Monday, November 18, 2013

Compaction Factor Test for Fresh Concrete

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  • Aim: To find out the compaction factor for the given fresh mix of concrete.


  • Theory: Workability of concrete is the ability/ease with which concrete can be mixed, transported and placed. This is a major factor which contributes to the other properties of concrete also. If concrete is workable enough then it can be compacted with less compacting effort.

 So there is a relation between the amount of work required to compact a given fresh concrete and the work-ability of the concrete. This relation is well suited for the concrete of the low water cement ratio. Slump cone test is also used to find out the work-ability of the concrete but only recommended for the concrete of higher work-ability. For less workable concrete(having less water cement ratio), compaction is standardized by various standards. 
  • Apparatus: Compaction factor apparatus/machine, weighing balance, cow/mixing tray.

Compaction Factor apparatus


  • Procedure:
1. Oil the inner sides of the top and bottom cone frustum.
2. Prepare a M15 concrete mix(1:2:4) using a water content of about 0.4-0.6 by weight of cement.
3. Fill the top cone frustum with the fresh mix of concrete and then open the gate and let the concrete fall on the second frustum.
4. Open the gate of second frustum and let it fall on the cylinder to fill up to top of cylinder. 
5. Measure the weight of the cylinder filled with the concrete(partially compacted) and then empty this.
6. Again fill the cylinder with the same sample of the concrete but this time, do the compaction using the mechanical vibrator to do the compaction. 
7. Again measure the weight of the cylinder filled with the concrete(Fully compacted).
8. Clean all the apparatus and put them at their places as before.

  • Calculations: 
Let weight of the empty cylinder = W1
Let weight of the cylinder with the partially compacted concrete = W2
Let weight of the fully compacted(Using mechanical vibrator) concrete with cylinder = W3
Compaction Factor = Weight of the partially compacted concrete/ Weight of the fully compacted concrete
i.e., Compaction factor = (W2-W1)/(W3-W1)


  • Result: The compaction factor of the given fresh concrete mix is .....
(Note: Relation between the compaction factor and work-ability is that higher the compaction factor higher is the work-ability. Theoretical maximum value of the compaction factor can be 0.96 to 1.0)


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Saturday, November 9, 2013

Concrete Slump Test


Aim: 

To determine the slump of a given fresh concrete mix sample.

Theory:

 Work-ability is the ease with which the given concrete mix can be mixed together, transported to the application place and can be placed/applied/compacted their within the initial setting time of cement. This property depends largely on the amount of water added i.e. water cement ratio of the concrete mix and kind of aggregates used. Generally work-ability increases with the addition of the water, however it results in the less strength of concrete. So we have to settle for an intermediate value of water cement ratio at which it may have the sufficient work-ability as well as sufficient strength.

Apparatus: 

Slump cone apparatus, cow tray

Slump Cone Apparatus:
This is the standard apparatus used to determine the slump of the  given fresh concrete. This is a metallic, conical shaped, of height 30 cm, 20 cm, and 10 cm as the larger and smaller diameters respectively at the far two ends. This is provided with two handles to lift it above the base.
Cow tray: This is used to mix the given materials in the given proportions (Water, Cement, Sand and coarse aggregates).


Procedure: 


  •  Oil the inner side of the slump cone apparatus(so that concrete do not stick to its sides)  and put it on the table/floor with the larger diameter base at bottom. 
  • After mixing the concrete,fill the slump cone with the freshly prepared concrete mix, compacting this in 3 layers, each layers tamped with the tamping rod giving 25 no. of blow. 
  • Remove the extra concrete overflowing from the top edge of the cone with the help of a strike.
  • Hold the two handles of the cone in hand and then lift the cone up leaving the concrete on the base. 
  • After the cone support is removed, concrete will fall under its own weight. The initial height of the concrete cone was 30 cm and after the fall, you have to measure the amount of fall from its top with the help of a scale.


Calculations: 

Initial height of the concrete = 30 cm
     Height after lifting the cone = h2.
 slump = 30 - h2 (cm)


Interpretations of Results: 

Types of slumps

  • Slump of concrete is interpreted as collapse slump, shear slump and true slump. 
  • If it is collapse slump then the concrete is too wet, and shear slump also indicates a poor mix, concrete of true slump is of value. 
  • A concrete of slump value 0 to 25 mm is used in the road mixes, 10-40 mm mixes are used in the foundations with light reinforcement, and 50-90 mm used for normal concrete works compacted with vibrations, slumps .10 cm are considered of higher work-ability.

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