The thermal performance of ground floor slabs—a full scale in-situ experiment

Abstract

A detailed description of a comprehensive in-situ experiment designed to monitor the thermal performance of real ground floor slabs is presented. In particular, the experiment was installed at the time of construction of a modern commercial building and subsequently monitored continuously for a one and a half year period. A detailed account of the design of the experiment and the instrumentation used is provided. Results achieved include transient variations of; heat flux through the floor slab, air temperature, ground temperature and ground moisture content. Automatic electronic data logging enabled readings to be taken at thirty minute intervals from most instruments. Therefore both the seasonal and diurnal response of the system were observed. Significant differences in summer and winter heat transfer processes are revealed. The results also show that ground temperature and moisture content may change significantly near the external wall of the building. Underneath the building, temperature and moisture content variations were relatively small. Therefore the presence of an edge effect near the outer boundary wall of the building is revealed. The ability of the ground to act as a thermal store is also indicated by the measured data. The thermal response of two ground floor slabs is presented. The first was constructed of normal weight concrete and the second of light weight concrete. The insulating effect of the light weight concrete is evident from the results obtained. The measured thermal data has been used to determine thermal transmittance values for each floor slab considered. These values have been compared to values calculated adopting the approach outlined in current design guides. In conclusion, the U value calculated using the CIBS guide was almost identical with the measured value for the normal weight concrete floor slab. However, the method could not be directly applied to the light weight concrete slab. The U-values calculated using the CEN draft document were found to be in reasonable agreement with the measured value for the normal weight concrete and in excellent agreement with the light weight concrete.