Federal Highway Research Institute

Relaxed hybrid

Evaluation of the technically and economically expedient renewal of a concrete surface with asphalt superstructure

In lower load classes, the concrete surface can be covered with asphalt depending on its structural condition.

The photo shows the relaxed pavement


Basic renewals of concrete pave-ments are usually carried out as full depth construction. In addition, in the absence of compulsory height points, renewal by overlay installation is an option – a technically and economically expedient method of construction. For this purpose, the concrete pavement must be relaxed in advance.

According to the guidelines for the standardization of pavement design (RStO), for heavily loaded sections the required layer thick-ness of the asphalt pavement should be designed according to the guidelines for the analytical pavement design of traffic areas with asphalt surface course (RDO Asphalt). Alternatively, in the lower load classes (Bk) – for example Bk 3.2 – the concrete surface can be covered with asphalt (total layer thickness 18 cm) depending on its structural condition.


The research project will be im-plemented by TPA GmbH and scientifically supported by BASt.

On the duraBASt, a concrete pave-ment with a thickness of 24 cm will be paved on the unbound layers. After the pre-defined 2-month lay period, the pavement is relaxed using an Impactor and covered with an asphalt package consisting of an asphalt base layer (10.5 cm) and an asphalt surface layer (3.5 cm).

The various construction activities will be monitored by BASt and prepared for later investigations by the installation of sensors. Load capacity measurements and compaction checks are carried out on the unbound layers. After the concrete has been constructed, both bearing capacity and ground penetration radar measurements are used again. Shortly before and after relaxation of the concrete surface and during asphalt paving, these measurement methods are also applied to the various layers.

Accelerated Pavement Testing (APT) is carried out by BASt using the standardised APT test program. The program is adapted to the project objectives. As a rule, the initial load of around 3 million rollovers is applied within 80 days and constantly monitored by measurement tech-nology.


Das photo shows the logos of the projectpartners

Within the framework of the project, characteristic values for the assessment of this type of maintenance and renewal measures for relaxed concrete pavements will be generated. These can support the design in the future. The aim is to reduce the layer thickness of the pavement. Thus resources will be conserved.

Construction site logistics can also be optimized: Fewer transports are required for demolition, but also for new construction.