Experimental investigations of effects of traffic-induced vibrations on young concrete

In the COUNT (Concreting under traffic) research project, the effects of traffic vibrations on young concrete have been extensively investigated. This research question has recently become more relevant since traffic closures often have to be imposed when bridges are renovated or widened, as there is a concern that the vibrations caused by traffic will damage the young (hardening) concrete. COUNT aimed to determine critical vibration values and underlying damage mechanisms based on three test series. Both harmonic and real bridge vibrations due to train, truck, and car traffic have been adopted, and the effects on the material concrete itself have been investigated, as well as on the bond behaviour between concrete and reinforcement, and the contact between existing and new concrete parts.
In test series 1, small samples of plain concrete were systematically excited by artificial vibrations during hardening. It was found that conventional traffic vibrations do not affect the strength of concrete per se. However, in test series 2, the experiments were repeated with reinforced specimens where the concrete was shaken while the rebar was fixed. It was found that the bond between the concrete and the reinforcement can be weakened if the vibrations are too great. Additionally, it was found that (differential) displacements are a more suitable measure for damage assessment than vibration velocity. Finally, in test series 3, entire concrete slabs were placed on a vibrating platform, simulating the traffic-induced vibrations of a bridge. The vibrating slab is in direct contact with a fixed slab of young concrete. Through these very realistic experiments, valuable insights could be found into the evolution of cracks, strength characteristics, and durability-related features.