Concrete : the new generation
Concrete is the world’s most ubiquitous construction material – but it comes with a hefty environmental cost. The cement and concrete industry must find alternatives that can make production greener.
An incredibly versatile material, concrete has proved essential to the development of urban life as we know it. Roads, bridges, towers, apartment buildings all depend on concrete. And its use is skyrocketing with the rapid growth of cities in developing and emerging countries. Contrary to popular belief, concrete and cement are not the same thing. Cement is a powder, which, when mixed with rock, sand aggregate and water, binds together into concrete. Although cements were used by the ancient Egyptians and Romans, the ordinary Portland cement we now rely on was only developed in the 1800s. Cement is the largest manufactured product on earth by mass, making it second only to water among materials consumed by humankind.
The environmental impact of concrete is heavy, however. The extraction of the sand and gravel we use as concrete aggregate leads to disappearing beaches, erosion and environmental issues like pollution or biodiversity loss. In Indonesia, entire islands have disappeared due to excessive sand mining, and sand mafias from India to Morocco illegally destroy coastal environments to feed the construction industry’s appetite for sand aggregate. The aggregate extracted in 2012 – estimated at 25.9 billion tonnes – would create enough concrete to build a wall 27 metres high and 27 metres wide circling the globe at the equator. But the most glaring environmental cost of concrete is from cement production. The cement industry is estimated to be responsible for up to 10% of global human CO2 emissions, making it one of the world’s largest industrial drivers of global warming.
Ordinary Portland cement is produced in a two-step process: raw materials, including limestone and other minerals are quarried, crushed and ground. This powder is fired in a kiln at 1450° C, where it fuses into small, hard nodules called clinker. The clinker is then ground together with other minerals to produce the powder we know as cement. Clinker production, says Detlef Heinz, professor of Mineral Engineering at the Technical University of Munich, is where the CO2 emissions come in. It’s a process he knows better than most, having previously worked in the cement industry. The clinker kilns not only burn huge quantities of fossil fuels but the chemical reactions involved in clinker formation also emit CO2.
Reducing the amount of clinker
The cement industry has long been aware of the gravity of the problem. In 2009 they released the Cement Industry Roadmap 2009, a plan for transitioning the industry to halve its global CO2 emissions by the year 2050. “There are potential ways to reduce the CO2 from cement production,” explained Heinz, “such as carbon capture and storage [CCS], where you capture the CO2 from the cement production, and then sequester it or use it as a basic material for new products like the synthetic natural gas CH4.” Could sequestering carbon and using carbon-neutral clinker kiln fuels solve the problem?
According to a 2016 report on eco-efficient cements from the United Nations Environment Program (UNEP), probably not. Although the report says CCS may be needed to reduce emissions enough to keep global warming to below 2° C, it points to better solutions in the short term. Heinz agrees: “I think the most important strategy, which Europe has been using for the last couple decades, is the use of secondary material to reduce the cement clinker component.”
Using less clinker in cement could significantly reduce the CO2 burden of concrete production. A proven example is the use of fly ash – a byproduct of coal burning – to replace normal cement clinker by up to 60%.
Another common alternative is the granulated slag formed in iron and steel blast furnaces, which can cut the requirement for traditional cement clinker by up to 80%.
These advances seem promising, but Karen Scrivener, head of the Laboratory of Construction Materials at the École Polytechnique Fédérale de Lausanne, has doubts about the viability of many of these clinker-substituting substances. Scrivener, who was one of the lead researchers behind the 2016 UNEP report, says there are serious issues with cement mixtures that use fly ash and blast furnace slag, the most common alternatives. “Today, the average rate of replacement worldwide is about 20%. The problem is that the supply is limited,” said Scrivener. “For blast furnace slag, we have about 8% availability compared to cement production, and the usable levels of fly ash works out to be about 8–10%. And, of course, if you’re talking about fly ash, which comes from burning coal, you have to ask how long we will keep on burning coal, which is a major CO2 contributor.”
Scrivener believes that her team in Switzerland may have found a better solution. With partners in India and Cuba, they have developed a calcined clay cement that can decrease clinker needs by half, cutting overall CO2 emissions by up to 30%. It is just as strong as normal cement, with properties that make it ideal for use in coastal environments where traditional steel-reinforced concrete structures are often subject to corrosion. They are testing the new cement in Cuba, where it will be used in the reconstruction following the devastation wrought by hurricane Irma.
In addition, massive amounts of this calcined clay can be found in mining tailing waste. “This means you don’t have to extract anything,” says Scrivener. “For example, in China, you have 10 million tonnes of it in a spoil heap in one site, two or three times the annual volume of cement use.” Other materials can be recycled and reused in concrete production, including old concrete itself. Lisbeth M. Ottosen, a professor specialising in resource recovery at the Technical University of Denmark (DTU), considers a used resource not as waste, but rather a resource input for the next process. She is a member of ZeroWaste Byg, an interdisciplinary research team that is investigating the reuse of construction materials for a zero-waste society.
Much of their research focuses on the potential of using bio ash from sources like Denmark’s wood-burning power plants. “It’s broader than just the reuse of concrete aggregate,” she says. “I focus my research on the resources which escape from the materials cycle as waste.” Another example is a DTU student research project in Greenland that uses the fibres from discarded fishing nets to replace the virgin polymers in fibre reinforced concrete. The fact remains that the lack of an alternative to sand and gravel aggregate for concrete is a huge problem in parts of the world like China. With around 200 tonnes of sand needed to build an average family house, construction booming and the environment suffering, the question stands – can we recycle concrete? Ottosen says it is common practice to reuse concrete in some European countries, but that this often involves “downcycling”, using it to create an inferior product for roading aggregate or “backfill” in areas like highway noise breaks.
With the EU setting a 2020 goal of recycling 70% of construction waste, it is debatable how much of this reuse will be in a degraded form – or instead “upcycled” into new concrete. Heinz says that using old concrete waste as a source of aggregate for new concrete might be a good solution, but there are still some issues that must be considered. Concrete is only a fraction of the total waste material from construction and only a fraction of this can be reused, especially since old concrete is sometimes contaminated. The recycling process itself also produces CO2.
And the aggregate available from recycling is not enough to meet the construction industry’s demand. “In Germany, you have annually something like 80–90 million tonnes of mineral materials like crushed concrete and bricks from the demolition of buildings,” Heinz says, “but we need up to 500 million tonnes of aggregate primary material for the production of construction materials.”
Buildings made of bamboo and hemp
We should instead consider alternatives to concrete – where applicable and available. “It will be necessary to go back to using renewable materials such as wood in situations where it is technically possible that they can carry out the same construction task as with concrete,” says Detlef Heinz. Although many large buildings require the use of concrete to ensure structural integrity, the argument for renewable or alternative construction materials in some cases is worth considering.
New methods to use renewable resources like timber and bamboo with only minimal concrete for specific purposes such as fire protection are emerging, in addition to alternative construction materials with concrete-like properties, such as rammed earth buildings and hempcrete – a material made from hemp fibres and a lime binder. In France hempcrete has been used in combination with concrete in a public housing project and a seven-story office tower. Petr Hajek, professor of Civil Engineering at Czech Technical University in Prague, says that new types of concrete are also proving important for areas where concrete-use cannot be eliminated. He points towards new variants like self-compacting-concretes, high- and ultra-high-performance mixes and concrete reinforced with textile meshes instead of steel. “These new technologies enable construction of more subtle concrete structures using less concrete, with better structural performance and durability, and lower environmental impact.”
Hajek says that even something as simple as optimising the shape of structural support can significantly decrease their weight and thus the amount of concrete needed. Or, going further, the use of non-steel reinforcement in the form of textile mesh made from carbon can enable the creation and use of thin, corrosion-resistant concrete shells. One such example is the Carbon Concrete Composite (C3) developed at Technical University Dresden, a woven-carbon fibre composite manufactured with lower energy consumption and CO2 emissions than traditional reinforcements. Similarly, a bamboo composite material developed by a team at ETH Zürich could replace steel-reinforced concrete, and potentially be used for beams, floor slabs and joints. And, as futuristic as it sounds, Hendrik Jonkers at Delft University in the Netherlands has invented a concrete that heals itself. Self-activating, limestone-producing bacteria in the concrete repair cracks, reducing the need for maintenance and thus new cement down the line. Like all the solutions above, there are drawbacks – in this case, higher short-term costs.
Scrivener is adamant that the cement industry must ditch this short-term mentality. “In the long term these are things that can save money, but we just have to get over the introduction barrier, and find incentives to introduce these changes.” Although it’s not clear exactly which path the cement and concrete industry will follow, one thing seems obvious: there is no magic bullet solution for making concrete more sustainable. “From making the cement to mixing it into concrete and using it in the building, in each of these steps we have possible savings. Only in putting together the solutions for each of these steps will we have the reductions we need.”
By Joe Dodgshun
Energy: Chasing waste
Between air conditioning, heating, lighting and hot water, buildings accounts for a huge proportion of the world’s energy consumption. Researchers, architects and urban developers are working on innovative solutions to fight the waste.
Is Europe an energy sieve? The continent’s property sector accounts for 40% of its energy costs and up to 40% of its CO2 emissions, similar to the US. The main reason is an ageing property inventory: pre-1945 buildings account for almost 30% of the total energy used by European buildings. In light of this, renovating old housing is at the forefront of efforts to cut down on waste.
The problem is that renovating an old house or flat isn’t all that simple. If not properly carried out, the work can damage a building, resulting in increased humidity, dampness and poor air quality. “The good news is that there are effective solutions,” says Vilhjalmur Nielsen of the civil-engineering department at the Technical University of Denmark (DTU). “A huge amount of research has shown that it’s possible to reduce energy consumption by at least 50% by insulating old buildings.”
“The real challenge lies in choosing the right methods and materials,” adds Frédéric Laroche, head of the Réhafutur project. “Renovation is a multidisciplinary issue.” This French project, which operates near Lille, involves testing a number of insulation techniques using eco-materials in a 350-m² house, working closely with all trades, from carpenters to electricians. The building is packed with sensors that provide data for long-term studies of each solution to identify which performs best over time.
A unique challenge with every building
To achieve large-scale success, renovating Europe’s properties requires bringing together expertise from engineers, businesses, public authorities, architects, universities, and more. This is the goal that DTU and its partners are seeking to achieve with their REBUS (Renovating Buildings Sustainably) project, launched in the spring of 2016 with funding of almost €11 million. It focuses on the renovation of social housing, which accounts for 30% of properties in Denmark.
France’s Biofluides company also hopes to have an impact on collective housing. Its solution is based on heat recovery from warm wastewater – which has an average temperature of 29° C – from residents’ showers, washing machines and dishwashers, which is then reinjected into the pump used to heat water for the building. The results are promising – the 70 buildings fitted with this technology require four times less energy to produce the same amount of hot water. As a result, residents have seen their energy bills fall by an average of 40%. Moving beyond housing, other buildings have their own challenges stemming from their environment or purpose. For instance, the energy requirements of a primary school are very different from those of a hospital open 24 hours a day.
The new departmental-archives building in Lille highlights this issue: proper preservation of its documents requires specific temperature and humidity levels. Instead of trying to maintain a fixed temperature through air-conditioning, the building, opened in 2014, was designed to allow the temperature to vary between 16° C and 25° C while preventing variations of more than one degree per 24 hours in order to avoid thermal shock, which could damage the archives. The building’s thickness, perfect weatherproofing, triple glazing and insulating carpentry, as well as its stainless-steel skin, which is designed to reflect solar rays, all play a part. The building is also capable of generating its own energy. With 300 m² of solar panels on the roof, it requires only a 16-kg cogeneration boiler to control the temperature over an area of 10,000 m².
Eindhoven’s green U
Across the entire campus, the Eindhoven Technical University (TU/e) is currently modelling a successful renovation. The work, which has already won a host of environmental awards, will transform the university’s main building into one of the greenest complexes in the Netherlands. The heating and cooling systems in most of the buildings will no longer be powered by gas but by one of Europe’s biggest geothermal systems. The system already stores heat and cold separately in a tank underneath the entire campus. Electricity will be supplied by solar panels, which will provide 500,000 MWh per year, equivalent to the energy used by 75,000 homes. Lit by a system of smart, low-power LEDs that can adapt to the needs of each visitor, the project is a new step forward in an energy strategy that has already allowed TU/e to reduce its gas consumption by more than 50% over 15 years.
Buildings’ energy efficiency is also part of a broader context. “More will happen in the next five years than has happened in the last 60,” explains Laurent Cantat-Lampin, sales director at RTE, the company responsible for managing France’s high-voltage electricity grids. “In 10 years, the amount of green electricity has tripled in Europe. However, grids aren’t designed to manage such flows, which are variable by their very nature.” This can cause absurd situations. “In Denmark, we have to stop some wind turbines on windy days because the grid isn’t smart or flexible enough to absorb the influx of energy,” says Henrik Madsen, head of DTU’s Computing and Applied Science department. Making better use of this energy means looking beyond the building scale and focusing on neighbourhoods or even entire cities. “The most sustainable, effective and cheapest solutions can only be implemented on a large scale,” says Madsen. This is particularly true when it comes to storage: at the building level, solutions for storing excess wind or solar energy are either impossible or too costly. At the neighbourhood level, they become profitable.
The good news is that Europe brings a wealth of assets to the table. “We’re ahead in terms of smart, automated and flexible systems that facilitate the integration, storage and distribution of renewable energy flows,” Madsen points out. As part of the CITIES (Centre for IT-Intelligent Energy System) project, he and his colleagues are working to help Denmark achieve its target of a fully renewable urban energy supply by 2050. This includes developing tools capable of managing all aspects of an energy system using forecasting, monitoring and optimisation. For example, the researchers have developed thermal-mass storage solutions within buildings, using the ability of certain heavy materials (such as concrete, brick and mudbrick) to store, then release heat or cold. “Used on a single site, the storage time can reach six to 12 hours. At the neighbourhood level, it can extend to two to three days.” The challenge lies in creating “energy-buffer” buildings that absorb variations in energy generation and consumption by acting on the heating and cooling systems in real time. This development will allow the much-discussed smart grids to constantly balance energy supply and demand. However, Nielsen points out that a number of obstacles still remain. “Making buildings smart enough to achieve the full potential of a smart grid isn’t always easy, technically speaking.” One limitation is the need to install extremely accurate sensors. The challenges are also financial, as it is necessary to develop standardised solutions, which are consequently less expensive on a large scale.
Across Europe, solutions that focus on entire urban areas are delivering convincing results. Launched in 2000, the BedZED project has completely transformed Beddington’s 2,500 m2 of homes, offices, retail premises, green spaces, and cultural and healthcare centres in South London. The result is an 88% drop in energy consumption for heating and 57% for hot water. Another iconic project is the Northern Harbour district in Copenhagen. Managed by DTU with government support, the EnergyLab Nordhavn project has a budget of €17 million and is helping the city cope with its growing population. Its work demonstrates how electricity and heating, energy-saving buildings and electricity transmission can all be integrated into a smart, flexible and optimised energy system. Nordhavn’s computerised control systems communicate with the energy systems in individual homes and offices right down to the radiators, heralding the smart cities of the future.
Urban heating, a major challenge
Temperatures in cities need to fall – and fast. But how?
Streets swelter, entire neighbourhoods are transformed into baking ovens, and residents are left exhausted by stifling heat. “Higher temperatures in cities are an increasingly noticeable issue, particularly at night,” says Thomas Auer, an expert on sustainable construction at the Technical University of Munich (TUM). “The heat produced by the sun, traffic and air-conditioning systems is absorbed by the tarmac and the walls of buildings during the day, and is then released at night.” Worsened by dark building exteriors and a lack of green spaces, which promote cooling, the formation of urban heat islands (UHIs) is becoming an issue of concern, especially with global warming set to further exacerbate this phenomenon. During the heatwave of 2003, which was responsible for the premature deaths of 70,000 people in Europe, the temperature difference between major European cities and rural areas exceeded eight degrees Celsius.
What can be done to alleviate this? “Reflective construction materials can minimise UHIs,” says Auer. Other researchers, like TUM’s Philipp Molter, are testing “skins” designed for use in the building envelope. In his laboratory, he and his team are working on the Flexcover project, which has created self-regulating windows modelled on human skin. Just like the skin’s pores, which can open and close in response to the temperature, they form an envelope that can “breathe”, adapting the building’s temperature to the conditions outside.
Rethinking urban design Even so, the real response won’t be found at the building level or even at the neighbourhood level, says Auer. Our whole approach to urban design needs a rethink, particularly when it comes to new districts. “Reducing urban density isn’t necessarily useful. However, we need to create a roadmap that we can use to adapt cities to the climate, to promote wind circulation in new urban areas using specially-designed corridors, like in Hong Kong, or to increase the surface area devoted to green spaces while ensuring these spaces are distributed across the city.” The transformation will take decades, but a number of major cities have already started. New York, for example, has painted the roofs of around 100 buildings white to reduce the amount of energy consumed. Since then, air-conditioning bills have fallen by 10% for five-storey buildings. What’s more, while Helsinki and Copenhagen are leading the way in Europe, Paris has been looking into urban renovation for around a decade, and Moscow has plans to redesign 3,000 major roads as part of My Street, a huge urban renovation project begun in 2015.[/quote_box_center]
By Jean-Christophe Piot
Automation: A glimpse of future building sites
Digital technologies can save time and money in construction, but the complexity of the processes will make automation difficult.
The robot looks like a pastry chef spraying cream on elongated pieces of pie. A white liquid shoots from the robot’s arm, swelling and solidifying until a wall-like structure begins to take shape. In Nantes in western France, the construction site of the future is a reality: robots are using 3D printing to build houses from digital blueprints. The white liquid is polyurethane, a plastic used as insulating material. The plastic forms two parallel walls between which concrete is poured, using the same technique, to form the basic structure. On the inside, the walls are then covered manually with plasterboard; on the outside, with plaster. Beginning next year, the 95-m2 house will house a family. It is a world première: although some buildings have already been built with 3D printing in China and Russia, these are not inhabited.
“The house meets all building regulations,” says Benoit Furet, project manager and researcher at the University of Nantes. “Ten more houses are already planned.” The advantages of this digital construction process: since neither concrete forms nor scaffolding are necessary, time and money are saved. The walls, for example, take three days instead of three weeks to build. “3D printing also makes building with curves easy,” says Furet. “This is important because buildings lose the most heat in corners. In the conventional building process, curves are often unprofitable because the formwork or the building material has to be manufactured individually or cut to size.” With the new technology, the house in Nantes has no right angles. In combination with the polyurethane, a 40% improvement in thermal insulation can be achieved.
Furet and his team are not alone. Several European projects show how robotics and 3D printing can be used in construction. As early as 2011, four flying robots demonstrated in a museum in Orléans how they could team up to build a six-metre-high tower consisting of hundreds of polystyrene building blocks. At the Dresden University of Technology, a research team is working on producing concrete in 3D printing, as in Nantes. The Technical University of Munich (TUM) is focusing on the purely mechanical properties of robots. In its “Hephaestus” project, the research team is developing a cable robot that can be used on building façades for both construction and maintenance. Controlled by a cable system mounted on the roof, the robot is especially useful for very tall buildings.
More complex than cars
According to British building company Balfour Beatty, construction in 2050 will have no workers, only robotic teams and drones supervising the site. All will be planned and controlled by algorithms that create 3D and 4D models. For the time being, however, construction remains highly dependent on analogue processes and, above all, manpower. This is no surprise for Thomas Linner of TUM’s Institute of Building Realisation and Building Informatics. “On one hand, there are often very large and heavy parts in buildings, where today’s robots quickly reach their limits,” he says. “On the other hand, local requirements must always be taken into account during construction, such as climate or specific material uses. And there are more variable components than in the automotive industry.” Nevertheless, he notes that automated construction has been steadily gaining ground. “This is due mainly to the new legal requirements for energy efficiency in Europe. Robots and 3D printing can save a lot of costs and material.”
According to Benjamin Dillenburger, professor of digital construction technologies at the Institute for Technology in Architecture at ETH Zurich, 3D printing raises basic questions. “Even today, there are automated processes for the prefabrication of certain components for standard buildings,” he says. “3D printing would not necessarily be profitable here.” In his view, this technology makes more sense for exceptional, non-standard models, as well as for specific construction steps. What happens when various digital functions are specifically used can be seen in the “DFAB House” project, which Dillenburger is currently working on together with several other professors from ETH Zurich at a research building (“NEST” of the Empa and Eawag, two Swiss research institutes) in Germany.
The load-bearing wall on the ground floor of the three-storey building is being built by a mobile robot, approximately two metres in size, using the so-called “mesh mould” method that researchers have developed from robotics, materials science and structural engineering. To do this, the robot first builds a steel wire mesh according to 3D models, into which concrete is then poured manually. Due to the grill’s fine mesh, the concrete cannot flow out; the wire framework thus serves as both formwork and reinforcement. As in Nantes, the supporting wall is not right-angled, but doubly curved. The formwork is made with a large-format 3D sand printer and then filled by hand with shotcrete. Printed formwork also makes complex shapes in concrete possible. The ceiling is geometrically highly differentiated, in order to distribute material only where it is structurally necessary. As a result, more than half the concrete can be saved. Another innovative construction step is the wooden construction of the two upper floors. These are prefabricated and assembled as part of the “Spatial Timber Assemblies” project in what is currently the world’s largest robotics laboratory in the architectural field at ETH Zurich. The 200-m2 house, to be completed in the summer of 2018, will serve as living and working space for guest researchers.
Assistants on the building site
Unlike emerging countries, Europe focuses on redevelopment, which accounts for 57% of all construction activity. Dillenburger particularly values the potential of 3D printing technologies here, since in most cases tailor-made solutions are required for recompacting or extension constructions. However, a key to efficient building renovation is powerful 3D digital models. Bernard Cherix of the École Polytechnique Fédérale de Lausanne develops such models with the aim of aligning them optimally to the needs of building renovation. He was involved in the renovation of Switzerland’s first skyscraper, the Tour Bel-Air in Lausanne. “To install new insulation and ventilation systems, it was important to simulate them in digital models,” explains Cherix. In addition, individual elements of the building were raised. Here too, it was necessary to simulate these steps to avoid distorting the building’s external appearance. These models allow for more efficient construction by eliminating errors and saving material. “3D printing and robotics can be helpful if used selectively,” says Cherix. In contrast, he sees great potential in the application of AI technology in digital 3D models.
Furet also sees hurdles for wider application of 3D printing and robots in building construction. “For now, we can build only single-storey houses with our robot. They would have to be lighter to be able to work on several floors.” He also sees limits in the construction of horizontal components.
Is the goal of a fully automated construction site too ambitious? Linner is uncertain. “Construction sites will eventually become like factories,” he acknowledges. “But I don’t believe in complete automation. Digital technologies will play a supporting role, but people will continue to be needed in the preparatory and subsequent work.”
By Robert Gloy
An expert discusses the challenges of creating a major district entirely with floating houses.
About 200 people live in the 91 floating houses of the Waterbuurt, a district in south-eastern Amsterdam. The first two phases of construction are finished; the third and final one is being prepared for the development of another 50 to 72 floating houses.
► Franco Pantano, the city’s advisor on floating buildings, describes the project’s challenges.
T. Can a large number of people with different cultural and social backgrounds live in a floating residential area?
Franco Pantano In general, the houses of the Waterbuurt are available for anyone willing to live here. In the current phase, the process of building the jetties and selling the houses will be done by the company with the best plan to do this according to the criteria set. We will start this selection process very soon. There are no restrictions on cultural or social backgrounds, and nor do future residents have to be familiar with living near water.
T. What are the main technical challenges in building floating houses?
FP. They’re related to water movement and load. How big are water level movements? How big are the wind waves or the waves from shipping traffic, and from which directions do they come? Another important challenge is providing the houses with basic services like drinking water, evacuation of waste water, electricity and telecommunications. These systems are usually engineered and built for houses on land, so we have to engineer the link between a floating house and the main systems on land. This link is more exposed to changing weather conditions throughout the year than a conventional underground connection. Because the Waterbuurt is a relatively small closed lake, with a surface of approximately eight hectares, we also have to take care of the water quality to guarantee pleasant living in the warm periods.
T. How do you do that?
FP. A system of primary dykes protects the Waterbuurt against water loads from the larger Lake Markermeer. Two sluices are located in the primary dikes system, enabling regulation of the water level and limiting the necessary bottom level. Because water-level fluctuations are limited, shorter mooring piles can be used. Thus, the mooring system can be realized with just two piles for each house. That is also sufficient to avoid contact between house and jetty, reducing loads on the jetties and resulting in lighter and less expensive construction. In addition, special pipelines have been engineered to have the same advantages as underground connections for the pipes going through the jetties.
By Blandine Guignier