Optimising the structural properties of bio-based cob materials

Haddad, Kamal, Lannon, Simon ORCID: https://orcid.org/0000-0003-4677-7184 and Latif, Eshrar ORCID: https://orcid.org/0000-0003-3982-6929 2026. Optimising the structural properties of bio-based cob materials. Presented at: Third RILEM International Conference on Earthen Construction (ICEC 2026), Lisbon, Portugal, 8-10 July 2026. Published in: Keita, E., Faria, P., Perlot-Bascoules, C., Perrot, A., Fabbri, A., Beckett, C. and Bras, A. eds. Third RILEM International Conference on Earthen Construction. RILEM Bookseries (77) Springer Nature, 10.1007/978-3-032-32331-6_62

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Abstract

Conventional construction materials such as concrete and steel carry high environmental costs, intensifying the need for lower-impact alternatives. Cob, an earthen composite of subsoil, water, and natural fibres, offers potential as a low-energy construction material, but its mechanical performance and dimensional stability remain insufficiently quantified for consistent structural use. This study examined how fibre type, fibre dosage, and initial water content influence the compressive behaviour and shrinkage response of bio-based cob mixes, with the aim of establishing repeatable composition?property relationships within the tested system. A multi-stage experimental framework was implemented. Subsoil grading was characterised by dry sieving, and fibre length distributions for barley straw and hemp shiv were quantified using computational machine vision. Based on these characterisations, 51 cob mixes were produced with controlled variations in fibre type, fibre dosage, and water content. Material-scale testing quantified dry bulk density, volumetric shrinkage, shrinkage ratio, and compressive strength (n = 3 per mix). A dedicated variability experiment evaluated the influence of fabrication controls and informed a refined protocol to improve repeatability. Ten optimised mixes were then assessed under standardised placement and consolidation into moulds and controlled drying. Organic fibres improved compressive response and shrinkage control, although the relationships were non-linear. Across the test matrix, compressive strength reached 1.58 MPa, while volumetric shrinkage increased up to 27.1% when initial water content and fibre dosage were excessive. In the final evaluation set, mixes with 20?25% initial water and moderate total fibre achieved the most consistent balance between strength and dimensional stability, with higher hemp-to-straw proportions generally outperforming straw-rich compositions. Mix H6S1W25 (6% hemp shiv, 1% barley straw, 25% water) delivered the highest overall structural performance within the tested set. These outcomes provide empirically grounded benchmarks for specimen-scale cob mix optimisation under controlled fabrication conditions.

Item Type:	Conference or Workshop Item - published (Paper)
Date Type:	Publication
Status:	Published
Schools:	Schools > Architecture
Additional Information:	RRS applied
Publisher:	Springer Nature
ISBN:	9783032323309
ISSN:	2211-0844
Date of First Compliant Deposit:	21 September 2026
Last Modified:	21 Sep 2026 15:30
URI:	https://orca.cardiff.ac.uk/id/eprint/189742

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