Compaction Concepts

Compaction ConceptsAsphalt, HF-rollers and concept comparison

Amplitude

The nominal amplitude is described as half the travel distance (vertical or horizontal) of the drum. As the counterweight rotates, the drum moves oppo-site to the counterweight. This means that when the weight is at its highest position, the drum is at its lowest point. The amplitude plays a great part in determining the maximum layer thick-ness for a roller. A high amplitude (and static linear load) enables the roller to work on a thick layer. Low amplitude limits the depth effect, but also reduces the risk of aggregate crushing.

MiscellaneousThe rolling speed has an influence on the compaction performance, it should not exceed5-6 km/h to assure adequate compaction. Often, lower speeds are required for the first passes on hot asphalt. Several other parameters affect the end-result of an asphalt job.Please refer to “Compaction and Paving, theory and practice” for further details.

Static linear load

The static linear load is calculated by dividing the part of the total roller weight carried by each drum by the width of the drum. Static linear load is normally presented in kg/cm, kN/m or PLI (pounds per linear inch). The static linear load has an influence on the ability of a roller to reach a high degree of compaction. In general, a roller with a higher static linear load will achieve a higher degree of compaction and /or deeper compaction.

Frequency

The vibration frequency must be selected in relation to the material to be com- pacted and the amplitude of the roller. Through research and experience it has been found that higher frequencies are outstanding for asphalt com-paction compared to lower frequencies. In general, if a lower amplitude is selected, the frequency should be increased to account for the loss in drum acceleration. This is the reason why you may run into inferior compaction results if using an asphalt roller with a too low frequency. High frequency also reduces the risk of rippling.

This folder describes the different compaction concepts available on the market. It is an objective comparison on how they function and perform. Several parameters affect the compaction performance of a roller and the most significant are described below. However, only practical testing can truly decide the performance of a roller or show the real difference between machines or concepts.

Compaction Parameters

Nutation

Four counter rotating eccentric weights generate a horizontal movement, transversal to the rolling direction as well as an oscillatory motion.

Oscillation

Two rotating eccentric weights placed away from the drum centre will generate an oscillatory motion of the drum. This means that, as opposed to the two previous, vibrating systems, the drum does not move its axis of rotation, but rather oscillates around it.

Directional vibration

By using two, counter-rotating eccentric weights in the centre of the drum it is possible to create a directional vibration. With a fixed amplitude, the direction of this fixed amplitude is varied from fully vertical to fully horizontal.

Static compaction

A static compactor works with the static linear load as the only compaction parameter. Compared to a vibratory roller the linear load has to be significantly higher to make the static roller an efficient compaction tool. Despite this, it has a limitation of about 50 mm when it comes to layer thickness. Variation in the static linear load is done by ballasting the roller.

Traditional vibration

The vibration is generated by one or more eccentric weights rotating on one shaft, centred in the drum. By changing the eccentric mass it is possible to generate different amplitudes. With this type of system the drum will vibrate in a revolving motion generating a “circular amplitude”.

Compaction Concepts

No ground vibration

Uncomplicated system

Well-known concept

Asphalt compaction on bridges

Thin asphalt layer compaction

Less ground vibration


Thin asphalt layer compaction


Less transversal ground vibration

Conclusion - traditional and directionalSimilar in compaction performance when operated manu-ally. However, the automated directional vibration system is questionable on asphalt, due to difficulties in measuring the stiffness accurately at the right depth. Too many alter-native settings confuse the operator. A less complicated system will, in general, cause a lesser need, and cost, for maintenance and repair. The directional vibrating roller only has one drum with this type of system. The other has a traditional vibration system, meaning that if one wants to run fully automated, one would have to shut the traditional

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vibration off and do twice as many passes as with both drums vibrating.

Static, oscillating & nutationSimilar concepts as far as compaction performance is con-cerned. The most economical way is to achieve compaction with the least complicated equipment, a static roller. They are all suitable for compaction of thin asphalt layers.

Limited depth effect

Number of passes required

Limited depth effect

Number of passes required

Drum shell wear

Complicated vibration system

Limited depth effectNumber of passes required on thick layersDrum shell wearComplicated vibration systemOnly one drum oscillating

Advantages Disadvantages

Suitable for all materials

Most effective method

Good depth effect

Achieves homogeneous compaction

Uncomplicated vibration system

Most effective method (vertical)

Good depth effect (vertical)

Thin asphalt layer compaction (horiz.)

Homogeneous compaction (vertical)

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Ground vibration

Drum shell wear (horiz.)

Ground vibration

Complicated vibration systems

Only one drum directional

35 40 45 50 55 60 65 70 75 80 85 90

Hot asphalt mix, maximum layer thickness 5 cm

Frequency (Hz)

100.5 %

100 %

99.5 %

99 %

98.5 %

98 %

97.5 %

2 3 4 5 6 7 8

101 %

100 %

99 %

98 %

97 %

96 %

7 km/h5 km/h


Low amplitude/High FrequencyNew applications have forced the development of new machines, as a result we have the higher frequencies available. The compaction effect depends on the amplitude and frequency (as well as the static linear load). If the amplitude is reduced, the frequency has to increase accordingly. This will create a machine with excellent compaction performance for thin layers on small amplitude (high frequency), and that works equally well on thicker layers on the high amplitude (lower frequency).

Rolling speedAn increased frequency does not mean that the compaction speed can be increased. Even though the impact spacing can be maintained at a higher speed with a higher frequency, the compaction result will suffer. There are also other problems, a high speed during the initial passes will create a bow wave that can result in cracks and/or bumps in the surface. Consequently, keep the rolling speed in the 3-6 km/h range. An even lower speed may be required for the initial passes.

Aggregate crushingThe use of larger aggregates in thinner layers increase the risk of aggregate crushing, particularly when using machines with traditional amplitudes and frequencies. This risk is significantly reduced when a small ampli-tude/high frequency machine is used.

Compaction performanceThe combination of low amplitude and high frequency is developed for compaction of thin asphalt layers. For this application it is the most suitable solution, both with regards to compaction performance, and to avoid aggregate crushing.

Vibratory asphalt compaction requires a lower amplitude and higher frequency than soil compaction. This is a well-known fact, ever since compaction of asphalt with vibratory rollers was first introduced in the 1950’s. Traditionally, the frequency has been around 50 Hz and amplitudes ranging from 0.35 up to 1 mm. However, since the trends in the asphalt industry are moving towards thinner layers and larger aggregates, the compaction equipment needs to develop accordingly. Thinner layers require smaller amplitudes to limit the depth effect and minimize aggregate crushing. To maintain the compaction effect the frequency needs to be increased. This is where the High Frequency asphalt compaction begins.

Asphalt compaction

N.B.: All graphs are based on results from full-scale testing of asphalt rollers on hot asphalt mix.

0 1 2 3 4 5 6 7 8

101 %

100.5 %

100 %

99.5 %

99 %

98.5 %

98 %

97.5 %

97 %

70 Hz40 Hz


0 1 2 3 4 5 6 7 8

101 %

100.5 %

100 %

99.5 %

99 %

98.5 %

98 %

97.5 %

97 %

70 Hz40 Hz


Growth of compaction for 40Hz and 70 Hz respectively (equal amplitudes). Note that the 40 Hz curve never reaches the required, minimum degree of compaction.No. of passes

Table 1

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Vibration Frequency:Amplitude: 0.21 mm

- Compaction of thin layers requires low amplitude. This, in turn, requires a high frequency to generate sufficient compaction energy.

- Low amplitudes minimize the risk of aggregate crushing.

- A high frequency does not allow higher rolling speeds, the compaction result may decrease below required levels if the speed exceeds 6 km/h. This means that more passes will have to be done.

- A roller with a low amplitude/high frequency setting for thin layers and a high amplitude/lower frequen-cy setting for thick layers is a very versatile machine. It can be used for base and binder courses as well as thin wearing courses.

Conclusions

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The graph shows the relationship between amplitude and frequen-cy and the desired degree of compaction at the 99 % line. The roller was tried with one, fixed amplitude and a variety of differ-ent frequencies. It shows that for a specific amplitude there is a frequency where the compaction is most efficient.

Table 3

Frequency (Hz)

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Amplitude: 0.21 mm

2 3 4 5 6 7 8

101 %

100 %

99 %

98 %

97 %

96 %

7 km/h5 km/h


Compare the number of passes needed to reach the required degree of compaction. If the speed is increased from 5 to 7 km/h, it will take 50 % more passes to reach the required minimum degree of compaction (six passes instead of four).No. of passes

Table 2

Amplitude: 0.23 mmFrequency: 70 Hz

Minimum requirement

Operating speed:

Minimum requirement

0 1 2 3 4 5 6 7 8 9

102 %

101 %

100 %

99 %

98 %

97 %

96 %

Oscillating Directed

Stone mastic 16 - 70/100, 40 mm

CC424HF

0 1 2 3 4 5 6 7 8 9

102 %

101 %

100 %

99 %

98 %

97 %

96 %

95 %

Oscillating Directed

Asphalt base 22 - 160/220, 70 mm

CC424HF

No. of passes

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No. of passes

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allThe CC424HF and the oscillating roller both

require six passes to reach the target degree of compaction. The directed vibration roller required eight passes to reach the target air void content.

Compaction concepts - comparison

Practical testing is the only reliable way to show performance differences between the concepts. These tests were carried out under similar circumstances and the compaction temperatures were closely monitored for all concepts to assure an impartial result. The tested concepts were the CC424HF with circular (traditional) vibration with a low amplitude and high frequency, directed vibra-tion (automatic) and oscillation. (N.B.: The oscillating roller has one drum with traditional vibration and one oscillating).

Asphalt mix SMA 16-70/100 AG 22 - 160/220

Max nominal particle size (mm) 16 22

Penetration (mm*10) 70-100 160-220

Required compaction, air void content (%) 2-5.5 3-8

Required compaction, degree of compaction (%) 97.3-100.9 96.9-102.1

Compaction temperature, min/max °C 90-135 80-125

Layer thickness, desired, mm 40 70

Layer thickness, mm 41-42 67-74

The CC424HF required six passes to reach the target degree of compaction. The automated directed vibration and the oscillating rollers both required eight passes to reach the target air void content.

Concept CC424HF Directed Oscillating

No. of passes required 6 8 6

Concept CC424HF Directed Oscillating

No. of passes required 6 8 8

Thin layer compaction proved to be more efficient with the circular vibration (low amplitude and high frequency) and the oscillating rollers. The auto-mated directed vibration roller required additional passes to reach specifications. However, the lower the amplitude of the machine during compaction, the lesser the risk of aggregate crushing in thin layers of asphalt. This proves to be an advantage for the Dynapac HF machines. The traditional vibration has a rapid growth of compaction on thicker layers

and is able to save two passes compared to the other compaction concepts. This means substantial savings in roller effort and machine input for any given job. It also proves the flexibility and efficiency of the HF concept. To have one, very low, amplitude for efficient compaction of thin layers without crushing and a high amplitude for compaction of thick layers to the correct degree of compaction is a winning concept.

Conclusion

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Dynapac Compaction Equipment AB, Box 504, SE-371 23 Karlskrona, Sweden. Tel: +46 455 30 60 00, Fax: +46 455 30 60 30We reserve the right to change specifications without notice. Photos and illustrations do not always show standard versions of machines.

The above information is a general description only, is not guaranteed and contains no warranties of any kind.

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Compaction Concepts