Byeong-Hun Woo and colleagues studied the effects of thermal conductive materials on the freeze-thaw resistance of concrete. They performed two experiments: freeze-thaw and rapid cyclic thermal attack in order to evaluate the thermal durability of concrete with thermal conductive materials. They showed that the graphite had a negative effect on the freeze-thaw and rapid cyclic thermal attack. While, the use of silicon carbide (50%) and steel fiber significantly improved thermal durability of concrete. Their study recently appeared in the Journal Materials.
Cold regions have two kinds of threatening factors for vehicle users. One is black ice and the other is pot-holes caused by the freeze–thaw cycle. Black ice makes the surface of the road slippery and causes traffic accidents. To prevent the generation of black ice, people use chemical salts such as CaCl2. However, chemical salts cause deterioration of concrete and reduce the service life of the concrete. While, the water present in the concrete mix freezes and this freezing causes deterioration such as such as cracking, scaling etc. This is called freezing and thawing. This deterioration occurs due to lack of air on the surface layer of concrete mass. Study showed that the combination of both black ice and chemical salts accelerate deterioration of concrete.
However, to overcome this problem, many studies tried to enhance the thermal conductivity of the materials. Like, some suggested the use of carbon nanofibers and carbon nanotubes, while others suggested the use of graphene as they have good thermal conductivities. But, they have certain limitations such as properties and cost.
Thus, to overcome this limitation Byeong-Hun Woo and colleagues now applied a substitution method. The reason behind using substitution method is that, the aggregates occupies more than 65% of the volume fraction. They used silicon carbide (SiC) as the substituting material and substituted it for 50% and 100% of fine aggregate in order to improve the thermal conductivity.
“Silicon carbide was chosen as the substituting material of the fine aggregate; as silicon carbide has good thermal conductivity and hardness, it is considered sufficient as a fine aggregate substitution material.”— they said
In addition, they used graphite at 5% of volume for enhancing the thermal conductivity, and the arched-type steel fiber for compensating the reduction in mechanical properties by the graphite. Furthermore, they used steel fiber (upto 1% vol. fraction) as the thermal conductive material because the steel fiber has a high level of thermal conductivity. However, there’s a risk if we apply all these various thermal conductive materials to the concrete, why? Because it would generate thermal damage by the difference in the thermal conductivity of each material in the cold environment, e.g., via freeze–thaw. Thus, it is necessary to verify or assess the thermal durability of concrete with thermal conductive materials, in conditions such as freeze-thaw.
For this reason, Byeong-Hun Woo and colleagues performed two experiments: freeze–thaw (FT) and rapid cyclic thermal attack (RCTA). Their concrete was made for application as road paving material, therefore, the FT resistance was important. In addition, cold regions usually change the air temperature very rapidly. Therefore, it was essential to performed RCTA test for assessing the thermal durability of concrete.
They found that, Arched type steel fiber improves the mechanical properties of concrete due to the anchorage effect. On the contrary, it was demonstrated that using graphite brought about a negative effect on the mechanical properties. However, graphite is a good material for improving the thermal conductivity of concrete. Therefore, the decrease in mechanical properties caused by using graphite could be compensated by using arched type steel fiber.
They also found that, SiC is able to be used as fine aggregate and has sufficient thermal conductivity. In addition, it was demonstrated through the thermal conductivity results that the steel fiber could be used as a thermal conductive material. The combination of SiC and steel fiber maximized the improvement in the thermal conductivity of concrete. Adding graphite also brought about an increase in thermal conductivity.
“Using 100% silicon carbide was considered the acceptable range, but 50% of silicon carbide was the best. Graphite decreased all the properties except for the thermal conductivity.”
Finally, it has been demonstrated from the results of the FT test and RCTA test that use of graphite is not suitable for FT and RCTA resistance. However, the arched type steel fiber showed a remarkable improvement of the FT resistance and RCTA. In addition, SiC compensated for the negative effect of graphite on the FT and RCTA.
“We suggest the content of graphite and use of other conductive materials should be carefully consider in further studies”— they concluded.
Featured image: Used thermal conductive materials. (a) Arched-type steel fiber; (b) SiC; (c) Graphite © Woo et al.
Reference: Woo, B.-H.; Yoo, D.-H.; Kim, S.-S.; Lee, J.-B.; Ryou, J.-S.; Kim, H.-G. Effects of Thermal Conductive Materials on the Freeze-Thaw Resistance of Concrete. Materials 2021, 14, 4063. https://doi.org/10.3390/ma14154063
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