Concreting under traffic: in-depth investigation of the effects of vibrations on young concrete

Regarding vibrations, extensive research has been conducted as far as the effects on both people and (historical) buildings are concerned. On the other side, investigations regarding harmful effects of vibrations on young concrete are very limited. However, transport authorities, construction companies and engineering firms are aware that traffic induced vibrations might afflict permanent damage to concrete structures when the vibrations take place in a certain time range after concrete casting. Regarding this “critical time range”, different values can be found in the literature ranging from 3 to 15 hours. Obviously, this depends on the specific hardening process of the concrete used, which is a function of its composition, the temperature, and other parameters. Therefore, bridges and viaducts often need to be closed for traffic in case that they have direct contact to new components made of concrete. Such a situation occurs frequently, e.g., when bridges and viaducts are reconstructed, extended, or partially restored.
The aim of the presented (ongoing) project is to investigate the influence of vibrations on young concrete in a more comprehensive and realistic manner than has ever been done before. Experiments with plain concrete as well as with reinforced concrete are being performed. Different concrete mixtures are used. Moreover, realistic situations of contact between existing vibrating concrete plates and newly cast concrete parts will be studied. A wide range of harmonic vibration signals as well as traffic induced signals with different intensities and frequencies are being applied. Following the vibration procedure, a variety of material parameters are determined on the test specimens as well as on reference specimens: compression strength, tensile strength, Young’s modulus, mass density. Laboratory tests are performed to identify flaws, cracks, and any changes in the concrete texture. The bond between concrete and reinforcement is investigated by means of pull-out tests. Experimental investigations are accompanied by analytical and numerical calculations. Results are verified by finite element analyses, in some cases also adopting multi-scale approaches.
Preliminary results indicate that plain concrete, in general, is not damaged by vibrations caused by common traffic situations. In most cases, density and strength quantities of specimens exposed to vibrations are, in fact, higher than those of respective reference specimens. So, it seems that vibrations acting on plain concrete within 3 to 15 hours after pouring have similar (positive) effects as vibrations induced immediately after pouring, in general. The latter fact is well known and is exploited on construction sites throughout the world on a daily basis (compaction by means of concrete vibrator). Nevertheless, this finding is interesting given the common (and most probably justified) cautiousness deployed by engineers in cases where new concrete structures are connected to existing ones exposed to vibrations. Hence, this finding urges the need to perform comprehensive investigations on reinforced concrete specimens as well as on contact surfaces exposed to vibrations.