Experimental investigations on the influence of the short-term load history on the creep behaviour of normal concrete

Abstract

In the pursuit of slender, material-efficient, and durable reinforced concrete structures, the time-dependent material behaviour of concrete presents a significant challenge for engineers in both academia and practical construction. The stress-dependent creep deformations are particularly noteworthy, often significantly exceeding the elastic response of the structure and introducing risks of inaccurate predictions due to substantial material uncertainties. These can compromise the usability or safety of load-bearing structures. Besides material technology and climatic factors, creep is primarily influenced by the load history. However, this factor is neither sufficiently understood nor has it been actively exploited to control the creep tendency of concretes and its variability. This article examines the influence of controlled preloads on the creep behaviour of concrete within an experimental test program. The study is based on the hypothesis that applying mechanical stresses, particularly during the early concrete age, can compact and homogenise the material’s microstructure, thereby positively influencing its time-dependent deformation behaviour. Concrete test specimens are subjected to short-term stresses corresponding to 30% and 60% of their compressive strength at an age of 7 days, followed by a stress-free hardening phase. In subsequent 100-day creep tests under sustained loads equivalent to 15% of the 28-day compressive strength, the preloaded specimens exhibit up to 20% lower creep coefficients, reduced variability between specimens, and enhanced viscoelastic properties compared to unloaded reference specimens. The reduction in creep tendency is found to increase with the intensity of the applied preload. Concurrent short-term tests reveal that preloading does not significantly affect the concrete’s strength or Young’s modulus.

Based on the experimental creep data, a first attempt is made to consider the influence of early age preloading on the creep behaviour with a prediction model based on the analytical creep function approach according to EN 1992-1-1 and preload-dependent model coefficients. The results are promising, but not conclusive due to the limited experimental data and the fact that the correlation with other creep-influencing factors has not yet been investigated. Nonetheless, the investigations help to better understand the influence of the load history on the creep behaviour of concretes and underscore the potential for controlling the creep tendency through tailored mechanical loading, offering new avenues for further scientific exploration in experimental and modelling studies.