EcoHouse Research

Matthew Smith (MEng, 2013)

 

This research is a predominately lab based project exploring the material properties of compressed earth blocks (CEBs) for use in a low-cost housing project in Quito, Ecuador as part of the EcoHouse Initiative. A literature review was conducted concerning stabilisers, their typical applications in CEBs, and the expected physical properties. It also looked at the wider context of CEBs: their economic cost, social acceptance in developing countries, and embodied carbon. Earth buildings are expected to achieve economic savings of 20-60% over traditional housing materials. There are recorded problems with social acceptability in development projects with initial attitudes being negative. However local research centres working to train the community in proper techniques can counteract this, while lobbying the government to promote earth buildings helps build wider acceptance.

A number of preliminary tests were conducted to classify the soil, determine whether the strength is isotropic, and explore methods of testing water resistance. The soil was classified as a poorly graded sand with less than 5% fines. Compressive strength tests on capped cubes showed that the compressive strength is isotropic, which simplifies meeting standards. Several tiles and 16 cubes were produced for preliminary trials with water resistance testing techniques. Both wet compressive strengths and a modified spray testing technique were trialled but both gave unusable results. Laboratory tests for water resistance were abandoned and the literature review was relied on for information.

Several dozen soil-stabilisers mixtures using cement, lime, pulverised fuel ash (PFA), ground granulated blast furnace slag (GGBS), and furnace bottom ash (FBA) were investigated. For each mixture 8, or 16 cubes were produced along with a tile. The cubes were crushed to determine the compressive strength and the tiles used to record the appearance for each mixture. The 28-day compressive strength of unstabilised soil was found to be ~1 MPa. The addition of cement increased the 28-day compressive strength to 4 MPa at 12% cement. PFA, GGBS, and FBA slightly improved the compressive strength, but there was no clear correlation in the results and more research is needed. Lime universally reduced the compressive strength of the soil to ~0.5 MPa whether used as a sole stabiliser or combined with PFA, GGBS, or FBA. This was in contrast to the literature that suggested compressive strength increases could be achieved with lime.

A mixture of 12% PFA, and 12% cement was chosen as the final mixture and 96 cubes were produced. The compressive strengths were normally distributed and the mean and standard deviation were found to be 10.27 MPa and 1.41 MPa, respectively. This gave a characteristic strength of 7.96 MPa, significantly above the target strength of 5 MPa.

The appearance of each mixture was recorded with a colour balanced photo of the tile, which was averaged to produce a hexadecimal RGB colour code for each mixture. This was a cheap and easy way to record the colour of mixtures that will be used for social acceptability studies in Quito this summer.

The relationship between compressive strength and density was explored and there was found to be no correlation. The literature suggested increased density would reduce permeability and increase strength so the result is unexpected and further tests should be carried out to confirm it. The importance of carrying out a standard Proctor compaction test on the soil was also questioned as the addition of stabilisers changes the properties of the soil. The moisture content at compaction and the density at 28-days of 224 cubes was plotted. The density was found to have a relatively small variation while moisture content ranged between 14 and 26% suggesting the optimum moisture content varied significantly depending on the stabilisers used. The consistent density shows that drop tests were a good method for determining the optimum moisture content of a mixture. However more research is needed to prove this conclusively and standard Proctor compaction tests should be carried out alongside drop tests for various mixtures to conclusively prove this.