Innovations in Concrete
By: Nicola Davies, Ph. D., freelance writer
This bridge column was built with a hybrid fiber reinforced concrete composite (HyFRC) developed by Professor Claudia P. Ostertag and her research group at the University of California, Berkeley, that has been exposed to seismic loading conditions. The material provides superior damage resistance due to crack control associated with fiber hybridization.
Advances in technology are resulting in more sustainable and cost-effective types of concrete.
CONCRETE IS THE most prolifically used building material worldwide, as well as one of the cheapest. Yet it also creates about 5 to 8 percent of man-made global CO2 emissions. Advances in technology therefore are being implemented to make it a more sustainable material. Developers, contractors, building occupants and the environment all stand to benefit from the development of new technologies such as geopolymer concrete, self-consolidating concrete and ultra-high performance concrete.
Durable Geopolymer Concrete
Dr. Erez Allouche, associate director of the Trenchless Technology Center at Louisiana Tech University, has been conducting research into geopolymer concrete, an innovative material characterized by long chains or networks of inorganic molecules. It relies on minimally processed natural materials or industrial byproducts like fly ash, a byproduct from coal-fired power plants, or granulated blast furnace slag to significantly reduce its carbon footprint. Replacing the Portland cement component of concrete with fly ash could save many waterways and thousands of acres of land from contamination by large-scale dumping of fly ash.
Geopolymer concrete (GPC) has been shown to offer greater resistance to corrosion and fire while exhibiting higher tensile and compressive strengths and lower shrinkage rates than conventional concrete. Allouche has shown that GPC can be sprayed, dry-cast and even extruded, making it usable in a variety of applications.
The tiny fly ash particles contain calcium, silica and alumina, and can easily fill the spaces between the larger particles of aggregate in concrete. In addition, because fly ash particles are spherical, concrete using fly ash requires less water to achieve a workable consistency.
Researchers estimate a reduction of 90 percent in greenhouse gas emissions over the lifecycle of this exceptionally durable concrete, which is projected to last hundreds of years. “Certain fly ashes can control alkali silica reactivity (ASR) for regions using reactive aggregate. The control of ASR results in longer-lasting concrete for pavement, bridges and buildings, reducing maintenance and repair costs,” says Steven Kosmatka, vice president of research and technical services for the Portland Cement Association.
The resilience of this concrete offers an additional advantage, since buildings made entirely of concrete are better able to withstand natural disasters. “Resilient concrete construction offers the greatest savings regarding maintenance costs over the life of a structure,” Kosmatka asserts. While the cost of manufacturing GPC is higher than that of traditional concrete (approximately $0.80 per block, compared to $0.60), the durability and reduction in carbon footprint outweigh the additional upfront cost.
Self-consolidating concrete (SCC) has the ability to produce smooth surfaces without any sign of honeycombing. Ideal for use in areas where reinforcement is congested or where formwork is highly complex, SCC flows well and is non-segregating. It fills the formwork without the aid of any mechanical consolidation.
SCC offers several additional advantages: placement and equipment costs are reduced; time is saved through quicker construction, quicker concrete truck turnaround and the elimination of the consolidation process; there is less noise, since no mechanical methods are needed to consolidate; and it has better hardening properties than traditional concrete.
The mixing method and timing used with SCC are critical, however, as they can affect slump flow and compressive strength. The coarse aggregate and water are put in first, then the cement and other powders are added, followed slowly by the fine aggregate and the water to trim the mix. While using SCC reduces overall production costs and construction time, the material costs and the need to use workers skilled in modified production practices result in higher costs than those for conventional concrete. However, with the increased use of this concrete and a resulting increase in skills in the labor pool, costs are expected to drop in the future.
Although costly to produce, see-through or light-transmitting concrete has energy-saving properties; light is conducted through optical fibers embedded in the concrete, which are extremely efficient, resulting in negligible loss of light. Furthermore, see-through concrete has aesthetic appeal, resulting in beautiful effects that can change the public perception of concrete’s opacity.
Fiber optic concrete panels can be used to bring light into a structure and to focus attention on certain aspects of a building. The insulating properties of translucent concrete will likely result in lower heating costs. It also has security benefits; it is so opaque that figures can be seen in an office or close to a perimeter fence without the use of surveillance cameras. And it may help reduce power costs. This technology is still experimental, rare and labor intensive; its use currently consists primarily of precast panels and blocks that are more expensive than traditional concrete. With more research, however, production costs are expected to decline.
Also known as ultra-high performance concrete (UHPC), reactive-powder concrete is made from silica fume, fine silica sand, Portland cement and quartz flour combined with a high-range water reducer, water and organic, synthetic or steel fibers. It can achieve compressive strengths of up to almost 30,000 pounds per square inch.
This concrete’s ultra-high strengths are achievable without the addition of coarse aggregates, making it very workable. High tensile strength is obtained by including synthetic, organic or steel fibers in the mix; the flexural strength can reach up to 7,000 pounds per square inch. Durability is achieved by a combination of ingredients selected for small grain size (600 micrometer maximum) and chemical reactivity.
Reactive-powder concrete or UHPC typically is used by specialty contractors to improve a secured facility’s resistance to explosion and is particularly suited to structures built to contain nuclear waste. It was also used to form the canopies for Calgary’s Shawnessy Light Rail Transit Station; the station’s 24 ultrathin shells are less than an inch thick (only 20 millimeters, or 0.79 inch). The cost of UHPC components is higher than that of conventional concrete, but this is offset by the elimination of reinforcement or supportive structures.
Reduced Maintenance Costs
Kosmatka believes that self-cleaning concrete containing titanium-based catalysts, a product that is not yet commercially available, is likely to have the biggest impact in terms of reducing maintenance costs over the lives of buildings: “Self-cleaning concrete would be my choice, or any technology that extends the life and durability of a structure. Less deterioration equals less maintenance costs.” He adds, “concrete mixes with low water contents [will also become] a cost-effective option, as they alleviate moisture problems with floor covering materials that are sensitive to moisture and shrinkage, reducing admixtures to reduce cracking.” The use of multigraded (micro- to macro-sized) fibers of plastic and metal helps to control cracking, improve ductility and increase durability.
Kosmatka is also in favor of self-healing concrete, which contains bacteria spores that close and seal tiny cracks when exposed to water and oxygen, resulting in improved durability and reduced corrosion of reinforcing steel. According to Kosmatka, the potential also exists for “concrete with tiny sensors that report the health of the concrete; for instance, the state of corrosion, humidity, shrinkage, chloride ingress or creep.”