Concrete Is Changing

Bendable concrete is a big improvement to conventional concrete’s frequent failures. These failures lead to repeated infrastructure and building repairs, using enormous quantities of material and energy and producing large quantities of greenhouse gas emissions (GHG).

Engineered cementitious composites (ECC), the technical name for bendable concrete, gains its flexibility and durability from polyvinyl fibers covered with a thin (nano thick) slick coating that allows slipping rather than fracturing when placed under stress. As ECC is gaining acceptance, research and empirical evidence is demonstrating that the microfibers and surrounding microcracks make the concrete self-healing. When air and moisture migrate into the hairline cracks, self-generating reactions of carbon mineralization occur, binding the micro cracks together. Basically, calcium ions inside the cracked concrete combine with moisture and carbon dioxide from the air, creating a calcium carbonate material similar to sea shells.

For some decades, the concrete industry has been trying to reduce its carbon footprint. One of the most successful strategies has been substituting flyash, rice husks or silica fume ash for cement, the main culprit in concrete contributing to GHG. This substitution reduces carbon emissions by essentially the same proportion as the cement is reduced. Many mixes replace 50 percent or more of the cement yet yield a stronger concrete. Fortunately, these substitutions work well in ECC mixes, thus producing greater strength, less carbon emissions and flexibility.

Another promising development is adding micro-sized wax capsules to the concrete mix. The phase-change wax in these capsules shifts from solid to liquid, or the reverse, between 73-74 degrees and stores or releases a good deal of thermal energy in the process (as when water vaporizes into steam or solidifies into ice). This strategy has been successfully deployed in drywall for many years to dampen temperature swings, helping keep building interiors cool and comfortable. Changing phases can occur an indefinite number of times. When embedded in concrete these microscopic capsules can keep the inside of buildings cool in summer and warm in winter.

ECC is also being modified to neutralize pollutants, thereby acting as an agent to clean dirty air. Concrete is commonplace in urban environments; embedding nano-titanium particles in concrete converts certain pollutants common in smog into inert salts when in direct sunlight.

Flexible, durable concrete with self-healing, pollution-reducing, temperature-modulating properties offers a promising vision: an urban infrastructure that responds to environmental change to create a lower energy, healthier and more livable lifestyle.

Can Concrete Really be Bent?

As with many other products and processes, biomimicry (learning and imitating the processes of nature) is transforming the world of concrete. Although in the early stages of being applied, concrete that is bendable without fracturing is now a reality. Concrete is the most ubiquitous building material on the planet, but it contributes between 6-7 percent of greenhouse gases and is thus a major contributor to climate change. It has great compressive strength but, when it cures, it becomes a hard, brittle material.

The idea for bendable concrete is borrowed from nacre (mother of pearl), the material that lines the inside of abalone shells. The main material in nacre--small, hard bits derived from calcium carbonate--is made flexible by the natural elastic polymer that surrounds and ties these small chunks together. This combination makes nacre both strong and bendable.

A number of universities around the world, including the University of California at Irvine, Stanford and the University of Michigan have been investigating the nacre model for concrete. By eliminating the coarse aggregate from the mix (gravel, sand and cement) and adding microfibers of silica, glass, steel and/or polyvinyl, they approximate the flexibility of nacre. The interfaces between these tiny fibers and the cement recreate the controlled slippage in nacre. Bendable concrete, technically called engineered cementitious composite (ECC) is not a single design mix but a broad range of design mixes. The precision of these formulae comes from the application of micromechanics theory.

Essentially, the microfibers create many pre-calculated microcracks. This contrasts with conventional concrete that develops a few large cracks that permit water intrusion, degradation of the reinforcing steel and, consequently, early rupture and failure under stress. The fibers and accompanying microcracks allow ECC to deform without catastrophic failure.

The advantages of ECC concrete are numerous: lighter weight (40 percent less); 300 times more flexible; superior seismic performance; less frequent maintenance and repairs, thus saving on costs; no need for expansion/contraction joints (e.g., on roads and bridges); and faster curing (7 days compared to 28 days).

The disadvantages are higher cost, the need for more skilled labor, and getting structural engineers to specify it when they have been taught that concrete cannot be flexible.

There are recently built bridges in Japan, Korea and the US using ECC. A 60-story skyscraper using flexible concrete for superior seismic performance is currently under construction in Japan. When our roads and bridges, which badly need fixing, get rebuilt, they can have a much longer projected life by using bendable concrete. The significantly greater durability of flexible concrete is the biggest sustainable improvement. Less frequent rebuilding of concrete’s failures also means big reductions of greenhouse gases.

Using Crushed Rock to Tackle Climate Change

A previous article focused on the need for large scale carbon sequestration with a look at a project in northern British Columbia that shows promise for meeting this challenge. Its approach takes biodiverse seed packets enveloped in biochar for nutrients and moisture retention and uses drones to spread these casings over wide areas to regenerate forests. This method of reseeding forests works especially well in remote, inaccessible terrain where replanting by hand is impossible.

Forests, or more specifically, the growing of trees, have been scientifically proven to pull carbon dioxide from the atmosphere. The calculations of some scientists, however, suggest that this natural process cannot achieve the scale of carbon drawdown required to offset our ever-growing carbon emissions. They cite the availability of land for forest restoration being the limiting factor. Consequently, another natural process is being considered to enhance and accelerate the storing of carbon not only in forests but on farms as well.  This process occurs when rain dissolves the carbon dioxide that is present in air creating a weak carbonic acid. If this acid falls on basalt rock, it reacts to form a carbonic mineral (calcium carbonate) that locks up the dissolved carbon for hundreds of thousands of years.

Basalt is the most common rock found on Earth’s surface. It is formed primarily from volcanic eruptions. Various forms of basalt are widely used in construction as aggregate in asphalt and concrete mixes and as base layers for highways and railroads. Although dense, this igneous rock crushes easily. Once pulverized into dust it can be spread relatively inexpensively on forest and farmland, making it readily available for rain to wash carbon out of the air and accelerate the process of sequestration. The understanding of this process, called “enhanced weathering” is not new, but because it speeds up a natural process it has only recently been explored for its potential to offset human-made emissions that are causing climate change.

The Future Forest Company, a recent start-up company, is conducting a trial of this speeded up weathering approach on a large birch and oak forest on the Isle of Mull in Scotland. Results of the trial will be known soon. If the data show the expected increase in carbon sequestering, then this accelerated weathering process could potentially capture gigatons of carbon dioxide when applied to forests and on farms around the world. Reseeding of forests is still needed, but enhanced weathering can supplement forest restoration and be applied to farmland as well.

Carbon Storing Building Materials

Since my recent article on constructing carbon storing buildings, many have asked for specifics on carbon sequestering building materials. Here is a partial list:

·         Cross-laminated timber (CLT) panels of varying dimensions made up of alternating layers of perpendicular boards. Because small diameter trees, pest damaged trees and even trees killed by wildfires are used in fabricating these members, forest resources are more fully utilized. In addition to being strong, stiff, stable and relatively light weight, CLT panels are highly fire resistant and hinder fire spread. CLT can often be substituted for steel, even in high rise construction.

·         All bamboo building materials. Bamboo is a fast-growing wild grass that takes carbon out of the air faster than other plants. When laminated into posts, beams, glue-lams and trusses (trade name Lamboo), it rivals the strength ratio of steel yet is more fire resistant without the use of ecologically unfriendly fire retardants.

·         Cal Star bricks and pavers.  These are manufactured from fly ash, a waste product, using a small fraction of the energy needed to fabricate other masonry products. Another technology, on the cusp of commercialization, is growing bricks at ambient temperatures using bacteria and biomass. Absorbing pollution and carbon is part of the process.

·         Hemp lightweight composite (building) blocks (developed by JustBioFiber Structural Solutions). The blocks are highly resistant to fire, mold and insect damage. Hemp products are top performers in the negative carbon materials classification.

·         Calplant MDF rice straw panels. These panels utilize a carbon sequestering waste material that normally is disposed of by farmers flooding their fields using large amounts of valuable water.

·         Low-carbon insulating materials: cellulose, fiberboard (Gutex Multitherm), hemp board, recycled denim and mushroom insulation. Ecovation is the brand name for mushroom insulation. It can be sprayed into wall cavities or seeded, filling the cavity in 3 days.

·         Ecosmart drywall. This product uses less energy, resources and water to manufacture, is fire resistant and lighter in weight, thus requiring less energy to transport.

·         Green concrete. Cement accounts for around 6 percent of greenhouse gas emissions (GHG). Green concrete focuses on 3 strategies: cutting GHG emissions, reducing inputs of natural resources, mostly by substituting recycled materials, and lessening air, land and water pollution related to its production. Ceratech, a US company, has created a feed mixture for cement that is 95 percent recycled fly ash and 5 percent renewable liquid additives, yielding an almost zero carbon footprint. Its concrete mixes reduce virgin resource inputs by 95 percent and water by half. This hydrated cement has superior properties to Portland cement, the industry standard. Another innovation is a cement that cures by absorbing CO2.

Incorporating these and other low or negative carbon materials in new and remodeled construction can substitute for many traditional building materials. Traditional materials like steel, concrete, aluminum and glass account for 11 percent of global CO2 emissions, according to a report from the UN Environmental Program. All materials listed above, except the bacteria grown bricks, are currently available and fit standard construction practices.