The right technique

Asphalt must be able to cope with the expected transport loads as well as load burdens resulting from weather and temperature stress. Hans-Josef Kloubert, re-known expert for compaction techniques, explains the key points in compacting asphalt.

Asphalt layers of a road pavement should be able to reliably carry traffic loads and it must be able to discharge these loads to the substructure or sub-base, in order to prevent harmful deformation. Besides the mix composition and the paving, the compaction of the mix is of utmost importance with respect to the quality and the service life of the road. For asphalt compaction you need the right technique, knowledge and experience.

Compacting asphalt already starts with the finisher – in case you use a finisher with low pre-compaction you probably need breakdown rolling, which includes the use of too heavy rollers or early compaction with vibration that might affect the evenness of the layer or even cause undesired displacements and misplacements in the material. If you use a finisher with high initial compaction you can start earlier using vibratory rollers, final compaction can thus be achieved by only a few roller passes. This is because vibratory rollers are very powerful, versatile and require considerably less passes than static rollers. The vibration reduces the internal friction of the aggregates in the mix, so that the interaction between deadweight and dynamic load increases the density. Besides the static linear load, other factors like vibrating mass, frequency and amplitude are also decisive for the compaction effect. But also the number of passes – too many might cause harmful loosening of material and disturbances in the structure of the asphalt layer.

Today a great part of it can be provided by systems using intelligent compaction, vibration systems that are automatically controlling, optimising and documenting compaction, a technique where BOMAG has lead the way for many years. The core of these systems is based on directed vibration, offering an enormous range of compaction performance and depth effect from low-vibration surface compaction up to enhanced depth effect vibration.

To control, optimise and document compaction, BOMAG developed the Asphalt Manager. A system that in automatic mode monitors compaction progress and adjusts compaction performance even many times a second, which eliminates drum bounce and operator error. Asphalt Manager requires no special user training and the control panel is self-explanatory. During compaction the dynamic energy leads into the pavement and is optimally and continuously adapted to the actual conditions. Grain damages and disturbances of the asphalt layer are effectively prevented and the system limits automatically compaction power for example, where there is a risk of over compaction. At the same time asphalt mix stiffness and compaction progress, surface temperature, travel speed, exciter frequency and selected amplitude are continuously displayed to the roller operator and can be printed out on-site using the data recorder. Due to this adaptability and the continuous control and optimisation rollers with Asphalt Manager are highly suitable for the complete spectrum of asphalt applications. Drivers can also select a manual mode. This can be used in the case of the compaction of layers on bridges or pavements close to buildings, where low-vibration compaction might be recommended. However, for up to 90 percent of all applications drivers use the automatic mode.

Asphalt is the 'royal league' of compaction where quality is dependant on a huge variety of factors. Modern intelligent compaction facilitates significant quality. 

Hans-Josef Kloubert is Head of Application Technology at BOMAG. For the past 20 years he has been BOMAG's application expert in soil and asphalt compaction and compaction measurement, deeply involved in the development of new technologies.