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Infrastructure Improvement Research using the Hot Disk TPS- Bridge Lifespan Optimization

Bridges are one of the most costly and maintenance intensive infrastructure components for governments to operate. Maintenance costs can run into the billions annually, providing strong incentive to improve the design and materials incorporated into the next generation of bridges to reduce future spending. In northern climates, freeze-thaw cycles can have a detrimental impact on the lifespan of bridges as they promote the corrosion of the steel frame. The expansion of concrete when it freezes causes cracks, which further enlarge once water seeps in and freezes. These cracks enable damaging liquids such as deicing components present on the bridge deck access to the steel frame, hastening its deterioration. 

In 2012, Sakulich and Bentz at the National Institute of Standards and Technology (NIST) performed research to determine whether the incorporation of phase change materials (PCMs) into the  bridge concrete could help reduce the number of freeze-thaw cycles. By becoming a solid at a certain temperature, the PCM would release energy that could keep the temperature of the rest of the structure above the melting point (Tmelt). To determine whether this was a viable option, Sakulich and Bentz (2012) tested the physical properties of four mortars containing PCMs. The Hot Disk Transient Plane Source thermal conductivity system was used for thermal conductivity measurement.

Thermal Conductivity Applications Confederation Bridge

Figure 1. Bridges are expensive to build and to maintain.1

Sakulich and Bentz (2012) studied three experimental mortars and one control mortar. PCMs can be introduced into the concrete through a pellet process or through a lightweight aggregate (LWA). Two experimental mortars were created using the LWA method, one mortar was produced containing PCM6, a paraffin wax, while the other contained PT4, a vegetable oil. The final mortar contained PT4 encapsulated in a polymer. Samples were produced in a cube form. The mortars underwent compression tests, heat release tests, and thermal conductivity measurements. Experimental procedures were performed in conjunction with extensive modelling using two programs, one from NIST and the other from the Indiana DOT. This modelling studied the effect that 12 different PCMs could have in this application, the geographic locations it would be effective in, and how much it would extend the lifespan of the structure.  

Thermal conductivity testing was performed using the Hot Disk TPS thermal conductivity equipment, which is capable of taking measurements between 0.005 and 1800 W/mK, After compression tests had occurred, the cubes were cut into three equal pieces for further analysis. The two outer pieces were sandwiched around a 6.403 mm Hot Disk sensor, and thermal conductivity and specific heat were measured (Figure 2). Tests occurred at room temperature (22-23°C), power output was 0.3 W, and tests lasted 10 seconds. A 45 minute rest period was observed between each test. Once tests were completed on dry samples, they were then placed in a lime water bath for 7 days, and measured again afterwards. A single sided sensor, the TPS-S, is also available for use with the TPS system and only requires one sample piece. This is an excellent option for difficult to cut or expensive samples.

Thermal Conductivity Applications Sensor And Set Up(1)

Figure 2. Set up used for thermal conductivity measurement using a two-sided Hot Disk sensor.

Thermal Conductivity Applications TPS S Upright

Figure 3. GIF showcasing the ease of use of the TPS-S.

Thermal Conductivity of PCM containing Mortars 

Sakulich and Bentz performed the physical properties testing to ensure that no quality was lost through the incorporation of the PCM into the mortar. Compression tests showed that significant strength was lost, which the researchers determined was the result of the low strength of the LWA. Thermal conductivity results revealed that the thermal conductivity of the PCM containing mortars was lower than that of the control mortar (Table 1). The dry mortar containing PCM6 had only 61% of the thermal conductivity value of the original mortar. Saturating the samples increased each thermal conductivity value by about 0.3 W/mK (Table 1). After saturation the thermal conductivity of the PCM6 mortar was 69% of the original. The encapsulated PT4 sample performed the best, with a thermal conductivity of 92% of that of the control. No obvious trends were seen in the specific heat data (Table 1). 

Table 1. Thermal conductivity and specific heat of four mortar samples tested by Sakulich and Bentz (2012) using the Hot Disk TPS thermal conductivity system.

Mix  State  Thermal Conductivity (W/mK)  Specific Heat  
(J/gK) 
Control Dry 2.67 0.62
  Wet 2.94 0.74
PCM6 Dry 1.63 0.47
  Wet 2.01 0.76
PT4 Dry 1.37 0.63
  Wet 1.78 0.8
PT4-Encapsulated Dry 2.47 0.87
  Wet 2.68 1.01

 

Predictive modelling using the two programs mentioned above indicated that the U.S. southeast and the Pacific Northwest have the weather conditions where prevention of the freeze-thaw cycle is possible using this method, and would add at least 1 year to the lifespan of the bridge. The models also identified an ideal Tmelt range between 3.55 and 6°C. This range depends strongly on the thermal conductivity of the composite as it determines how cooling occurs. A high thermal conductivity  partnered with a PCM with a Tmelt in the optimal range would promote even temperature distribution through the bridge deck and prevent freezing.  However, outside of this range, composites with a lower thermal conductivity perform better since a high thermal conductivity would lead to more freeze-thaw cycles. 

Sakulich and Bentz (2012) conclude in this paper that there are PCMs on the market that can perform well in this application. However, the issues of loss of compressive strength and optimization of the Tmelt/ thermal conductivity relationship must occur in order for this to be viable. Experimental testing on bridge decks outside of predictive modelling needs to occur as well to move the project forward. This project is an excellent example of how the Hot Disk TPS system can be used to tackle complex infrastructure issues.  

Learn More About Hot Disk Transient Plane Source (TPS)

The Hot Disk Transient Plane Source (TPS) technique allows for precise thermal conductivity measurement of a huge array of materials ranging in thermal conductivity from 0.005 to 1800 W/m∙K . TPS is capable of measuring bulk and directional thermal properties of solids, liquids, pastes and powders.

References:

Nova. 2008. Smart Bridges: Expert Q&A. PBS (Online). Available at: http://www.pbs.org/wgbh/nova/tech/lynch-structural.html 

Sakulich, A.R., Bentz, D.P. 2012. Increasing the service life of bridge decks by incorporating phase-change materials to reduce freeze-thaw cycles. Journal of Materials in Civil Engineering. 24: 1034-1042.  
Available at: http://ascelibrary.org/doi/abs/10.1061/%28ASCE%29MT.1943-5533.0000381?journalCode=jmcee7 

Photo References: 

1- http://www.steveheckbert.com/wp-content/uploads/2013/06/Confederation-Bridge-PEI-canada-69877_550_371.jpg

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