There are important issues to investigate in order to make an informed decision whether or not to use rammed earth for a project. These are site logistics, geologic hazards, building design, available soils, client preferences, manpower availability and construction budget. A feasibility study is comprised of an interview, site evaluation, and review of preliminary plans or concepts. A typical work product is a written report listing findings and recommendations.
Value engineering is an ongoing process that takes place during concept design and design development, in collaboration with the client, architect, and structural engineer. The object is to identify opportunities to reduce cost and complexity while retaining the intent and aesthetic of the architecture. In some cases, budget considerations may precipitate significant programatic changes. In others, minor modifications to wall dimensions or structural details can result in valuable cost reductions. In every case, the fees associated with value engineering services can be saved in construction efficiency.
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.
Upon satisfactory completion of the trial mix design program, larger sample tablets can be produced to serve as design aids to the architect and interior decorator. In some cases, mock-up panels built on site may be useful to confirm lift depth, strata configuration, and color in relation to site conditions. Mock-ups can serve as benchmarks against which to compare the final wall installation.
Pre-construction design and evaluation services are performed on a fee basis under contract with the architect or the project owner. Subsequent services such as construction planning, formwork design, equipment provisioning, and wall installation supervision are negotiated and performed in conjunction with the project general contractor or concrete subcontractor. They can be provided on either a fee basis or fixed cost, depending on a range of circumstances.
Construction planning involves designing a strategy for completing the work: forming system, material processing and stockpiling, production expectations, manpower requirements, scheduling, and interfacing with other trades. Typically, an REW supervisor will meet with the contractor’s team prior to groundbreaking to review plans and to establish a tentative project schedule. A decision will be made regarding a preferred forming approach, mobilization strategy, and manpower needs. If requested, shop drawings of the forming system will be produced along with a take-off list of forming components. As site preparation and work on foundations progresses, periodic confirmations of scheduling updates are expected with eight-week, four-week, and two-week look-aheads obligatory.
As forming gets underway, an REW supervisor will make site-visits once or twice weekly to confirm
accuracy and stoutness of the formwork as well as to address specific issues that may need resolution. Questions can be addressed regarding electrical, plumbing, or HVAC services; framing layouts or mounting hardware. Installation of wall material cannot take place until formwork has been completed to specifications and approved by an REW supervisor.
Equipment provisioning and installation services are customized to each project. Factors that influence the planning are site access, project size, available manpower, schedule and budget. The REW business model is to train contractors in the skills particular to mastering the rammed earth method. The company goal is to incrementally expand the roster of knowledgeable contractors in order to build confidence within the marketplace, and thus promote the growth of the industry. Rammed earth is an ideal material for use as one component of a sustainable building system. Its widespread application is currently limited by a shortage of experienced practitioners.
In developing countries, where labor rates are low and purchased materials expensive, building rammed earth totally by hand is the most affordable and socially responsible approach. An entire project is completed with a single set of simple forms, shovels for mixing, buckets for delivery, and hand tampers for compaction. Two REW trainers and a crew of four to six local masons are sufficient. Production rates average 1-1/2 cubic feet per man per hour.
In regions with higher labor rates, the use of equipment for mixing, delivery, and compaction can significantly increase the speed of construction. Which pieces of equipment to mobilize and how many rammers to put into use are decisions influenced primarily by site access, construction schedule, and available manpower. A smaller project may be best suited for a skid steer tractor for mixing and delivery with two pneumatic rammers for compaction. A larger project could benefit from a proportioning mixing machine, articulated conveyors for delivery and five or more air-powered rammers. Production rates for mechanically assisted rammed earth installations with a full manpower compliment can approach 40 cubic yards of placed material per day.
A meaningful proposal for training and installation supervision can only be developed after a thorough review of the project, mix design formula, and a clear understanding of the expectations of all parties involved.