Not dense enough? Check the temperature before adding passes

There is a popular belief on job sites: if the density is not there yet, throw a few more roller passes at it. It sounds reasonable, right up until the mat has cooled. After that, extra passes mostly add to the fuel bill while the density barely moves.

Hot mix is a bit like cooked rice. Warm rice packs into a ball with one good squeeze; cold rice falls apart however hard you press. Compaction works the same way: it has to happen while the mix is hot enough for the stones to shift and lock together.

Watch the temperature, not just the pass count

The temperatures for starting and finishing compaction vary with the binder and the mix, so one pair of numbers will not fit every job. Thin lifts, a cold base and wind all cool the mat quickly and shrink the time you have to work. The safest approach is to set the compaction window on a test strip with the actual mix.

The breakdown roller can stay close behind the paver once the mix is stable enough. If the mat shoves in front of the drum or shows fine cracks, do not reach for more force straight away. Check temperature and roller settings first; that is usually where the problem is.

A fast roller leaves a washboard

A vibratory drum hits the mat in time with its vibration. The faster the roller travels, the fewer impacts each metre of road gets, and at some point the surface starts to ripple like a washboard. At typical frequencies, a brisk walking pace of about 4–5 km/h is the usual working speed, but check the manufacturer's guidance for your machine.

When the rollers fall behind the paver, the first instinct is to speed them up. That lowers density and adds ripples at the same time. It works better to bring in another roller or slow the paver down so everyone keeps the same rhythm. A paving train is a team, not a race.

Bigger amplitude is not always better

High amplitude sounds powerful, but it does not automatically give better results. In many cases it is better to start with high frequency and low amplitude, then adjust. Thick lifts and stiff mixes are where high amplitude really earns its keep.

On very thin lifts, under about 25 mm compacted, static rolling or vibration off is usually the way to go. Vibrate hard on a thin lift and the drum bounces like a motorbike on a bumpy road: stones crack and the surface ripples. Settings that work well on a thick lift should stay on the thick lift.

One reading does not tell you much

An infrared gun tells you the temperature of exactly one spot, the one it is pointed at. A cold streak half a metre away stays invisible. A thermal camera or a sensor bar behind the screed shows the whole picture. Cold streaks often turn up where the paver stopped to wait for a truck, or from the last of the load in the truck bed.

When you see a cold streak, note the chainage, compare it with truck arrival times and paver stops, and check density at that exact spot. Surface temperature is a warning sign; it does not replace a density test.

Density has to be measured

On the test strip, measure density after each group of passes to find the point where it stops rising. That gives you the rolling pattern for the job. Once the mix has cooled or reached its limit, more passes will not push density any higher, so look at timing, temperature and the rolling pattern instead of adding passes.

The rolling log should record roller type, vibration setting, speed, passes over each point, temperature and test results. Agree on how to count passes from day one: going over a point and coming back over it counts as two. Sounds obvious, yet plenty of site meetings have argued about exactly that.

If you are doing everything right and the roller still vibrates weakly, the exciter sounds odd or the frequency will not come up, the problem is in the machine. The Vu Gia service team is happy to come out and check your roller with your crew.