Linear logarithmic model for concrete creep : II. Prediction formulas for description of creep behaviour

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.Validerad; 2003; 20080523 (ysko)