Long-term performance of bonded concrete overlays subjected to differential shrinkage

Includes bibliographical references (leaves 231-243).The performance of bonded concrete overlays relates mainly to the resistance against cracking and debonding. The associated failure mechanisms are largely a result of differential volume changes between substrate and overlay. The objective of this research was to develop analytical tools to facilitate the design of bonded overlays subjected to differential shrinkage. The experimental programme included the identification of fundamental strain characteristics and bond strength development of composite members in relation to different interface textures and overlay materials. Existing analytical models for the prediction of strains and stresses in bonded overlays were evaluated. Results from the experimental work indicated that existing models, which are based on simple beam theory, are deficient in modelling overlay strains in a realistic manner. The degree of overlay restraint was found to depend far less on relative sectional dimensions of substrate and overlay as commonly assumed. Based on fundamental aspects concerning strain characteristrics of bonded overlays identified through experimental tests and numerical simulations, an analytical prediction model was developed based on localised strain conditions at the interface

Robustness improvement of fresh concrete and mortar performance for challenging casting environments with focus on sub-Saharan Africa

For various concrete applications, the early properties are a major specification, determining the functioning, the mechanical properties, and the durability of structures. The rheological properties of cementitious systems are depending on the water-cement interaction as well as on interactions of chemical admixtures with the cement hydration. The cement hydration, however, is strongly affected by the environmental boundary conditions, such as the climate or the available equipment for the proportioning and dispersion of concrete. This paper presents strategies for the robustness improvement of fresh properties of cementitious systems for challenging and particularly warm climate conditions. After explaining the basic mechanisms affecting the workability, practical conclusions are drawn with special attention on the circumstances of sub-Saharan Africa, where the climatic conditions are difficult and the casting technology is often limited. Solutions are suggested, how despite the disadvantageous circumstances, highly elaborate engineered concrete and mortar can be applied safely and with consistent quality