Index and Engineering Properties of Oregon Cob

Abstract

Cob is an earthen building material comprised of sand, clay, straw, and water used for millennia to construct dwellings. Although cob construction largely died out during the nineteenth century, it is experiencing a revival in England and the Pacific Northwest of the United States. Little scientific research has investigated the engineering properties of cob, knowledge of which is important for modern-day design practices and code requirements. Researchers at Oregon State University investigated six different Oregon cob mixtures using a series of standard soils and concrete tests adapted for this material. The objectives were to characterize the constituents, to establish estimates for the magnitude of, and degree of variability in, the mixture properties, and to develop correlations between the engineering properties and mixture composition. Results indicated low to moderate variation in basic mixture properties (i.e., unit weight, moisture content, and sand equivalent), moderate variation in strength properties, and high variation in the elastic modulus. Several reasonable correlations were found between shrinkage, compressive strength, elastic modulus, and sand equivalent and between flexural strength and fiber tensile strength.

Cob, a vernacular earth construction process in the context of modern sustainable building

“…Centuries after centuries, they optimized the use of available natural resources, according to geographical context and societal evolutions, and developed local constructive cultures [1,42,48]. As a consequence, embodied energy of earth construction is very low in comparison to other materials conventionally used in construction [1,2,9,10,16,46,[49][50][51][52][53]. For example, embodied energy of a wall made of earth is about 20 times less than that made of hollow cinder blocks [9,10].…”

“…For example, embodied energy of a wall made of earth is about 20 times less than that made of hollow cinder blocks [9,10]. Earth construction offer other benefits: better social impact [5], low greenhouse gas emissions [6,9,10,46,49,54], high thermal mass [1,19,30,49,52,[55][56][57][58][59][60][61][62][63], good indoor air quality [1,7,9,10,49,51,54,[64][65][66][67] and reversible clay binding allowing a complete and low-energy recycling [10,16,31,49,50,[52][53][54]. As these local constructive cultures are a source of inspiration for anti-seismic…”

Experimental Campaign on the Use of the Flat Jack Test in Cob Walls

“…Every layer was tamped by hand, pressing with the fingers to try to fill all the voids. This preparation procedure is similar to the one implemented by Pullen and Scholz [20].…”

Conceptual narratives, ecological heeds, social purposes and subjective ends of natural building movement

“…Building material with cob as one of its traditional names is roughly comprised of sand, soil (with significant clay content) and straw in various ratios [39]. Contemporary explorations of this material and building technique in United States (whence its popularity was dominantly promulgated from) is related to cob traditions of Wales and southwestern England -particularly Devon [40] and to US-based work of Welsh architect Ianto Evans.…”