It’s one of the most impressive feats in modern engineering, and crossing the world’s longest sea bridge – the 55km (34 miles) Hong Kong-Zhuhai-Macau bridge, which opened in October 2018 at a cost of $20bn (£15.9bn) – certainly has its benefits. But impressive as it appears, this mammoth construction project, like so many others, has come at a cost.
No less than one million tonnes of concrete were used in the eight years it took to build the bridge. It was this concrete that invaded the habitat of the critically endangered pink dolphin, and is thought to be the reason that dead dolphins washed up on nearby shores while the population near the bridge plummeted by 60%. Of course, dolphins weren’t the only victims – habitats are destroyed and countless other marine species are affected when large amounts of concrete are poured into the ocean.
Destruction of this kind is often the cost of using concrete – the most widely used manmade material on Earth. With three tonnes per year used for every person in the world, there are few parts of the planet that concrete hasn’t reached. The production of concrete is also a huge emitter of CO2. At least 8% of humanity’s carbon footprint comes from the concrete industry, mostly from the production of cement – one of concrete’s principal components. The cement industry generates around 2.8 billion tonnes of CO2 per year – more than any country other than China or the US.
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In the oceans, concrete is the main construction material, accounting for more than 70% of coastal and marine infrastructure such as ports, coastal defence structures and waterfronts. In China, for example, around 60% of its coast is effectively concrete. Similarly, more than 14,000 miles of the US’s coastline is covered in concrete.
In China, around 60% of its coast is effectively concrete. Similarly, more than 14,000 miles of the US’s coastline is covered in concrete
“Concrete is damaging in the ocean because, to put it in place, natural ecosystems are destroyed,” says Alex Rogers, director of science at REV Ocean, a not-for-profit company studying ocean health and raising awareness of global impacts on the marine environment. “Concrete is a conventional material, everyone understands it, and it is low cost. But really, in this day and age, when we’re looking at much more sustainable ways of carrying out development – whether it’s coastal reclamation or other forms of building – we should be looking at alternative materials that have a lower impact on the environment.”
Concrete walls are an easy way to bolster the coastline from the sea, but the environmental impact of using this material is huge (Credit: BBC)
Those alternatives might already be here. Among them is a substance called ECOncrete, developed as an eco-friendly concrete. Co-founded by marine ecologist Shimrit Perkol-Finkel, ECOncrete produces bio-enhancing concrete products intended to protect and rejuvenate coastlines and marine resources. This is achieved by using a mixture made almost entirely of by-products and recycled materials, and is thus nearly carbon neutral. This mixture is combined with up to 70% slag cement (a by-product of the steel industry which has a low carbon footprint) and is beneficial for marine concrete thanks to its high chloride resistance. The overall result is a low-carbon concrete.
By retrofitting concrete surfaces with eco-tiles and panels that are designed with greater surface complexity, it gives room for marine life to colonise
But it is the nature of the concrete-water interface that makes the most immediate difference for local wildlife. Typically, the smooth concrete surfaces of artificial seawalls are not inhabitable for marine organisms. However, by retrofitting them with eco-tiles and panels that are designed with greater surface complexity – grooves, ridges and crevices – it gives room for marine life to colonise and hide from predators and harsh conditions, ultimately improving the biodiversity of the seawalls.
An example of where this appears to have worked is Hong Kong, where shortage of space has meant reclaiming land from the sea has been common for many years. In areas of reclaimed land in the city’s New Territories region, work is ongoing to retrofit four existing artificial concrete seawalls with different types of eco-engineered fixtures: seawall eco-tiles, eco-panels and “armour” units designed to protect tidal pools.
A preliminary test of eco-tiles in the western waters of Hong Kong found that the number of marine species had doubled to 12, compared with the number of species found on artificial seawalls without such fixtures. “The results were very positive, showing that the eco-tiles with enhanced complexity could significantly increase marine biodiversity,” says Kenneth Mei-Yee Leung, a professor of aquatic ecology and toxicology at Hong Kong University.
The bio-blocks are designed to mimic the intertidal zone – the area where the sea meets the land at high and low tides – in a bid to provide a more suitable habitat for marine species
Work is now underway to introduce Hong Kong’s first eco-shoreline. Part of a 130-hectare reclamation project in the city’s Tung Chung area to provide additional land for housing, the new eco-shoreline will use concrete bio-blocks designed to serve as shelter for marine animals such as crabs and molluscs, to recreate natural habitats lost to land reclamation. “As a marine ecologist, I hope the project will be a great success in promoting marine biodiversity,” says Leung, “while also providing a nice and multifunctional shoreline for people to enjoy and appreciate local marine life.”
Wildlife-friendly blocks could reduce the harm to marine life by land reclamation (Credit: ECOncrete)
A 15-metre (49ft) installation has already been set up as a test site for the eco-shoreline that will eventually stretch 3.8km (2.4 miles), and is expected to be completed by 2023. The bio-blocks are designed to mimic the intertidal zone – the area where the sea meets the land at high and low tides – in a bid to provide a more suitable habitat for marine species.
Furthermore, unlike traditional concrete, which is highly alkaline, the specially designed concrete that is used for fabricating bio-blocks has a pH value (a measure of the acidity of substances) near to that of sea water, which helps to promote the growth of intertidal marine species such as crabs, molluscs, clams, mussels and oysters. The relatively neutral pH value of the bio-blocks is attained by replacing some of the ordinary Portland cement used in conventional concrete with alternative cementitious materials, such as slag cement, which has a secondary benefit of producing lower CO2 emissions.
“The seawater pH level is around 8, which is suitable to most marine organisms,” says Leung. “However, normal concrete has a pH of 12-13 which is not favourable to the colonisation of marine life. Therefore, concrete-based eco-engineered bio-blocks are commonly made with lower surface pH (pH 9 to 10) that are more suitable for marine organisms.”
ECOncrete’s products are already in use across eight countries and six different seas, from seawalls in Hong Kong to the Port of Rotterdam. “Our vision is that in the future, all man-made structures in coastal and marine environments will be designed and built with environmentally sensitive technologies,” says Perkol-Finkel.
Adding ridges to concrete blocks is thought to help boost colonisation of marine life, but how effective it is depends on the nuances of the local environment (Credit: ECOncrete)
But not everyone is convinced that eco-friendly concrete bio-blocks are the solution.
Beth Strain, a lecturer in marine biology and a project leader for Australia’s National Centre for Coasts and Climate, says the evidence for eco-friendly concrete as a better surface for marine organisms is very mixed. “In some cases, authors have reported that these materials benefit corals or biofilms, whereas in other studies there have been no benefits for marine organisms.”
Strain says at the moment there is not enough proof of concept to really see where these eco-friendly concretes do and don’t work. “It can be location specific. We did an experiment in 15 harbours around there world with the same type of concrete surface complexity.” Largely, the results were positive, but there were some locations where it didn’t actually work. In Penang, Malaysia, for example, using concrete with a complex, creviced surface that would theoretically retain moisture and be better for organisms to latch onto made no difference. Strain thinks this is because of the area’s typhoons, which make the area very wet already, so the crevices of the concrete aren’t a big help. “There is a degree of difference and each location will have its own environmental challenges,” says Strain.
Nature-based solutions provide you with a solution that essentially self-repairs and keeps up with that sea level rise – Alex Rogers
Louise Firth, a lecturer in marine ecology at University of Plymouth, agrees that the results are mixed. “Those producing eco-concrete purport that lower pH in concrete is better for marine life,” she says. But in her recent research, currently under review, her team found that in Singapore and the UK, a lower pH didn’t make a difference to the colonisation of marine organisms.
But while eco-friendly concrete may require more testing, there are other potential solutions, such as bio-cement, which can be particularly useful for coastal hardening. Bio-cement is formed by taking sand, or other forms of aggregate, and then adding bacteria and urea, a component of urine. The urea triggers the bacteria to secrete calcite – a form of calcium carbonate – binding the mixture together into a solid material similar to limestone.
“Bio-cement is certainly a more interesting technology in terms of improving the sustainability of coastal defences and coastal hardening,” says Alex Rogers, the REV Ocean’s science director. “It has something like one-third of the CO2 production of normal concrete, and of course you can mould it to make it more biologically friendly.”
Not to be confused with bio-cement, is another alternative: bio-concrete. This is where bacteria called Bacillus pasteurii is actually encapsulated and added to the concrete, along with a form of starch that serves as its food. The bacteria stay dormant in the concrete until a crack forms and air gets in. This change wakes the bacteria up, and they begin to eat, grow and reproduce. In doing so, they excrete calcite, which bonds to the concrete, fills the crack and seals it up. So in essence, this type of concrete structure is capable of self-repair.
Natural habitats such as saltmarshes are an effective protection from the sea, and support a wide range of species (Credit: Alamy)
Then there are nature-based solutions for coastal defence as well – like seagrass meadows, coral reefs, mangrove forests, salt marshes and other forms of wetland. These types of habitat tend to increase in height naturally and they can also retreat further inland as sea level rises to provide constant protection. Going forward, nature-based coastal defences might be the best solution as hard concrete structures in the ocean are likely to encounter increasing problems as a result of climate change. Scientists say we’re going to see more than one metre of sea level rise within the next 80 years, causing traditional coastal defences to fail.
“Building hard concrete structures that don’t naturally adapt to increasing sea level could create an additional problem that periodically those defences will have to be built up or even replaced,” says Rogers. “Whereas nature-based solutions provide you with a solution that essentially self-repairs and keeps up with that sea level rise.”
In the UK, in extensive areas along the east coast, it’s now recognised that it’s not economically viable to continue defending the land using hard concrete structures because they only get broken up, undermined and destroyed as result of changes in the climate. Instead, there is a move towards favouring nature-based solutions, using some concrete where it’s essential.
One benefit of nature-based solutions is the huge biodiversity associated with them – in the UK these natural defences can be home to mussels, marine molluscs, fish, shellfish, salt-tolerant plants like glassworts, and seaweeds such as kelp. These areas are all also massive CO2 sinks, says Rogers. “Salt marshes have something like 40 times the CO2 absorption potential of tropical rainforests on a per area basis.”
From multiple perspectives, it appears that nature-based solutions are definitely the way forward. “Coastal engineers are going to have to change,” says Rogers. “They can’t economically afford to keep replacing concrete defences that are being undermined and destroyed by rising sea levels and increasingly extreme weather.”
Rogers believes that these engineers really need to take onboard the fact that nature can be used as a viable and long-term strategy for coastal defence and possibly even coastal reclamation.
Ultimately, of course, in an ideal world the best solution might be no reclamation. But until there’s a cost-effective alternative to concrete, or more widespread use of nature-based solutions, this material looks firmly set to stay.
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