Gulf Construction Online - Dynapac unravels paving secrets

Compaction & Paving Equipment

Dynapac unravels paving secrets

01 September 2005

Swedish compaction equipment manufacturer Dynapac, has in its recent study found that traditional low frequency, high amplitude tandem vibratory rollers are less suitable for compacting the new thinner surfacings of up to 40 mm thick.

“The combination of low frequency and high amplitude, needed to achieve the desired density and depth of compaction of the thicker layers, have a tendency to over compact thinner layers and considerably increase the risk of crushing the larger aggregates used in the new and much thinner stone mastic asphalt surfacings,” says a company spokesman quoting the study.
Using its comprehensive research, testing and training facilities situated at its International High Comp Centre in Sweden, Dynapac has embarked on an extensive programme to design, develop, and produce a completely new family of tandem vibratory rollers specifically for compacting the thinner stone mastic asphalt surfacings, says the spokesman.
“Our research and development engineers were already aware of the five main parameters, which have varying degrees of influence on compaction; these being drum amplitude, frequency and static linear load, the rolling speed and number of passes,” he says.

Static linear load
Static linear load – the weight of the roller carried on each drum divided by the drum length – has perhaps the greatest influence on a roller's ability to reach a high level of compaction. Generally a roller with a high static linear load achieves the specified density in fewer passes.

Drum amplitude
Drum amplitude also plays a major part in determining the maximum layer thickness a specific roller can successfully compact. A roller's nominal amplitude is half the total vertical or horizontal travel distance of the drum and is generated by a counterweight rotating inside the drum. When the counterweight is at its highest position, the drum is at its lowest point, and vice versa. Consequently the relationship between the weight of the drum and the counterweight affects the roller's drum amplitude. A heavy drum with a light counterweight will generate low amplitude and a light drum with a heavy counter weight will generate high amplitude.
A high amplitude and high static linear load enables a roller to work on a thick asphalt layer.
Through research and testing Dynapac engineers have established that as compaction develops and builds up in a thick layer, the roller must be able to switch from high to low amplitude in order to properly compact the upper part of the layer.

Frequency
The frequency of drum vibration – another important factor in quality compaction – must be selected in relation to the amplitude of the roller. Dynapac has established that frequencies around 50 Hz (300 vpm) and above are outstanding compared to lower frequencies and in combination with suitable amplitudes.
Generally if low amplitude is selected, the frequency should be increased to account for the loss in drum acceleration that is a result of the low amplitude. “This is a possible explanation why contractors may experience inferior compaction when using an asphalt roller with suitable amplitude but with a frequency considerably lower than 50 Hz,” he points out.

Rolling speed
Rolling speed does not have as great an influence on compaction as the static linear load, amplitude, but is more influential than frequency. Rolling speed has a greater influence on compaction than frequency, if the frequency is optimised according to the amplitude.
A smaller variation in frequency, approximately within 10 per cent, has very little influence on the compaction result. A high rolling speed during the initial passes will create a bow wave in the asphalt that can result in cracks and bumps in the surface. Lower speeds are sometimes required for the first passes on hot asphalt to avoid transverse cracking and roll out the bow wave. But there is no economic benefit to roll at a low speed, as it is not possible to correspondingly reduce the number of required passes.
Tests and trials have shown that rolling speed should not exceed 6 km/h to ensure adequate compaction. If speed is increased from 5 km/h to 7 km/h it will take 50 per cent more passes, six instead of four, to reach the required minimum level of compaction.
Dynapac's engineers focused on combinations of the four main parameters during their research into finding practical solutions for compacting the new thinner stone mastic asphalt wearing courses. Following exhaustive tests and trials, Dynapac established that the best solution for the successful compaction of thinner asphalt surfacings, while at the same time minimising the risk of crushing the larger aggregate, required a combination of low drum amplitude of about 0.2 mm and high frequency oscillation of about 70Hz to generate sufficient compaction energy.
This was a complete reversal of the high amplitude and low frequency required for the compaction of thicker layers.
In 1996 Dynapac launched the CC222, and CC232 – the first low amplitude, high frequency hydrostatically driven tandem vibratory rollers introduced specifically to cater for the thin stone mastic asphalt surfacings. Since then Dynapac has steadily increased the range of these low amplitude high frequency rollers.
The company has also been developing a family of tandem vibratory rollers with dual amplitude and dual frequency. These extremely versatile, dual-purpose tandem vibratory rollers, such as the CC422 and CC522 allow contractors to compact traditional thick base and binder courses and thin wearing courses with just one machine.
The dual purpose roller can be initially set on the high amplitude and low frequency setting to achieve the required density and depth of compaction in the thicker base and binder layers. The same roller can then be simply switched over to the low amplitude and high frequency setting to complete the compaction of the upper part of the thicker layer and compact the final thin overlaying wearing course. They can also operate as deadweight machines without vibration to perform, if required, a final sealing finishing pass of the wearing course.