Asphalt paving crew talk: from tack coat to finish roller

It is two in the morning on a closed highway lane, and the paver is crawling forward in a cloud of steam. The operator on the back platform yells, "More head on the left, and nobody touch the wings!" A raker shouts back that the joint is low. Somewhere in the dark a roller operator asks, "What's the gauge say?" A paving crew talks fast because the mix is cooling every second, and once it cools, the chance to fix anything is gone. Here is what they are saying, from the first spray of tack to the last roller pass.

Getting the road ready

Most night work is mill and fill: a milling machine grinds off the worn surface, sweepers clean up, and new mix goes down the same shift. The grindings go back to the plant as RAP, old pavement recycled into new.

Before any mix goes down, a distributor truck sprays tack coat, a thin film of asphalt emulsion that glues the new layer to the old. Emulsion goes on brown, because it is asphalt droplets suspended in water. When the water leaves and the droplets fuse, it turns black, and that change is the break. "Let it break" is the standing order, and a haul truck that backs over brown tack picks it up on its tires. On raw gravel base, the spec may call for a prime coat instead, a thinner spray meant to soak in.

The trucks

The mix comes from the hot plant, either a drum plant mixing continuously or a batch plant mixing a few tons at a time in a pugmill, to a recipe called the JMF. Each truck carries a load ticket with the tonnage, mix and time, and the dump man collects them while waving trucks back to the paver.

Most loads arrive in an end dump, raising the bed into the paver's hopper. A belly dump instead lays a windrow on the road ahead, and a live bottom trailer walks the mix out on a conveyor floor. On big jobs an MTV sits between trucks and paver, remixing the load and feeding it without a pause.

Before the truck backs in, someone will shoot its temperature with an infrared gun. Haul trucks get a tarp to hold in the heat, and a load that shows up too cool, or crusted with cold chunks, is a cold load that goes back. Too hot is its own problem: blue smoke curling off the bed means the binder is cooking. Crews also judge the oil by eye. A load that looks shiny and wet is fat; one that looks dull, brown and harsh is dry.

Behind the paver

The paver takes the mix, carries it back on conveyors, and spreads it across the width with augers in front of the screed, the heated plate that floats on the mix and lays the mat. The person running it is the screwman, and the job is mostly about consistency.

The screed floats, so mat thickness comes from its angle of attack, which is set by raising or lowering the tow point. On a long pull the grade sensor that moves the tow points rides a ski, an averaging beam that ignores small dips and rises in the road below. In front of the screed sits the head of material, the roll of mix the augers keep pushing ahead. Keep the head steady and the mat stays flat. Let it rise and fall and the screed rides up and down with it, building in waves.

The hopper's folding sides are the wings. Flipping them after every truck dumps the cooled, coarse mix from the corners onto the conveyor in one shot, which shows up as segregation, an open, rocky patch in the mat. A thin stripe of the same down the middle is a center streak, the signature of the auger gearbox. Rough, pulled lines in the surface are tearing, usually a cold screed or a cold load.

Each pull lays one lift, and a road gets several: a binder course underneath, a surface course on top.

Joints and handwork

The weakest spot in any pavement is the longitudinal joint, where one pass meets the next. Good joint matching sets the new mat to overlap the old edge by about an inch and stand just high enough for the rollers to bring it down flush.

That is where the raker earns the money. With a lute, the raker will bump the joint: push the overlap off the cold side back onto the hot edge so the roller can pinch it tight, rather than scattering it across the mat. Where new pavement has to taper into a driveway or a gutter, the raker will feather it down to almost nothing.

Rolling

Right behind the paver comes the breakdown roller, usually a vibratory steel drum doing most of the compaction while the mix is hottest. A pneumatic roller follows, its rubber tires kneading the surface tight, and a finish roller makes the last passes to erase marks. The rolling pattern, how many passes, in what order, at what spacing, gets worked out on a control strip at the start of the job.

Rollers have their own vocabulary of trouble. Pickup is mix sticking to the tires or drums and tearing out of the mat, and the cure is warm tires and a light mist of release agent, never diesel, which softens the binder. A tender mix shoves sideways under the drums in a certain temperature range, and the operator has to wait it out. Fine hairline cracks opening under the drum are checking. The mat also drops as it compacts, and the roll-down has to be planned into how thick the screed lays it.

Behind the rollers walks the QC tech with a nuclear gauge, shooting density as a percentage of the mix's rice value. The next morning a crew cuts cores, and those numbers, rolled into a pay factor, decide whether the contractor gets a bonus or a deduction.

Where it shows up later

Every mistake made in the dark comes back in daylight. Low density lets water in, which leads to raveling and, eventually, a pothole. Mix that is too fat ends up bleeding or rutting in the wheel paths. A bad joint unzips first. And a low spot nobody caught becomes a birdbath after the first rain.

Five words to learn first

• Tack: the glue between layers. Let it break.
• Head: the roll of mix in front of the screed. Keep it steady.
• Screwman: the person running the screed, and the one yelling.
• Bump: what the raker does to the joint.
• Shoot: take a reading with the gauge or the heat gun.

With those five, the yelling at two in the morning starts to sound like what it is: a crew racing the temperature of the mix.