Longitudinal joints: smooth at handover, open after the rains

The longitudinal joint is usually the first part of a road to fail, and it fails quietly. At handover it is smooth and looks great. A few rainy seasons later it starts to ravel, the edge opens up, and water finds its way down into the layers below.

The catch is that smooth does not mean dense. The eye may approve; the density gauge does not always agree. This article covers the most common case: a hot lane paved against a lane that has already cooled.

Prepare the edge before the next lane

Keep the edge of the first lane straight, or if it curves, curve it evenly so the next lane can hold a steady overlap. The screed end gates should seal well so mix does not escape under the edge. When you extend the screed, the conveyors and augers have to deliver mix all the way out, or the outer edge will always run short.

Before the hot lane goes down, coat the vertical face of the cold edge with tack or joint adhesive as per the approved method. A tack coat on the layer below does not do this job, because the vertical face is where the two lanes actually have to bond.

The hot side needs enough mix

The hot lane must go down slightly higher than the cold edge, by the amount the rollers will press out of it. For ordinary dense-graded mixes that extra is often around a quarter of the thickness, but the real number comes from a test strip. That extra height is there to be compacted, not a lip to be raked away.

If the hot side is laid level with or below the cold edge, the drum ends up riding on the cold lane before the hot mix next to the joint is dense. This is called bridging. The roller keeps rolling, the vibration keeps humming, and the one strip that needs compaction most gets the day off.

Overlap just enough, rake less

The hot lane only needs to overlap the cold edge by two or three centimetres, depending on the joint type and the test strip. Too little and the joint stays open; too much and someone has to rake it.

A raker flicking spare mix from the joint out across the new lane looks very hard-working. Unfortunately it is hard work in the wrong place: the joint loses exactly the mix it needed, and the scattered material leaves rough patches on the new mat. With the right thickness and overlap, there is almost nothing left to rake.

Where the first pass goes

When a cold lane supports the joint, a common approach is to make the first pass on the hot lane with the drum edge about 15–30 cm from the joint. That pass pushes the hot mix toward the joint. The next pass overlaps onto the cold lane by around 10 cm to close it up. Rolling straight onto the cold edge from the start just gives you bridging again.

An unsupported edge, with no lane beside it, is a different story. Rolling too close tends to push the edge out and crack it lengthwise. Tune the rolling pattern for unsupported edges separately on the test strip instead of copying the joint pattern.

Test the joint on its own

Check density at the joint separately, as set out in the quality plan, rather than assuming it from mid-lane results. A good number in the middle of the lane says nothing about the joint. If the joint comes in low, look at four things: mix on the hot side, overlap, temperature during rolling and where the first pass went.

An open joint loses material bit by bit under truck wheels and lets water into the layers below. A smooth joint after the finish roller is a good sign, but it is not proof of density. You have to measure it.

A good joint also needs a paver in good shape: end gates that seal, augers that reach the edge, a grade sensor that tracks the old lane properly. Vu Gia has the parts and the technicians for these jobs; get in touch and someone will come out to take a look.