The Impact of Vegetable Fibres on the Shrinkage and Mechanical Properties of Cob Materials

Abstract

This study examined the shrinkage rate and mechanical properties of cob samples. Cob is a natural building material composed of clay, water, and varying amounts of plant fibres. The red and beige cob materials in this study containing 3% and 6% wheat fibres were manufactured by hand with clay, bulk fibres (short and long fibres), and a 25% water ratio (water/clay) in order to make their manufacture and use on construction sites feasible and simple. The reference samples were mixed with clay, 25% water, and 0% wheat fibre. The mechanical properties were assessed through compression and flexural tests after 28 and 120 days. The results showed that the fibre addition decreased the bulk density of the composites from 1902 kg/m3 to 1264 kg/m3. The compressive strength increased from 1.8 MPa to 4.57 MPa for the red clay samples and from 1.65 MPa to 4.61 MPa for the beige clay samples at 28 days. The compressive strength of each mixture decreased slightly with age for the red and beige clay samples, respectively. Conversely, the flexural strength increased with age for the samples reinforced with 3% and 6% fibres. The results also showed that the cob samples can deform without breaking. Increasing the fibre content in the mix resulted in a significant reduction in the shrinkage rate and an increase in the mass loss rate during thermogravimetric analysis. This analysis showed a total mass loss of approximately 5.64%, 6.12%, and 44.87% for the red clay, beige clay, and fibres, respectively. An average volume shrinkage of 1% was observed for the samples with 6% fibre content. The cob discussed in this article can be used as a filling material. In large quantities, it can be made by hand, with feet protected by boots, or with the use of a mixer. The environmental benefits are considerable, as the raw materials are renewable, and the manufacturing process is less energy-intensive.