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Comment by dyauspitr | Hacker News Reader

I didn’t exactly understand the take away from this video. In the first test he does, the river sand concrete is three times stronger than the desert sand concrete, but in the second test, the desert sand concrete is 10% stronger than the river sand concrete. What exactly is going on?

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It's normalising for different things.

If you normalise for the the same ratio of cement / water / gravel / sand then sharp-sand (crushed rocks) is stronger.

If you normalise for workability (i.e. the concrete flows and can be worked easily) you need to add additional water to sharp-sand, which means that the smooth-sand (e.g. river-rocks) end up stronger.

That’s what I mean, he doesn’t really test for absolute strength. Which one is stronger if the right amount of water for the corresponding concrete is added?

He explained it in the video. The amount of water was adjusted so the viscosity (well, workability) was identical between the samples in the second test. Less water in concrete has a huge positive influence on strength, and rough sand needs more water to be workable. Thus, for comparably workable samples, rounded sand concrete with less water was actually stronger.

But given the right amount of water, which one has the higher absolute strength?

Define "right" amount of water. There is no singular correct amount of water in a concrete mix. It is dependent on the properties you want, the materials you have available, how things are going that day, etc.

Just enough water for the given concrete to maximize for absolute strength.

I'm sure if you have an easy form like a regular cylinder that you just have to fill from the top, then the sharp sand is stronger in an absolute sense.

It's when you have complex forms, logistics considerations on site, curing considerations like heat, using pretensioned steel rods, etc. Then you start to run into these workability tradeoffs and "strongest == best" is no longer true.

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