Precast’s resiliency makes construction engineers take notice.
By Mark Crawford
Three years ago, McCann Concrete Products in Dorsey, Ill., decided it was time to introduce a new product line. Though the company has been in business for more than four decades, mainly in the department of transportation market, it was struggling with increasing cutbacks from customers in Illinois, Missouri, southern Indiana and Iowa.
Many forces – natural and man-made – threaten our national infrastructure. Earthquakes, hurricanes, tornadoes, explosions and terrorist attacks can all destroy vital structures like bridges, highways and dams. As a result, engineers are increasingly turning to precast concrete because it is strong, durable, nonflammable, inorganic and, most of all, resilient. Depending on project requirements, precast concrete can be designed to resist a wide range of loads and impacts such as tectonic forces and hurricane-strength winds.
“Precast concrete is a high-performance material that provides three important characteristics – versatility, efficiency and resiliency,” said Bryant Zavitz, vice president of product development for Tindall Corp., a precast company in Spartanburg, S.C. “The structural soundness of precast provides protection against numerous hazards. Extensive research has resulted in innovatively designed structures that provide resiliency, even during extreme load events such as earthquakes.”
Structures made from precast rarely suffer complete damage during a catastrophic event, which means they can be rebuilt more quickly. For example, bridges and highways that can remain open after a hurricane or earthquake greatly improve rescue efforts and minimize suffering.
Resiliency is, of course, important for all structures, regardless of scale. However, it is especially critical for larger infrastructure, people depend on such as dams, highways,
bridges and towers. These structures must be as resilient as possible to withstand challenging events, which requires innovative thinking and design. Below are three examples that illustrate how engineers are taking advantage of the resiliency and versatility of precast concrete to make structures that can survive some of nature’s toughest challenges.
Bridges are especially vulnerable during earthquakes. Examples include the San Francisco-Oakland Bay Bridge collapse in 1989 and the 1994 Northridge earthquake in the San Fernando Valley, which caused more than $20 billion in damage.
Engineers continue to look for new ways to build bridges that can better withstand tectonic events, and precast is often at the forefront of their thinking. Engineers at the University of Nevada-Reno Earthquake Engineering Laboratory recently constructed and tested a 50-ton, 70-foot-long bridge section that was built on top of three 14-foot-by-14-foot hydraulic driven shake tables. They then subjected the bridge to shaking that simulated the Northridge earthquake. The bridge underwent a remarkable 9% drift, moving more than 6 inches off center at the base, and returned to its original position with little or no damage.
M. Saiidi, professor of civil and environmental engineering at UNR and leader of the research team, attributes much of this success to the innovative materials and design elements incorporated into the bridge. These include copper-based and nickel-titanium shape memory alloys, rubber, carbon-fiber-reinforced polymer shells and plastic hinges.
The shape memory alloys, which were embedded in the precast, comprised less than 1% of the total building materials, “yet were very important in eliminating any permanent tilt of the bridge,” Saiidi said. “The plastic hinge elements were placed in the critical part of the column where there is considerable deformation. It is in the column plastic hinges that most of the earthquake energy dissipation takes place. We took that part of the column and made a separate element so that it can be replaced.”
High-performance precast concrete collars are often used to anchor pipelines that cross ocean bays and harbors. Garden State Precast, a Wall Township, N.J.-based precast company, is currently working on a project in Massachusetts that consists of attaching precast concrete collars to a sewage pipeline that is lowered to the bottom of Marblehead Bay. The previous anti-flotation system, designed to last 50 years, had a much shorter life cycle because the collars corroded faster than expected in the saltwater environment.
The project requires 350 dual collars and 185 single collars. Single collars are used for shallow depths while dual collars are required for deeper placement.
“The collars are required to hold the pipe in place on the bottom of the bay,” said Paul Heidt, engineering manager for Garden State Precast. “They are specially engineered to be heavier on the bottom so they stay stable going into the water.”
A special mix of high-performance concrete was used to minimize the transmission of chlorine ions from the seawater into the concrete, thereby reducing the rate of corrosion of the steel reinforcement inside the collars. The mixture included CNI, a calcium nitrite-based product designed to inhibit the corrosion of steel in reinforced concrete, as well as fly ash and microsilica.
The dual collars are bolted together around the pipe. A rubber saddle between the pipe and the concrete provides grip, keeping the collars from sliding down the pipe as it is lowered into the water. Corrosion is further reduced by using fiberglass bolts and nylon structural inserts at securing points to further protect the bolts from saltwater.
“With these extra anti-corrosion measures, we fully expect this anti-flotation system to last at least 100 years,” Heidt said.
Wind energy reaches new heights
Utility-scale wind turbine towers usually are made of steel and are about 260 feet tall. Taller towers are desirable because they capture more sustainable wind energy. However, building taller steel towers becomes problematic from a transportation and logistics perspective. For example, some of the parts that would be required for taller towers, if carried by truck, would be too large to fit under standard bridges. As a result, engineers are investigating precast concrete as an alternative material.
“Precast has several advantages over steel that become more important as hub heights increase,” said Christopher A. Palumbo, vice president of business development for Tindall Corp. “One advantage is fatigue performance and service life. Concrete does not have the same issues that steel does in terms of fatigue, and therefore, concrete can provide a much longer service life than steel, with very little additional investment.
“Secondly, the use of concrete for wind towers is a more efficient use of material, since the mass of the concrete tower provides stability for the turbine that would need to be included in the concrete foundation of a steel tower of the same height.”
A research team at Iowa State University, led by Sri Sritharan, professor of civil, construction and environmental engineering, has developed a new tower known as the Hexcrete tower. Built from precast concrete, it will allow heights of 300 feet and higher. Key components are precast ultra high performance concrete, high-strength concrete segments and high-strength prestressing strands. With funding from the Department of Energy and Iowa Energy Center, the ISU team and its partners are designing towers to reach hub heights of 460 feet. These new heights will lead to wind energy production in parts of the country where it was not considered feasible in the past.
The tower consists of six exterior columns. Concrete panels completely enclose the tower interior and allow it to act as a composite structure during loading, without the need to be rigidly connected to the foundation. The columns and panels are segmented into manageable sizes for easy transportation and on-site assembly. Sritharan believes a tower life span of 40-50 years is not unreasonable.
“The design of steel towers is governed by fatigue load, which is tied to duration of life chosen for the towers, typically about 20 to 25 years,” Sritharan said. “Concrete towers are not controlled by fatigue; UHPC provides excellent durability properties. Our tower is more durable than steel and can be easily tailored to meet any turbine size and tower height.
“It will also save costs on transportation, and we are in the process of establishing an optimal erection procedure and construction schedule. Construction costs will also come down as more towers are built.”
Research and development teams continue to embrace novel concepts and materials to substantially improve the resiliency of precast concrete. These include using materials that are not normally used in precast construction, such as metal-based shape memory alloys, rubber and carbon fiber.
“For example, we are studying the feasibility of carbon fiber-reinforced polymer tendons and ultra high performance concrete to develop precast bridge columns that are earthquake-resistant, have little or no damage during earthquakes and are durable,” Saiidi said.
According to Saiidi, many states are following new research results intently and are eager to adopt proven accelerated bridge construction methods. This is especially true for those states in active seismic areas.
“We have solved the problem of survivability. We can keep a bridge usable after a strong earthquake,” Saiidi said. “With these techniques and materials, we will usher in a new era of super-earthquake-resilient structures.”
Sritharan is hopeful that more mechanical engineers will learn about precast concrete and how it can be used over a wide range of structural applications – often with improved results and lower overall lifecycle costs compared to other materials.
“This lack of interest or awareness of precast is partly due to the lower strength of concrete compared to steel, and their inexperience with concrete,” Sritharan said. “However, with the introduction of UHPC with 29,000 psi strength, we have a material that has the best of both concrete and steel, but needs newer design concepts to be fully utilized.”
Mark Crawford is a Madison, Wis., based freelance writer who specializes in science, technology and manufacturing.
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