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CCAT terrace plasters 2017

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Engr305 Appropriate Technology page in progress
This page is a project in progress by students in Engr305 Appropriate Technology. Please do not make edits unless you are a member of the team working on this page, but feel free to make comments on the discussion page. Check back for the finished version on May 15, 2017.


The Campus Center for Appropriate Technology (CCAT) is an organization at Humboldt State University. This group is focused on finding old and creating new solutions to common problems. Appropriate technology means they are small-scale applications that have communal knowledge of how to solve a problem. Our project focuses on the continued work at CCAT to create a durable plaster made from up-cycled materials for use on a group of garden terraces. The scope of this project is to find a new plaster mix that will focus on one section of the terrace to find its overall durability. This project will be carried out by Wes Ponting and Sarah Cooper in association with Lonny Grafman for ENGR 305 during the spring 2017 academic year.

Problem statement[edit]

The objective of this project is to find a new plaster mix that will focus on one section of the terrace to find its overall durability. Our project focuses on this continued work at CCAT to create a durable plaster made from up-cycled materials for use on a group of garden terraces. An attempt will be made to not use cement in our final plaster mix to achieve a very low overall carbon footprint.


Below is a list of the criteria for this project. Compiled from interviews with CCAT co-directors Kelsie, Austin Anderson and with the help of project leader Connor Kennedy. This list should reflect the criteria from these sources along with suggestions from other interested parties.

Criteria Constraints Weight
Longevity Withstand the Arcata Climate. Last more than a few months or to be able to last through the winter. 10
Life Cycle Analysis Embedded energy of the materials used. What approach is the most sustainable within our given parameters . 9
Functionality Holds up dirt within garden beds. Succeeds in covering the Earthbags & holding up the Woolcrete top in an attractive and appropriate manner. 8
Educational Aspect Materials used must go through testing process to find strength and physical properties. This data will be used to select the final plaster mix to be used for terrace's. 7
Aesthetic Must be visually pleasing – terraces should be all relatively the same look and color if possible. 6

Literature Review[edit]

Below is a review of relevant information for the plaster terrace project at CCAT.

Plaster Basics[edit]

Plaster is a mixture of powders and crushed materials that are then mixed into a slurry and applied to a surface. Once the surface hardens the the plaster will go through a chemical process and begin to harden. [1] There are many different types and mixtures of various plasters used - some are better for interior use, some for exterior use, some most apt for humid weather and some that last the longest in dry weather. Strength, permeability, resistance to weathering, and embodied energy are all aspects to consider when deciding the right mixture for a plaster project. Henry, Mike. (please put this source w/correct footnote) "Binders for Natural Plasters (Part 1)."The Sustainable Home" N.p., 25 June 2014. Web. 31 Jan. 2017.[2]

Types of Plaster[edit]


Gypsum plaster is a unique type of plaster because of it's fast setting time as well as its ability to expand rather than shrink as it cures. In turn, it can reduce the amount of cracking as well as be used without the addition of sand or fiber. It's also one of the most ecological plasters, as it can be heated as low as 350 degrees Farenheit to create a binder. [3] Gypsum plaster, also known as "plaster of Paris" was used as a widespread building material in Europe after large deposits of gypsum were discovered around Paris. Gypsum is a plaster that can be shaped and molded fairly easily, as well as being a natural fire retardant. Because of this, many Europeans began coating the interiors of buildings with the material. [4]

One of the main issues with gypsum plaster, however, is that it's not very breathable and thus not usually recommended for external uses in damp places. [5] It is also very weak in terms of strength, and thus must be used with stronger plasters such as lime to increase its longevity. (please put this source w/correct footnote) Henry, Mike. "Binders for Natural Plasters (Part 1)."The Sustainable Home" N.p., 25 June 2014. Web. 31 Jan. 2017.[6]


Lime plaster is praised for its strength, flexibility, permeability, and weather resistance. It is obtained from the sedimentary stone lime stone, which is composed of coral, shells, and nodules of “kunkar". [7] Lime is an incredibly well balanced binder, it is relatively strong, but still flexible enough to move with natural buildings. It also has a low rate of cracking [5]. Lime plaster is very breathable - meaning it "allows air-borne moisture to travel freely through - it does not trap moisture inside a wall system. Without moisture buildup, biodegradable materials, such as wood or straw, are protected from decomposing." [8] Fat limes, or lime that does not slake easily, set slowly in air, and not at all underwater, is the most common type of lime used for lime washes and plaster projects. However, it is most effective if mixed with a proportion of Portland cement.

However one downside is that it can be relatively hard to work with. It is difficult to apply evenly, and must be used in thin coats and is not recommended for filling in big gaps. It can also burn your skin if not using gloves. [5]


The main benefit in using cement are its waterproofing components that lead to a longer lifetime before maintenance. Cement plaster is also easily accessible, durable, and offered at a relatively low cost in abundance. [5].

The biggest issue with cement plaster is large carbon footprint, however. It has an incredibly high embodied energy thus making it not the most green option when thinking in terms of sustainability. [9] Although it is a very strong binder, it can also be very brittle and is prone to cracking, especially in environments where there may be shifting of the structure. [5]

Additives to Plaster[edit]

In addition to making adding cement to the mix, other ways to strengthen the plaster is through the use of material additives. the are many different materials that can be used, as well as the size and overall consistency. Materials such as plant fluff, hair and especially manure have been used to create very strong plaster [10]

Climate Concerns in Humboldt[edit]

One of the main concerns for this project is the harsh weather that often occurs in Humboldt County, "Because of the moisture and moderate temperature the average relative humidity is high. [11]. The main reasons previous attempts at updating the plaster terraces went awry was because of the excessive rains and flooding occurring in the area. With this in mind, our main goal is to try to find the perfect plaster mixture that can live through the long days of wet and humid weather.

Performance Testing[edit]

Earthen materials being used for buildings has been occurring for thousands of years. Adobe construction can be traced back to 8300 B.C., used for constructing the walls of Jericho [12]. During this time building standards, methods of design and testing of those materials progressed. In 2010, the American Society for Testing and Materials published a comprehensive list of standards as well as testing methods for earthen materials[13]. Another source for testing methods is Earth Construction: A Comprehensive Guide [14], this resource will aid in both types of test that can be performed as well as information on other plaster types and their durability in different climates. According to an article in the Appropriate Technology magazine, "Simulation test in the laboratory, such as the spray erosion test, can only be indicative because factors such as changes in scale, influence of true climatic conditions, building usage and maintenance practices are not easily replicated. One of the most realistic simulation methods is to expose small test-walls to natural conditions." [15]. In addition to testing current plaster mixes, Long term data on old plaster quality will be gathered to attempt to capture this temporal aspect.To finalize this research, creating a data table showing the physical properties of plaster mixes will benefit both our project as whole and increase the knowledge about earthen materials at CCAT.

Existing Terrace at CCAT[edit]

The current terraces at CCAT have had many coats of different plaster mix applied to them including wool, corn cobs, straw mixed with various ratios of sand, clay, gravel and cement. The only plasters that can last in a wet and humid environment are shown to be those mixed with some amount of cement. In addition the Woolcrete that was used has also held up to the climate.


Examples of Other Systems[edit]

One example of an existing plaster and earth bag system that we stumbled upon was this Hermit's Dome done by James Fox, an emergency shelter dome aimed at protecting individuals from tornadoes and other natural disasters. The system had similar components to the CCAT earth bag garden terrace - it's shaped and insulation was made up entirely of sandbags. It was then wrapped around in chicken wire and then plastered with a 3 parts earth 1 part Portland cement mix. After the application process, Fox watered it every four hours for the next week, since such a large amount of plastic cement was prone to cracking during the curing process. [16] (and add picture)

Another example we found was an eco-building group creating a earth bag building in Pyrenees. For the external part of the building, they chose to use a lime-sand plaster since the walls would be affected by bad weather and needed to be incredibly durable. They made their plaster using this recipe:

1 bucket sieved earth + 2 buckets sand + 1 bucket lime + ½ bucket flax fibre (to prevent cracking) + Water

The group also noted that the lime-drying process was a slow one, and during that time it needs to be protected from direct sun and drying wind during and after plastering.[17]

(and add pics)

Client Criteria[edit]

According to our clients at CCAT, one of those most important aspects of the terrace plasters is longevity. The plaster must last longer than the previous attempts to update the terraces. Another critical component of the plaster is that it withstand Humboldt climate. That means that through all the rain and floods and humidity, the plaster will still be durable enough to last through anything Humboldt County may throw it's way. Another aspect that we will try to accommodate is making all the plaster the same color, or matching it up with the existing terraces that are already in good condition. [18]


Dates Task
Feb 1 - March 1 Planned out testing strategy and goals. Gather materials and mixed test plaster batches and begin longevity test.
March 2 - April 1 Gather data on different test plaster batches, decide on appropriate mix.
April 2 - May 1 Carry out full terrace plaster repair with chosen recipe.
May 2 - May 15 Project follow up with client; assessing completion according to client criteria.

Materials and Equipment Budget[edit]

This project is being financed by CCAT through specific funds given to them by Humboldt State University's ENGR 305 class. The cost of this project does not exceed the budget set aside for the class and does this by using donations from CCAT, as well as selecting base materials that would be affordable to those who would attempt to re-create the project on their own.

Quantity Material Source Cost ($) Total ($)
1 Yard3 River Sand Wes Green Landscaping 40.00 40.00
1 Yard3 Beach Sand Wes Green Landscaping 40.00 40.00
2 Yards3 Fill Dirt Wes Green Landscaping 25.00 50.00
8 Bags @ 47 lbs. each Quickcrete Hardware Store: Mill Yard or Ace 10.00 80.00
3 Bags @ 50 lbs. each Lime Type: S (Hydrated) Hardware Store: Mill Yard or Ace 48.00 144.00
1 Bale Hay Feed Store: A & L Feed and Pet Supply 20.00 20.00
Misc. Items Tools & Gloves Provided by CCAT 0.00 0.00
1 Day Cement Mixer Rental United Rentals 118.00 118.00
Total Cost $492.00


  3. Henry, Mike. "Binders Part 2: Gypsum and Lime." The Sustainable Home, 20 Feb. 2015. Web. 31 Jan. 2017.
  4. "Gypsum Plaster." Minnesota Lath & Plaster Bureau. N.p., n.d. Web. 30 Jan. 2017.
  5. 5.0 5.1 5.2 5.3 5.4 Kessem, Amira, and Scott Howard. "DIY Plaster: Eco-plaster and Traditional Waterproofing Open Source Hub." One Community, n.d. Web. 30 Jan. 2017.
  7. Stern, Peter. "Limes, Mortars, and Plasters." Field Engineering: A Guide to Construction and Development Work in Rural Areas. N.p.: n.p., n.d. 55-60. Print.
  8. Koko, Sigi. "Lime Plaster." Down to Earth Design, n.d. Web. 30 Jan. 2017.
  9. Henry, Mike. "Binders for Natural Plasters (Part 1)."The Sustainable Home" N.p., 25 June 2014. Web. 31 Jan. 2017.
  10. 2017. Ecocosas.Com. Accessed February 1 2017.
  12. ASTM E2392 / E2392M-10, Standard Guide for Design of Earthen Wall Building Systems, ASTM International, West Conshohocken, PA, 2010,
  13. ASTM E2392 / E2392M-10, Standard Guide for Design of Earthen Wall Building Systems, ASTM International, West Conshohocken, PA, 2010,
  14. Houben, Hugo, and Hubert Guillaud. Earth construction: a comprehensive guide. London: Intermediate Technology Publications, 1994.
  15. Levrat, Maison. "MUD PLASTERS AND RENDERS." Appropriate Technology, June 1, 1999, 1-4.
  16. Fox, James. "The Hermit's Dome." Earth Bag Building. N.p., n.d. Web. 31 Jan. 2017.
  17. "Eco-building in the Pyrenees, from Earth Bag Foundations to Plastering." Making Sense of Things. N.p., 04 Nov. 2011. Web. 30 Jan. 2017.–-eco-building-in-the-pyrenees-from-earth-bag-foundations-to-plastering
  18. Summers, Kelsey "CCAT Terrace Plasters" Interveiw by author. January 31, 2017