Concrete: New Technology, New Approaches

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August 11, 2010

Concrete: New Technology, New Approaches (Continued from p. 44 of the October 2008 issue of BD+C)



                  
Once you've read this special report, take the AIA Exam to earn 1 AIA HSW learning unit. (one-time registration required)

 


Reed Business Information is a Registered Provider with the American Institute of Architects Continuing Education Systems. Credit earned on completion of this program will be reported to CES Records for AIA members. Certificates of Completion for non-AIA members are available on request.
       This program is registered with the AIA/CES for continuing professional education. As such, it does not include content that may be deemed or construed to be an approval or endorsement by the AIA of any material of construction or any method or manner of handling, using, distributing, or dealing in any material or product. Questions related to specific materials, methods, and services will be addressed at the conclusion of this presentation. 

 

Nasvik classifies self-consolidating concrete as one of the new state-of-the-art products in the marketplace. “It has enjoyed wide acceptance in the precast concrete industry and is beginning to be used by cast-in-place contractors, especially wall contractors,” says the editor of Concrete Construction. “Most decorative contractors have had little experience with SCC, but it can greatly improve the look of decorative work. And because SCC produces high early strength, molds can be stripped more quickly.”

The PCA also notes that because SCC material flows well and consolidates into small voids, it’s particularly useful for more detailed, decorative work, such as for ornate, neoclassical columns and moldings.

In addition to particle-packing mixes, reactive powder, and SCC, a number of other concrete innovations are being used in the field. Mortarless concrete block is gaining popularity thanks to advances in built-in interlocking systems, as well as the use of recycled materials to enhance the sustainability and aesthetics of unit masonry, such as post-consumer glass.

Another advance in cast concrete is high-ductility concrete. Similar to reactive powder mixes, these concrete formulations offer high strength and greater flexibility once cast. These products also incorporate metallic or organic fibers to achieve these properties. “Compressive strengths as high as 33,000 psi and flexural strengths as high as 7200 psi are claimed, and under extreme loading, the material exhibits ductile behavior more similar to steel than conventional reinforced concrete,” states Amphion’s Iano. “Ductile concrete also exhibits low porosity, reduced creep under sustained loading, and reduced drying shrinkage.”

Translucent concrete, made by adding carefully oriented optical fibers to the concrete mix, has been turning some heads. In fact, one of these products was deemed one of the “coolest inventions” a few years ago by Time magazine, and it was featured in an exhibit at the National Building Museum. Since then, the application has been seen in architectural applications for walls and floors and in many concrete sculptures.

“It’s most common application is for interior non-load bearing applications,” relates John Hrovat, OAA, LEED AP, senior associate and manager of architectural design for Albert Kahn Family of Companies, Detroit. “However, companies are continuing development of this product and a structural variation could hold incredible opportunities as a future building system.”

Emerging Ideas in Reinforcing
The parallel emergence of advances in strategies for reinforcing—including new materials and designs—is hardly surprising, notes Neil Watson, editor of the United Kingdom-based Concrete Society’s Concrete magazine:












       
Insulating concrete forms, also known as ICFs, provide high levels of insulation as well as unexpected benefits, such as good blast resistance. Photo: Courtesy The Concrete Centre
               

“Reinforcement is fundamental to structural concrete. From straightforward reinforcing bar to prefabricated components, manufacturers and fabricators are continually extending the options available to solve problems, reduce costs, and increase durability.”

According to the Concrete Reinforcing Steel Institute (CRSI), “High-strength, low-cost, long-lasting reinforced concrete has proven itself as the material of choice for ensuring the greatest return and best value for building owners and their tenants well into future decades.”

Among the latest trends in steel reinforcing, CRSI’s Gibbons lists:
• The use of reinforcing bars with higher yield strengths.
• Computer programs for fabrication bar lists.
• Computer generation of placing drawings.
• Automated bar fabrication machines that speed production and lower on-site labor costs.
• Coiled steel reinforcing bars.
• Welding of shop-fabricated steel reinforcing-bar assemblies

Gibbons also points to advances in structural engineering and design, such as improved shear connectors for concrete slabs at concrete columns, steel reinforcing-bar couplers, increased use of epoxy-coated bars in both mats of bridge decks, and two-coat epoxy-coated bars.

Many of these techniques have become common practice for leading commercial and institutional Building Teams. A few others are just emerging, such as: 1) carbon-fiber reinforcing mesh; 2) reinforcing mats, sometimes called “reinforcement carpet”; 3) stainless-steel-clad rebar; and 4) MMFX, the acronym for microcomposite steel reinforcing. All of these technologies make construction easier and improve the durability and performance of the installed concrete.

Carbon-fiber reinforcing mesh. A novel carbon mesh product, branded as CarbonCast, uses a noncorrosive, high-strength carbon fiber grid as a replacement for conventional reinforcement. In addition, the system is lightweight, relatively easy to work with, and allows similar designs to use less concrete material.

According to Dave Cook, VP with the Clyde Companies, a large construction firm based in Orem, Utah, the product “eliminates wire mesh reinforcement, reduces shrinkage and cracking, doubles impact resistance, and triples fatigue resistance.”

“Carbon fibers have an elongation of 1.5%, while steel has an elongation of 6%,” says Todd Jackson’s, a foundation repair expert and associate member of American Concrete Institute’s Committee 440 on Fiber Reinforced Polymers. Because of the advantage in tensile strength, he feels the technology can be a good substitute for steel. “If you repair a wall with steel reinforcing, the wall could move in an additional 6% before the steel would come into play. If the wall is reinforced with carbon fiber mesh, it won’t move as the fiber material is much stiffer.”























              
A post-tensioning process underway: New techniques in post-tensioning include the use of intumescent materials in the post-tensioning process to increase the fire resistance of concrete structures. Photo: Courtesy Structural Systems
            

Reinforcement carpet. Another relatively lightweight reinforcing material is known as “reinforcement carpet,” a method that the PCA has pointed out can help reduce shipping and erection costs and improve the speed and precision of concrete mat construction.

Quickly and simply manufactured, these reinforcement rolls are precisely spaced and sized, welded to flexible metal banding, and rolled around reinforcement hoops that are later discarded. “As well as saving labor, the system requires less steel than conventional reinforcement systems and the mesh is easy to lay,” according to the U.K. Concrete Society’s Watson. “The fivefold increase in fixing speed can lower construction time, leading to a significant reduction of total costs.”

Galvanized and stainless clad rebar. Another issue with conventional steel rebar is corrosion due to naturally occurring environmental conditions. Billions of dollars are spent annually “to reconstruct or repair structures whose design life has been either shortened or eliminated as a result of corrosion, or through loss of aesthetic value or functional obsolescence,” say the Purdue researchers.

One solution is the use of galvanized and stainless-clad rebar. These rebar materials penetrate hardened concrete to dramatically reduce corrosion by 65%, extending the service life of the facility. In addition, it provides a protective layer on both the anode and cathode parts of the steel, according to the Purdue team: “This protective layer further acts to displace chlorides from the steel. The product can be used as an admixture in the placement of new concrete or topically applied to existing structures.”

Microcomposite steel reinforcing. Another corrosion-resistant alternative is MMFX, or microcomposite steel reinforcing. These materials are engineered at the atomic level to improve installed strength, energy absorption, toughness, brittleness, ductility, and formability, as compared to conventional carbon-steel reinforcing. The favorable strength characteristics of these materials can reduce construction costs and the quantity of materials needed. In some cases, MMFX will also simplify the placement of concrete, as a result of greater bar spacing, in heavily reinforced concrete structures, according to the Purdue researchers.

In addition, MMFX is not a coated material, so its monolithic composition is unaffected by field handling, as compared to coated products that frequently suffer damage on the construction site, or require touch-up recoating. MMFX can also be used by crews trained in standard field rebar fabrication procedures, whereas some coated products call for offsite cutting and bending, as well as special end-caps or other protections. Lastly, MMFX is safe: There are no unusual safety hazards for field erection, which has been one complaint about epoxy-coated bar.

Another “nano” innovation on the horizon is “self cleaning” concrete. By applying a “nano technology additive to the concrete, it reacts with sunlight to repel stains commonly associated with pollution and grime. This technology also has a compelling side benefit in that it improves air quality in the immediate area around the installation,” explains AKA’s Hrovat. “The possibilities are intriguing in that this technology can be used as part of new construction, part of a paving system and even can be applied as a retrofit coating to an existing façade.”

Reliable, Novel Concrete Structural Techniques

When it comes down to the structural design of concrete buildings, designers benefit from a number of systems and approaches for walls. For example, ever since its introduction to the United States a few years ago, aerated autoclaved concrete (AAC) block has become popular for its green qualities and light weight. At the same time, insulating concrete forms (ICF) offer a quick, easy option for building highly insulated, strong walls. Another approach, tilt-up construction, is pushing the limit of heights and sizes of the wall sections that can now be erected—as high as 100 feet, enabling greater efficiencies.

One of the most appealing features of lightweight aerated autoclaved concrete is that it utilizes less material and energy to produce as compared to other concrete materials, as approximately 80% of its volume is made of residual air voids and pockets. In fact, about 12 cubic feet of finished AAC material can be produced from three cubic feet of raw material slurry, according to the International Masonry Institute. Other advantages include longevity and energy efficiency of properly erected walls, as well as conventional, cost-effective installation methods. Units made of AAC can be large and precisely dimensioned, meaning that less on-site adjustment is needed for a given project, thus adding to jobsite productivity. In terms of sustainability, AAC offers the unique combination of high R-value, thermal mass, and air tightness, all built into the durable structural elements.

For simplifying construction, however, the PCA maintains that it’s “hard to find an easier system than ICFs.” ICFs are wall systems produced from hollow foam blocks or panels stacked into the desired enclosure shape. 

“The forms are filled with steel-reinforced concrete to form a solid structure, sandwiching a heavy, high-strength material between two layers of light, high-insulation foam,” notes the PCA. “The resulting walls are air-tight, strong, quiet, highly insulated, pest and fire resistant, and durable in the face of even the harshest weather.” Because the industry produces a wide variety of ICF shapes and sizes, it is a commonly used system for industrial buildings, hotels, government facilities, schools, shopping centers, and religious facilities.

ICF construction provides high R-values, low air infiltration, and high thermal mass, which helps account for the 25-50% energy savings that ICF-constructed facilities boast as compared to similar wood-framed or steel-framed facilities, according to the Glenview, Ill.-based Insulating Concrete Form Association. 

“It’s not just about energy efficiency any more,” says ICFA director Joe Lyman. He says that the U.S. Department of Defense has been investigating ICF technology for its resistance to blasts. “We’ve found through blast demonstrations that ICFs lessen the impact of air pressure placed on the lateral face of a wall,” says Lyman. “When an air blast hits the wall, it compresses the EPS foam against the concrete.” Amphion’s Iano adds that other benefits claimed by ICF system manufacturers include design flexibility, acoustic and thermal control, jobsite safety, cost efficiency, and ease of installation.

As for tilt-up construction, perhaps most compelling is the ability to erect large-sized panels, which accounts for its popularity in the construction of big walls for industrial buildings. CRSI estimates that about 20% of all U.S. industrial structures have been constructed using the tilt-up method, in addition to a large share of warehousing and distribution facilities.

In recent years, tilt-up construction has become popular for more vibrant building types, too: schools, office complexes, and retail centers. “New finishes and forming techniques have allowed designers to incorporate texture, color, openings, articulation, and curves into a building system that was once thought of as the mainstay of big-box buildings,” says Lemay. “Now tilt-up is even making big box buildings more attractive than ever.”

While not generally considered a decorative concrete application (see accompanying box, “The Decorative Side of Concrete”), tilt-up panels can be treated with ornament, color, and other enhancements. According to the Tilt-Up Construction Association, these options include:
• Colorings added to the concrete mix.
• Textured paints.
• Applied textures produced by form-liner cast surfaces.
• Exposed aggregate.
• Mechanical tooling surface treatments.
• Surface forming, sometimes in combination with trompe l’oeil painting, to suggest three-dimensional forms.

Another recent enhancement to tilt-up, according to the TCA, is the use of intumescent materials in the post-tensioning process to increase the fire resistance of concrete structures. By incorporating a layer of reinforced intumescent coating between an inner and outer layer of high-density polyethylene sheathing, the coating swells during fires to effectively insulate and protect the substrate layer.

Yet another novel technique is external post-tensioning, known as EPT, for added strength. With EPT, tendons are attached to the outside of the concrete structure and then tensioned, applying a vertical, upward force to the bottom of the floor. “This method allows for the application of large upward forces at practically any location in a sagging floor and also increases shear strength and helps eliminate further deflection,” says John Criger, P.E., VP and technical manager for VSL, a Hanover, Md.-based company that designs, manufactures, and installs post-tensioning and structural strengthening systems. “They can be applied to many types of structures and, in particular, provide effective strengthening reinforcement for retrofits.”

Whether it’s decorated, precast, post-tensioned, reinforced, cast in place, or tilted up, concrete is not only a building material of choice, but it is changing with the times. From sustainability to versatility to durability, concrete has much to offer. Perhaps that’s why the PCA calls it “the most widely used construction material on earth,” with the “longest lifespan of any traditional building material.”
 
About the authors

C.C. Sullivan is a communications consultant and author specializing in architecture and construction. Barbara Horwitz-Bennett is a writer and contributor to construction industry publications.












































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Reed Business Information is a Registered Provider with the American Institute of Architects Continuing Education Systems. Credit earned on completion of this program will be reported to CES Records for AIA members. Certificates of Completion for non-AIA members are available on request.
       This program is registered with the AIA/CES for continuing professional education. As such, it does not include content that may be deemed or construed to be an approval or endorsement by the AIA of any material of construction or any method or manner of handling, using, distributing, or dealing in any material or product. Questions related to specific materials, methods, and services will be addressed at the conclusion of this presentation.


         
 

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