Paper by HUBER BECKHAUS WIEGRINK SCHIEßL from ISCR 9th 2004 Instanbul Turkey
Depending on the environmental conditions during casting and hardening, actual temperature and moisture gradients in concrete pavement induce strains. These strains lead to deflections of the slabs or, when restrained, to stresses and in some cases, mainly in combination with traffic loads, to cracks. Concrete with recycled concrete aggregate (RC-Concrete) is more susceptible to shrinkage and swelling than adequate ordinary concrete and is therefore more sensible to hygral strains and stresses. In two layer pavements RC is used in general in the bottom layer, but different hygral (or thermal) expansion coefficients of top and bottom layer can implicate a greater deflection tendency due to the bimetal strip effect. In order to determine the influence of recycled aggregate on upward and downward curling of concrete pavement slabs, beams were subjected to different temperature and moisture gradients under laboratory conditions. To investigate the behavior under real conditions, several roads (one/two layers, with/without RC-aggregate) were examined in different seasons and weather conditions. These results will be compared to the surface profiles taken directly after setting. Laboratory observations show that concrete produced with recycled aggregate does not differ significantly in behaviour from ordinary concrete, when submitted to alternating moisture conditions. The first observations in practice confirmed the expectation that the deformation of field slabs, mainly on account of substantial restraint, is always lower than the values of the laboratory specimens. So the hygral effects and with them the influence of RC in pavement concrete - even if covered by a top layer with natural aggregate concrete seems to be negligible in comparison to the temperature effect.