We need to begin construction of our zero energy house by deciding how it will be built. This decision will impact our design, materials and process. In making this decision there will be tradeoffs and consequences that must be carefully analyzed and understood. No design or method can be entirely perfect. After examining our options, we will choose the method that works best for the goals of this project.
In part one we examined rammed earth construction techniques
In part two we examined cordwood and strawbale construction
Today we will be talking about two construction methods that make extensive use of concrete. So before we begin let’s take a moment to examine this material. Concrete is a mixture of aggregate and sand held together by a binder. The aggregate is generally some form of gravel and the binder is some form of cement. When reinforced with either rebar, wire mesh or fibers, concrete is one of the strongest readily available materials in existence. Concrete unfortunately has a very high carbon footprint which results from the production of cement. The main compound in most cement mixes is calcium oxide (CaO). This compound is produced from naturally occurring calcium carbonate (CaCO3). The extra carbon dioxide molecule (CO2) is given off in the process. Concrete is generally used as the foundation for most buildings. Its high compressive strength allows it to carry and evenly distribute the load of the structure. Insulated Concrete Forms and Monolithic domes use concrete as a structural element to construct the entire building.
Insulated Concrete Forms
In this system, modular blocks or ‘forms’ are stacked to create the formwork for a wall. This formwork is then filled with concrete which sets together with the blocks to form the wall. Concrete has high thermal mass but poor insulation, thus insulation is generally built into the block forms. The blocks themselves and the accompanying insulation can be made in a variety of ways from a variety of materials. Many different companies offer a variety of insulated concrete forms. Generally the forms are made from either expanded polystyrene or a cement bonded composite material. The insulation can be provided by the polystyrene form itself or by additional fiberglass or mineral wool insets.
Quad-Lock Youtube Channel. Contains many tutorials and overview videos.
Monolithic Concrete Domes
In this system concrete is sprayed onto a dome shaped form to create a single, monolithic concrete dome. An inflatable airform is generally used as the starting formwork. A grid of rebar is then placed around the airform. Typically, polyurethene foam is then sprayed over the rebar grid. This foam shell serves as insulation and as a more complete, rigid form to which the concrete can be applied. Concrete is then sprayed to complete the structure.
Strength and Longevity:
Since these structures are made of mostly concrete they are extremely strong. Monolithic concrete domes are especially durable. Reinforced concrete domes have survived tornadoes, hurricanes, earthquakes and even military grade bombs! When constructed properly they have the potential to persist unharmed for centuries.
In each of these systems, insulation is naturally built in as part of the construction process. The naturally low surface area of domes also limits energy loss through the exterior.
There are many negative environmental impacts to consider. As discussed earlier, the production of concrete creates a massive amount of carbon dioxide. Insulated concrete forms also use either additional cement or expanded polystyrene in production of the forms. Monolithic construction uses polyurethane spray foam. All of these materials are environmentally damaging.
In both methods some advanced equipment and training is required. Large concrete mixers or Concrete spraying pumps are necessary. Setting up the formwork and applying the concrete properly requires special methods and techniques. This equipment and training requires either additional preparation or additional cost.
GeoPolymers, Lightweight Concrete and Other Considerations
The overall design of monolithic concrete construction has potential. The problem thus far has been either an inefficient implementation or a lack of good materials. Lately there has been a lot of work devoted to finding an alternative to cement binders. Such materials are collectively categorized as geopolymers. Geopolymers have the potential to offset the major environmental disadvantages of conventional concrete. Several companies have already made significant advancements in this area.
Gigacrete replaces the majority of cement with industrial byproduct wastes such as fly ash or blast slag. Novacem and Grancrete have successfully used Magnesium Oxide (MgO) in place of cement which can be produced from readily available magnesium silicates.
It is also possible to replace the aggregate of conventional cement with a lightweight insulating material such as recycled paper or hemp. Doing this greatly increases insulation value but decreases structural strength. If insulating concretes could be used successfully in monolithic construction, petroleum based foams would no longer be needed. Finally, simpler DIY equipment for mixing and spraying concrete is becoming available. The mortarsprayer for example has been successfully used in a variety of applications where more complex concrete guns and pumps had previously been required.
The obvious best case scenario would be some form of easily sprayed, insulating geopolymer concrete. However both lightweight concretes and geopolymers are very new. More testing across a variety of areas is required.
Monolithic Concrete construction has great potential and some very compelling benefits. With current methods and technology however the environmental cost is simply too great. In addition, the specialized equipment and skills currently needed will likely drive up the cost.
An insulating geopolymer concrete combined with an easy application method would most definitely be a game changing development.