Testing Your Cob Mix
Sep 16, 2022
5 minutes
—Michael G. Smith and David Wright
The new cob code requires builders to test their cob mixes for shrinkage, compressive strength, and, in some cases, modulus of rupture. The shrinkage test is the most straightforward and can easily be done onsite by the builder. All you need is a box made out of 2x4s. The inside dimensions of the box should be 24 inches long, 31 ⁄2 inches wide, and 31 ⁄2 inches high. The box can have a wooden bottom, or it can be left without a bottom and placed on a flat surface. Line the box with thin paper or plastic sheeting before filling it completely with cob, screeded flat and troweled smooth on top.
Leave the cob in the box to dry, a process that
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