Road building stars

Experts explain the procedures, on-board technologies and equipment that ensure a compact, smooth and uniform asphalt mass in road projects

As anybody knows, most of safety, comfort and durability of a road depend on the quality and the way that the asphalt was poured. First, to ensure long life, the pavement has to offer stability to vehicles, to support heavy weights and to ensure low permeability to water. To obtain these results, compaction is one of the most important phases of pavement construction. After all, there is no help to use a good-quality asphalt concrete if technical mistakes occur in this phase. It is exactly in this point that compactors—a classic equipment of road construction used to compact the track, eliminating the spaces located inside the layer (“voids”) to ensure a firm, smooth and uniform pavement—come in.

There are basically three types of compactors. The first is the tandem roller, equipped with two smooth cylinders that may be vibratory or not (in this last case it is called static). Multipurpose, this equipment may compact all types of asphalt mixtures. In the static mode, the weight of the rollers is responsible by the compression. In the vibration mode, the compaction is generated by an eccentric (out of center) weight that turns in high speed inside the cylinders, making them move up and down in an amplitude (vertical distance from the pavement) of 1 to 1.5 mm and a frequency of 30 hz (30 times per second). These movements generate shock waves that reduce internal friction between the aggregates and the particles of asphalt mass, intensifying and making easier its compaction.

Another common type of compactor available in the national market is the tire static compactor. In this case, only the weight of the equipment is responsible by the compaction work. According to the experts, compactors with an even number of tires—what ensures the same distribution in each axle—are the most efficient. To prevent marks on the paved surface (left by the space between two wheels), the wheels of one axle are placed in a way to cover the spaces of the other axle. In other words, there is an overlapping between front and rear wheels, to cover 100 percent of the compaction width. Finally, the third type includes the combined compactors, which have a vibratory (or static) cylinder in the front and tires in the rear axle.

PROCEDURE

Generally, the construction of a road starts with the earthmoving of the area that will become the asphalt bed. Next, after making the sub-base and base,  the asphalt concrete is placed on the soil and pre-compacted by a machine called asphalt finisher, which uses a system of vibratory heated plates. But the compaction of this layer of the pavement is effectively carried out by the rollers. This demands to establish an appropriate sequence of works. “The correct procedure is, at first, use a vibratory roller to compact the thickest part and, in sequence, a tire compactor to carry out a surface finishing, what is called sealing, explains Juliano Gewehr, expert on product and application of Ciber Equipamentos Rodoviários, dealer of the brand Hamm in Brazil. According to him, the wrong sequence is used several times. “Only the tire compactor or only the vibratory roller is used”, says the expert. “But the correct is to use both compactors in synchrony.”

However, the president of Bomag Marini Latin America, Walter Rauen, has a little different information. “The sequence of passing rollers varies in accordance with the contractor and the quantity of equipment”, emphasizes him. “Generally, the tandem roller comes just after the asphalt finisher. The tire compactor, which is the main responsible by the compaction, comes in sequence. Finally, another tandem roller carries out the final finishing removing the marks left by the wheels.”

The expert of Caterpillar for the paving market, Paulo Roese, contributes to the debate synthetizing the differences between both types of compression. “Static rollers have as main feature to carry out the compaction through their own weight”, explains him. “Vibratory rollers have a higher compaction power and tire compactors have as their main feature the flexibility of increase or decrease the pressure on the pavement through  reduction or addition of ballast or through changes in the tire pressure. Combined rollers are a good alternative for some works where they can join the benefits of both types.”

In any case, the temperature of the asphalt mass plays an important role in the sequence of compression. According to Gewehr, the entire process has to occur at a temperature between 100ºC and 140ºC. “The asphalt leaves the plant at an average temperature of 165ºC. And is losing temperature during its transport and pouring”, explains the executive. “Then, the ideal range for compaction is around 140°C, since it hardens when the temperature comes near to 100ºC. Of course the compaction in lower temperatures is feasible, but problems may occur. For example, when the vibratory roller passes on the already cold layer, cracks and fissures may occur. That is why the tire compactor has to be the last machine to pass on the finished surface.” Vibration also cannot be used on bridges and viaducts, since it may damage their structure.