Effect of straw fibers addition on hygrothermal and mechanical properties of carbon-free adobe bricks: From material to building scale in a semi-arid climate

Abstract

To achieve net-zero carbon emissions in a building's lifecycle, a comprehensive approach is required, addressing both embodied and operational carbon footprints. To support this goal, this study assesses the environmental, hygrothermal, and mechanical characteristics of different carbon-free adobe bricks bio-sourced with straw and evaluates their impacts on improving the energy performance of a one-story case study building located in a semi-arid climate. At the material scale, the mechanical performance, hydrothermal behavior, and durability of carbon-free straw-reinforced adobes, with weight percentages of 0, 1, 2, 3, and 4%, at different sizes: long straw (100–200 mm) and crushed straw (5–25 mm) were studied. The results show that incorporating straw into bricks increases porosity, enabling the production of lightweight bricks that comply with Moroccan standards for lightweight structures, possessing a density lower than the accepted limit of 1750 kg/m³. Furthermore, the thermomechanical properties of reinforced adobes were considerably improved compared to non-reinforced ones, with a gain in mechanical compressive and flexural strengths of 31.2% and 184%, respectively. However, it is important to note that in the case of adobe bricks reinforced with long straw (BLS), the findings revealed that, unlike other thermomechanical properties, long straw has a negative effect on compressive strength. Additionally, thermal conductivity decreased by 93.8% and 86.8% for a 4% inclusion of long and crushed straw, respectively. At the building scale, the indoor hygrothermal comfort was evaluated with respect the hot, semi-arid region in southern Morocco. The results show that the reinforced adobe material is able to reduce the indoor temperature of the case study building from 3 °C to 12 °C during the summer time, compared to the outdoor temperature, with a time lag of up to 5 h between the interior and exterior. Moreover, it successfully maintained diurnal indoor air temperature fluctuation within 2 °C. In addition, indoor relative humidity remained in the recommended 40%–65% by ASHRAE Standard 62.1–2022, over 78% during the monitoring period, even when the outside relative humidity fluctuated significantly (5–95%). Finally, a Morris-based sensitivity analysis was carried out on nine design parameters of the case-study building to assess the impact of varying the thermal properties of reinforced adobe bricks on cooling and heating energy performance, respectively. Thermal conductivity was found to have a significant impact on cooling energy demands and a lesser influence on heating demands. The study also indicates that straw-reinforced adobe bricks can improve cooling energy performance by up to 17.8% without compromising the overall embodied carbon of the building envelope.