Mat thickness won't settle? Put the screed crank down

Every paving crew has seen this one. Someone checks the mat, finds it a few millimetres thin and gives the depth crank a turn. Ten metres later it is still thin, so another turn, just to be sure. A bit further on the mat suddenly comes up thick, and the crank goes back the other way. By the end of the shift the road has a gentle swell to it and cars cross it like they are on a ferry.

The screed usually is not broken. It just reacts slowly: every adjustment takes a stretch of road before you see the result. And the crank is only one of many things that change the thickness.

Adjust, then wait

The screed is not bolted to the paver. It is towed on two long arms and floats on the mix, more surfboard than mould. Change its angle and it does not jump to the new thickness; it drifts there. Old hands reckon the paver has to travel about five tow-arm lengths before a change has fully played out. With 3 m arms, that is roughly 15 m of road.

So make one small adjustment, then wait. Have a drink of water, look at the sky, ask the roller driver how the family is doing. Just keep your hands off the crank. Turn it again before the last change has landed and the corrections stack up, and the finished surface will faithfully record every bit of impatience.

Steady beats fast

When paver speed changes, the push of the mix against the screed changes, and so does the thickness. Every time the paver stops to wait for a truck and pulls away again, the screed tends to settle a little and leave a transverse mark. The rollers can work on it all day; it sticks around like a biro mark on a white shirt.

Pick a speed the plant and the trucks can actually keep up with, not the top speed in the brochure. The brochure never has to wait for a truck. A paver that runs slow and steady all shift beats one that sprints and then sits idle. The rollers need to keep pace too. Nobody hands out medals to a paver that finishes miles ahead of its own crew.

Check the auger chamber before touching the screed

• Keep the mix in front of the augers at a steady level, around the centre of the auger shaft. Flooded one minute and starved the next means the screed gets pushed hard, then soft, and no operator alive can hold the thickness steady through that.
• Check the feed sensors on both sides and how each conveyor and auger is running. If one side keeps running short of mix, fix the feed system. Cranking the screed on that side just puts out a fire that will be back tomorrow.
• A low streak down the middle of the mat usually means mix is not getting under the auger gearbox. Check that the reverse flights in the centre are there and fitted the right way round. Reversed flights still spin with great enthusiasm; they just send the mix somewhere else.
• When you add screed extensions, extend the augers and end gates to match. Otherwise the outer edges run short of mix, and someone reaches for the screed crank again.

Measure in the right place and write it down

The mat right behind the screed is still as fluffy as a fresh sponge cake, so it has to go down thicker than the design thickness to end up right after rolling. For ordinary dense-graded mixes the extra is often around a quarter, but the real number comes from a test strip of that exact mix. Leave the last job's factor with the last job.

One small habit saves a lot of arguments: log loose and compacted thickness by chainage, along with every screed adjustment. When the site meeting gets heated, the notebook does the talking. If the loose mat is steady but the compacted thickness still goes up and down, look at the rolling pattern or the base underneath. Cranking the screed all day will only get you a sore arm.

If you have worked through all of this and the thickness still will not sit still, the screed may have a real problem: worn tow points, tired leveling cylinders, or a grade control that has lost its focus. Give us a call. The Vu Gia service team is happy to come out and take a look with your crew. We will bring our own water.