> 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

> 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.