National research results on the thermal conductivity of concrete in case of fire

Introduction

The first part of this article gave you a rough idea of the magnitude of the thermal conductivity λ of concrete moves. This second part provides you with some background information that is not evident in the standards. It is discussed here which factors influence the size of the thermal conductivity. An important national research project is discussed. The results of some international research work can be found in a concluding third part of this series of articles.

Research of Prof. Dr.-Ing. habil. Ulrich Schneider

As early as 1982, Prof. Dr.-Ing. habil. Ulrich Schneider (1942-2011) presented the results of extensive research on the behavior of concrete at high temperatures. If you want to see the original document, you can use this download here as a pdf file (in German). Prof. Schneider considered the temperature range from 20°C to 1350°C and examined, among other things, the thermal conductivity of concrete in this temperature range.

Prof. Schneider points out that it is difficult to determine the thermal conductivity of concrete experimentally. This also applies to the interpretation of the results. This explains why the measurement results for the thermal conductivity of temperature-stressed concrete vary greatly (see page 30 in the source cited above). Nevertheless, Prof. Schneider was able to work out the following basic relationships:

Thermal conductivity is roughly proportional to the moisture content of the concrete. This means that wetter concrete conducts heat better.
The better the aggregate conducts heat, the higher the overall thermal conductivity of the concrete.
Cement stone has a lower thermal conductivity than normal concrete aggregate (this does not apply to lightweight aggregate). Thus, so-called lean concrete with a low cement content tends to have a higher thermal conductivity.
If concrete is reheated after an initial heating and cooling, its thermal conductivity is reduced.

From these relationships, Prof. Schneider was able to derive how the thermal conductivity changes depending on the fire temperature:

At low temperatures, the thermal conductivity is high because the water present in the concrete conducts heat well. In addition, the structure of the concrete is still undamaged and can therefore conduct the heat well.
At temperatures around 100°C, the thermal conductivity drops for the first time because part of the water in the concrete pores has evaporated.
In the temperature range from 100°C to approx. 300°C or 400°C, the thermal conductivity drops further because the concrete dries out even more and cracks appear, which damage the structure and make heat conduction more difficult.
At even higher temperatures, conduction should increase slightly because heat transport is by radiation rather than conduction.

Overall, therefore, the thermal conductivity of the concrete decreases as the temperature of the concrete body increases. This relationship corresponds to the information in Eurocode EN 1992-1-2 (see the first part of this article).

The comparison with the research results of Prof. Schneider also shows that the information in the Eurocodes applies more to normal concrete. These are on the safe side for lightweight concrete, since the thermal conductivity is lower. On page 30 of Prof. Schneider's research work, a picture can be seen that lightweight concrete has a thermal conductivity in the range of approx. λ=0.2 to λ=0.5 ^W/_m⋅K indicates. This is significantly lower than the value given for standard concrete in the standard.