Cracking of pavements from concrete effects of alkali (ASR), Phase II: Laboratory investigations on the effects of pre-stressing and external alkali supply

Duration: 2006-2008

Cracking of concrete pavements has been observed recently and is often associated with ASR. The observed cracks are more pronounced in the main lane than in the less loaded second lanes and hard shoulders. As part of Phase I of this research project, in-situ investigations were conducted on cracked and uncracked concrete pavements. However, the following equally important questions could not be quantified in more detail:
•  How do changes (microcracks) imprinted in the concrete structure by preloading, whether caused by external traffic loads or by load-independent restraint stresses, affect the development of ASR and further cracking?
•  To what extent does an external alkali supply (de-icing agent) intensify ASR in concrete pavements?
In Phase II of this research project, the superposition of thermal / hygric stresses or load stresses with stress gradients from an ASR was therefore verified by testing. In summary, the tests at the Technical University of Munich showed that rapid, abrupt deformations are significantly triggered by temperature differences. After twenty alternating storage cycles, however, no damage due to ASR could be detected on the concrete specimens used. The investigations at the Ruhr University Bochum showed that as a result of the cyclic pre-damage caused by, for example, 5 million load cycles, the rel. dyn. modulus of elasticity dropped by around 10%. This decrease could also be measured in-situ and is associated with a degradation in the microstructure of the concrete. Furthermore, the whitewash tests showed that with increasing pre-damage, the NaCl solution penetrated up to approx. 40 % (at 5 million load changes pre-damage and 2 million rollovers) deeper into the concrete structure than without corresponding pre-damage. Thus, the degree of pre-damage present before rolling has a significant influence on the penetration behavior of the NaCl solution. At the Bauhaus University Weimar, ASR investigations were carried out on pre-damaged and undamaged concrete with and without external alkali supply. The results of the ASR performance test for the concrete with alkali-sensitive rhyolite splits showed a clear influence of the external alkali supply. The influence of cyclic/dynamic pre-damage was not as significantly expressed in these tests, but this may have been due to the nature of the pre-damage and the limited duration of the tests. Nevertheless, after 12 test cycles, the ASR-induced elongation of the mechanically pre-damaged specimens was around 30% greater than that of the non-pre-damaged specimens. In the present test scenario, this corresponded to a time advantage of about one cycle (3 weeks).