Coral relief - concrete reefs

Materials World magazine
2 Apr 2012
artificial coral reef made out of concrete

Concrete domes are making coral homes. Eoin Redahan looks at the science behind artificial reefs.

Almost anything can become an artificial reef — a shopping trolley, a mattress spring bed or a scuttled ship — but the most effective coral reefs require more assiduous design and material selection.

Before a Maricuda Reef Ball gives home to octopuses, corals and other creatures, it is attuned to the proclivities of local species and the given environment. Oysters, for example, like to eat cement with a high pH (it helps their shells grow). Red groupers like to live on ledges, and some smaller creatures enjoy little recesses. As such, domes will be designed with different features and chemical compositions.

According to Todd Barber, Chairman of Maricuda, based in Florida, USA, the best artificial reef is one that mimics a natural reef in both species diversity and population density. ‘When certain biological goals are desired, such as enhancing the natural settlement of corals, even the surface textures must be specific.’

‘It must be inert,’ he adds. ‘It cannot allow chemicals to leach into the water that could hurt or alter the way marine life grows naturally, and the surface pH has to be the same as that of seawater.’ For the past 20 years, Barber and his colleagues have submerged more than 500,000 Reef Balls in 70 countries. The dome shape is predominant as it offers better angles for sunshine and currents and is more stable in rough conditions.

The Reef Balls, which range in size from about 14–3,600 kilogrammes, are manufactured using fibreglass moulds. The moulds are pinned together around inflatable polyform buoys to create the dome shape. The concrete is poured into the moulds and cured before they are taken apart for the application of surface textures. After curing on land for 30 days, the inflatable parts are reinserted into the assembled dome, which is towed to the deployment site.

Inevitably, composition and design will vary according to the region. In areas where tropical storms are frequent visitors, a denser aggregate can be used (for example, river gravel instead of limestone) to make the domes heavier. The production process can also be tweaked to alter weight. ‘The hole in the centre of the reef buoy inflates’, Barber says. ‘If you inflate it less, the walls of the Reef Ball are heavier. Occasionally, we have to put in anchoring systems. If we’re trying to protect a beach from erosion, (in the Caribbean in particular) you might have pilings, fibreglass rebar or concrete pile anchors.’

Material availability is also an issue. ‘The materials in two places could be different,’ he says. ‘Volcanic rock could be available in one place and limestone in another. Limestone is good; you want it exposed. So, you might use a concrete that makes those rocks stick out where the animals can grow on it. Volcanic rock, on the other hand, can be too smooth and slick for corals to grow on. So, we may use a different type of concrete that coats the outside of those rocks and seals them in, so the animals have something to attach to.’

The composition of materials, such as Portland cement, varies in different countries. To address this, the admixture must be adjusted to improve the cement’s pH. Other challenges are harder to foresee. Barber has worked on islands without fresh water, and has had to adapt using salt water. In a recent project in Venice, Italy, they had to contend with freezing conditions, which modifi ed the concrete’s behaviour and hardened the inflatable rubber moulds. In Georgia, USA, the team came across a type of granite that wouldn’t accommodate coral growth under any circumstances. ‘Some of the stuff had been sitting on the bottom of the sea for the past 100 years. Nothing was growing on it and no one could explain it, but we had to use a different aggregate.’ Considering the company is undertaking projects in Bahrain, Poland, the Gulf of Mexico and Montserrat, these material challenges look certain to persist.

Barber and his colleagues are constantly looking at new ways of improving the Reef Ball. One such technique has been applied to create natural-looking, ‘lumpy’ reef moulds in Australia. ‘We have developed a dissolving concrete mix that allows a designer to create any shape by casting a part with dissolving concrete and placing it inside the Reef Ball mould,’ he says. When the concrete dissolves, the new shape is imprinted inside the Reef Ball.

Alternative materials are also being assessed. US-based company Ceratech in Virginia has pioneered a new type of concrete using a high grade of fly ash, rather than Portland cement, that produces fewer CO2 emissions and sequesters carbon. ‘We’re looking at this as a possibility’, he says. ‘But I don’t want to put too much emphasis on the material, because there’s a lot more to designing reefs.’ Just ask a broody octopus. 

How to build an octopus lair

  1. Select a Reef Ball — a concrete dome dotted with holes.
  2. Take one glass Coca-Cola bottle.
  3. Place the bottle upside down at a 70º angle into one of the holes in the Reef Ball.
  4. Fill concrete completely around the bottle, making sure that no light can get in.
  5. Submerge the dome and wait. The female octopus will go into the dark area, clean out the bottle and lay her eggs inside it.
  6. Enjoy your octopus lair.

Please note: Lairs must be at least four metres apart to avoid fighting octopuses.