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[BIPIN PATEL]

Why Concrete cube is required to submerged in Water prior to Testing age. but in the field we only sprinkle water to structural member (i.e. Column/Beam/footing), we didn't submerged structural member on site. so how we can compare strength of cube and Structural member.

[sspawar]

Dear Bipin

Your Question is often asked by many engineers:
Its very genuine and common among civil engineers.

The answer is here:
As Per Indian standards guidelines Cubes are soaked/dipped in water.
和测试和认证okay-ed当它的力量comes equal to or greater than Fyck+4 or Fyck+0.825*SD- which ever is higher. (IS 456)
And Concrete is said to be okay-ed at work/ in structure - at its design strength (Fyck).

for example
IF M30 is required Concrete strength as per design for a particular structure.
Cube strength must come 30+4 =34 MPa while at site /at work in structure its strength is considered as 30 MPa.
Cube is cured and tested differently ( in better way) as per IS guidelines, that's why 30+4 = 34 is minimum required strength
Where concrete in structure is cured differently ( in possible and specified way) that's why acceptable strength is only 30.
As per code Core cut sample of Concrete from structure when tested its strength is acceptable as 30 (average) and not 34.

Hope point is clear.

Regards
S S PAWAR

[sspawar]

As per Amendment 3 fck+3 or fck+0.825*SD whichever is higher

[vikram.jeet]

TMS , the target mean strength of concrete is higher than Characteristic Strength to assure the concrete produced at site in whole or part is not found wanting in strength .

Studies on curing by various methods indicate that dipping continually in water is best , of which only cubes or precast members can be cured . Next is the ponding of water on horizontal surfaces of floors , roofs and provide nearly same curing if ponding is retained ; For vertical surfaces of walls and rc columns , instead of spraying sprinkling water intermittently (generally continuity is not maintained at sites unless strict supervision) , it is always better to cover members surfaces with wet clothes (earlier jute funny bags) and sprinkling water to keep wet is more effective way.

But definitely cube strength requirements is higher as discussed and elaborated nicely by Er Pawar since it is cured by dipping as ideal way. Actual str members are cured in different ways and over and above , are left to mercy of Man which may/may not cure effectively.

[suraj]

Keep cubes on slab also-Not banned in addition to codal requirements
For your satisfaction, place & preserve one sample of cubes on slab & cure accordingly, as slab is cured
Test these cubes alongwith conventional preserved cubes & compare results
These shall give same results & better try it
I have experienced, as such long back

[sspawar]

Dear Surajji
Its quite possible and might happened but it is in special cases.
Not necessarily in general or in all the cases.
There are many factors which may affect the result.
Apart from curing, compacting method, weather condition etc , differ the results.
现场数据集可以作为证明了一样concrete mix is poured and at the same time, but filling cubes and and pouring a structure element are differently controlled process and after caring methods are also largely different.
Regards

[suraj]

Curing
In 2004, I was asked by Qatar Petroleum Company, which is number 1 Petroleum orgnaisation to have samples of concrete cubes cure in same conditions as slab was cured, which instruction I did not oppose, for Contractual Engineer has got contractual right to instruct for whatever satisfaction to an extent, instruction does not contravene engineering
I wrapped cubes in hessian cloth & instructed foreman to keep a watch that these cubes & slabs are perfectly cured all days & nights
Codes do not mention such requirement, but Contract conditions empower engineer for issuing such an instruction

Even, concrete cores can also, be taken for additional satisfaction
It is an issue, related to achieve quality & there can be no refusal to such instructions

Engineers should earn confidence on respective activities & in Bharat India, engineers do lack in confidence to maximum extent
We should not all times be confined to what Code mandates, but what good industrial practice requires

I have even experienced failure of cubes in 1997 on 8th slab level of a commercial building located in Abu Dhabi UAE & it took 6 months for me in capacity as CE for those projects to plead whatever arguments had been required, per BS & ACI & ultimately, slab dismantling order was revoked & approval received from Engineer

One should have certain coordinative thinkng & thorough studies of requirements

Mistakes do happen on projects, when work goes to controls of those, who got no such skills, but all involved & one cannotcontrol each proces of all operations & activities

That is where Method statement becomes a proferssional working tool for supervision & prosecution

[yatin_joshi]

[suraj]

Curing & Cubes-Concrete Cube Testing & Approval

Indeed, cube results validate strength of concrete element, yet this is not real measure of concrete compressive strength

It is a mandate on Codes that cubes must be taken for certain limited cubic meters quantity of concrete or certain minimum quantity, depending on members/elements to be poured

It must be noted that cubes indicate representative samples of concrete
Cubes results are valid only, when cubes pass mandated compressive testing

In case, cubes fail, which state confirms that relevant concrete element is defective qualitatively, leading to rejection of element in question, unless given codal conditions under ACI 318 are met

Evaluation of real concrete strength of concrete is resolved only, by conducting concrete cores, in line to BS related core extract sample requirement

All other tests, such as slab loading or hammer test etc are only indicative & it is up to engineer in-charge, how to reach equivalent cube strength in case of cubes failure

Requirement of concrete curing is that defined temperature must be maintained, during concrete curing

Even, if cubes are allowed to be placed on slabs to encounter real conditions, & curing is done per slab requirement, there is no problem at all to an extent, curing conditions are met

Yes, keeping cubes in water is codal requirement, even then certain major pours are cured thermally, without allowing any water application on concrete, but cubes are sampled there as well

Even auto claving is conducted for early use of concrete elements

[vikram.jeet]

Acceptance /rejection criterion of concrete cubes is clear in various National Codes .
But concrete is a material produced at site and variation in cube strength test results is obvious.

Instances are there where structural designer's were compelled by higher authorities to to recheck the design with slightly lower strength , say for M25 , it was asked to recheck on design for a strength at which cubes passed , say M 23. This was requested to avoid huge dismantalling in event of rejection.

But such instances were rare and for marginal shortfalls.

Cannot say the above rechecking is against spirit of agreement , but marginal shortfall for strength was also recovered from
construction agency ,by the authorities , as learnt .