Soil evaluation and mix design formulation

Soil evaluation and mix design formulation:  Historically, rammed earth walls were built of soil taken directly from the site.  All walls from a given region had similar color and texture.  This was also true during the early years of the resurgence of rammed earth in the end of the 20th century.  Ochre colored lateritic soils in Western Australia; tan and beige sandy gravels from the American Southwest; Permian red clay, salmon-colored rhyolites, and gold decomposed granites from Northern California each carried a distinct regional aesthetic.  As appreciation of the art of rammed earth has advanced in recent years, builders skilled in the method have expanded their soil horizons to produce walls that do not necessarily reflect the same regionalism of earlier rammed earth. 

Ecological responsibility dictates that soil from the construction site be given first priority in consideration as a component in the formulation.  In nearly all cases, a percentage of site soil can be used in the mix design.  Soil amendments, whether selected for composition, color or texture are prioritized based on their proximity to the site.

The goal of the soil evaluation service is to identify mineral aggregates available in the immediate vicinity of the building site and to determine their suitability for use in a mix design formulation.  Typical tests conducted are a sieve analysis (ASTM C-136) to determine grain size distribution and an Atterburg limits test (ASTM D 4318)  to determine the plasticity index.  The soil is classified according to the Unified Soil Classification System (USCS).  Based on the test data and the soil classification, a preliminary mix formulation can be predicted and an estimate of the type and percentage of amendments calculated.  Selection of appropriate amendments will be based on proximity, suitability to create an optimum mix design, color, texture, and cost. 

Mix Design Formulation is the follow-up to soil evaluation.  The object is two-fold:  to investigate formulations that will produce finished walls that meet the expectations and aesthetic preferences of the client and architect; and to insure that the combination of ingredients will produce walls that meet or exceed the strength and durability specifications of the structural engineer and building officials.

The process by which an acceptable mix design is achieved involves the production of small test blocks and test cylinders which are cured and then tested in a compression machine.  The small blocks serve as architectural samples for study and refinement.  The cylinders provide a record of the compressive strengths to be expected in the finished walls.  In some instances a single trial mix design set will be sufficient to meet the requirements.  In others, a second or possibly third set of trial mixes will be undertaken before the desired results are obtained.