Control of thermal cracking in young concrete is of great importance to ensure a desired service lifetime and function of a structure. Young concrete is here defined as the period up to approximately 100 days after casting. Making reliable predictions about thermal stresses, and thereby cracking risks, the creep behaviour forms an important part of the material modelling. Up until now few studies have been made to investigate how different creep modelling influences calculated thermal stresses. Existing creep models for young concrete are often pure mathematical expressions with no direct relation to the material behaviour and thereby complicated to understand and use in a more practical context.In this paper a new basic creep model primarily aimed for early age purposes is outlined. The formulation with its model parameters, which have an easy to understand meaning in the material behaviour, is based on piece-wise linear curves in logarithm of time and therefore denoted the Linear Logarithmic Model (LLM). Comparison with experimental creep data and other more commonly used creep formulations for young concrete is made to achieve an opinion about the accuracy of the new model. The new model is a flexible and robust formulation that can model the behaviour of both young and mature concrete. The robustness enables it to make reliable creep modelling with very few test data.Another advantage with the LLM formulation is that the appearance of negative relaxation in linear viscoelastic modelling is very small and negligible with respect to thermal stresses. This means that the original formulation may be used directly in a thermal stress analysis for young concrete without any adjustment for negative relaxation.The LLM formulation shows very good agreement directly with experimental creep data and indirectly with measured thermal stresses, whereby the formulation has been used to model the viscoelastic behaviour of the concrete. The formulation also has the best correlation with experimental data compared to other commonly used creep models that have been analysed in this paper.

A reliable modelling of the young concrete creep behaviour is of great importance for consistent thermal crack risk estimations that shall contribute to assure a desired service lifetime and function of a structure.All-embracing creep tests aimed for thermal stress analyses are often very time consuming and thereby also costly to perform. Therefore thermal stress calculations in everyday engineering practice are often performed with standard sets of creep data involving no or very limited laboratory testing, which increases the error of the crack risk predictions and consequently also affect the design safety margins. The need for formulations that based on limited test data can make reliable predictions about the creep behaviour of hardening concrete is thus quite evident. This paper is a direct continuation of a previous study by Larson and Jonasson (2003) where a new concrete creep formulation called the Linear Logarithmic Model (LLM) was formulated. Here creep prediction formulas based on the LLM formulation are established and evaluated. It is shown that general model parameters can be established whereby the long-term creep behaviour is clearly dependent on the modulus of elasticity with larger creep deformations for lower E-modulus.An average error related to creep of 15 percent is what can be expected from most thermal stress analyses that are performed with standard sets of creep data today. By use of the prediction formulas based on the proposed LLM formulation for creep compliance it is possible to reduce the error by almost two thirds (2/3) only by adding the results from a test of the modulus of elasticity at the age of 28 days. For more advanced applications, where even better accuracy is required, it is recommended that at least a creep compliance test is performed at two loading ages, of which one at the age of 28 days.

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