Where the four thousand pounds comes from
There is no mystery in the number. Concrete is stone, sand, cement and water, and that mixture weighs in the neighborhood of 150 pounds for every cubic foot of it. A cubic yard is a cube three feet on a side, which is 27 cubic feet. Multiply and you are at about 4,000 pounds.
Mix design moves it a little. More stone or a heavier aggregate nudges it up, air entrainment and a lighter aggregate nudge it down, and a cured slab weighs slightly less than the wet mix did because some of the water has left. None of that swing is big enough to plan differently around. If you are working out what your pour weighs, use two tons a yard and you will be close enough for every decision that matters.
The weight shows up before the concrete does
A ready mix truck is not heavy because it is a big truck. It is heavy because of what is in the drum. Ten yards of concrete is roughly 40,000 pounds on its own, and the truck is underneath that. Nothing that normally uses your drive comes close, not the pickup, not the tractor, not the delivery van.
That is the whole reason access is a real conversation and not a formality. Red clay softened by a wet week does not hold that weight, and a thin layer of gravel over soft ground can look completely solid right up until a loaded truck commits to it. A steep pull asks even more, because the drive is fighting gravity as well as the load. On the hillside lots this region hands you, that adds up fast.
Choosing where the truck sets that weight down
Once the truck is on site it has to stop somewhere, and where it stops is a decision worth making before pour day rather than in the moment. Solid ground, ideally an existing drive or the road shoulder. Away from anything buried that would rather not have twenty tons of concrete parked on top of it, which around here usually means a septic field, a leach line or a water line.
When there is no good answer, the truck does not have to get close at all. Our pump is trailer mounted, so it tows in on a trailer instead of driving in loaded. The truck parks where the ground will take it, we run 2.5 or 3 inch hose the rest of the way, and the pour happens without a loaded wheel ever crossing the part of your property you were worried about. A hose full of concrete is still light enough for two people to move, and it sits on the surface rather than pressing into it.
The finished slab has weight too, and the ground has to hold it
Run the same arithmetic down to slab thickness and it gets useful. At 150 pounds per cubic foot, 4 inches of concrete weighs about 50 pounds for every square foot of surface. A 20 by 20 patio is 400 square feet, so that pad is roughly 20,000 pounds of concrete sitting on whatever you put under it. Ten tons, permanently.
That is why the subgrade and the stone base under a slab are not an upsell. The native red clay here does not hold still. It takes on water, it lifts when that water freezes and it draws back as it dries out, so twenty thousand pounds of slab sitting directly on it is resting on something that moves every winter. Crusher run stone, laid in lifts, gives that weight a base that drains. Compaction is what turns those lifts into a base instead of a pile of loose stone, and a slab on grade fails at the bottom far more often than it fails in the concrete.
The weight does not disappear when the slab comes out
Tear out is the other end of the same number. An old 4 inch drive is still about 50 pounds a square foot after it is broken up, so a small driveway coming out is several tons that has to be loaded and hauled somewhere. That is why removal is priced on its own line on a written estimate instead of being folded quietly into the new pour.
It is also worth knowing before you plan a weekend of demolition. People underestimate broken concrete constantly, because a piece you can just about lift makes the whole slab feel manageable. Work out the square footage, multiply by 50 pounds, and decide from there.