Precast concrete gives designers and project managers control in the face of many uncontrollable variables in extreme situations.
By Evan Gurley and Kirk Stelsel
It’s not uncommon to hear a specifier say precast concrete is the best material for a job. But on certain projects there is no other product that comes close. Examples include custom jobs or situations where extreme circumstances come into play – sometimes both.
In the following examples, specifiers used precast concrete to mitigate risks to human life, high levels of traffic, major threats or harmful environments.
The perfect storm (shelter)
As evidenced by the devastation caused by major storms from coast to coast, resilient construction is rarely a priority and people and businesses suffer the consequences. The Institute for Business and Home Safety estimates 25% of small- to mid-sized businesses do not reopen following a major disaster.1 The loss of life and commerce to a community is often catastrophic. Unfortunately, it often takes learning these hard lessons before an investment is made in a shelter or other resilient construction.
Mike Vaughn, P.E., is the president and general manager of Vaughn Concrete Products headquartered in Henderson, Colo. His company manufactures a wide range of storm shelter products. Vaughn has heard from numerous customers who received thank you letters from employees following installation of a precast shelter, but there’s one he’ll never forget. After exhibiting at a garden show in Oklahoma City, Okla., he reached out to a local livestock auction to see about setting up his display there rather than hauling it back home.
“I told the guy I needed about 100 feet of space at his facilities to set up these shelters,” Vaughn said. “He said, ‘What are you going to pay me?’ and I said, ‘It’s not about what I’m going to pay, it’s about what you’re doing for your people.’”
That didn’t resonate, so the two agreed to a price. Two years later, a large tornado went right through the property.
“Nine of his people are alive only because of those storm shelters,” Vaughn said. “He called about 10 days after that and was quite humbled and said he got calls from the families of eight of the nine employees telling him how much they appreciated him putting in that display so they had a place to go. On that phone call, he ordered a new shelter to be built right in the middle of his facilities.”
Extreme weather events are just one example of the many situations in which the performance of precast concrete cannot be matched.
Big rig–resistant walls
Champion Precast in Troy, Mo., makes a range of anti-terrorism and security products to impede everything from crowds to large vehicles. Its products are even used for training by the military. The company regularly supplies Fort Leonard Wood with different sizes of T and Alaska barrier products used during live-fire exercises to recreate scenarios soldiers will face overseas.
Bollards manufactured by Champion Precast sit outside the iconic Gateway Arch in St. Louis to obstruct vehicular traffic and protect pedestrians. Each is 8 feet, 3 inches tall with about 4 feet of that buried. The company casts the bollards with two chamfers and inserts so the customer could hang a large anchor chain between the bollards for looks.
“Precast concrete is a superior barrier material due to its great design flexibility and aesthetic compatibility with urban settings,” said Jon Ohmes, production coordinator. “They can be used for everything from simple tasks such as crowd control or parking to uses such as securing a business to avoid terrorism.”
One of the largest security projects the company has worked on is a security wall for a nuclear power plant. The company manufactured two rows of wall panels installed in an offset pattern that surround the plant. Each wall panel is 4 feet tall, 5 feet deep and 9 feet long. The company determined the length so it could ship two units per load. The panels have a double cage and while they are not architectural, the company poured them with self-consolidating concrete for aesthetics and finished them with a chamfered side.
To test the wall, the customer ran a fully-loaded semi into it. The test moved one wall enough to change the specification to add a second row. A panel was also taken to a quarry for an explosives test. In this extreme condition, no other product could provide the security and longevity of a precast concrete wall.
Sitting in traffic or enduring endless detours is painful. Thankfully, the precast industry has pioneered a modular solution for road construction. Precast concrete pavement allows owner agencies to balance accelerated construction and rapid renewal, structural capacity, durability, quality, geometry, aesthetic versatility and reduced life cycle costs.
Precast concrete pavement is starting to gain national attention. In 2014, four jobs in Southern California alone totaled nearly $30 million. Caltrans bid a slab replacement project in Santa Barbara County on U.S. Route 101, one of the longest state highways in California. The highway faces high traffic volume at all hours. U.S. 101 also happens to be part of a state-recognized safety corridor that acts as an emergency route for the military. The highway has only two lanes in each direction, so closing down more than one lane was out of the question and would require a major incident report by the state.
Pre-Con Products used 680 cubic yards of concrete to manufacture precast individual slab replacement panels. The panels are equal to the width of a lane and were placed directly on the graded base with voids filled by grout. Panel thickness criteria came from the existing pavement thickness, except when the existing thickness was less than the minimum threshold.
“The main reason for selecting precast pavement for this project was the need for a rapid construction,” said Mehdi Parvini, Caltrans senior pavement engineer. “The other concern for the project was a limited work zone. With precast pavement, the space required for the construction zone would be reduced.”
Parvini added that precast projects are ideal when there’s a limitation for road closure. Closing high-volume roads for long curing times is not practical. With precast pavement, the road is closed during a night shift and reopened in the morning.
Jason Kline, Caltrans resident engineer, said this was District 5’s first time using precast pavement for rehabilitation. And while Kline said there were some challenges, it worked as planned.
“District 5 will be pursuing using precast pavement for rehabilitation and repair projects in the future,” he said.
Pat Camp with Pre-Con Products has been manufacturing precast pavement for many years, and is extremely excited about the traction this product is gaining in Southern California and across the country.
“Precast concrete pavement, in my opinion, has the potential to be one of the biggest markets ever in the history of precast,” Camp said. “It really has the potential to be as big as anything precasters have ever seen.”
Camp emphasized that having good working relationships with Parvini, Kline and the contractor allowed for the project to be a success. He also stressed that doing extensive homework to make sure the product works in the field is essential.
“Since precast pavement is in its infant stage, any precast pavement project that goes wrong will give this technology a black eye and we can’t allow that as an industry,” he said.
With extreme levels of traffic, no product can minimize disruptions like precast concrete pavement.
Precast wave blockers
The Schuyler Heim Bridge, completed by the Navy in 1948, is one of three that connects Terminal Island to the mainland in Los Angeles County. It is named for Commodore Schuyler F. Heim, commanding officer of the Terminal Island Naval Base throughout World War II.
In 1994, it was determined the bridge was in need of seismic retrofit improvements, but replacing the bridge was more practical and cost-effective. The removal and replacement of the bridge and the construction of new on- and off-ramps using various precast concrete elements is making and breaking waves.
The $210 million replacement project started in October 2011 and completion is estimated for early 2017. In addition to creating a new fixed-span bridge that meets current seismic standards, the project also adds 42 feet in width in the form of standard shoulders and a southbound auxiliary lane. The minimum vertical clearance of the bridge will be 46.9 feet over the mean high water level, allowing for accommodation of the new 45-foot fireboats.
“This is an exciting project to work on because of the complexities of constructing a project in water but also because of the Heim’s historic past and current importance to goods movement throughout the state and the country,” said Hammer Sui, Caltrans resident engineer. “We want to make sure that it remains a vital piece of infrastructure well into the future.”
Of all the bridge elements, the ones subject to some of the most extreme conditions are the precast concrete isolation casings. The casings protect the bridge columns from the aggressive ocean environment, seismic loads and abrasion from passing vessels. Precast isolation casings were selected by the designer on the project over steel casings as they are more durable and have superior impact and abrasive resistance.
Concrete has been used in seawater applications for decades with excellent performance. However, special care in mix design and material selection is necessary. Warren Taylor, president of Pro-Cast Products Inc. located in Highland, Calif., said special provisions such as the use of epoxy-coated rebar, epoxy-coated bar couplers, galvanized ladder rungs and silica fume were all used in order to combat the aggressive corrosive environment. Using precast concrete, the isolation casings are able to resist weathering, chemical attack and abrasion while maintaining their engineering properties.
Pro-Cast manufactured 220 pieces to construct the 55 isolation casings. Heights of the casings varied from 13 feet to 25 feet and the heaviest precast piece was 20 tons. A good working relationship between the precaster, MCM Construction and the Caltrans engineers led to a successful installation of the precast components.
Dealing with extremes
Precast concrete products protect humans, infrastructure and equipment from extreme environments on a daily basis. They also make projects in extreme conditions easier to manage. And it’s in these situations that precast concrete products exhibit the breadth of their capabilities to designers and specifiers.
Evan Gurley is a technical services engineer with NPCA.
Kirk Stelsel is NPCA’s director of communication and marketing.