Optimizing the Thermal Conductivity of Bedding Materials

We test the thermal conductivity / the thermal resistivity of bedding materials and backfills for cable trenches and geothermal applications as a function of moisture content and compaction, and we determine how to improve the thermal properties of a given material.

Material Composition

Bedding materials consists of a solid grains (matrix) and pore volume. The highest matrix thermal conductivity of natural materials has pure silica sand (6.5 to 12.5 W/mK), while the thermal conductivity of calcareous sand is significantly lower. Hence the shares of calcium carbonate and silica determine if a sand is suitable as a bedding material. Thermal conductivity tests help to select an appropriate material for your application.

Additives

Additives like cement or bentonite increase the density, but can even lower the thermal conductivity of the base material because their thermal conductivity is usually lower than that of silica sand. Thermal conductivity tests help to decide if additives make sense for a particular application.

Grain Size Distribution and Compaction

The grain size distribution determines how well a sand can be compacted. The best compaction and the highest thermal conductivity value is achieved with a wide grain size distribution. Fine grained materials lacking coarse components do not reach high thermal conductivity values. Thermal conductivity of bedding materials must be measured at the same degree of compaction which will later be used in the field.

Moisture Content

Moisture content has a significant influence on the thermal conductivity of a porous material. Either thermal conductivity tests must be made at several different water contents in order to register the complete thermal conductivity vs. moisture content curve, or the worst case only is measured (i.e. the maximum thermal resistivity, which is reached when the sample is completely dried).

Our Services

You either send us samples produced with the desired degree of compaction, or the base materials and we prepare the samples. We test the thermal conductivity in the completely dried state (lowest value) or at different water contents as requested by the customer. After preparing the samples they are saturated and thermal conductivity is measured. Then the samples are gradually dried and additional thermal conductivity tests are made at different water contents.