Cracking of concrete pavements - Effects of alkali-silica reactions (ASR), Phase I: In-situ investigations on concrete pavements with/without cracks

Duration: 2005-2007

In recent years, cracks have been detected in various concrete pavement lots all over Germany. In most cases, no clear crack cause could be assigned to these cracks. Several causes, both load-dependent and load-independent, can be considered for these cracks, which can overlap in time and space. In this context, these cracks are repeatedly associated with an alkali-silica reaction (ASR). However, it is unclear to what extent such ASR contributes significantly to cracking in concrete pavements. In order to get closer to this question, in Phase I of this research project, in addition to targeted literature studies, in particular evaluations of construction files and relevant ASR investigations on selected concrete pavement lots (with and without cracking) were carried out. The results showed, among other things, that cracking tended to increase with increasing temperature during the production of the concrete pavements (above about 15-20°C). Similarly, in one case where very unfavorable hygric conditions (high groundwater level) were present, extreme cracking had occurred. In addition, more intensive ASR was observed in core samples taken from already crack-damaged roadway lots than in samples taken from undamaged roadway lots, although both showed comparable ASR potential in terms of concrete composition. It can be deduced from this that the actual crack initiation is significantly more influenced by thermal / hygric constraint stresses as well as load stresses than by ASR, the latter contributing mainly to damage propagation in the presence of initial cracks (also microcracks). In the investigations, an external alkali supply led to greater strains than a purely moist storage without such external alkali supply. This increase in elongation was particularly pronounced when the specimens were taken from stretching lots that had already been damaged by cracking. The drill core investigations also revealed different strains and thus a different ASR potential for concretes with different aggregates (granodiorite/greywacke, quartz porphyry), for which the other boundary conditions (in particular the alkali input by the cement) were the same. However, even with the exception of one section batch (BAB A10-8 with 2.89 kg/m³ total alkali content), no ASR to a damaging extent could be detected in the presence of alkali-sensitive aggregates if the total alkali content in the concrete was kept below 3.0 kg/m³.