As environmental threats deplete the world’s reefs, precast concrete offers cost-effective and workable methods for restoring them.
While it’s not unusual for precast concrete to be a component of green commercial building projects, often helping architects and builders earn points for LEED certification, precast has also been lending a hand on the environmental front through natural habitat-restoration projects. In recent years, environmentalists around the world have been incorporating precast concrete into the construction of artificial reefs, restoring habitat for marine life and protecting fragile coastlines from storm devastation.
Why restore “living shorelines”
The Nature Conservancy is involved in several such projects, including a number of oyster reef restorations in the Gulf of Mexico. According to Amy Smith Kyle, project manager for the Louisiana Field Office Coastal Conservation, scientists at the Nature Conservancy have found that 85% of the world’s oyster reefs have been lost. “These reefs provide a lot of ecosystem services,” she says. “They make for fantastic fish habitats, and they’re natural coastal buffers that help break up wave energy.” Oysters are also filter feeders, meaning their presence in the ecosystem improves water quality. “They’re essentially living shorelines,” Kyle adds.
The importance of oyster reefs to the ecosystem, where they provide habitat for juvenile fish, shrimp and crab – and protect human populations on the coast during hurricanes and other severe weather events – has led The Nature Conservancy to initiate a number of reef restoration projects along the northern Gulf of Mexico. The conservation group is placing artificial reefs where oyster reefs have historically been located. The Conservancy is testing a number of different models of reef restoration, some of which include precast concrete.
Testing precast in Vermillion Bay
In Vermillion Bay off the coast of Louisiana, the Conservancy is testing precast concrete in its restoration of oyster reefs along the National Audubon Society’s Paul J. Rainey Sanctuary. The Conservancy is using a patented Oyster Break system developed by Wayfarer Environmental Technologies based in Hunt Valley, Md. Wayfarer’s Mike Turley explains that the system basically looks like Olympic rings sitting in the water. The system of rings is designed to perform several functions, including protection of the coastal shoreline from erosion.
The precast concrete rings measure 5 ft in diameter. The first phase of the project was completed last October with the placement of 670 linear ft of rings. The second phase began in the summer of 2011 with the installation of another 685 ft, and the third phase will add 1,700 more ft.
“A big part of the this project is trying to see what types of oyster reef restoration projects work best in which habitats,” Kyle explains. In other areas of the Gulf, the Conservancy is testing triangular steel frames filled with recycled oyster shells in mesh bags that interlock with one another. The Louisiana State University Agricultural Center is monitoring the various oyster reef restoration projects and will measure the biologic responses and shoreline change for three years following construction.
To attract oysters to the precast rings, Wayfarer adds a proprietary organic mix to the concrete. “We have a high attraction rate,” Turley says. “Oysters tend to attach themselves at the ends of the shelves, so you actually end up with higher-density reefs. The bigger the structure gets, the bigger its capacity to deal with storm surge.”
Wayfarer is the provider of the molds for the precast rings, and with each project, the company brings the molds to the local area and uses local precast concrete manufacturers to cast the rings, which weigh in at one ton apiece. To install the rings, contractors use an airboat with a crane boom or standard barges with excavators that position the rings in the water. The ring design in Vermillion Bay consists of six segments of 225 linear ft each with a 100-ft break in between each segment.
For the Vermillion Bay project, Wayfarer has been working with Houston-based Oldcastle Precast Inc. Turley says artificial reef projects offer a significant opportunity for precast manufacturers. “We see a need for higher production output from precasters,” he says. “Some of these projects will be quite large.” Turley says his company is currently bidding on a 2,000-unit reef project.
Why precast works
The reef production process is fairly quick. Turley says precast manufacturers pour concrete into Wayfarer’s forms, which can then be broken in two to three hours. “That avails itself to just-in-time delivery,” he points out, “and also reduces standing inventory requirements.” The rings are generally ready to ship within 10 to 14 days. Turley says he can get any reef project going in as little as 30 days.
While the Conservancy is testing multiple possible solutions for oyster reef restoration, Turley feels precast concrete offers some distinct advantages. “You get much better consistency and strength from precast, and it also minimizes transportation costs,” he says. “All we need is a batch line. We can take our forms to local precasters.”
Precast also offers flexibility in mix components. “We can use different size stone and admixtures, allowing us to custom design for particular habitats and customers,” Turley notes. His company is able to fine-tune the concrete mix profiles based on local aggregates.
Turley says precast also offers weight capacity that keeps the rings in place. They are not easily shifted by waves or storm surges, but at the same time, the rings offer a lot of flexibility in design. “Our biggest competitor is rock,” Turley says. “Imagine putting 2,000 pounds of rock in 2 ft of water.”
The ring design also provides for wave attenuation, and despite the natural habitat benefits of artificial oyster reefs, Turley says the first goal of the reefs is shoreline protection. “As a wave comes across the structure, it goes circular around it and into it, taking a lot of the beat out of the wave.”
While it’s still too early to make any definitive assessments on the effectiveness of the precast concrete oyster reefs in the Gulf, Kyle says oysters are attaching in Vermillion Bay. “They seem to really like the texture of the product,” she says.
Reef balls offer solutions worldwide
Precast concrete rings aren’t the only method of artificial reef restoration being used. The Reef Ball Foundation, based in Athens, Ga., has helped place precast concrete reef balls in more than 4,000 reef restoration projects in some 60 countries around the globe. Their projects include not only oyster reef restoration but coral propagation and planting systems, estuary restoration, mangrove restoration, and beach erosion control projects.
Like Wayfarer, the Reef Ball Foundation in partnership with Reef Innovations Inc. in St. Cloud, Fla., has a variety of molds that are shipped to the reef restoration site. A local precaster then fills them with concrete, micro silica and aggregate, which cures for 24 hours. Reef Ball Foundation executive director Kathy Kirbo says that in addition to the molds, the foundation also sends its clients additives like microsilica, which increases the reef balls’ durability and strength and establishes resistance to corrosion. Adva Flow is also added to the mix to keep the reef ball pH low so it’s close to that of seawater. Microfibers added to the concrete create a rough texture to encourage organisms to attach. With most of the weight at the bottom of the balls, they are also less likely to shift during severe weather events.
The foundation offers reef balls in a variety of sizes from as little as a few pounds up to 6,000 to 7,000 lbs, according to Kirbo. They use the heaviest balls along coasts for breakwater to shift sand. Kirbo says the balls are especially useful in coral reef restoration because they mimic the limestone boulders to which organisms attach.
The Nature Conservancy recently employed reef balls at three sites in and around Mobile Bay in Alabama. “Some have been in place for over a year,” says Jeff DeQuattro, coastal project manager for the Alabama chapter of The Nature Conservancy. He says the beauty of reef balls is that they’re easy to move and can be placed using low-impact equipment in coastal marshes.
Currently, the Conservancy has placed a total of 3,500 reef balls in and around Mobile Bay, using 220-lb to 250-lb balls as well as smaller 150 lb balls. “The Reef Ball Foundation and Reef Innovations have been around quite awhile and have had success with oyster attachment,” DeQuattro says. He says the reef balls provide particularly good habitat for fish, since they are hollow and filled with holes.
“Oysters are attaching to some of them,” he adds, though he admits the Conservancy has seen greater attachment rates with bagged oyster shells and rebar cages filled with oyster shells, because oysters like to attach to other oysters. However, DeQuattro says precast concrete reef balls definitely have other advantages. He says precast concrete is the most cost-effective method for reef restoration because it requires the least amount of labor to install. He says it’s also an ideal solution for areas that are closed to oyster harvesting, because they don’t have to use recycled oyster shells to create the artificial reefs, thus saving the shells for areas that will produce oyster harvests.
“In the old days of artificial reef creation, people put down trash, tires, ships and cars,” Kirbo explains, “but those things all shift under water and have toxins, too,” adding that precast concrete offers a much more environmentally friendly and stable option for reef restoration.
Deborah Huso is a freelance writer who covers home design and restoration, sustainable building and design, and home construction.