How to do concrete sampling and cube casting

[sachin0102]

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.

Sir,
Can like cubes, aggregate Settle in structure components also??

 

[AjithS]

Wht is Accelrated curing of cube ... Nd how to get early strenght of cube?

Another name Boiling water method

 

[SANJAY K Pandey]

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

Workability of concrete is depends on size and density of reinforced steel in RCC member. Using the high slump concrete in slab is not depend on thickness of slab. Slump of 200 mm is very high and will be not good for durable concrete. Slump is based on W/C ratio and no case W/C ration should more the 0.45.

 

[krishnareddy1039]

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

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
View attachment 1656

Jerking cube from out side to release entrapped air.
View attachment 1657

Inserting trowel on the face of mould to ensure smooth finish and releasing entrapped air.View attachment 1658

Finishing of cube top surface.
View attachment 1659

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.
[/QUOTE

If we cast 50 cum of concrete do we have to take 1+2+3+4 samples or only 4 samples please clarify?

View attachment 1656

Jerking cube from out side to release entrapped air.
View attachment 1657

Inserting trowel on the face of mould to ensure smooth finish and releasing entrapped air.View attachment 1658

Finishing of cube top surface.
View attachment 1659

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.

 

[krishnareddy1039]

If we cast 50 cum of concrete do we have to take 1+2+3+4 samples or only 4 samples please clarify?

 

[krishnareddy1039]

If we cast 50 cum of concrete do we have to take 1+2+3+4 samples or only 4 samples please clarify?

 

[Dnyan Deshmukh]

If we cast 50 cum of concrete do we have to take 1+2+3+4 samples or only 4 samples please clarify?

Its really very confusing to understand and make use of IS Codes, but IS codes are just basic guidelines to make sure you do everything correctly.

Whatever written by me is as it is from IS 456 and i too confuse on the same due to last line 51 and above – 4 plus one additional sample for each additional 50 cum.

It makes it difficult to understand the whole meaning of whatever written above. else someone would have conceived it as 1st sample, 2nd sample, 3rd sample and 4th sample but last line put a full confusion.

Again if you read a note in same table, it says one sample for one shift at RMC or concrete production unit as minimum frequency.
it also state frequency can be as agreed between both parties.

so that is the reason i mentioned in first post.

(Better to have you own set standards for frequency of sampling, more samples lead to more authentic data on concrete properties)

You need to set your own frequency for sampling of the concrete base on your site specific requirement which are mostly a balanced concrete mix design.

Say, you want to test 3 days strength for de-shuttering of slab - 1 set
you want to do de-shuttering of beam at 7 / 14 - 2 set
You want to check for acceptance - 1 set.

Now you are using concrete with most economical design where low strength gain can be observed in concrete and sometimes instead of achieving strength at 28 days, you are achieving at 35 days or at 42 days and it is acceptable to all. - you need 2 sets

now you need to check in between days when you not getting strength required - like on 4th or 5th day for slab depending on what is indicated in 3rd day test - 1 set.

Same for the beam - 1 set.

So if you make a tally, i have listed total 8 sample for one scenario. this 8 samples means 8 x 3 = 24 cubes or specimens.

Now say, you are going to place the concrete in structure which is 80 cum, the frequency of sampling will become 80 / 8 = 10 cum, it means you need to cast a 3 cubes or specimen (sample) at an approximate interval of 10 cum to collect a samples which represent whole concrete.

Now if the concrete Quantity is say 160 cum, your approx frequency will become 1 sample at 20 cum.

I hope you have understood the sampling.

You can also follow IS code 456 or IS 4926 or you can set your own which will be in-between or best suitable for your Project Scenario.

In all cases 28 days strength is ultimate criteria for acceptance or rejection of concrete unless and otherwise there is mutual agreement (between concrete supplier and purchaser - again this involves structural consultant) on acceptance of concrete for lateral age strength where highly optimised concrete mixes are use.

 

[krishnareddy1039]

OK thank you for the brief.....................................................

 

[Mariammal.S]

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

Yes as per Is 416 code previsions each layer should be tamping at 25 blows,

 

[Mariammal.S]

Yes as per Is 416 code previsions each layer should be tamping at 25 blows,

IS 456

 

[Ganeshkiller222]

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

Due to Heavy compaction ..it's leds to sperate the concrete materials( due to differences in specific gravity of materials) CA settle the bottom,then FA settle the above CA,then cement settled the top of FA thats y ...we need to compact the concrete moderate compaction that's y they give the limitations of compaction

 

[K. BASKARAN]

Dear Sir,

very fine In nuclear power project they are using temracture concret why they are using this type of concreteing work

K. BASKARAN