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.
Pros/Cons
Pros
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.
Energy
Efficiency:
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.
Cons
Environmentally
Harmful:
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.
Specialized
Equipment/Training
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.
Final
Thoughts
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.
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