Testing the Thermal Conductivity of Asphalt
NOTE: the following blog post highlights recent work conducted in our lab in advancing the modified transient plane source technique the TCi employs in offering a faster, easier, and more reliable technique in the thermal conductivity characterization of asphalt materials. A more in-depth capture of this work is to be presented at the International Thermal Conductivity Conference scheduled for June 2011 in Quebec, Canada.
Asphalt concrete is generally considered to have three main structural elements: aggregates, bitumen (binder) and the contact layer between them. In addition to these components, it is necessary to include air and moisture which fills capillary porous spaces. Each of these elements has its effect on thermal properties of the system known as asphalt concrete.
The mechanical behaviour of asphalt concrete is highly temperature dependant. It is relatively “soft” at high temperatures and susceptible to rutting under the traffic, whereas it is brittle at low temperatures resulting in thermal cracking. As a result, pavement deterioration is aggravated in countries such as Canada with wide annual temperature swings. Asphalt concrete tends to expand with increasing temperature and contracts with decreasing temperature. The ability to predict the thermal properties in pavement structures has generally been a major concern in such seasonal frost areas.
A material’s thermal properties (thermal conductivity, thermal diffusivity, heat capacity) has a significant effect on the distribution and variation of temperature in a body.
The thermal properties of asphalt materials have been studied for many years. Although the literature on measurement techniques of thermal properties of pavement materials is extensive: ASTM C177, ASTM C518, ASTM C1045; steady-state methods have obvious limitations. Traditional steady-state methods are inconvenient due to the time (usually several hours) required to obtain a measurement and their restricted size of testing samples. More advanced, transient-state methods have been proposed and tested on asphalt pavements. A fast, simple and accurate method of analysis for thermal properties as offered by the TCi Thermal Conductivity Analyzer is highly desirable.
The picture below illustrates the setup of the TCi sensor in testing a sample of asphalt. As the TCi sensor only requires a single-sided interface, the user has greater versatility in how they wish to test the sample and do not require two samples to take the measurement as with the traditional transient plane source method or guarded hot plate technique.
A concern with the traditional transient plane source technique is the requirement to sandwich the sensor between two samples. It is often not convenient or practical to have two flat smooth surfaces. With such approach there is also concern that the measurement is actually an average of two samples and thus it is not possible to get accurate representative data of a single sample.
These concerns are eliminated and the contact issues halved in reducing the sample requirements to a single side with the TCi sensor. A thermal grease is also used with the TCi as a contact agent to optimize further the accuracy of the measurement in limiting the effect of contact resistance. The thermal grease is very easy to apply and is typically applied in similar fashion with guarded hot plate testing and other similar steady state thermal conductivity characterization techniques.
A comparison of results for the thermal conductivity of light weight (LW) and normal weight (NW) asphalt concrete samples obtained from three most common methods are presented in the table below.
The results obtained from C-Therm TCi system (MTPS) show a good correlation with the values obtained using the older less convenient methods. Sample results for the light weight asphalt ( LW1 and LW2) and the normal weight ( NW2) provide a better than 1% agreement. Sample results for NW1 differed by approximately 6% which is likely due to positional bias of where the TPS sensor was located in relation to the multiple sample locations tested with the MTPS sensor. One advantage to the TCi is the ability to look at the relative homogeneity of the sample in understanding how this impacts the localized thermal conductivity performance.
Ultimately - three methods to determine thermal conductivity of asphalt concrete were compared. The C-Therm TCi System, using a modified transient plane source method offers greater ease of use and versatility while delivering equivalent results to the alternative measurement techniques.