Concrete Cube Casting Procedure and frequency | Sampling of Concrete

Concrete Cube Casting Procedure and frequency is required to achieve Compressive strength result of concrete, Sampling of concrete and Concrete cube casting shall be done as per IS Code. So we described here how to the sampling of concrete and concrete cube casting procedure and frequency as per IS Code.

 

What is concrete cube?

Concrete cubes are the representative of those concrete structure for which we are casting cubes. We conduct compressive strength test on these concrete cubes and we get compressive strength value what required for a related concrete structure.

 

Why Concrete Cube Casting required?

As we described, concrete cubes represent the compressive strength of related structure from which we have taken a concrete sample and cast cubes. So that we cast concrete cubes.

 

Sampling of Concrete procedure IS Codes

IS Code 1199 for concrete sampling.

 

Concrete Cube Casting Procedure IS Codes

IS Code 516 for Concrete Cube Casting.

Concrete Cube Casting Procedure and frequency

 

Concrete sampling procedure for cube casting

Sampling of concrete for any grade shall be truly representative of the same mix. 

Concrete sample shall not be less than 0.02 cubic meter in volume. Concrete mix sampling shall be done from various points in the batch.

 

There are two ways of concrete sampling

1. Concrete Sampling from transit mixture

Take a minimum of three concrete samples while discharging, about the time one quarter, one half and three quarter.

 We should not take a concrete sample from the first batch while discharging. 

We should take concrete mix sample on a non-absorbent tray.

 

2. Sampling of concrete at the time of placing

We should take a concrete sample while placing and collect from at least five well-distributed positions by avoiding segregation. We should not take sample from the edge.

 

Concrete Cube Casting Apparatus and Tools Required

  1. Concrete cube moulds of size ( 150 mm * 150 mm * 150 mm )
  2. Temping rod ( length – 600 mm, dia – 16 mm )
  3. Trowel or scoop
  4. Non-absorbent tray ( capacity minimum 0.02 cubic meter of concrete )

 

Precautions Before Concrete Cubes Casting

  1. Use only clean concrete cube moulds otherwise clean it before use.
  2. Ensure that all cube mould’s screw is properly tight.
  3. Apply de-shuttering oil/agent in the inner surface of cube moulds.

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Concrete Cube Casting Procedure

  1. Take the concrete sample as we described before.
  2. Mix concrete sample to avoid any segregation.
  3. Concrete cube shall be cast in three approximately equal layers.
  4. Start pouring the concrete mix in three cube moulds up to 50 to 60 mm height for the first layer.
  5. Start temping minimum 35 times maximum 50 times by temping rod in a uniform manner over the cross-section of the mould.
  6. Then pour the second layer approximately 50 mm and temp between 35 to 50.
  7. While temping, temping rod should not insert the previous layer.
  8. The same procedure shall be repeated for the third layer.
  9. Now the top surface of the concrete shall be finished with a trowel or English trowel.

Concrete Cube Casting Procedure and frequency, sampling of concrete

  1. If any trace or gap formed by temping rod, struct on the side of cube moulds to avoid surface voids too.
  2. Place a glass plate or metal plate on the top of concrete cubes to stop moisture evaporation.
  3. If you are using a vibration table for cube compaction, the time for vibration should not more than one minute for each layer.
  4. After casting, concrete cubes shall be placed on vibration-free place or firm ground.
  5. Temperature and Humidity shall be maintained where concrete cubes are placed after casting,

temperature 27 +/- 2 degree centigrade, related humidity 90 percent.

  1. After 24 hours, de-mould cubes, the marking shall be done and shift the concrete cubes in cube storage tank.
  2. The temperature of cube storage tank water shall be 27 +/-2 degree centigrade.

 

Concrete Cube Sampling Frequency

Minimum 03 concrete cubes shall be cast for each transit mixer or as per concrete structure quantity.

 

Concrete Cube Casting Frequency as per IS Code

Quantity of Concrete (cum)Number of cube required for 28 days
1 – 53
6 – 156
16 – 309
31 – 5012
51 – 10015
101 – 15018
151 – 20021

(Mentioned in MORTH 5th revision, section 1700.)

for each 50 cum increment, o3 concrete cube shall be increased.

1 Sample = 03 cubes, This concrete cube frequency for only 28-day compressive strength.

If 03 days or 07 days compressive strength required, additional concrete cubes shall be cast as per engineer’s instructions.

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Viva Questions on Sampling of Concrete, Frequency and Cube Casting Procedure

Q.1  What is the minimum concrete quantity requirement for cube casting?

Ans. We have to take a minimum of 0.02 cum of concrete for concrete cube casting purpose.

 

Q.2  Why are only 35 strokes required each layer to cast the cubes? ( I faced this question in three interviews.)

Ans. In IS code (IS 516)  mentioned that minimum 35 strokes required for each layer, minimum not maximum.

        So we can give more than 35 strokes per layer depend on our concrete mix grade and concrete quality.

For example:

       If your concrete is cohesive and mixed well then during cube casting it’s enough to stroke 35 blows ( how you will know).

See carefully while temping throughout the mix with temping rod in cube, when coarse aggregate going to sink or disappear in concrete mortar, that’s the point to stop temping. 

And it takes minimum 35 strokes. Indian standard code writers are brilliant to understand that.

So now if our concrete mix is not so cohesive, low slump or not well-graded, in that case, it can take up to 40 or 45 strokes for each layer during cube casting.

 You will be shocked if I will show you one of my experience

 

We casted three cubes with M60 grade concrete.

For the first cube, we applied 40 strokes in each three-layer.

For the second cube, we applied 60 strokes in each three-layer

and for the third cube, we applied 80 strokes in each three-layer.

Let’s see the result

concrete cube casting

After 5 days we got this result.

As you can see, by increasing strokes in cube we are getting less value of compressive strength.

 

Any other question? just ask in comments. We are waiting.

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Read more:

Compressive Strength of Concrete at 7 days, 28 days Calculation & Formula
Concrete Meaning, Properties, Strength, Advantages & Construction
M30 Grade of Concrete Mix Design Calculation Procedure
Concrete Mix Design Calculation for M40, M60 as per IS Code

 

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