Experimental study of compressive and tensile creep of substrate and overlay concrete

A popular method for rehabilitating or strengthening a RC structure is to add a concrete overlay. The durability of a RC structure strengthened by a concrete overlay is influenced to a large extent by restraint effects between the concrete overlay and the adjacent substrate concrete. They are mainly caused by shrinkage of the overlay concrete, which is restrained by the substrate concrete. Hence, tensile stresses are generated in the overlay whereas compressive stresses develop in the adjacent substrate concrete. In the desirable case of an intact concrete overlay, these stresses are reduced by creep strains. Nevertheless, in unfavorable cases they may trigger failure mechanisms like cracking or local delamination of the overlay. The intention of the present research proposal is to develop a sound basis for the evaluation of such restraint effects by an experimental study of compressive and tensile creep of substrate and overlay concrete, respectively, aiming at the prevention of cracking of concrete overlays or delamination of concrete overlays from the substrate concrete. The experimental program comprises compressive and tensile creep tests in terms of the age at loading and of different drying conditions as well as associated shrinkage tests. Comparison of basic creep strains and drying creep strains in compression and in tension, measured on cylindrical specimens of the same size, will allow a deep insight into the time-dependent behavior of substrate and overlay concrete in compression and tension, respectively. The specimens will be loaded at concrete ages of 2 days, 7 days and 28 days at 30% of the respective compressive or tensile strength. In order to improve the knowledge concerning the expected nonlinear creep behavior of overlay concrete in tension, additionally tensile creep tests will be performed at loading levels between 60% and 80% of the respective tensile strength. Furthermore, the time-dependent moisture content will be monitored by means of a non-destructive measurement system in the samples for investigating compressive and tensile creep as well as in the samples of the associated tests for autogenous shrinkage and drying shrinkage. Comparison of the time-dependent moisture content and the respective deformations will allow a detailed investigation of the Pickett effect under compressive and tensile loading by means of numerical simulations. Finally, the results of the proposed research project will be a valuable basis for the calibration of numerical models for studying the time-dependent behavior of concrete overlays added to existing concrete structures. The latter models allow simulating numerically the interplay of hardening, shrinkage and creep of the overlay concrete and shrinkage and creep of the substrate concrete and, consequently, they allow evaluating the restraint effects between the concrete overlay and the substrate concrete.

The increasing age of existing bridges and the steadily increasing traffic volume during the last decades pose huge challenges on the maintenance of existing bridge structures. A popular method for rehabilitating and/or strengthening reinforced concrete (RC) bridges is to add a concrete overlay. The durability of a RC bridge strengthened by a concrete overlay is influenced to a large extent by restraint effects between the concrete overlay and the adjacent substrate concrete. The restraint effects are mainly caused by shrinkage of the overlay concrete, i.e. a volume reduction of the overlay concrete due to hydration and drying of the overlay concrete. As shrinkage of overlay concrete is restrained by the substrate concrete, tensile stresses are generated in the overlay whereas compressive stresses develop in the adjacent substrate concrete. Tensile stresses in the overlay concrete may induce the initiation and propagation of cracks, resulting in damage of the bridge structure. In the desirable case of an intact concrete overlay, these stresses are reduced by creep, i.e. by the time-dependent increase of deformations under constant load without cracking. A primary aim of rehabilitating or strengthening a structure is to ensure an intact concrete overlay. Thus, the present research project focused on creating the basis for the evaluation of restraint effects between substrate concrete and overlay concrete. For this purpose an experimental study of the time-dependent material behavior of substrate and overlay concrete was conducted. It comprised the evolution of temperature of the hardening overlay concrete, the water adsorption and desorption of overlay concrete, shrinkage and creep tests for sustained constant stresses in compression and tension. The tests were performed on specimens subjected to different drying conditions and different ages at loading. The results of the research project were published in international scientific journals (Strain, Materials, Bauingenieur, Materials Today: Proceedings) and they were presented at scientific conferences (58. DAfStb-Forschungskolloquium in Kaiserlautern, 2nd International RILEM/COST Conference on Early Age Cracking and Serviceability in Cement-based Materials and Structures in Brusseles, 6th ECCOMAS European Conference on Computational Mechanics & 7th European Conference on Computational Fluid Dynamics in Glasgow (ECCM-ECFD 2018)). The results are a valuable basis for calibrating models for the numerical simulation of the time-dependent behavior of concrete structures, strengthened by concrete overlays. The latter models take into account the interplay of hardening, shrinkage and creep of the overlay concrete and shrinkage and creep of the substrate concrete. Thus, the restraint effects between the concrete overlay and the substrate concrete can be evaluated.