My heavy equipment guy said we should put "3 inch minus" down on a new driveway. What is it, and is it a good choice?
Interestingly, it seems that different localities tend to use different jargon for stone sizing, with not much standardization. There is no government agency that regulates it. :)
Regardless, since the sources of stone tend to use sieves with defined size holes to sort the stone, this tends to tell you the size of the stone to use. So around here, #3 stone means that which will fit through a sieve with 3 inch holes. In some cases, the stone will already have been run past smaller sieves.
I will suggest that stone with dirt in it will tend to support the growth of weeds, encourage tree roots more, etc. And since these stones tend to be more round, with few sharp edges, they don't really pack that well for a driveway base.
I have also seen that "crusher run" tends to pack very solidly. This is crushed limestone, run through the same sieves to size the stone. The sharp edges of crushed stone prevent things from sliding around, and the presence of smaller crushed stone fills in the interstices, locking things together even more. In fact, #3 crusher run seems to be a common base for driveways around here, with smaller sized crusher run on top as a surface. Our gravel driveway is made of exactly that.
So I would not be at all surprised if the heavy equipment guy was in fact referring to #3 crushed limestone for a base.
It means anything that will fit through a 3" x 3" screen grid, including large stones, smaller stones, all the way down to dust. It's something that would typically be used as a deep base to support a finer top layer of gravel.
A search of the internet reveals that it is "rocks, dirt and stones 3" and under." From the discussions I've found, it should only be used as a base at that size. 3/4" minus is the most recommended.
Old post but i thought i'd add my two cents. Granted the definition can vary from region to region, but i don't think any of the above got it right. Any time I have dealt with any aggregate in the Midwestern states described as X" minus it has always been crushed stone. As it is crushed it is screened so that only material of the desired size makes it to the desired deliverable. So yes 3" minus means that the largest stone will be 3", but it will be very angular since it has been crushed and FULL of material less than 3". If you driveway has slope that is prone to erosion this is an excellent choice - as it will not wash away in the majority of cases. In my experience it does 'pack' and tree roots and weeds aren't going to grow better here than any where else... Other driveway choices are 21A and 22A. Either choice is good, but determine if either has had a component of clay added to them. The clay helps bind and shed water vs pass water through...which is most, not all, cases it better for a driveway.