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The CRI Proposed Cob Testing Program

Why a Cob Testing Program is needed

  • Current popular standards for Cob are based on empirical knowledge
  • Earth as a building material is highly variable
  • Current structural data for Cob is limited, incomplete, and unorganized
  • A scientific understanding of Cob's structural properties is lacking
  • Coherent and factual engineering standards for Cob are needed for it to be accepted into the standard Building Code

In order for Cob to be accepted, a program of basic research must be undertaken. Cob as a structural system needs to be subjected to a rigorous series of accurate tests which measure the nature, strength, and performance of cob as a building material and technique. A set of realistic cob construction standards with hard data to back them up can then be developed and used to form the basis of a reliable cob based section for the Building Codes.

The purpose of the testing program

  • Verify or disprove current empirical cob standards
  • Establish standards to deal with the variable nature of earthen materials
  • Establish the strength properties of Cob as a material and method of construction
  • Establish factual cob construction standards which will withstand the forces which effect buildings
  • Test both standard and nonstandard building forms constructed with cob

The purpose of this testing will not be to negate any of the current hard-won empirical knowledge about cob which now exists, but rather to put measured truth to it. Nor will such testing deny the versatile possibilities of cob. It will simply deal with the hard fact that when cob can be factually shown to withstand the various forces that affect buildings it can then be accepted into the building code.

Success requirements of the testing program

  • Adequate funding must be available
  • A basic Cob Research Institute facility and staff needs to be developed
  • The major testing needs to be done within a sanctioned testing facility

For a cob testing program to be acceptable to code authorities it needs to be done within a sanctioned testing facility. Adequate funding is critical to making this happen. An abstract of the Proposed Cob Testing Program is described below as a beginning vehicle in the search for funding.

Test Program Description

Analysis of Found Earth

Study a broad series of native found earth samples to:

  1. Develop a data base relative to sample constituent content for further testing
  2. Develop a simple and reliable method of field testing to determine constituent content of any given found sample
Mix Tests

Perform a series of mix tests based on percentages of constituents (e.g. clay, aggregate, moisture, straw) to:

  1. Develop correlation's of working mix cohesion and dry state cracking based on sample consistent percentages
  2. Develop a data base of mix cohesion and dry cracking relative to mix constituent percentages for further testing
Strength Testing

Perform a series of strength tests based on Mix Test samples that do not crack when reaching a dry state. The first two test series to be:

  1. Compressive strength
  2. Rupture (bending) strength
  3. Develop an initial strength data base relative to these two tests

Perform a third series of tests 5 & 6 based on highest strength samples. Vary straw (or other possible fibers) content, species, and length to:

  1. Develop a further strength data base relative to straw (fiber) content variables.

Perform a fourth series of tests based on best performing previous results:

  1. Shear strength - test wall panel samples to determine in-plane lateral strength
Attachment Tests

Perform a series of tests of common attachment methods. Tests to be based on best performing previous samples from previous test results.

  1. Test four attachment methods to failure
  2. Develop a standard for the attachment methods based on pullout strength performance
Seismic Resistance Tests

Use data gathered to this point to predict how a theoretical structure will perform in a seismic event. Perform a series of shake table tests on either 1/2 scale or full scale sample structures. The goal of these tests is to determine seismic resistance strength of possible building structural configurations and compare to predicted performance. Test sample structures at varying intensities and duration until the structure fails. The potential tests to include:

One-story Structures
  1. Test three possible plan configurations. Test structures to consist of walls only
  2. Repeat the same series of three tests for those structures that fail. These test structures to have a continuous perimeter foundation and concrete or wood bond beams at their wall tops
  3. Repeat the same series of tests with structures containing roof diaphragms attached to top of walls
Two-story Structures

Perform a repetition of tests 12, 13, & 14 on two story test structures

The Testing Program is to be modified as necessary based on results as the testing proceeds. Total number of tests to be determined based on results.