By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Nov 25 2024A recent article published in Construction and Building Materials experimentally investigated the hygrothermal properties of two clay materials, red and beige clays, reinforced with wheat fibers using the traditional heavy cob technique for modern wood frame construction.
Study: Natural thermal and hygrothermal regulation with heavy cob for low carbon construction. Image Credit: Brian A Jackson/Shutterstock.com
Background
Geo-sourced materials are essential in modern construction due to their low carbon footprint and recyclability. Raw earth is an abundant building material with low grey energy; it is locally available, easy to work, and meets several environmental sustainability criteria. Moreover, earth materials can regulate internal humidity due to their hygroscopic properties.
However, the main drawback of earth materials is their vulnerability when in contact with water. Adding plant fibers such as flax, hemp, or wheat can increase the resistance of earth materials to moisture and adverse weather conditions. Meanwhile, among various methods used to make earth materials, the traditional technique of constructing with cob stands out for its simplicity and ecological characteristics.
Accordingly, this study explored the influence of moisture content on the thermal and hygroscopic properties of wheat fiber-reinforced clay specimens manufactured using the heavy cob technique. Cob, a natural material, has several sustainable advantages over conventional building materials.
Methods
Red and beige clay powders, ranging in size from 0 to 63 µm, and wheat fibers were used to prepare test specimens. The mineralogical composition of clay powder was determined using X-ray diffraction, X-ray fluorescence, and thermogravimetric analysis. Notably, the fiber content in the clay mixtures was varied as 0, 3, and 6 wt.%.
While the fiber-free red and beige clay samples were prepared with a 25 wt.% water/clay ratio, the fiber-reinforced samples had fibers soaked in water with a water/fiber ratio of 205±15 wt%. Subsequently, the clay/water mixture was made with 25 wt.% water before adding the moistened fibers. Rectangular and cylindrical samples were produced from these mixtures using the traditional cob production method that employs raw fibers without chemical treatment.
The wheat fibers’ water absorption and desorption behavior were examined in distilled water at different temperatures (23, 40, and 50 °C). Meanwhile, the open porosity of the clay samples was determined by the combined gas pycnometer method using helium and the non-wetting oil immersing method using olive oil.
The thermal properties of the clay mixtures were measured in a dry state and NaCl solution. Additionally, their water absorption capacity was determined using the capillary absorption method (partial immersion in water for 24 h) and moisture absorption method (in NaCl for 48 h). Moreover, the water vapor permeability was measured using the wet and dry cup methods.
Temperature variations using the prepared materials were examined using a bi-climatic chamber and by constructing mini-walls (80´80´15 cm3) Notably, two cities were chosen for this study, Djibouti (20 to 45 °C) and Johannesburg (-5 to 35 °C).
Results and Discussion
The fiber-reinforced clay mixtures exhibited increasing thermal conductivity, specific heat, thermal diffusivity, and thermal effusivity with increasing water content. This was attributed to the replacement of air in the samples’ pores air in the dry state with water vapor when exposed to increasing relative humidity.
The moisture content of the clay samples reached equilibrium in four to eight days with varying relative humidity, exhibiting the relative humidity regulation capability of the fiber-reinforced and fiber-free samples. Additionally, according to the sorption test results, fiber-reinforced samples could be exposed to very high relative humidity (80–95%) for over two weeks without risk of mold after a visual inspection.
The clay specimens exhibited an excellent moisture buffer value, which decreased when the relative humidity was between 20% to 50% and increased when it was between 50% to 95%. The same trend was observed for the coefficients of moisture effusivity and specific moisture capacity, except that of moisture diffusivity, which decreased with increasing relative humidity.
The fiber content significantly influenced the hydric properties of the clay specimens. However, it did not impact the samples’ water vapor resistance factor and equivalent moisture penetration depth.
The 15 cm thick cob wall demonstrated the ability of fiber-reinforced clay mixtures to maintain a comfortable indoor temperature range of 19.5 to 23 °C for one week, even when exposed to extreme temperatures of -5 and +45 °C. Therefore, the prepared composite materials were considered suitable for constructing walls with a 15 cm or greater thickness.
Conclusion
Overall, the researchers thoroughly analyzed the thermal properties of wheat fiber-reinforced red and beige clays as a function of moisture content and hygroscopic behavior at varying relative humidity. Clay mixture samples with 3 and 6 wt.% fiber yielded good hygrothermal properties due to their fiber-derived ability to regulate high relative humidity without the risk of mold.
However, the study determined only the samples’ apparent water vapor permeability, not considering the influence of surface film resistance. Evaluating the actual water vapor permeability of cob materials and their dependence on water content is essential to comprehend their reaction to external humidity.
Journal Reference
Kabore, A., Laghdir, A., & Ouellet-Plamondon, C. (2024). Natural thermal and hygrothermal regulation with heavy cob for low carbon construction. Construction and Building Materials, 451, 138832. DOI: 10.1016/j.conbuildmat.2024.138832, https://www.sciencedirect.com/science/article/pii/S0950061824039746
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