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.

Building with Wood

For several decades there has been a debate in the building industry as to whether wood frame or steel frame construction is more sustainable—wood being a renewable material, while steel has recycled content, often incorporating 70-80 percent old automobiles. Perhaps the debate is finally being decided due to a panel technology called cross-laminated timber, CLT for short. Developed in Europe in the 1990s, it is only recently gaining popularity here.

A CLT panel usually consists of 3, 5, 7 or 9 layers of kiln-dried boards stacked in alternating directions, bonded with structural adhesives and pressed to form a solid, straight, rectangular panel. Surprisingly CLT has good fire-resistant properties: it is hard to ignite and once lit resists fire spread. Because the layers are oriented perpendicular to each other, the CLT panels are exceptionally strong, stiff, stable, relatively light weight and able to handle load transfer on all sides. They can be used for walls, floors and roofs in a single building system, or used interchangeably with other wood products.

Most commonly CLT panels are 40-60 feet long but can be as much as 100 feet. They are up to 18 feet wide and any thickness up to 20 inches. These panels are widely used in Europe, Australia, Canada and Japan. The possibility of large panels is revolutionizing how 10, 20 and 30-story buildings are being built. Currently an 18-story, 400 student residence (174 feet high) at the University of British Columbia is the largest CLT structure, but a 24-story tower is under construction in Vienna and a 35-story building in Paris is in the works. The most ambitious proposal to date is London’s CLT framed, 80-story Oakwood Tower.

Not only are CLT panels frequently made using small-diameter trees, but also can use less desirable wood from pest damaged trees, or even trees killed by wildfires, without compromising the panel’s overall integrity. These small, less-than-perfect inputs to panel manufacturing are leading to better utilization of forest resources. Pulling out small and medium sized timber, as well as dead trees, contributes to healthier forests.

Processing these culled trees into CLT panels, which then get incorporated into buildings, sequesters carbon from the atmosphere. CLT not only emits less carbon dioxide during the manufacturing phase but the finished buildings then help sequester carbon for longer periods. Scientists estimate that buildings made with these materials result in a 25-30 percent reduction in global warming potential compared to those made with traditional materials—concrete, masonry and steel.

Because they lend themselves to design versatility, fast installation, reduced waste and good thermal and seismic performance, CLT can reduce construction costs by up to 50 percent. Perhaps the biggest advantage, however, is sequestering carbon while creating healthier and more resilient forests.