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Comment by gtvwill | Hacker News Reader

Yo it's cement + aggregate makes concrete. As in concrete is the sum of two or more products. Concrete doesn't grip anything. Cement binds and aggregate shape doesn't count for bugger all.

You use tie in rods of rebar to bind one slab to another, not the aggregate because the aggregate does sh*t all for structural strength. It's more for the compressive strength.

Tbh you can make a form or concrete out of perlite and cement. It's light af, great for ship hulls when reinforced with steel. Still concrete,just doesn't need any sand.

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> Cement binds and aggregate shape doesn't count for bugger all.

This isn't true at all.

Using your example of rebar, this is why any piece of rebar has lugs down it; to bond better with the cement. If you use a smooth cylinder of steel, it won't bind to the cement as well and you wind up with a weaker finished product. The ribs of the rebar increase the surface area and make the rebar-cement bond stronger.

Same thing with desert sand. If you have rounded sand grains, you've got reduced surface area per unit volume, and thus reduce the strength of the bond between the aggregate and the cement. That weaker bond becomes a fracture point.

Thanks. Yeah, I've poured a few slabs and built a few walls, but construction engineering is waaay out of my normal competencies, so I appreciate the vocabulary corrections.

You say "aggregate shape doesn't count for bugger all". So why does river / crushed sand work, and desert sand doesn't? It sounds more like there are different aggregate mixes / types of aggregate material that work for particular applications (hadn't thought that through, but it makes perfect sense), and that "rough" sand is better for the most-common concrete use-cases. Do you know of specific types of concrete where "smooth" sand would be preferred?

Someone covers it further down. There's a whole bunch more to the strength of concrete than the aggregate.

Man last major concreting I did smooth sand would have been awesome. We were already trying to use the finest grade sand we could find. It was on mad max 4. Making concrete not look like concrete (think underwater caverns, cliffs, waterfalls). So smooth is awesome, makes it easy to texture to look like different types of weathered rock.

All the structural strength was from steel rebar with chook mesh wired to it. The crete and scratch coats were about 100mm thick at the most.

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