Brick vs Cob Brick Testing

Brick vs Cob Brick Testing

March 5, 2019 0 By admin

Background Information

One thing that I have become increasingly interested in is natural building materials. One of type of building material is know as cob. Cob are bricks made from earth. The are made by combining clay, sand, and straw. They are cured by drying them out. It works best when it is hotter out and you can use the sun to cure them. There are many articles on them and seem to have a ton of benefits. Some of those benefits include:

    Can be locally sourced and great reduce cost and energy required for brick
  • Do not require high heat ovens/kilns
  • Help regulate temperature of the building because they absorb moisture well
  • Reduce waste related to production of blocks b/c waste is earth
There are of course downsides to this kind of building/block. One of which is in general the strength is not as high as kiln fired brick. Also cob bricks don’t contain some of the constituents that other bricks do that make them a very solid building material like cement in some cases. Since the cob blocks are not as stable as modern bricks it can cause weakness in your building and require more maintenance. I will be using the the recipe from this site for cob blocks Diy House Building. I liked this one for what I will be doing.

Hypothesis and Experiment Design

To go down the path of designing this like an lab experiment we will use cob bricks made by me using the dirt from my backyard and pine straw from the trees I have in my yard. I will be using bricks from Home Depot as the with the following specs.

  • Compression Strength:1900 psi
  • Weight:4.1lbs
  • Dimensions:2inx3inx7in
  • Cost:0.90 USD/brick

This is a basic red brick that is nothing special. In this experiment we will be testing how the bricks hold up as walls for what is called a grow hole. A grow hole is from a book on building underground greenhouses by Mike Ohler called THE EARTH-SHELTERED SOLAR GREENHOUSE BOOK. They were one of his first underground growing tests. They are 2-3 feet deep and filled with top soil and manure. For this I may use some of my homemade compost instead of manure. They are then covered by a panel that allows sunlight in. I will get a polyethylene sheet to cover and build a greenhouse like covering to test the effectiveness of the blocks within the ground. How will this weather the bricks? What kind of issues will arise? I will post pictures of the holes for this after on make them to this post and social media ask I make them.

Hypothesis: The cob bricks will do no better than a conventional brick and show some kind of weathering within the first couple of months.

The six variable classes for this test will be:
  • Control: Regular Brick in ground by itself
  • Variable 1: Cob in ground by itself
  • Variable 2: Regular in ground with plastic film between brick and earth
  • Variable 3: Cob in ground with plastic film between brick and earth
  • Variable 4: Regular brick in ground with a water drainage system surrounding it.
  • Variable 5: Cob in ground with a water drainage system surrounding it.

the red brick will be the control because I have seen the results of a brick being in the ground a long time. That is pretty much nothing except getting dirty. I am just really interested to see how well the cob holds up. How will it hold up in heavy rain? The plastic film is based on a recommendation from the book mentioned above and I want to try it out because Mike Oheler was a pioneer in underground living. If this ends up working out it will be a huge win. Making bricks from site materials for essentially only the cost of labor would be amazing!

I invite any of you reading this to try this yourself and share your results in the comments below or hit McCormick Growers on any of the social media platforms below. Can’t wait to see what you come up with!


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