TCkit™ - Thermal Conductivity Kit

The following Application Note addresses the testing of concrete using the C-Therm TCkit™.  The document highlights the use of bulk thermal conductivity testing on porous samples with the TPS method.  

Researchers, engineers, and students in the field of concretes, cement, geopolymers, and similar materials need to tune their products’ thermal properties to the application in question, to optimize energy efficiency and control thermal strain. Too low thermal conductivity can result in a large temperature gradient between opposite ends of the material in the application, resulting in thermal strain and cracking. On the other hand, very high thermal conductivity can make it difficult to control the internal temperature.  

To maximize efficiency in construction, a variety of lightweight concretes have been developed to decrease the weight of walls and foundations without losing much strength. However, these lightweight concretes often are more porous than their standard analogs, resulting in changes to the effective thermal conductivity. To counteract these changes and ensure the new lightweight formulations will still pass heat-transfer requirements, additives are often added to tune the thermal properties.   

This document highlights the TCkit’s performance capabilities in testing concrete samples.   

EXPERIMENTAL 

TCkit Testing of Concrete

In testing the thermal conductivity of concrete, two similar pieces should be obtained and the TCkit’s TPS Flex sensor is sandwiched between the two pieces, as shown above. Note that the effect of contact resistance is addressed within the method and procedure outlined in the ISO standard for TPS testing.  The TCkit does not, however, have the Wheatstone bridge circuitry specified in the ISO standard as it relies on a standard Keithley Source Measure Unit for its data acquisition.  Should researchers wish to achieve the higher sensitivity outlined in the ISO standard, they would be advised to consider C-Therm Trident thermal conductivity instrument. .[1]

Figure 2. Ten measurements on concrete in different locations with the C-Therm TCkit.

Between each test, there is a waiting period of approximately 5-10 minutes in allowing the sample and sensor to return to equilibrium.

As concrete is typically heterogeneous, it is recommended to test the sample in multiple locations establishing a composite average thermal conductivity as illustrated in the Figure above.    The results match well with the expected value for concrete and demonstrate the effectiveness of applying the TCkit in testing concretes, cement, asphalt and other similar materials.

References

[1] International Standards Organization (ISO). 2015. ISO 22007-2: Plastics — Determination of thermal

conductivity and thermal diffusivity — Part 2: Transient plane heat source (hot disc) method.

The following Application Note addresses the testing of samples at sub-ambient conditions using the C-Therm TCkit™.

Many groups want to test their materials at sub-ambient conditions to get a better idea of the thermal behavior of the material under the conditions of application – for example, material for an Arctic installation might be tested at -30°C to ensure it will withstand the harsh conditions without losing thermal performance. The TCkit™ can be used to test materials in sub-ambient conditions, using an optional thermal chamber and test clamp assembly.

Figure 1 – Ice has a thermal conductivity over 3X higher than water

TESTING ICE
Sample preparation was straightforward for the testing of ice: The sensor was submerged in degassed deionized water in a paper cup and then frozen overnight. As the ice cracked on freezing, cracks were filled with more deionized water and then the sample was refrozen to ensure good thermal contact. The wooden sticks were used as a placement aid to ensure the sensor alignment remained good during sample preparation and testing.

Following crack-filling and re-freezing in a thermal chamber, the paper cup used to prepare the sample was removed, yielding the sensor embedded in a block of ice as shown below.

Figure 2 – C-Therm Flex TPS Thermal Conductivity Sensor Embed in Ice

Thermal conductivity test data generated with the C-Therm TCkit is presented in Figure 3.

Figure 3 – Thermal Conductivity of Ice

In this case, the reference data set is an aggregate of 4 literature data sets on the thermal conductivity of ice.[1] The average of the four is reported here. Error bars represent the relative standard deviation of the experimental and reference data set. At all four temperature points, the agreement between experimental and literature data is better than the aggregate precision of the data sets, indicating very good agreement between the experimental data and the test data.

This application highlight demonstrates the effectiveness of applying TCkit in characterizing the thermal conductivity of samples at sub-ambient temperature conditions.

References:

[1] Ratcliffe, E.H. The Thermal Conductivity of Ice New Data on the Temperature Coefficient. Philosophical Magazine 1962, 7, 1197–1203.;
Alexiades, V.; Solomon, A.D. Mathematical Modeling of Melting and Freezing Processes; Hemisphere Publishing: Washington, 1993.;
Lunardini, V.J. Heat Transfer in Cold Climates; Van Nostrand Reinhold: New York, 1981.; Waite, W.F.; Gilbert, L.Y.; Winters, W.J.; Mason, D.H. Estimating Thermal Diffusivity and Specific Heat from Needle Probe Thermal Conductivity Data. Review of Scientific Instruments 2006, 77, 044904.