Pour day: concrete words from batch plant to broom finish

The first truck rolls in before sunrise, drum turning, and the foreman meets it with a flashlight, reads the ticket, glances at the chute and says, "Run a slump before he dumps. If it's soup, send it back. If it's a hot load, same answer." A pour runs on a clock that starts at the batch plant and does not stop until the surface is cured, and every word on site is about beating that clock. Here is the vocabulary, in the order the day unfolds.

Before the first truck

By pour day the formwork is set, oiled and braced. Inside it, the rod buster has tied the rebar or laid out sheets of WWR, and propped it all up on chairs or dobies so the steel ends up at the height the drawings call for, not lying in the dirt. The inspector walks it with a tape, checking cover, the depth of concrete between the steel and the surface. Too little and the bar rusts; too much and it does not do its job. Under a slab on grade that will get finished flooring there is a vapor retarder, and anything a later trade will connect to, anchor bolts, weld plates and sleeves, goes in now as embeds.

The order itself was placed days ago, in yards, against a mix design the engineer approved. Order too little on the last truck and you pay a short load fee. Order too much and you get to figure out where to dump it.

At the truck

Every truck carries a batch ticket showing the mix, the yards and, most importantly, the time it was batched. Concrete starts reacting the moment water hits cement, and most specs want the truck empty within about ninety minutes.

The tester runs the slump test: fill a metal cone, pull it straight up, measure how far the pile sags. Four or five inches is a normal slab or wall mix; eight or nine is soup unless an HRWR put it there. For outdoor work in freezing country there is also an air meter reading, because air entrainment is what keeps the slab from flaking apart after its first winter. Then the tester casts cylinders, the samples that will be crushed later to prove the strength.

The eternal argument happens here. The finishers want it wetter, because wet concrete is easy to spread. The engineer wants it dry, because every gallon of added water raises the w/cm ratio and costs strength. The usual peace treaty is an admixture: an HRWR makes the mix flow without extra water, a retarder buys time on a hot day, and an accelerator speeds things up in the cold. A load that starts setting early is said to kick, and nobody wants to be holding a chute when it does.

Getting it in the forms

On a small job the truck chute reaches. On a big one, a boom pump unfolds over the site, or a slick line of steel pipe snakes up to the deck. Small lines want a pea gravel mix, and any line can plug, which stops everything while the pump operator beats on the pipe looking for the jam. Where neither chute nor pump reaches, a buggy hauls it.

Once the mud is in the form, someone works the stinger, an internal vibrator that shakes out trapped air. That is consolidation, and it is a judgment call. Too little and the stripped wall shows honeycomb or bugholes. Too much and the stone sinks while the paste rises. Walls go in lifts, and the next lift has to land before the last one sets, or you get a cold joint. The nightmare is a blowout, a form that bursts and lets a truckload of concrete out across the site.

Strike off and float

On a slab, the crew starts to screed: drag a straightedge across the forms to strike the concrete to height. Right behind comes the bull float, a wide float on a long handle that smooths the surface and pushes the stone down. Then everyone waits.

What they are waiting for is bleed water, the sheen that rises as the solids settle. Work it back into the surface and you get a weak, dusty top. When the sheen is gone and a boot leaves a quarter-inch print, it is time for floating, which brings up the cream, the fine paste the finishers actually shape. Edges get an edger. The middle gets worked from kneeboards or, without stepping on it at all, with a fresno. On a hot, windy afternoon someone sprays an evaporation retarder between passes, because a surface that dries too fast opens plastic shrinkage cracking before the slab has even set.

The finish

Big floors go to the power trowel. The first passes run with a pan clipped on to flatten; later passes run bare blades, pitched steeper each time. Warehouse floors get a burn, troweled until the surface turns dark and glassy. Trowel too soon, though, and you seal air and water under the skin, which comes back as blistering.

Outdoors, a slick surface is a lawsuit, so sidewalks and driveways get a broom finish, a broom dragged across for grip. A slab with entrained air should never be burnt at all.

Curing starts the moment the finishers step off, often with a sprayed curing compound that holds moisture in so the cement keeps hydrating.

Then the sawcut. Concrete shrinks and it will crack; the only question is where. A control joint tells it where, and the early-entry saw cuts it within hours, while the slab is still green concrete. Wait too long and the slab picks its own line, a wild crack running right across the panel.

After the pour

The drivers rinse their chutes at the washout and go home. A week later the lab does its first break, crushing cylinders to check compressive strength. The number that matters is f'c, the strength the engineer specified, usually checked at 28 days. Low breaks start meetings. Then the owner walks the slab after the first rain and finds the one bird bath nobody saw.

Five words to learn first

• Slump: how far a cone of fresh concrete sags, the field check on consistency.
• Bleed water: the sheen you wait out before finishing.
• Stinger: the vibrator that shakes the air out.
• Control joint: where the slab is told to crack.
• Break: the test result that settles every argument about the mix.

Know those and the shouting on pour day turns into instructions you can follow.