What is 1" minus recycled concrete mean and does it also include stone sizes smaller than 1"?

Question

What does 1" minus recycled concrete mean? Does this also include stone sizes smaller than 1"?

Answer 1

Concrete is a mixture of cement (Portland Cement), water and aggregate (rocks).

Strength of concrete is largely based on the aggregate: size, blend, grade (hardness), etc. Crushed rocks are used in concrete and they are graded by size. Yep, they pour rocks through a sieve and sort them by size...down to sand. Then they mix the various sizes together so they can compact. (If you used all the same size they would not compact and would not be as strong.).

The term “minus” refers to the smaller rocks that are used and the 1” is an indication of the largest rock used in that “batch”. (The largest I’ve used is 3” minus, but 4” minus is common.)

Contractors like smaller aggregate (3/4” minus) for slabs because they don’t want a big rock sticking out of the surface to trowel around. They use larger rock mixes like 3” minus for footings because it’s cheaper and no one will ever see the finished product.

Recently everyone wants to “recycle” used building materials rather than dispose of them at a landfill, especially concrete because concrete has a very large carbon footprint. The trouble with recycled concrete is that we don’t know where the aggregate came from so we don’t know it’s “hardness”. Normally when we mine rocks from a river or mountain, it can be tested (crushed) and we know how hard it is. But recycle concrete can’t be crush tested because it has cement attached to it and does not give a true reading.

I don’t like recycle concrete and don’t use it. If they tell you it’s the same, ask to see the ASTM crushing test.

Yeah, there are other ingredients too, like air entrainment, fly ash, binders, etc., but this is just the basics.