Several materials don’t fit neatly into the other material categories and include fibers, filler material (such as wood chips, chopped tires, and other by-products), and lightweight synthetic particles. Numerous types of materials have been used in concrete to gain specific properties (such as energy absorption or extreme light weight). Concrete is also used as an inert material (when hardened) to encapsulate waste materials, such as radioactive waste from reactor decommissioning.

Fiber-reinforced concrete

Fibers in the concrete mixture contribute to the longevity of a concrete structure primarily through crack control and apparent reduction. Short standard fibers help reduce plastic shrinkage cracking during placement and curing. Some long fibers can bridge structural cracks, and can help spalling resistance (particularly under fire or blast loads). Many of the fiber types are made of recycled materials from other industries. Carpet waste, recycled steel, tire cord, and many others have been used.

A specific type of fiber-reinforced concrete, known as engineered cementitious composites (ECCs), can replace some reinforcement and provide a lighter-weight member. These fibers also help the concrete exhibit strain hardening with increased ductility. Cracks are distributed among the fibers, and ECC maintains very low crack widths (which helps corrosion resistance and long-term durability).

Fillers in concrete

Materials such as wood chips and chopped/shredded tire can be used as a filler material in concrete. The addition of these materials can also have benefits for certain applications. For instance, concrete made with rubber from tires can be beneficial in applications requiring vibration damping (such as foundations for machinery or equipment) and in applications with impact loads (such as barriers).

Photocatalytic materials

Photocatalytic concrete was used at the Dives en Misericordia Church in Rome, Italy. Photo courtesy of Essroc Cement Corp.

A relatively novel tool in the effort to combat air pollution is the use of photocatalytic materials (Van Hampton 2007; Portland Cement Association 2009). These chemicals, most notably titanium dioxide (TiO2) particles, can be mixed with cement (or added as a separate ingredient in concrete), which then provides the concrete with an inherent ability to convert some pollutants (for example, NOx to nitrate, SOx to sulfate, and oxidation of some organics) when exposed to sunlight (or other ultraviolet light sources). The process is somewhat analogous to photosynthesis. Because the photocatalytic materials are catalysts, they are not consumed in the reactions, and will continue to work at reducing pollution.

These agents also help maintain the light-colored (and often brilliant white) appearance of concretes, and are often referred to as “self-cleaning concretes”. The breakdown of organic materials decreases staining of the concrete, which reduces maintenance and maintains reflectance in supporting heat island mitigation.

Lightweight synthetic particles

Expanded polystyrene spheres can be used as an ultra-lightweight additive. The Elemix^® concrete additive business (www.elemix.com) has produced a specific type of this aggregate known as Elemix^® XE additive that has negligible water absorption (the particles are hydrophobic) and negligible chloride contribution. The spheres have a maximum diameter of 1/8 in. (3 mm), with an average bulk density of 1.45 lb/ft3 (23.2 kg/m3). The thermal resistance of the concrete produced with this additive is improved due to the lower density. Additionally, lab and field performance was improved in a number of other areas as well: freezing-and-thawing resistance, cracking resistance, and pumping. A new acceptance criteria sets forth the testing to evaluate the use of lightweight synthetic particles in structural concrete.

Waterproofing concrete

Waterproofing agents of various types have been added as a surface layer for many years. A new take on this concept from Hycrete, Inc. (www.hycrete.com) is to use an admixture to the concrete that makes the concrete hydrophobic while also forming a protective barrier around reinforcement. The combination of keeping the water out and protecting the reinforcement can provide a very effective way to inhibit the corrosion of metal in concrete.

CO₂ sequestration

A loose pile of Elemix^® XE concrete additive. Photo courtesy of Syntheon, Inc.r

Innovative methods for CO₂ sequestration in concrete and for reducing the amount of CO₂ from cement production are gaining ground. The projects are based on the concept of CO₂ reduction through lowered emissions, by sequestration, or both. Innovative technologies, such as CO₂ sequestering cement and aggregate, are also coming into the market.

One sequestration method is an accelerated concrete curing that uses CO₂ in precast plants (refer to www.cleantech.com for more information). The plants can use the CO₂ they produce to divert to the curing, or could be situated near a large CO₂ source such as a power plant. The process essentially carbonates the concrete, but this is not appropriate for every type of concrete member. For the right application, however, this method can provide more than a tenfold reduction in curing time, resulting in large energy savings (and reducing CO₂ emissions), along with providing a potentially CO₂-negative precast plant.

A new technology can use brines, waste water, and flue gas in a process to produce a limestone aggregate, capture CO₂, and provide fresh water (refer to www.calera.com for more information). The process uses pollutants and salt water to make a calcium carbonate material that can replace a portion of cement in concrete. For every 1 ton (0.9 metric ton) of material made, there is potential to sequester 0.5 tons (0.45 metric tons) of CO₂. Several companies are continuing to develop this new technology by using waste materials for the partial replacement of cement while sequestering CO₂ or in producing alternate binders.

Another potential technology being explored by the cement industry to capture and reduce CO₂ is algae bioprocessing. Pilot testing is under way at one North American plant and several others are in discussions with multiple technology companies active in this field. Plant stack gases would be directed to closed algae
bioreactor vessels where CO₂ is converted to biofuels and biomass. This fuel could be recycled as plant fuel, used to replace diesel in mobile equipment, or as a feedstock for bioethanol (in lieu of agricultural feedstocks).

Citations

Portland Cement Association, 2009a, “Photocatalytic (Self-Cleaning) Concrete: Bibliography of Selected Publications,” LB032, Portland Cement Association, Skokie, IL, 14 pp.
Van Hampton, T., 2007, “Smog-Eating Concrete May Soon Cover U.S. Buildings,” Engineering News-Record,
V. 258, No. 9, 49 pp.