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[suraj]

Curing of Concrete
In ACI 318, when we dealt with failure of slab in 1997/98, on an average criteria was mandated for approval of concrete, when cores had been tested
Minimum acceptance for single cube was 85% characteristic strength
It is true that all cubes give varying results, even if, RMC is used
RMC of course, gives higher than required strength, yet provision must be complied with
Not, only cores were tested, but also, Sulpha capping compound was tested
All core  dimensions were checked & prepared for testing
We had used 3 laboratories for testing cores at different times of course, under certain dispute with Engineer
It was really, great experience for me in those days, wherein we had been involved with repetitive hammer test, load test, core tests over entire slab
Only, long term curing helped in that instance, because concrete technology at that time was not fully known
It was all, due to site lack of curing, due to certain lazy engineers & front line workers
Core is best answer to ensure factual results, but long term curing may also, help in certain cases, when concrete is more than 56 days old
There should be no requirement of design check, if variation occurs say 1 or 2 N

[sspawar]

Dear Sefians
It may be a wise idea to get rechecked, the actual strength is passing with the required design strength or not, rather to dismantle the subjected structure.
But
As per IS 456 Core test result are acceptable as follows:

17.4.3 Concrete in the member represented by a coretest shall be considered acceptable if the averageequivalent cube strength of the cores is equal to at least85 percent of the cube strength of the grade of concretespecified for the corresponding age and no individualcore has a strength less than 75 percent.

For example If M 25 grade of concrete is design grade of concrete for a particular structure.
In core test if it averagely obtained as 85% of 25 = 21.25
Here Designer may reject the concrete and the subjected structure, if suppose he rechecked and get the design requirement at least at M 23,  

But Code allows to accept.


Regards

[Dr. N. Subramanian]

[suraj]

Field Engineering - History of 8th slab failure & approval-Abu Dhabi-1997
I worked as Chief Engineer in this organisation
Issue was curing
Refer to attachment
It was long back posted on Sefi as ell

[suraj]

Field Engineering+Concrete+Curing

Purpose of curing is to prevent evaporation of water from hard concrete
If water can be allowed to stay within concrete, curing is effected successfully
When codes were initiated long back, concrete technology did not reach to higher levels & codes were drawn by those, probably, who never ever worked in practical fields
Due to development of concrete technology, various methods of curing have been introduced
Concrete curing by water, by curing compound, thermal curing, autoclaving etc.
If concrete temperature can be maintained within prescribed limits, it is not necessary that concrete must stay on contact with water
If hardened concrete temperature can be maintained say 27 deg C & around, by whatever means of curing, it is effective
If cubes can also, be maintained in that environment, it serves equivalent purpose
If foregoing methods can work on real elements, why not on samples?
Those, who have authority on project site can experiment, using various methods on concrete cubes
Use curing compound on saturated cubes & let cubes stay till these are tested
Saturate cubes & cover with saturated hessian, further covered with polythene sheet, so that no air enters & make sure that cubes remain in this condition, till testing is conducted
Apply steam on cubes, if equipment / device is available & then test
Keep cubes on works & cure as works are cured & then test
All thee experiments would reveal facts
Why have speculation, about some literature that is not real
Foregoing should be a parallel testing of course, in agreement with responsible supervising engineer or responsible competent authority for purpose of research, provided, defined procedure is also, conducted