the assessment of the rheological behavior of concrete, these parameters are redefined in terms of forces involved in the transmission of mechanical stresses on the concrete.
The fresh concrete is subjected to normal and shearing forces during its handling and placing.


FACTORS AFFECTING RHEOLOGICAL PROPERTIES OF CONCRETE


The rheological properties of concrete are affected by the mix proportions, i.e. the amount of each constituent, the properties of the ingredients, the admixture content, the mixing amount and the time after mixing. The factors affecting the rheological properties of the concrete are explained below:

  • 1.      Mix Proportion of Concrete Affecting the Rheology
  • 2.      Consistency of Concrete Affecting the Rheology
  • 3.      The hardening and Stiffening of Concrete Affecting the Rheology
  • 4.      Shape and Texture of Aggregate affecting the Rheology
  • 5.      The Grading Aggregate Affecting Rheology
  • 6.      Maximum Aggregate Size Affecting the Rheology
  • 7.      Admixtures in Concrete Affecting the Concrete Rheology


Mix Proportion of Concrete Affecting the Rheology


The concrete mixture is proportioned so as to give the desired workability during construction and to assure that the hardened concrete will have the required performance characteristics. The concrete mix that has coarse aggregate in excess amount will have enough amount of mortar in order to fill the void system. This will result in the loss of cohesion and mobility. This mix is considered harsh and requires a large amount of effort to place and compact the mix. One of the reasons behind the same will be low air content. An increase in entrained air can also alleviate the excessive use of fine aggregate. This increase in entrained air or the use of excessive fine aggregate will result in the mix to be harsh and difficult in the compaction of the mix. A higher amount of fine aggregate demands large cement because of the greater surface area. So more cement is required in order to obtain the desired mobility. This might result in the increase in the drying shrinkage and then crack.
In practical conditions, the fine aggregate and the cement is proportioned in an extra amount so that the desired mix criteria is obtained.

Consistency of Concrete Affecting the Rheology


The consistency of the concrete, as measured by the slump test is a relative indicator of the relative water content in the concrete mix. An increase in the water content or the value of the slump above the desired amount in order to achieve a workable mix help to produce fluidity and also decrease the internal friction. The reduction in the cohesion within the mixture increase the potential for segregation and the excessive bleeding. So it is observed that the water content more than that is required will not bring any improvement in the rheological properties. Bringing a lower slump value or water content will reduce the mobility and the compatibility which will minimize the difficulties that are concerned with the placement and the consolidation.
When considering the rheological properties, an increase of 1% of air entrainment will result in an equivalent increase of 1% fine aggregate which is an increase of water content by 3%. A dry mix will result in the loss of cohesion and increase in the dry segregation.

The Hardening and Stiffening Of Concrete Affecting The Rheology


There are many sources in order to increase the hardening of the cement concrete. Use of elevated temperatures or accelerating admixtures, or cement deficient gypsum or the rapid hardening cement can help in hardening of the cement. These methods will reduce the mobility of the concrete. The use of dry and porous aggregate will rapidly increase in the workability by absorbing the water from the mixture or by the increase of surface area that is required to be wetted.

Shape and Texture Of Aggregate Affecting The Rheology


The rheology of concrete is greatly influenced by the shape and texture of the aggregate used in the concrete mix. A Higher percentage of voids are formed by the use of rough and angular fine aggregates. These voids will hence demand large fines to fill them and asked for higher water content. The angular fine aggregates will increase the internal friction in the concrete mixture. Angular aggregates will ask for large water content compared with the well-rounded natural sand.

The Grading Aggregate Affecting Rheology



Good workability is obtained with the help of well-graded aggregates. If there is any absence of aggregates i.e. The gap graded system or there is no systematic change in size distribution, the workability of the mix will be affected greatly. This effect is more shown in the fine aggregates than in coarse aggregates. The increase in the fine aggregate increases the need for water that will make the concrete mix sticky. Fine aggregates to coarser ones will reduce the cohesion making the mix harsh. This will increase the chances of bleeding. In order to maintain workability, it is necessary to adjust the grading of the fine aggregates.

Maximum Aggregate Size Affecting the Rheology


The increase in the maximum size of aggregates will reduce the fine aggregate content that is required in order to maintain the workability. This will reduce the surface area that is to be wet. This will hence increase the cement content for a constant water-cement ratio.

Admixtures in Concrete Affecting The Concrete Rheology


Among different admixtures that is used, the most important admixtures that will affect the rheology are the plasticizers and the super plasticizers, air entraining agents, retarders and the accelerators. These admixtures can be used in three ways:

  • 1.      To increase the workability by maintaining the strength and durability in long term basis
  • 2.      To give workability with lesser amount of water without compromising the strength
  • 3.      To gain workability and strength with a lesser amount of cement content, this should also link with the durability considerations.
The plasticizers that are available commercially bring a reduction of water content by 10% without bringing any kind of detrimental effect.


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