Concrete compresive strength prediction 1

[arawinda]

Dear friends, usually we crush test cubes at 7, 14 and 28 days after casting and use attached figure to verify the compressive strength. But sometimes we missed exact date. May be we have to crush it on 10th or 29th day due to practical matters occurred at site. So I want to know if there is any formula to calculate compressive strength at any age. Please help...!!

 

[Ron]

There is no magic formula because each mix is somewhat unique. There are too many variables for such an equation.

Further, there is no point in testing to the exact desired compressive strength. Concrete design doesn't work that way.

While we use a specific design strength for our designs, the goal of the mix design is to achieve that value within a reasonable tolerance and predictability. Such predictablity comes from specific strength gain curves for each mix design; however, not all ready mix suppliers provide such curves. If the concrete is mixed on site, there is little predictability of strength gain until enough testing is done to validate the design.

If you use 35 MPa for your design, your mix design should be proportioned to achieve that design strength in 28 days, typically. There are other design strength days; however, 28 days is the norm.

If you are using conventional portland cement concrete with Type I cement (US designation) and without much in the way of admixtures, you can predict that the concrete will achieve about 70 percent of its design strength in 7 days. This is a generalization, of course, but based on generalized strength gain curves from copious amounts of data over the years for various mix designs.

If that value is not achieved in 7 days, it becomes a "flag" for us to watch the strength gain of those test specimens. Use such generalizations carefully, as they might not be accurate for your specific mix design.

 

[rapt]

It will be very mix dependant.

You could plot your results form a series of previous tests on the same type of concrete and determine if there is a sensible relationship for the concrete mixes you are using!

 

[arawinda]

Thanks Ron & Rapt.....

 

[jayrod12]

I also honestly feel that an odd day or two different from the prescribed break day should not affect the final product any. It has been explained before that the 28 day strength was apparently derived from payroll requirements.

 

[allgoodnamestaken]

If you are getting the 28 day strength because you need it to verify contract requirements, you should be making sure you're breaking said concrete on 28 days or later.

If you're breaking samples on another day, the question is why? To make sure you can remove support/scaffolding/forms and start the next floor? Because of bad communication and timing with your testing lab?

 

[TLHS]

I generally don't care if you do your 28 day test late. Realistically, I don't care if it actually happened at 28 days, I just care that the concrete eventually hit design strength before it went into actual service.

The tests before 28 days are really just to flag whether crazy things are happening and to verify that it's safe to partially load if necessary.

 

[KootK]

For horizontal, suspended concrete, I care that the tests are done on time. For slabs, deflection performance is correlated to the amount of slab cracking that occurs during construction. I want to know what I can expect from f'c and Ec during the reshoring process. That said, like Jayrod, I'd be unlikely to sweat a day +/-. My only concern would be for overall quality control and contract document compliance.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.