Evaluation of Eurocode 2 approaches for thermal conductivity of concrete in case of fire

Abstract

The thermal conductivity of concrete is the property most influencing the temperature distribution within structural elements in case of fire. In this paper, the influence of concrete components (aggregates, moisture, cement) on the heat transfer within the concrete matrix is discussed with focus on the thermal conductivity. The current version of Eurocode 2 fire part (EN 1992‐1‐2) provides simplifications due to applicability of design method. One of these simplifications is the assumption of two limit functions (upper and lower curve) to describe the thermal conductivity for elevated temperatures of ordinary concrete. In each country, the value of thermal conductivity may be set by the National annex within the range defined by lower and upper limit. In this paper the impact of the forementioned simplifications is described and a new approach of a single temperature dependent function to describe the thermal conductivity (“mixed‐curve”) is evaluated in contrast with the upper and lower curve of thermal conductivity. The results of 23 international fire tests with elements of ordinary concrete, performed between 2012 and 2017, are compared to numerical results using the three different approaches for thermal conductivity. The results are evaluated by using functional analysis. Further, the model uncertainties of EN 1992‐1‐2 regarding neglecting reinforcement steel and moisture transport are investigated. The results show that the “mixed‐curve”‐approach of thermal conductivity is capable to describe the temperature distribution within concrete structural elements in case of fire. Moreover the influence of reinforcement steel on the temperature distribution can be neglected as well as the influence of moisture transport especially when evaluating the temperatures at the fire‐opposed surface.