2425 s2 - Biobased Building - Cob's water resistance

Cob, a traditional earthen building material, is composed of clay combined with sand, water, and straw. The combination of durability, affordability, and thermal efficiency has made cob construction a popular choice that has been around for centuries. However, the main limiting factor of cob comes from its high water absorption capacity, as it causes the material to break down and erode, leading to reduced durability over time. The porous framework of cob material makes it easily damaged by exposure to rainwater, especially in damp or flood-prone locations. Given the growing emphasis on sustainable and biodegradable materials, research into natural biopolymer-based modifications, such as chitosan and alginate, presents a viable alternative for enhancing the durability and performance of cob in construction applications. The primary goal of this research is to explore how chitosan and alginate modifications affect the water resistance and durability of cob.

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About this project

Start date:	02/03/2025
Project description:	Cob, a traditional earthen building material, is composed of clay combined with sand, water, and straw. The combination of durability, affordability, and thermal efficiency has made cob construction a popular choice that has been around for centuries. However, the main limiting factor of cob comes from its high water absorption capacity, as it causes the material to break down and erode, leading to reduced durability over time. The porous framework of cob material makes it easily damaged by exposure to rainwater, especially in damp or flood-prone locations. Given the growing emphasis on sustainable and biodegradable materials, research into natural biopolymer-based modifications, such as chitosan and alginate, presents a viable alternative for enhancing the durability and performance of cob in construction applications. The primary goal of this research is to explore how chitosan and alginate modifications affect the water resistance and durability of cob.
Outcomes:	This research tested natural coatings to improve the water resistance of cob, a traditional mix of clay, sand, and straw. External coatings worked better than internal mixing, with linseed oil alone (on straw-free samples) showing the lowest water absorption. Surprisingly, this outperformed biopolymers like chitosan and alginate. Straw increased strength and structural integrity, even though it absorbed more water. Coatings with beeswax and chitosan best prevented water rise, and beeswax mixes showed high water repellency, though this didn’t always reduce absorption. In short, linseed oil alone already improves water resistance, and when combined with straw, the cob becomes both more durable and water-resistant. These findings highlight the potential of biobased materials for enhancing traditional building methods. Recommendations include testing long-term performance, effects of different water types, combining coating methods, evaluating mechanical impacts, and better understanding straw’s structural role.
Commissioned for:

Marianna Ansiliero de Oliveira Coelho (Lectoraat Biobased Bouwen)