Compressive Strength of Cement Cube

<...the seven day period is from the day mortar mould is prepared (first day) or from the day (second day) when curing in water is started ?...>

That would be determined by the end Client's requirements, which cannot be seen from here.

Maybe it would help if you inquiry/contact the University of Wisconsin-Madison.

The had a 100 year Cement study.

They may even have information available online.

Its the same rule that you apply to the 3 days!

ASTM C109 / C109M - 16a Standard Test Method for Compressive ...

How to Perform ASTM C109 Cement Mortar Compression Testing ....

Well you see you got your time since batching, and then you have your setting time, and then curing time...

Since the batching and setting take place within an hour, the question seems moot...

Not so.

Finishing and curing are not directly related.

Final set and curing aren't either.

Curing may not be complete for months after pouring and hardening to the point of firmness.

Specific, and well understood, chemical reactions continue but the degree of cure after the periods mentioned may be sufficient to indicate final cured properties.

Waiting months for the tests is not practical.

For the OP: the period starts when "The mortar of cement ,sand and water is prepared and is placed in moulds."

Ok I see it's a technical explanation of the test itself....so then the time period to be measured either 3 or 7 days begins when the cubes are submerged in the clear water...The first day is dismissed in the test...

http://www.cement.org/for-concrete-books-learning/concrete-technology/concrete-construction/curing-in-construction

Correct, although your example is one of a number of sample preparation methods. Some do wait 28 days to test.

The fact remains that for the purposes of this compression test, the first day is not included in the count.

So, the correct answer would be 8 days after mixing and pouring into cube mold.

I don't know about the OP, but I'm confused! In #7 you say the period starts when "The mortar of cement, sand and water is prepared and is placed in moulds."

In #9 you say the correct answer would be 8 days after mixing and pouring into cube mold which would be 7 days after taking out of the mould (agreeing with SE's #8)

If the client hasn't specified, and there isn't accepted practice, I'd be inclined to measure time from coming out of the mould, as longer time should give better strength results.

That was more in response to SE's #5 which said the time period was moot.

Relying more on chemistry than the specified test, I assumed that the time would start when the reactants were mixed together and the chemical curing reaction began.

I'd always defer to the requirements of the specification controlling the preparation/testing procedure.

There, that's as muddled as I can make it.

Get, read, and follow the specification.

How is the strength of these cubes compared to the whole wall poured with mortar.

By the use of safety factors to relate cube test strength to allowable design compressive stress. I don't have details but civil engineers will know. If cube strength is say 25N/mm², typical design stress might be 10N/mm² if memory serves.

Design tensile stress is usually taken as zero.

Please don't feed the troll.

When the concrete is cast on site. That is the day you will take your samples. Curing starts immediately after casting and finishing. Placing them in a damp cabin is also a form of curing. Covering them with a rag, or splashing them with water is for instance, also a form of curing.

They are placed in a water bath for optimal curing at optimum temperatures.

Concrete will start gaining strength from the very instant it is mixed with water, unless there are admixtures to prevent that from happening.

When a bridge is under construction, and you placed the concrete in-situ, you would for instance want to know when it is safe to strip the formwork, that would be when the concrete is strong enough to support it's own weight + safety factor. Curing starts immediately after casting and finishing, and your cubes are a test to confirm what the strength development would be in such a concrete element.

You have to therefore measure the 7 days from the very day the concrete was placed in the mold (cast).

Unless the controlling specification says otherwise.

That is what none of us really know, apparently.

When in doubt, do the safe thing.

In a way it amused me seeing the curing tests done. As a cement manufacturer we did 3, 7 and 28 day tests. At the most the cement would be in the bulk storage silos for a couple of days before being shipped out. By the 7^th day the cement would have been cast, 28 days and there’s building stood on it. By then it’s a bit late to tell them its failed the physical test.

Back in college, one of the professors told us that exact thing had happened in the construction of a (skyscraper).

The issue was (resolved) by letting the remaining construction be limited to five or six fewer stories of top office space of the original total design height.

The building owners were not amused...

At one point, in my early Civil Engineering career, I conducted such testing on such cubes which were 3-day, and 7-day, samples of (quick-set) concrete, where the extent of curing was very critical...