HIGHLIGHTS
• Exposure of concrete to significant changes in weather, temperature and to chloride-based salts used to melt snow, can result in damage to its microstructure.
• Self-heating concrete that utilizes a phase change material has been developed to minimize damage to concrete during periods of cold weather and snow. 
• An outdoor study using two types of self-heating concrete was conducted over a three-month period during the winter. A self-heating concrete based on a phase control material infused in the porous structure of a lightweight aggregate was found to be very effective in melting snow and ice. 

Manufacture of concrete is an energy intensive process, and the use of this material in demanding outdoor applications limits its long-term viability. Lubricants such as gear oils and greases are required to ensure the equipment used in the production of concrete. Research is ongoing to find approaches for utilizing concrete in a more sustainable fashion. 

A previous TLT article¹ describes work done to produce and evaluate a better performing concrete that exhibits triboelectric properties. Imbedding an auxetic polymeric lattice as a reinforcement in the cement matrix improves the physical and mechanical properties of the material. The polymeric lattice is prepared from thermoplastic urethane and polylactic acid. Upon application of a stress such as an automobile passing over the concrete on a highway, electricity can be generated by this mechanical excitation process. One potential application is for this intelligent concrete to self-sense structural changes. 

Concrete is subjected to significant changes in weather and temperature when used in applications such as highways. During periods of cold weather and snow precipitation, concrete can be exposed to snowplows and deicing salts, which are used to clear roads. Robin Deb, doctoral candidate in the department of civil, architectural and environmental engineering at Drexel University in Philadelphia, Pa., says, “Conventional methods for snow removal that are both mechanical and chemical are energy intensive and can cause damage to concrete. Freeze-thaw cycles can occur due to changes in temperature. Ice formation present in concrete pores can generate internal pressure and cause microcracking formation and growth with repeated freezing and thawing cycles. Chloride-based salts used to melt snow and ice have been found to react with byproducts of the cement hydration process. The result is damage to concrete microstructure via formation of expansive chemicals (i.e., oxychlorides).” 

One approach for dealing with these negative consequences is to modify concrete so that it can exhibit sufficient heat-transfer properties to achieve self-heating characteristics when placed in a wintry environment. Deb says, “Self-heating concrete has been developed through the incorporation of a phase change material (PCM) in the concrete matrix. PCM will undergo a phase transition from a liquid to a solid state in a specific temperature range. The type of PCM can be tailored for specific applications.” 

Deb indicates that three requirements for adding a PCM to concrete are a high enthalpy, minimal supercooling effect and ease of incorporation into concrete. High enthalpy means that a large amount of energy is required/released to affect a phase change in a PCM. Supercooling occurs when a material retains its liquid state below its melting point. This phenomenon has to be minimized to ensure the PCM is effective. 

Deb and his colleagues including Dr. Yaghoob “Amir” Farnam, associate professor in the department of civil, architectural and environmental engineering at Drexel University, conducted studies with self-heating cement using the paraffin, n-tetradecane, as the PCM. He says, “We conducted a series of lab experiments on various construction materials to find the best process for using a PCM in concrete. This included formulating concrete with the PCM and different additive mixes and also varying the water:cement ratio used to optimize the design.” 

The researchers obtained promising results with two types of PCM-based concrete and initiated a study to evaluate them under real-world conditions. 

PCM-LWA and MPCM 
The concrete types utilized in an outdoor study were PCM-LWA (PCM infused in the porous structure of a lightweight aggregate [LWA]), and a microencapsulated PCM (MPCM). Deb says, “LWA is a unique lightweight aggregate slab that is derived from volcanic ash. It exhibits a very porous structure that facilitates PCM absorption and confinement in the concrete matrix. This enables PCM-LWA to exhibit excellent heat-transfer properties in the desired temperature range for the real-world study.” 

MPCM is prepared by incorporating PCM into a polymeric resin that is hydrophobic. Deb says, “This resin exhibited unfavorable interactions with the hydraulic cement particles leading to agglomeration and inferior mechanical performance characteristics. For this reason, use of a MPCM concrete mix that meets the strength criteria for road paving cannot be achieved.” 

Deb indicated that the objectives in optimizing the concrete mixes was to achieve the highest possible amount of heat release during conditions of snowfall and severe freeze-thaw exposure conditions, incorporate the maximum amount of PCM in the concrete mix and produce concrete with high strength and good durability.” 

Slabs that were 762 millimeters (mm) x 762 mm x 203 mm in size were prepared from PCM-LWA and MPCM. A third slab with a concrete mix design that is used in pavement construction was prepared as a control. Figure 2 shows an image of the slabs situated outdoors in the Philadelphia area. 


Figure 2. The experimental setup used to evaluate self-heating concrete under real-world weather conditions during the winter is shown. Figure courtesy of Drexel University.

The researchers installed Styrofoam thermal insulator panels on the four sides of each slab to allow for one-dimensional heat transfer. A series of five Type T thermocouples were placed at a spacing of 50 millimeters to measure temperature change. 

The study commenced in December 2021 with initial results covering an evaluation period through March 2022. During this period, the three concrete slabs were subjected to eight major events of snowfall. At these times, all of the slabs were covered with snow and/or ice. Deb says, “We found that the PCM-LWA was effective in melting snow and ice at a temperature range between 4°C and -13°C. In our view, this is a major breakthrough in self-heating concrete.” 

Deb reports that the outdoor study is still in progress and the researchers are still collecting data. He adds, “Future work will entail doing forensic analysis to better understand how the PCM derived concrete performed under outdoor conditions.” 

Ultimately, the researchers would like to demonstrate the effectiveness of self-heating concrete on a larger scale by placing concrete slabs with dimensions of 100 meters x 100 meters in parking lots, roadways or sidewalks. 

Besides the sustainability benefits, development of this technology should make it easier for those of us having to shovel snow in the future. Additional information can be found in a recent article² or by contacting Farnam at yf338@drexel.edu.

REFERENCES
1. Canter, N. (2023), “Intelligent concrete,” TLT, 79 (7), pp. 12-13. Available here.
2. Deb, R., Shrestha, N., Phan, K., Cissao, M., Namakiaraghi, P., Alqenai, Y, Visvalingam, S., Mutua, A. and Farnam, Y. (2024), “Development of self-heating concrete using low-temperature phase change materials: Multiscale and in situ real-time evaluation of snow-melting and freeze–thaw performance,” Journal of Materials in Civil Engineering, 36 (6), https://doi.org/10.1061/JMCEE7.MTENG-17048.