Fresh Concrete And Workability

What is Fresh concrete?

Fresh concrete or plastic concrete is a freshly mixed material which can be molded into any shape.

Workability

 A concrete is said to be workable if it is easily mixed, transported, placed, compacted and finished without any segregation”. Workable concrete is the one which exhibits very little internal friction between the particle and or which overcomes the frictional friction resistance offered by the formwork surface or reinforcement.

Factor affecting workability

          The factors helping concrete to have a more lubricating effect to reduce internal friction for helping easy compaction are given below.

      a) Water Content

      b) Mix Proportion

      c) Size Of Aggregate

      d) Shape Of Aggregate

      e) Surface Texture Of Aggregate

      f) Grading Of Aggregate

      g) Use Of Admixture

a) Water Content:

          The water content in a given volume of concrete will have a significant influence on the workability. The higher the water content per cubic meter of concrete, the higher will be the fluidity of concrete, which is an important factor affecting workability. 

At the work site, the supervisor who is not well versed with the practice of making good concrete, resort to adding more water for increased workability. This practice is often resorted to because this is one of the easiest corrective measures that can be taken at the site, but with an increase in water content strength of concrete decreases.

B) Mix proportion

                 Aggregate/cement ratio is an important factor influencing workability. The higher the aggregate/cement ratio, the leaner is the concrete. In lean concrete, less quantity paste is available for providing lubrication, per unit surface area of aggregate & hence the mobility of aggregate is restrained.

On the other hand, in case rich concrete with lower aggregate/cement ratio, more paste is available to make the mix cohesive & fatty to give better workability.

c)Size of aggregate

                The bigger the size of the aggregate, the less is the surface area and hence less amount of water is required for wetting the surface and less matrix or paste is required for lubricating for wetting the surface to reduce internal friction. For a given quantity of water and paste, bigger size of aggregate will give higher workability.

d)Shape of aggregate

                  The shape of aggregate influences workability in good measure. Angular, elongated or flaky shaped aggregate makes the concrete very harsh when compared to rounded aggregate or cubical aggregate.

Contribution to better workability of rounded aggregate will come from the fact that for the given volume or weight it will have less surface area & less void than the angular or flaky aggregate. Not only that being round in shape, but the frictional resistance is also greatly reduced.

 e)Surface texture

                 The influence of surface on workability is gain to the fact that the total surface area of roughly textured aggregate is more than the surface area of smoothly rounded aggregate is more than the surface area of a smoothly rounded aggregate of the same volume. 

From the above discussion, it can be inferred that roughly textured aggregate will show poor workability & glassy textured aggregate will give better workability. A reduction of interparticle frictional resistance offered by smooth aggregate also contributes to higher workability.

f)Grading of  Aggregate

                  This is one of the factors which will have maximum influence on workability. A well-graded aggregate is the one which has the least amount of voids in a given volume. Other factor being constant, when the total voids are less , the excess paste is available to give lubricating effect.
With an excessive amount of paste, the mixture becomes cohesive & fatty which prevent segregation of particles . The aggregate particle will slide past each other with the least amount of compacting effort. The better the grading, the less is the void content & higher the workability.

g)Use of Admixtures

             Of all the factor mentioned above, the most important factor which affects the workability is the use of admixture. Plasticizers & super plasticizers greatly improve the workability. Use of air entraining agent being surface active, reduces the internal friction between the particles.

Measurement of workability

               The following test were conducted to measure workability

  Slump Cone Test

  Compaction Factor Test

  Flow Test

  Kelly Ball Test

  Vee Bee Consistometer Test

    

      SEGREGATION

Segregation of concrete is the separation of ingredients of concrete from each other.

In good concrete, all concrete aggregates are evenly coated with sand and cement paste and form a homogeneous mass. During handling, transporting and placing, due to jerks and vibrations the paste of cement and sands gets separated from the coarse aggregate. 

If concrete segregates during transit it should be remixed properly before depositing. However, a concrete where initial setting time is over, should not be used.

           The separation can be of three types as mentioned below.

1. Coarse aggregate separating out or settling down from rest of the matrix (Type-1 segregation)
2. The paste separating away from the coarse aggregate (Type-2 segregation)
3. Water being lowest in specific gravity, separating out from the rest of the material (Type-3 segregation, called Bleeding)

    Causes of Segregation of Concrete: 

     1) Use of high water-cement ratio in concrete. This general happens in case of concrete mixed at site by unskilled workers.

    2) Excessive vibration of concrete with mechanical needle vibrators makes heavier particles settle at bottom and lighter cement sand paste comes on top.

    3) When concreting is done from height in case of underground foundations and rafts, which causes concrete to segregate.

     4)Transporting concrete mixes for long distances.

     5)Poorly proportioned mix, where sufficient matrix is not there to bind the aggregates

     6)Dropping concrete from more than 1.5m.

    Prevention of Segregation of Concrete:

Wherever depth of concreting is more than 1.5 meters it should be placed through temporary inclined chutes. The angle of inclination may be kept between 1:3 and 1:2 so that concrete from top of chutes travels smoothly to bottom. The delivery end of chute should be as close as possible to the point of deposit.

Vibration of concrete is one of the important method of compacting. It should be remembered that only comparatively dry mix should be vibrated. The vibration is continued just for required time for optimum results. If the vibration is continued for long time, particularly in too wet mix, it is likely to result in segregation due to settlement of coarse aggregate.

            While finishing concrete floors or pavement, with a view to achieve a smooth surface, masons are likely to work too much with the trowel or float immediately on placing concrete. This immediate working on the concrete on placing , is likely to press the coarse aggregate down, which results in the movement of excess of matrix or paste to the surface.

            The tendency of segregation of segregation can be remedied by correctly proportioning the mix , by proper handling, transporting, placing, compacting and finishing.
 

                  Bleeding

Bleeding is a form of segregation in which water present in the concrete mix is pushed upwards due to the settlement of cement and aggregate. The specific gravity of water is low, due to this water tends to move upwards. Bleeding ordinarily occurs in the wet mix of concrete.

The Prime factor for bleeding in concrete is the high dosage of Water cement ratio. Higher water-cement ratio weakens concrete and leads to excessive bleeding.

The bleeding in concrete is not harmful if the rate of evaporation of water is equal to the rate of bleeding. Normal bleeding is quite good, and it enhances the workability of concrete.

When the concrete is fully plastic, bleeding may not cause much harm. However, concrete is still in the plastic stage later it is subsidized and compacted.

          How bleeding effects concrete stability

1. 1) As mentioned above, water move upwards in bleeding, sometimes with this water certain amount of cement moved along with water to the concrete surface. When the top surface is worked up with the trowel, the aggregate goes down and cement paste forms at the top surface this is called ‘Laitance in concrete.’ Due to the formation of Laitance, structures may lose its wearing capacity and decreases its life.
2. 2) Water while moving from bottom to the top, forms continuous channels. Due to this channels, concrete becomes permeable and allow water to move, which  forms water voids in the matrix and reduces the bond between aggregate and the cement paste.
3. 3) Forming of water at the top surface of concrete, results in delaying the surface finishing. 
4. 4) Concrete becomes permeable and loses its homogeneity. 
5. 5) Excessive bleeding breaks the bond between the reinforcement and concrete.  

      How the bleeding in concrete is controlled:-

1. 1) Bleeding in concrete is controlled by Adding  minimum water content in the concrete mix. 
2. 2) Encouraging the use of air en-training admixtures in the mix. 
3. 3) By adding more cement in the mix.