What causes concrete pump blockages (hint: it's rarely the pump)

Ask a builder what causes concrete pump blockages and he'll point at the pump. Ask an operator who's cleared a few hundred of them and he'll point everywhere else. The pump is usually the last innocent party in the chain. Blockages are made in the mix, the layout, the rubber and the wait — the pump just delivers the bad news.

The mix

Concrete pumps on its grout — the paste of cement, water and fines that carries the rock and greases the pipe wall. Bony mixes, gap-graded aggregate, harsh sand and low-fines specials don't have enough paste to do the job. The rock runs dry, bridges in the line, and locks. Same goes for anything that's been flogged around on the agi too long on a hot day. Slump dies, friction climbs, and the line stops being a pipe and starts being a mould.

Grout loss

Every leaking clamp and worn seal bleeds grout under pressure. It looks harmless — a bit of milk dribbling at a joint. But you're draining the lubrication out of the mix stroke by stroke, and somewhere down the line the rock is running dry because of it. Tight clamps and fresh seals aren't tidiness. They're blockage prevention.

The line itself

Kinked rubber. A hose flattened under a truck wheel. Worn sections gone soft and ballooning under pressure. Reducers necking down too hard, too late in the run. Tight bends stacked together. Every one of them is a narrow point where the flow changes and rock stacks up. Most of them were visible at 6am, before the first cube, when fixing them was free.

The wait and the air

Concrete sitting still in a line is concrete going off in a line. A long gap between loads on a summer pour will cook the run from the inside. And a hopper allowed to run low sucks air — the next stroke slams a dry plug down the line and can turn the end hose into a weapon while it's at it.

The prime you skipped

A dry line steals the grout out of the first load. Skip the prime, or short-change it on a long run, and the first rock arrives at the far end with nothing to ride on. Plenty of mystery blockages in the first ten minutes of a pour are just a prime that never happened.

The foreign object

Then there's the stuff that should never be in there. Dried lumps knocked off the hopper grate. A glove. In the first Operators Club module — No One Is Coming To Save You — it's a chunk of rubber conveyor belt off the batch plant, jammed halfway down a four-inch line. Rubber on rubber. No hammer ringing that one out. Push, stop, bash, repeat, for as long as it takes.

The pattern is simple: almost every blockage was booked in before the pump made its first stroke. Check the mix. Check the line. Check the seals. Keep the concrete moving. The pump can only push what you give it.