Posted on November 26, 2020
Imagine the strength and durability of concrete. Now imagine that it bends - enough, at least, to absorb shock and reduce cracking. Bendable concrete intrigues researchers for many reasons. The possibilities for its use are many.
Engineered cementitious composite (ECC) has been around for more than a quarter-century. The enhanced material delivers decreased maintenance costs and increased lifespans. It may very well solve some of the country's infrastructure challenges.
Certain kinds of fiber-reinforced ECC concrete mimic the behavior of nacre. Nacre is a hard yet resilient substance that lines the inside of an abalone shell. Its magic derives from crystalline calcium carbonates arranged into flat hexagonal tiles. A flexible, gummy substance holds them together. It allows the layers to glide laterally across one another when stressed.
Fine aggregates are more common than coarse. Fiber-reinforced concrete often contains about two percent fibers by volume.
Early ECC concrete was too expensive to fabricate or too costly to import. Today, there are many ways to make ECC concrete more cost-competitive:
- Less material due to the same strength at reduced thicknesses
- Savings from joint-free construction
- Reduce or eliminate steel reinforcement
- Reduced maintenance expense
Dr. Victor Li pioneered research into bendable concrete exhibiting nacre-like resilience. In the 1990s, he found that tiny, scattered fibers in the concrete mimic the effect.
Both cost and energy consumption limit the widespread acceptance of bendable concrete. Recent breakthroughs address early concerns. For example, researchers at Australia’s Swinburne University developed a cement-free formulation. It requires less energy to produce, cutting CO2 emissions as a result.
In the United States, researchers launched a multi-year project in 2017. Louisiana State University (LSU) researchers are working on bendable concrete. Compared to regular concrete, it has two times the flexural strength and 300 times more deformation potential.
The team tested 30 different mixes. They used different kinds of fiber, sand, recycled crumb rubber and fly ash. One of their formulations is already used to repair sidewalks on the LSU campus.
To reduce costs, the LSU team used fine sand from the nearby Mississippi River. Fly ash from nearby sources replaces 75 percent of the cement. PVA fiber is available and affordable throughout the country.
Bendable Concrete: Key Characteristics
What are some of the important qualities of bendable concrete?
Capacity for self-healing
In regular concrete, water tends to exploit hairline fractures. Freeze-thaw cycles tend to expand tiny fractures into visible cracks that weaken structures. Experiments at the University of Michigan demonstrate how ECC concrete can self-heal. In hairline fractures, extra dry cement reacts with CO2 and water to form calcium carbonate. In the lab, one to five wet-dry cycles healed cracks 60 micrometers wide.
Reduced water permeability
Sealants waterproof traditional concrete, when desired. By comparison, ECC concrete itself resists moisture. This adds further to its crack-resistant nature. Fine aggregates and waterproof fibers combine to dramatically reduce permeability.
Resilience under tension
Fibers enhance ductility. Certain types deform five percent or more under tension without losing strength. As a result, bendable concrete outperforms its traditional counterpart in vibration-prone environments.
Flexible Fiber Options
A wide range of both natural and synthetic fibers have already been closely examined by researchers. Here are some of the possibilities:
Polypropylene fibers help hold the mix together. This reduces bleeding rates while slowing the rate at which coarse aggregates settle. Longer drying times reduce shrinkage.
Fibers restrict the expansion of hairline cracks. No slump modifications, no special equipment. You must determine the quantity and length of fibers (longer fibers for larger aggregates). Longer fibers increase the bond. However, longer fibers do not always distribute evenly. During mixing, the movement of aggregates shears the bundles of polypropylene fibers. This creates individual fibers or smaller bundles of fibers.
Polyvinyl alcohol (PVA)
Kuraray, a Japanese company, pioneered in the mass production of polyvinyl (PVA) fibers. These fibers have been used in many applications, including concrete. During hydration and curing, the fibers chemically bond with the cement. PVA-ECC has high ductility and tensile strength.
PVA fibers used in ECC concrete are about a third to a half-inch long and half the thickness of a human hair. Coated fibers slide rather than break. Proper fiber disbursement is important to avoid a “hairball effect.” PVA fibers add significant strength to concrete. Eight ounces of PVA fiber per cubic foot delivers the same strength gains as five pounds of glass fiber.
Reinforcing concrete with natural fibers is also attractive. It is an affordable and sustainable solution. One study notes that “Jute also is one of the most affordable natural fibers and is second only to cotton in the amount produced.” Similarly, sisal is a readily available natural fiber. The trick is to deliver the desired performance.
Carbon-fiber reinforced concrete features carbon fiber in a polymer matrix. The carbon fibers may be of organic origin. One example is lignin, an organic polymer derived from paper or ethanol production. Carbon fiber can also come from petroleum-based products like polyacrylonitrile (PAN).
Carbon reinforcement in concrete is about one-quarter of the weight of steel. It saves as much as 70 percent in greenhouse gas emissions.
Bendable Concrete: Uses
Interest in bendable concrete will only increase. The industry is always looking for ways to deliver a more sustainable product. Cost reductions will drive acceptance.
Safety also drives interest. The tensile strain capacity of fiber reinforced concrete is many times greater than that of regular concrete. This makes it very resilient when earthquakes strike. It withstands a certain amount of shaking and vibrating without weakening. PVA-reinforced concrete reduces vertical shear.
Roads and bridges
An aging transportation network concerns leaders in both the public and private sectors. In 2018, the American Society of Civil Engineers rates U.S. infrastructure by state. Pennsylvania’s infrastructure grades ranged from B to D minus. The state’s overall grade was a C minus. ECC concrete may demonstrate the durability and resiliency required for infrastructure upgrades.
The Pennsylvania Aggregates and Concrete Association discusses the latest developments in the industry. For further help, please contact us.