Slump is how a concrete mix specifies its workability: a stiff mix might be ordered at 25–50 mm, a typical structural pour at 75–100 mm (3–4 in), and a fluid one at 175–200 mm. In ACI 211.1 mix design the slump is one of the three performance choices made up front — alongside target strength and maximum aggregate size — and it is the slump, not the strength, that drives the water demand.
The ACI water table reads directly from slump and aggregate size: a 75–100 mm slump with 20 mm stone needs about 193 kg of water per cubic metre, rising to 202 kg at 125–150 mm and 211 kg at 175–200 mm for the same aggregate. Because the water-cement ratio must hold for strength, every extra kilogram of slump-driven water drags cement with it — wetter workability is paid for in cement and cost, not just in water.
The specification is closed out in the wheelbarrow: ACI practice scales the paper design to a 0.03 m³ trial batch precisely so the actual slump and density can be measured against the target and the proportions adjusted before trucks are ordered.