A lower ratio of sand and aggregate to cement will lead to a stronger concrete.
What are the reasons for using a 'weaker' higher ratio mix? Is it just cost?
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Does this concern a DIY project you are doing that involves concrete? We can answer What is the proper strength of concrete that is appropriate for my project "Why not always use a stronger mix ratio for concrete?" Best practice is to use what is necessary and appropriate for the job. (unless you are guitar player, then you have your amps special made to go up to eleven instead of ten.)– Alaska ManCommented Apr 3, 2021 at 17:00
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Yes it's a diy project and Im dealing with very small amounts (few buckets) of concrete. I want it to be pretty strong, I dont have an exact requirement, but it is such a small amount I'm wondering why I wouldnt just use the strongest ratio possible.– JayCommented Apr 3, 2021 at 17:37
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2 Answers
Adding excess Portland cement will cause the concrete to shrink more and accelerate the hardening process making it even more brittle than it normally is.
My Dad used to always say there are 2 types of concrete. The kind that had cracks and the kind that will.
Cement really has no strength because it is brittle add something like sand and rock for it to bind together and it has some strength. Add rebar to further bond and it is even stronger.
The problem is over time it continues to hydrate making it more and more brittle. This process can take over 100 years to become a real strength issue with proper ratios used in the original mix. Excessive cement can accelerate the process where failure is in a hand full of years compared to proper ratios.
Have you ever seen a bag of cement that got wet it is a solid “brick” but less than 10 years they were spalling and crumbling we had about a dozen bags that we used as riprap I noticed it crumbling at about 5 years and by 10 years it was like shale.
Using excess cement in your concrete will shorten the useful life of whatever you are making.
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So are some standard ratios specifically designed for high strength but shorter life? E.g highest standard ratio is 1:0.25:0.5 for M40 mix, this seems to have a high amount of cement, would you consider this 'excess'?– JayCommented Apr 3, 2021 at 17:51
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@Jay There are different mix ratios for different strengths with the same durability and longevity. With proper mix, the concrete getting stronger with time due to the slow process in hydration (gaining bond strength). The old method of proportioning was by volume (cement:sand: gravel) - 1:3:6 (2000 psi), 1:2:4 (2500 psi), the amount of water shall be just adequate to allow for good workability (consistency). I wouldn't recommend the old method for something important though.– r13Commented Apr 3, 2021 at 18:46
More cement and less water make the concrete stronger ; But major strength comes from steel rebar and aggregate. "Six bag mix" is the standard for good concrete ( 6 cement bags to a yard of concrete mix ). These may all be moot points ; cement is not an adhesive - it does not "stick" to the previous day pour. As described , between each of your batches will be a plane of weakness ( AKA - a crack).
answered Apr 3, 2021 at 20:24
