From simpler, strong coupling beams and self-healing concrete to eco-concrete and greater fly ash content, researchers are developing exciting real-world solutions for greener, more sustainable concrete applications.
By Sue McCraven
Going green or “pro-environment” has often been considered a politically correct position – but not a sound business decision. Times have changed. Backed by federal dollars for energy-efficient, durable projects and spurred by new sustainable technologies, a good business decision today is one to build green at every opportunity. Today, thanks to structural and environmental engineering research programs, concrete producers can take advantage of new technologies that are valid green solutions for rebuilding infrastructure for the 21st century. In this article, Precast Solutions looks at four emerging technologies that support more competitive, sustainable and durable products for the precast concrete industry.
NEW, MORE DURABLE COUPLING BEAM EASIER TO FABRICATE
Precast concrete coupling beams for high-rise structures in earthquake-prone areas are difficult to fabricate because the traditional design is characterized by intense congestion from required steel reinforcing bars (see Figure 1). But that could change with new research from engineers at the University of Michigan. UM researchers simulated the effects of a large earthquake in the structural engineering laboratory to test their new technique for constructing high-rise reinforced concrete buildings. The engineers used steel fiber-reinforced concrete to develop a better kind of coupling beam that requires less steel bar reinforcement and is much easier to construct (see Figure 2).
Coupling beams connect the shear walls of high rises around openings such as those for doorways, windows and elevator shafts, and these required openings can weaken the structure’s ability to withstand a seismic event. Their proposed design procedure for coupling beams in a core-wall structural system passed the test, withstanding more lateral deformation than an earthquake would typically demand.
“We simulated an earthquake with lateral displacements larger than those likely to be caused by the maximum credible earthquake and our test was very successful,” said James K. Wight, UM professor of civil and environmental engineering. “Our fiber-reinforced precast concrete beams, fabricated here at the structural lab, behaved better than the beams in use today,” he added.
“Fabricating a precast coupling beam without the intricate reinforcing bar skeletons of past designs means easier fabrication for precast producers and a simpler installation in the field,” said associate professor Gustavo Parra-Montesinos.
Researchers eliminated a substantial amount of reinforcing bars by using a highly flowable, steel fiber-reinforced concrete where the fibers are introduced during the concrete mixing process. The fibers are about 1 inch long and about the width of a needle. Remy Lequesne, a doctoral student, indicates the importance of this research for the precast industry, saying that, “Instead of constructing a skyscraper in the time-consuming and labor-intensive procedures used today, we envision this new coupling design being fabricated off site and delivered ready for installation.”
Engineers tested a 40-percent replicate of a 4-story coupled-wall structural system in December 2008 by applying a peak load of 300 thousand pounds against the building, pushing and pulling it with hydraulic actuators (see Figure 3). Measuring the drift (or motion) at the top of a building compared to the base, researchers found that the structure with the new coupling design easily withstood a drift of 3 percent; in a large earthquake a similar building is likely to experience a drift of 1 percent to 2 percent. This new coupling beam could provide an easier to fabricate and less expensive way to brace buildings in earthquake-prone areas around the world. Researchers are now working with a structural design firm to install the beams in four high rises soon to be under construction on the West Coast.
SELF-HEALING CONCRETE EXTENDS SERVICE LIFE
“Self-healing concrete invented at UM is a new type of concrete designed for enhanced service life in concrete structures through its ability to self-heal when damaged,” says civil and environmental engineering professor Victor Li. Without human intervention, in just the presence of water and carbon dioxide (CO2), small cracks in the concrete heal themselves (see Figure 4). “A handful of drizzly days would be enough to mend a damaged bridge made of this new substance,” claims Li.
Self-healing is possible because the material is designed to bend and crack in narrow hairlines rather than break and split in wide gaps as does traditional concrete. In other words, this new, self-healing concrete is damage-tolerant and very durable. Under tensile loading, explains professor Li, “this concrete shows a stress-strain curve that resembles that of a ductile metal: first a yielding followed by a strain-hardening response.”
Even during overloading, the cracks in this new material stay very narrow. The tensile ductility of this new concrete is as much as 300 to 500 times that of normal concrete with a tensile strain capacity of 3 percent to 5 percent — a stretching of specimens almost 5 percent beyond the initial size, enough to catastrophically fracture traditional concrete (see Figure 5).
During strain-hardening, the material undergoes controlled micro cracking with crack widths as tight as 50 microns or less. The tight crack width combined with the tailored ingredients allow self-healing of these micro cracks when the material is exposed to water and air.
“We were excited to find that when we load the concrete again after it heals, it behaves just like new, with practically the same stiffness and strength,” Li says. “Self-healing of crack damage recovers initial stiffness lost when the material was damaged. The material can be damaged and still remain safe to load.” Engineers found that cracks must be kept below 150 micrometers, and preferably below 50 or 60 micrometers (about half the width of a human hair) for full healing.
To accomplish this, Li and his team improved the bendable engineered cement composite (ECC) that has been in development for the past 15 years. More flexible than traditional concrete, ECC acts more like metal than glass. Traditional concrete material is considered a ceramic: brittle and rigid, it can suffer catastrophic failure when strained in an earthquake or by routine overuse.
But flexible ECC bends without breaking. It is studded with specially coated reinforcing fibers that hold it together. Li’s ECC recipe ensures that extra dry cement in the concrete exposed on the crack surfaces can react with water and carbon dioxide to heal and form a thin white scar of calcium carbonate. Calcium carbonate is a strong compound found naturally in seashells. In the lab, the material requires between one and five cycles of wetting and drying to heal.
The self-controlled tight crack width without dependence on steel reinforcement for crack control, along with self-healing functionality, enables this material to slow the penetration of aggressive agents such as chloride ions. A more impermeable concrete cover delays the initiation and propagation of steel bar corrosion. Self-healing concrete is expected to enhance concrete infrastructure durability and service life. Selfhealing ECC has been demonstrated in the laboratory to show repeated healing under multiple cycles of imposed damage.
“Our hope is that when we rebuild our roads and bridges, we do it right, so that this transportation infrastructure does not have to undergo the expensive repair and rebuilding process again in another five to 10 years,” Li said. “Also, rebuilding with self-healing bendable concrete would serve to protect our environment by reducing the energy use and carbon footprints generated in building our concrete infrastructure.” Bendable concrete can be both cast-in-place and precast. Large elements have been manufactured in precast facilities in Japan and Australia.
ECO-CONCRETE REDUCES CO2 EMISSIONS
Civil and environmental engineering professor Hwai-ChungWu states that: “Recent advances in addressing two imminent problems facing the construction industry, namely calling for more durable materials and reducing CO2 concentration, has led to the development of high-performance eco-concrete.”
Several versions of eco-concrete have been developed and researched in the department of civil and environmental engineering at Wayne State University (WSU) in Detroit. Two of the most promising candidates for replacing portland cement are 100 percent fly ash-based and magnesium-cement.
Eco-concrete is compositionally tailored for high performance, low cost or sustainability; many industrial wastes can be recycled as source materials. All potential source materials can be processed at a much lower temperature as compared with portland cement, greatly reducing the generation of CO2 from ordinary portland cement production.
At present, the production of ordinary cement accounts for 5 percent to 7 percent of global anthropogenic CO2 emissions. In addition, eco-concrete may have a significantly higher absorption rate of CO2 from the ambient environment than traditional concrete. Future construction or replacement with eco-concrete would have a very positive impact on reducing the current environmental CO2 levels.
Eco-concrete can be further enhanced mechanically by fiber reinforcement. High ductility and crack resistance are the major benefits provided by ecoconcrete, unlike the material characteristics of normal concrete (see Figure 6). These new eco-concretes are truly maintenance-free and are expected to be very durable.
GREENER CONCRETE USING FLY ASH AND GEOPOLYMERS
“We’ve won twice!” exclaims Melanie Kueber, civil engineering and materials PhD student at Michigan Technological University in Houghton, Mich. (see Figure 7)“ If we can produce the energy we need with U.S. coal resources and utilize the byproduct of coal combustion — fly ash — to produce sustainable concrete, we can win twice,” says Kueber. “We can reduce the amount of fly ash going to landfills and at the same time reduce significant amounts of CO2 atmospheric pollution from cement production.” This is sustainability in action. Using fly ash in concrete is a key project right now at Michigan Tech.
“Secondary material use, particularly fly ash, to save the environment and reduce landfill space is a priority, because we know that electricity generation by coal is going to be around for a while,” explains Kueber. “We have ways to use industrial byproducts, and we are discovering ways to increase their use.” This researcher’s realistic, can-do attitude is part of the university’s strong partnerships with industry and government.
How much fly ash can we feasibly use in concrete? Kueber explains that fly ash has been used in concrete since the 1930s. Currently, when used in highway concrete, it makes up about 5 percent to 15 percent of the total cementitious material used. The vision is to raise the use to 25 percent in the near future, with government and industry targeting 50 percent by 2010.
“Implications of higher use of fly ash in concrete are huge,” claims Kueber. In the past two years alone, 72 million tons of fly ash were generated by coal combustion in the U.S. Only 44 percent of that amount was reused beneficially. Half of that amount went into concrete.
Using fly ash as a component in concrete can have significant economic and environmental benefits, but several problems facing the concrete industry must be addressed by researchers working on this project. As an example, carbon in coal fly ash adsorbs air-entraining chemicals from the fresh concrete mixture, thereby compromising the concrete’s air-void system and resulting in less durable concrete.
Also, each fly ash source is different depending on the coal fuel burned, the fuel blending at the power plant and the various boiler burning conditions. Because individual fly ash sources will have varying material characteristics, achieving specified concrete properties at higher levels of fly ash substitution becomes problematic.
With projects like Michigan Tech’s fly ash project, improved material choices through research will lead to economic solutions that can provide for an infrastructure with an increased service life. Higher usage of fly ash in highway applications could pave the way for other industrial uses.
As of today, two LEED points that can be attained through the use of fly ash in concrete, under the category of Materials and Resources Credits 4.1 and 4.2 – Recycled Content. This credit is awarded when the sum of post-consumer recycled content and one-half of the pre-consumer recycled content are at least 10 percent of the total value of the materials used in the project by weight. Reusing fly ash in concrete falls into the category of pre-consumer recycled materials.
Another cement alternative Michigan Tech is looking into is geopolymer cement. Geopolymer cement is a portland cement replacement made through fly ash activation with alkali solutions. Typical Type I portland cement production is very energy intensive and emits high amounts of CO2 into the atmosphere. Production of geopolymer cement emits approximately 80 percent to 85 percent less CO2, and can potentially utilize an increased share of the coal fly ash generated, making it a more sustainable alternative to portland cement.
Sue McCraven, NPCA Senior technical consultant, is a civil engineer, technical writer and editor, and environmental scientist who has contributed numerous articles and studies to prominent scientific journals.