An increasing number of road bridges in Germany shows deficits in terms of structural safety and durability. Replacing these structures by new ones is cost-intensive and requires considerable material and human resources, which are hardly available to the necessary extent given the size of the task to be accomplished. A look at neighbouring countries shows that the repair or strengthening of bridge superstructures by means of thin reinforced cover layers made of ultra-high performance concrete (UHPC) represents a cost-effective and durable alternative to reconstruction. The special suitability of UHPC for this field of application results from the impermeability of its microstructure and its resistance to all forms of chemical and physical attack, so that UHPC can also be used for bridge decks without further protective measures. In this aspect as well as regarding its mechanical properties, UHPC differs fundamentally from normal and high strength concrete. In the first part of this article, the various possibilities of applying UHPC with existing bridges are presented and the special material properties are explained. The present second part of the article deals with the production of UHPFRC, the application on-site and quality assurance measures. As with part 1, the information can be used, for example, as a basis for project-related approvals, for developing or supplementing technical guidelines and for drafting the construction contract.

Recent concretes like ultra-high performance concrete (UHPC) are comprised of large quantities of fines following a well-defined gradation to fill the voids between coarser particles, such as cement grains. The filling of voids displaces water, which positively influences the flowability. As the rheological properties of these fresh concretes depend mainly on the forces acting between the fines due to their high specific surface areas, understanding of these forces has become crucial. In this study, the atomic force microscopy technique of colloidal probes has been used to study the adhesive forces acting between individual silica particles placed in superplasticizer and electrolyte solutions, as silica exhibit the largest fraction of inner surface in common UHPC mixtures. Pairs of individual amorphous glass particles were approached and retracted from each other. Using this technique and silica particles as a model system, several commercial superplasticizers could be evaluated regarding their influence on the interparticle adhesion.

Within the last decades, the performance of concrete has been significantly improved by applying different kinds of micro- and nanoparticles and by applying analytical methods from fundamental research disciplines that had not been used for construction materials before, e.g. atomic force microscopy. One prominent result is Ultra-High Performance Concrete (UHPC) with its steel-like compressive strength which allows for slender but nevertheless very long lasting and thus sustainable concrete structures. On top of that several research projects performed at the University of Kassel, which this contribution likes to review, aim at making concrete an impervious, ceramic-like, acid resistant multifunctional “smart” material with added values by further changing its nano- and microstructure and/or applying reactive coatings e.g. with the ability to degenerate air pollutants.

For decades, damaged asphalt or concrete pavements in the U.S. were repaired by applying the whitetopping“ technique. This measure covers the damaged pavement with a new layer of a thickness between 15 and 25 cm, in order to re-establish its viability and safety to traffic. In a research project by the University of Kassel in cooperation with the Federal Highway Research Institute (BASt), it was investigated whether this construction method could be more durably and sustainably realised by using a conventional road paver, if only 6 to 8 cm thick layers of steel fibre reinforced High Performance Concrete (HPC) or steel fibre reinforced Ultra High Performance Concrete (UHPC) was used. First, the road construction was dimensioned with a finite-element program. The appropriate thickness of the structure was designed considering the contribution of fibres and the mesh reinforcement and to restrict the crack width to 0.1 mm only to prevent corrosion of the reinforcement. Then the structural behaviour was determined under fatigue load on a model structure in the laboratory. After a sufficient capacity was confirmed, a first test road – a parking lot at the German freeway A2 – was built in 2008.