Guide
What Slump Do You Need for Pumping Concrete?
Slump is the most argued-about number in concrete. Here's what it actually tells you, what it doesn't, and why water is never a free fix.
Ask ten blokes what slump you need for pumping concrete and you'll get ten different numbers and at least one argument before smoko. Here's the short version: most pump work in Australia runs on concrete ordered at 80 to 100mm slump. Boom pumps with big steel deck lines will happily push the stiffer end. Small line pumps dragging rubber up a driveway want it workable. But the number is only half the story. And it's the half most people get wrong.
What the slump test actually measures
Slump is one test, done with one cone, at one moment in time. Fill the cone, lift it, measure how far the concrete slobbers down. That's it. It tells you how wet and workable the load is right now. It does not tell you whether it will pump.
A bony mix short on sand can hit 120mm slump and still pack solid in your reducer. A properly graded mix at 80mm will slide through a hundred metres of line without a grunt. Slump measures consistency. Pumpability comes from the whole mix: the fines, the grading, the stone. Don't confuse the two.
The numbers that matter on the day
A typical order for pumped residential work: 100mm slump, 20mm aggregate, N25 or N32. And remember the plant has a tolerance. The standard gives them roughly 15 to 20mm either side of what you ordered, so your "100 slump" load can turn up at 85 and be perfectly legal. If your job only works at exactly 100, you've planned it too tight.
Slump also dies in the drum. Hot day, long haul, forty minutes waiting behind the crane. A load that left the plant at 100 can hit your hopper at 70. Order for the conditions on site, not the conditions at the plant.
Water is not a free fix
The temptation is always there. Load turns up stiff, builder's pacing, so someone yells for water. Understand what you're actually doing. A splash of 10 litres a cube can move slump 20-odd millimetres, and it takes strength with it. Every litre past the mix design is MPa walking out the gate.
Most dockets carry a water allowance. Inside it, fine. Past it, the plant's warranty on that concrete is gone, and whoever signed the docket owns the result. If it cracks, if it dusts, if the core test fails at 28 days, the conversation comes back to that signature. Don't let a builder in a hurry make you the bloke who signed away the slab.
And if every load needs water to move, that's not a weather problem. That's a mix problem. Ring the plant. Fix it at the source, not at the hopper.
The operator's rule
One of the 33 Rules Built Under Pressure says it plainly: know what's going into your hopper, because once it's in there, it's yours. Slump is your first read on a load. Learn what good concrete looks like coming off the chute, how it folds, how it holds an edge, and the cone just confirms what your eyes already told you.
The number isn't the skill. Reading the load is.
