psi (pounds per square inch)
The US unit for concrete strength. Residential flatwork commonly falls in the 3,000 to 4,000 psi range, with higher numbers used for structural and exposed work.
What does psi (pounds per square inch) mean?
A psi figure by itself is incomplete, since it means nothing without a test age and a test method. Higher psi generally comes from a lower water-cement ratio, which also improves durability, so specifying strength is often an indirect way of specifying density. Paying for very high psi on a residential slab rarely helps as much as paying for proper thickness, base, jointing, and curing.
In practice
Suppliers price by mix, and stepping a driveway from 3,000 to 4,000 psi is usually a modest per-yard upcharge that buys real durability.