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Concrete Strength Grades

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#1 Paul


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Posted 17 April 2017 - 02:24 AM

My father was a general contractor before he retired. But, for years during my life, and many years before, he was a masoonry contractor. As a boy, I learned - if I recall correctly, that mortar mix was 1 sack of mix, 15 shovels of sand, and about 5 gallons of water, in the mixing machine. I don't recall what it was for concrete, as that was usually delivered by truck, to his work sites. So, I am glad I finally - at the age of 50, am learning about the other mix he used on many of the job sites I worked as a boy.
Anyway, while doing some research for my projected footings, I started learning a few things about concrete that I wanted to share with those of you who may not know.
Concrete strength is rated in grades. C15, C25 and C30, for examples. The grade of the concrete is the concrete compression resistance after it has cured for a bit under a month. 28 days, actually. But, who is counting? 
The table below will show the parts of cement, sand, and gravel to add, to make the various mixes above, and to achieve the strengths necessary for a given project.


Cement  :  Sand  :  Gravel  :  Grade
     1        :     2      :      3        :   C30 (very strong)
     1        :     2      :      4        :   C25 (strong)
     1        :     2      :      5        :   C15 (general purpose concrete)
For the money, I would just as soon use C30 for everything, than a lower strength mix. But, that's just me. I would rather make sure my concrete pour is going to be stronger than I need it, than possibly weaker. Pretty difficult to go back and pour it over, once the construction job has been completed.
The variable C is equal to the same number in Newtons, per square millimeter, or N/mm2. For the yanks in the classroom, who use the Imperial System of measurements, this would be Pounds per square inch, or PSI. So, C30 would be 30 N/mm2, or 3,059,000 Kilograms of force per square meter kgf/m2, or 4,351 PSI
Pretty damned strong. A lot stronger than I thought it would be. 

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#2 JohnOBohn


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Posted 18 April 2017 - 09:28 AM

   Those are your basic materiel formulas and have been forever and a day.    HOWEVER not the only consideration in concrete styrenth.

    The amount of WATER is the real determining factor in concrete strenth.    The more water added, the weaker the final result.

    More water makes easier mixing but shitty concrete.    

    You have seen the crappy building in the Phils.   Because the lazy Filipinos add way to much water because it is EASIER to mix/pour.    Same with their hollow blocks.    In all fairness, this applies to all 2nd & 3rd world countries.

   Quality concrete is NOT supposed to be the consistency of a pot of chicken noodle soup.    It should just barely  pour.

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#3 andy


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Posted 19 April 2017 - 12:11 AM

I agree with John, but this is Asia, and hot! You have to work fast with your mix, if not it will be wasted. You can afford to put a little extra water, depending on the job, to give you a little time for the work.
At least here you don't have to worry about the worst thing for concrete "frost".
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#4 Oz Jon

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Posted 23 April 2017 - 02:55 PM

You are quite right Paul on compression strength 3000-6000 psi is typical of good average concrete.

[1000psi is about 7MPa = 7million N/sqm]


But John is also right that the the strength is pretty much inversely proportional to the water to weight ratio.

The minimum quantity of water required to enable placing the concrete makes the strongest concrete.

If air temperature causing quick setting is a problem, best to mix/place smaller batches if you want the best strength for your chosen cement aggregate mix. A concrete vibrator is very helpful for placing dryer mixes.


Back in my student days, we made many different concrete test samples with different aggregate grading mixes, different cement ratios and different water ratios. 28 days later we compression tested them to failure.

With everything optimised we got the best samples up over 10,000psi - that's not your everyday concrete!

Cement ratio and water ratio were the key factors controlling strength.


Many people don't know that concrete is very weak in tension. Typically 300-700psi.


That's why re-bar is needed to take care of any tension stress - the re-bar can handle up to about 30,000psi before yielding.

But it needs to be placed in the right place in the concrete.

Placed where tension stress will occur (but with a minimum of about 1" concrete cover to avoid later rusting from moisture penetration).


Figuring out where tension stress may be in your slab/wall/etc is a job for an Engineer or experienced tradesman. If you haven't got access to one, think about what the forces on the slab may be and figure out which parts are likely to be in tension.


The "fix-all" fallback solution is to put in 2 layers of steel (1 top and 1 bottom/each side) in a slab floor/path/wall etc.

Edited by Oz Jon, 23 April 2017 - 03:19 PM.

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