Posted on August 05, 2021
Insulated concrete form (ICF) structures are low-maintenance, energy-efficient and long lasting. ICFs have been used to build structures as high as 22 stories. When it comes to home construction, interest only accelerates in an era of high lumber prices.
ICF Components and Sustainability
As ICFs gain favor, it is useful to look at the potential for using more eco-friendly components.
By weight, ICF walls are mostly concrete. Low-carbon concrete mixes make ICFs more sustainable. Concrete made with Portland-limestone cement improves CO2 emissions by about 10 percent. Concrete injected with CO2 is another possibility.
Supplementary cementitious materials (SCMs) also reduce carbon emissions. ASTM International defines an SCM as “an inorganic material that contributes to the properties of a cementitious mixture through hydraulic or pozzolanic activity, or both” (ASTM 2015). Popular SCMs include slag cement, coal fly ash and silica fume. There are also ternary mixtures. These combine ordinary Portland cement (OPC) with other materials in the binder fraction.
By weight, rebar is the second major component in an ICF structure. Epoxy-coated “black” steel rebar is an inexpensive, durable way to reinforce concrete. Steel rebar is mostly fabricated from recycled steel.
The epoxy coating helps protect against corrosion and oxidation threats. Corroding rebar swells, increasing the tensile load on the concrete. When this happens, cracking and spalling may follow. Gaps speed the deterioration of both rebar and concrete, necessitating repairs or replacement. Regular rebar is also vulnerable when exposed to salts and aggressive chemicals.
Today, the quest for sustainability drives interest in using other products to reinforce concrete.
Continuous basalt fiber
Fiberglass-reinforced (FRP) polymer
Manufacturers make FRP rebar from glass fiber mixed with a polyester resin. It is rust-free. It is also electrically and thermally non-conductive. Workers can tie it off and chair it up like steel. FRP minimizes maintenance and repair, making lifecycle costs competitive. It is a strong, lightweight product that also reduces transportation costs.
FRP rebar costs more than the traditional black steel alternative. However, it delivers twice the tensile strength at one-fourth the weight. One FRP manufacturer asserts that “slabs exposed to cyclic loads last 20 times longer than steel reinforced slabs.”
Bamboo composites are of interest because bamboo grows rapidly and it sequesters carbon. Woven-strand bamboo (WSB) is an engineered product that is the subject of ongoing study. In Singapore, researchers fabricated WSB from bamboo split lengthwise. The long, thin strands get carbonized, dipped in a water-based adhesive and pressed into molds.
The microbar alternative
Microbars reduce or even eliminate the need for rebar reinforcement. However, engineers must certify the use of microbars for structural reinforcement. For one 16-story project, horizontal reinforcement was reduced up to the fifth floor. There was no need for traditional reinforcement from floors six to 16.
As ICF Builder notes, steel microbars are an alternative to traditional rebar. Contractors add zinc-coated, inch-long strands of twisted carbon steel to concrete mixes. One manufacturer claims triple the fatigue resistance. It also cites a 33 percent increase in flexural strength.
Microbars made of basalt fiber aggregate are a natural alternative to rebar and microbar made from steel. Basalt is an abrasion-resistant igneous rock that’s readily available in certain regions. It is inherently resistant to fire, acid and corrosion. Basalt fiber has 2.5 times the strength-to-weight ratio of traditional steel rebar.
Expanded polystyrene (EPS) insulation
Of course, the other primary component of an ICF is the EPS insulation. EPS closed-cell foams have been popular for more than 50 years. EPS is versatile, cost-effective and high-performing. It is 98 percent air and 100 percent recyclable. Today, fabricators use EPS in everything from bicycle helmets to insulation materials.
Expanded polystyrene (EPS) is in a family of polymers called “thermoplastics.” They soften when heated and harden when cooled. Companies make EPS from styrene, one of the many light components derived from crude oil. The thermal conductivity of EPS is very low, and it does not absorb moisture.
Although virgin EPS beads are petroleum-based, EPS insulation cuts energy use. The longevity of ICF walls also affects its net environmental impact.
In an ICF wall, webs act to brace the inner and outer EPS layers, creating the space for the concrete. They are often made of recycled polypropylene. For example, Amvic says its webs are entirely fabricated from recycled material. This makes the blocks “60 percent recycled material by total weight.”
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