AU103.1 Clay subsoil

Clay subsoil for a cob mix shall be acceptable if the mix it produces meets the requirements of Section AU103.4.

AU103.2 Sand

 Sand or other aggregates such as, but not limited to, gravel, pumice and lava rock, when added to cob mixes, shall yield a mix that meets the requirements of Section AU103.4.

AU103.3 Straw

Straw for cob mixes shall be from wheat, rice, rye, barley or oat, or similar reinforcing fibers with similar performance. Before mixing, the straw or other reinforcing fibers shall be dry to the touch and free of visible decay.

AU103.4 Mix proportions

 Cob mixes shall be of ny proportion of refined clay or clay subsoil, added sand (if any) and straw that produces a dried mix that passes the shrinkage test in accordance with Section AU103.4.1, complies with the compressive strength requirements of Section AU106.6 and complies with the modulus of rupture requirements of Section AU106.7.

AU103.4.1 Shrinkage test for cob mixes

Each proposed cob mix of different mix proportions shall be placed moist to completely fill a 24-inch by 3 1/2-inch by 3 1/2-inch (610 mm by 89 mm by 89 mm) wooden form on a plastic or paper slip sheet and dried to ambient moisture conditions, or oven dried. The total shrinkage of the length shall not exceed 1 inch (25 mm), as measured from the dried edges of the material to the insides of the form. Cracks in the sample > 1/16 inch (1.5 mm) shall first be closed manually. The shrinkage test shall be shown to the building official for approval before placement of the cob mix onto walls.

AU103.5 Mixing

The clay subsoil, sand and straw for cob shall be thoroughly mixed by manual or mechanical means with water sufficient to produce a mix of a plastic consistency capable of bonding of successively placed layers or lifts.

AU103.6 Installation

Cob shall be installed on the wall in lifts of a height that supports itself with minimal slumping.

AU103.7 Dry joints

Each layer of cob shall be prevented from drying until the next layer is installed, to ensure bonding of successive layers. The top of each layer shall be kept moist and malleable with one or more of the following methods:

  1. Covering with a material that prevents loss of or holds moisture,
  2. Covering with a material that shades it from direct sun, or
  3. Wetting.

When dry joints are unavoidable, the previous layer shall be wetted prior to application of the next layer.

AU103.8 Drying holes

Where holes to facilitate drying are used, such holes shall be of any depth and not exceeding 3/4-inch (19 mm) in diameter on the face of cob walls. Drying holes shall not be spaced closer than ten hole-diameters. Drying holes shall not be placed in braced wall panels. The design load on load-bearing walls with drying holes shall not exceed 90% of the allowable bearing capacity as determined in accordance with Section AU106.8. Drying holes shall be filled with cob before final inspection.

AU103.9 Adding roof loads to walls

Roof and ceiling loads shall not be added until walls are sufficiently dry to support them without compressing.