Although asphalt is widely used and accepted as the go-to material for constructing our road systems, scientists are always looking for ways to improve the thermal and mechanical characteristics of it. One major downfall of asphalt is the amount of heat it absorbs during daylight hours due to its low solar reflectance value (0.04-0.06). This characteristic means that asphalt can reach temperatures of up to 70°C in the sun, which can cause deformations in the road surface and intensifies the urban heat island effect present in large cities. Phase change materials (PCMs) have the ability to moderate temperature increases by undergoing a phase change and storing energy at a specific temperature. Chen et al. (2012) performed research to investigate the effect of PCM additions on the thermal and mechanical properties of asphalt, in an effort to reduce the temperature increase experienced by road systems.
Figure 1. Asphalt temperature can soar during hot summer days.1
Chen et al. (2012) selected two PCMs for their research, PCM-L, and PCM-Z. These PCMs were combined with conventional asphalt to create samples. A sample of conventional asphalt was also used as a control during testing. Thermal property investigations included measurements of thermal conductivity, thermal diffusivity, and heat capacity. Mechanical property studies involved indirect tensile strength, dynamic stability, and low temperature cracking resistance.
The Hot Disk Transient Plane Source Thermal Constants Analyzer was used by Chen et al. (2012) for all thermal property measurements. This system is a powerful piece of equipment that is capable of measuring thermal conductivity between 0.005 and 1800 W/mK. Absolute measurements of thermal conductivity are made simultaneously alongside thermal diffusivity and specific heat measurements. The Hot Disk sensor acts as both the heating element and the temperature sensor, and the accompanying equipment calculates the thermal properties based on the information relayed by the sensor. A sample set-up using the two sided sensor is displayed in Figure 2, where two identical pieces of a sample are sandwiched on either side to ensure good thermal contact. A single sided sensor is also available (Figure 2). This sensor performs the same measurements as the two-sided sensor, and is an excellent option for clients who do not have two sample pieces available.
Figure 2. The test set-up used by Chen et al. (2012) is on the left, while an illustration of the single-sided sensor is on the right.
Thermal Conductivity of Asphalt Containing PCMs
Thermal conductivity values obtained by Chen et al. (2012) using the Hot Disk TPS revealed that the two PCMs had different effects on the asphalt. Samples containing PCM-L had a higher thermal conductivity than the control samples, and samples made with PCM-Z had lower thermal conductivities. Lower thermal conductivity than the control is desirable, as less heat is conducted from the asphalt surface to subsequent layers.
Mechanical property measurements revealed that the addition of PCMs had a negative effect on the strength of the samples, with tensile strength and resistance to permanent deformation decreasing in samples containing PCMs. However, the sample containing PCM-Z had the best results in cracking prevention tests. Therefore, the researchers concluded that PCM-Z could have potential as an additive to reduce the temperature increases of asphalt.
This project illustrates the importance of effective thermal conductivity measurement to advancing research and knowledge of thermal properties. As we continue to strive to improve the materials that are a crucial part of our everyday lives, the Hot Disk TPS is a powerful tool that ensures thermal research is accurate and reliable.