Recently we went to the Department of Civil Engineering at the University of Aveiro to test various adobes, made from different soil mixes, under compressive strength. The opportunity was given to us by Professor Humberto Varum and we were helped with the tests by the Engineer António Figueiredo. We’d like to thank the two.
Our goal wasn’t a rigorous scientific test but rather a simple test so we could determine witch mix can produce the adobes with the best behavior under compressive strength.
To make the adobes we had four materials: clay collected free of charge from about 8 km away from the building site; sandy soil – with little clay – collected on site; bought sand and horse manure that we also had to buy. We did six different mixes and in half of each of them introduced the horse manure. Thus we tested 12 groups of 2 adobes each (for a more rigorous test they should have been at least 3) 6 with manure and 6 without so we could also carry out a comparative test by analyzing this variable alone. The following image illustrates the various proportions and results in MPa (mega pascal). The graph shows the results of both adobes tested for each mix.
The adobes with greater resistance (excluding those with a greater concentration of manure) were made from a 50-50 mixture of clay and sandy soil plus the horse manure. This means they are optimal from both a structural stand point and an economical one because we can make them from free material. This will be the mix we’ll use.
Regarding the manure the results confirm our choice to use it. Specially in clayey soil 20% of manure increases the compressive strength by about a third (the effect is less prevalent in sandier mixtures). In the mixture with 33% of manure the results are really encouraging as it made the adobes much more resistant. Besides the adobes are left with a lower density (lighter) which is also better in structural terms. In the future we want to take deeper look into this factor, finding the limits and the optimum value.