How to do concrete sampling and cube casting

Dnyan Deshmukh

Honorary Member
Staff member
Sep 7, 2017
2,048
3,505
Pune
How to do concrete sampling and cube casting
If sampling and cube casting goes wrong, end result of it goes wrong and can create a panic.

Concrete cube casting is most important activity as we judge the concrete in place by testing the cubes casted for it.

Sample concrete can be taken from following locations for cube casting

At Transit mixer – when concrete is unloaded to placing location / concrete pump hopper through transit mixer. You can take direct sample concrete from the transit mixer in a tray or in wheel barrow.
The sample concrete should be taken such a way that, it represents total concrete of transit mixer i.e. – take at start of unloading, take another when its half unloaded and take the sample when it is almost to empty.
Mix this concrete in a tray or wheel barrow to get uniform homogeneity in mix.

At Placing location – mostly in case of pump concrete and in slabs, where you choose to take sample at placing location. When the concrete is pumped and poured in slab, you need to take sample from various locations from poured concrete quantity i.e. – take some from center, middle and from end portions of poured concrete.
Mix this concrete in a tray to get uniform homogeneity in mix.


At Production Location - This is mostly in cases where weight batcher is installed at site to produce small amount of concrete for column and wall casting. You can take sample from mixer drum same way as described for Transit Mixer.

It is important to cast the cubes of concrete when its apparent slump is around 100 to 120 mm.

The cubes sizes are small compare to actual structure sizes and hurdles in pouring for which the extra slump is designed. Waiting for drop in slump does not actually modifies the property of concrete as the slump gain was due to addition of admixture and with time the cells are breaking down to allow cement to react with water.

If you fill the cubes at higher slump, chances are more to get aggregates settle down at bottom and the slurry comes at which will leads to weaker layer in cube and easily gets peel off under compression and indicates failure in taking further load.

If you cast cubes at very stiff or at lower slumps, it will lead to improper cube casting due to more efforts required for compaction and also the de-moulded cubes will scare you by its finish faces.

From each sample, 3 cubes need to be casted as per following procedure;

The cubes are generally of 150mm, and casted in 3 layers.

Cubes should be properly oiled to ensure easy de-moulding, the oiling should be such a way that, oil should not be accumulate or visible in thick formation. Very thin light coat of oil should be applied on it.

Do check dimensions of cubes for its correctness, it is one of the culprit in failures or boosting strength parameter.

The sample concrete should be poured in 3 cubes up to 1/3rd height.

It should be compacted with 16mm round tip rod by tamping it for 35 to 40 times.

Then next layer should be filled in all three cubes and again tamp for 35 to 40 times.

Then remaining layer should be filled and tamped same way.

After tamping / compacting each layer – do insert trowel from face of mould and give some jerks on each face. Also tamp mould from outside. This ensure smooth finish of cubes and escaping of entrapped air at faces.

The top layer should be filled slightly more in order to compensate shrinkage and allowing better finish.

Cubes top should be finished smooth and emboss with details of concrete by any sharp tool. The minimum following details should be written on it

Grade of Concrete
Location
Date
Cube Number

In above details cube number is most important detail, if written improperly. You can’t do statistical analysis as you will not be able to find actual root cause of increased and decreased strength variation.

You can also stick a paper note on it with all details, if its late night work and does not get time for embossing details on it (paper will also be remaining there without getting damaged, even if we take out cubes after 28 days from curing tank).

Some photos in action
Tamping of concrete layer
Tamping.jpg

Jerking cube from out side to release entrapped air.
Jerking.jpg

Inserting trowel on the face of mould to ensure smooth finish and releasing entrapped air.insert trowel.jpg

Finishing of cube top surface.
finishing.jpg

After 16 to 24 hours of cube casting, it can be de-moulded and put into curing tank for curing by recording details of it.

For sampling frequency, you can follow any standards or you can make your own.

IS 456, Do suggest following frequencies of sampling. It also states, in all cases only 28 day’s cube results are considered for acceptance or rejection of concrete (Better to have you own set standards for frequency of sampling, more samples lead to more authentic data on concrete properties)

For concrete quantity between 1 to 5 cum – 1 sample should be taken

For concrete quantity between 6 to 15 cum – 2 sample should be taken

For concrete quantity between 16 to 30 cum – 3 sample should be taken

For concrete quantity between 31 to 50 cum – 4 sample should be taken

51 and above – 4 plus one additional sample for each additional 50 cum.
 

Shubham Gomare

Junior Member
Mar 1, 2018
26
18
Pune
sir,
First of all thanks for such nice information & arranging them in very good manner
My question is many times in case of flow concrete we didnt get the required flow though we had used same proportion as mentioned in CMD. So is it good to add some extra admixture, what is the best way to solve this condition
 
  • Like
Reactions: Sujit

Mehboob

Junior Member
May 29, 2018
33
2
Pune
Thank you so much sir. I have a doubt about tamping.sir it is 25 blows per layer as per procedure? Pls answer the query sir
 

Mehboob

Junior Member
May 29, 2018
33
2
Pune
But as per IS 1199-1959, 25 strokes why is this difference, n why 35or25 strokes why not 30 or 40 pls explain anyone pls Dynan sir
 

Shubham Gomare

Junior Member
Mar 1, 2018
26
18
Pune
But as per IS 1199-1959, 25 strokes why is this difference, n why 35or25 strokes why not 30 or 40 pls explain anyone pls Dynan sir
For slump cone 25 no of blows per layer and cone should be filled in 4 layers.
For preparation of specimen for compression test mould should be filled in 3 layer and no of blows should not be less than 35 per layer

If you read carefully (IS 516 clause no 2.10.1) it is clearly said that no of blows should not be less than 30 means u can give more also purpose of doing this is concrete in mould must get properly compacted
 

Shubham Gomare

Junior Member
Mar 1, 2018
26
18
Pune
[
For slump cone 25 no of blows per layer and cone should be filled in 4 layers.
For preparation of specimen for compression test mould should be filled in 3 layer and no of blows should not be less than 35 per layer

If you read carefully (IS 516 clause no 2.10.1) it is clearly said that no of blows should not be less than 30 means u can give more also purpose of doing this is concrete in mould must get properly compacted
Dnyan sir please correct me if I'm wrong
 

Mehboob

Junior Member
May 29, 2018
33
2
Pune
For slump cone 25 no of blows per layer and cone should be filled in 4 layers.
For preparation of specimen for compression test mould should be filled in 3 layer and no of blows should not be less than 35 per layer

If you read carefully (IS 516 clause no 2.10.1) it is clearly said that no of blows should not be less than 30 means u can give more also purpose of doing this is concrete in mould must get properly compacted
You are 100% correct but my question is why 35 blows only
 

Dnyan Deshmukh

Honorary Member
Staff member
Sep 7, 2017
2,048
3,505
Pune
You are 100% correct but my question is why 35 blows only
Because it is stated in IS code.

We refer it to follow a basic guidelines.

You can change that value to single tamp or say 25 or any number you wish base on your mix design and R&D done by you.

If you are not sure about it, then you can always follow the basic guidelines.
 
  • Like
Reactions: Mehboob

Mehboob

Junior Member
May 29, 2018
33
2
Pune
Because it is stated in IS code.

We refer it to follow a basic guidelines.

You can change that value to single tamp or say 25 or any number you wish base on your mix design and R&D done by you.

If you are not sure about it, then you can always follow the basic guidelines.
Got it thank you so much sir
 

Gokulnathitd

Newbie
Dec 10, 2018
3
0
Tamilnadu
Dear sir ,
If I am doing pile concrete , there will be 160mm to 180 mm slump at site. Can I cast cube on later?
Or slump will affect the cube casting.
 

Dnyan Deshmukh

Honorary Member
Staff member
Sep 7, 2017
2,048
3,505
Pune
Dear sir ,
If I am doing pile concrete , there will be 160mm to 180 mm slump at site. Can I cast cube on later?
Or slump will affect the cube casting.
Best way of casting cube is, by reducing the slump of concrete and making it between 80 to 120 mm.

why we should keep sample in tray / wheel barrow / any equipment which can hold sample to reduce the slump of concrete?

The slump of the concrete which is achieved is not by adding water in concrete but by dosing admixtures in it to improve the performance of it, in terms of workability.

Technically, keeping sample in a tray or wheel barrow and waiting for reduction in effect of admixture performance does not cause any issue or affect the quality of concrete.

so if my concrete is of 200 mm slump and i took a sample and waited for some time after which slump is reduced to 80 or 120 mm, does not make any difference in properties of concrete other than workability. as we had admixture in concrete whose effect got reduced over time and made the workability to go down does not make any change in properties of concrete, its still same but required much more efforts for placing and compaction if we need to pour low slump concrete in actual structure.

Concretes are almost designed with slump in mind of 0 to 30 or 30 to 80 mm (when we test the mix without using admixture for increasing performance), so if you produce same concrete without adding admixture into it, you will find, the concrete is not only unworkable, but cant even be loaded into transit mixers from RMC Plants.

The admixtures are playing very important role in Today's produced concrete and achieving very less water cement ration to get more out of concrete with optimum consumption of cementitious material.

Now coming back to your point, why i should not cast the cube at high slump like 180 or 200 mm or should we do casting of cube at that slump.

When it comes to slump / workability of the concrete - this factor is purely dependent of providing comfort level in placing and compaction for workmens. and achieving defect free concrete structure by ensuring passing of concrete in all congested areas.

When it comes to concreting, the sample concrete and concrete poured in structure is same.
But the shape and hurdles in structure are completely different from the concrete cube moulds.

Pouring the same type thing in that two environment can produce a different results, for example

structure may not be have slurry on top and aggregates settled down at bottom, but if we placed concrete with high slump in cube moulds, it will lead to slurry at top and aggregates gone at bottom leading to artificial failure of cube, which indicating wrong scenario than the actual concrete.

so it is always better to reduce the slump of the concrete which is going to be poured in cube moulds.

Does reduced slump of concrete, increases the strength of the concrete?
Absolutely not, because you waited to let admixture effect get over and not played with water cement ration.

Cube casted with slump between 80 to 120 may look like you are getting higher results for concrete, but they are actually a correct result for the concrete designed with parameters in mind.

if you are producing concrete without using admixture, then you can cast cubes directly with produced concrete.
If concrete is produced without using admixture and you found slump above 120 mm then either its a mockery of concrete mix design or you are really consuming high quantity of cement.

so best way of casting a cube is wait for reduction in slump of concrete, where slump is achieved by dosing admixture in it, high slump may lead to settlement of aggregates at bottom and slurry at top, producing wrong result of the concrete.
 
  • Like
Reactions: M_U_K_U_L

shafi12344

Apprentice
Jan 17, 2019
10
2
duabi
As Per BSEN ( BS ) for making 100 mm cube 3 layer 25 blows and 150 mm cube 3 layers 35 blows , Tamping rod using for making cube in this picture is wrong according to BSEN(BS)
 

Dnyan Deshmukh

Honorary Member
Staff member
Sep 7, 2017
2,048
3,505
Pune
As Per BSEN ( BS ) for making 100 mm cube 3 layer 25 blows and 150 mm cube 3 layers 35 blows , Tamping rod using for making cube in this picture is wrong according to BSEN(BS)
The rod shown in picture is correct and i think you haven't work at construction project or lab yet.
For your kind information, those photos are taken at India's one of the top Most company and biggest brand which does not even required introduction and its MNC, though i never worked there but got chance to visit there Project and have some snaps.
 

shafi12344

Apprentice
Jan 17, 2019
10
2
duabi
Workability of the concrete will be change according to the location where the concrete is poured or depends up on the structure for example if you poured the concrete in the slope, Low slump concrete will be used , if you poured concrete in a slab of a bridge (more than 1.5 m meter ) should use high Slump concrete (200 +/- 25 or 175 +/-25 , 150+/-25) even in pile also , etc.... . If you use low slump concrete in a pile or retaining wall or slab of the bridges (will change depends on height ), segregation, Honeycomb etc .. like these problem want to face in the site.

Sampling (Casting Cube, cylinder or beam) is a very important process while concreting because samples we are talking for testing is the representative of the concrete poured in the site , it means at which condition you pored the concrete in the site at that condition you should take the sample, once you take sample form the truck or mixer , should finish the entire processes (From temperature to casting cube ) with in 15 minutes (no Standard specifies that, to reduce slump of concrete for sampling (Casting Cube, cylinder or beam) . Slump will not affect the strength of the concrete. If the follow the mix design properly while producing the concrete and the sampling procedure is correct, will achieve the design strength ).

These Samples will use for the further study of the poured concrete in the site (Strength, Durability, so and so )

To perform Slump test 3 layer 25 blows According to international standards (ASTM AASHTO, DIM, BSEN like that). i am not aware of the INDIAN standard
 

Sbanary

Apprentice
Jan 17, 2019
14
6
India
Minimum cylinders should be 6 nos
2 cylinder - 7 days
2 cylinder - 14 days
2 cylinder - 28 days

What will happened if you take 1 sample and it's failed how you will verify the concrete strength.. Later.. You need to break the concrete. So it's safer to take minimum of 6 nos cylinder and 7,14,28 days crush test.