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University of Leeds concrete architecture

The concrete set

The University of Leeds is known for its concrete architecture and our experts are studying the material from the very bottom up to pave the way for a stronger, more sustainable future.

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It’s under your feet, under your car, under your bed and might be holding up the roof above you. The prevalence of concrete, nearly as ubiquitous as the air around us, might explain why we only notice the material when we think it looks terrible. But does concrete deserve its reputation for being dull or ugly?

Walk around the University of Leeds campus and you’re sure to develop an opinion about the colossal concrete precinct that covers the majority of the southern campus – on a sunny day, you’ll see the blue sky reflected endlessly in the plate glass windows and if it’s raining you might seek shelter along the skyways and corridors as they blend in with the grey.

Less obvious, perhaps, are the advances in architecture and planning that won the concrete offices, laboratories, libraries, lecture halls and aerial walkways grade-II listed status and the technology contained in the material itself.

The foundations of concrete

The story of modern concrete begins in the city of Leeds, and continues today with the extensive research conducted in the University’s School of Civil Engineering. From the way the atoms arrange themselves in cement and the chemical reactions necessary to make concrete set, all the way up to how buildings are built, how we maintain them and how we tear them down, the University is uncovering how we can make concrete stronger and more sustainable.

Later this year, the School of Civil Engineering will become home to a new Leeds Centre for Ageing of Infrastructure Materials (part of the National Centre for Infrastructure) as well as the Neville Centre of Excellence in Cement and Concrete Research. With these two initiatives, the University places itself at the forefront of current research into the understanding and implementation of concrete. It might even be changing the drab perception of concrete by revealing how advanced and dynamic the material really is.

Professor Phil Purnell

“Everything about concrete spans the nanoscale to the megascale. We must get the nanostructure right to get the microstructure right, to get the macrostructure right all the way up to getting the building right. We want to find out how we do what concrete already does, but better. ”

Born in Leeds

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Concrete vocab

  • Cement: the binder - after mixing with water, it sets, hardens and adheres to other materials.
  • Aggregate: coarse material such as sand, gravel, crushed stone, recycled concrete.
  • Concrete: a mixture of aggregate, water and cement that hardens over time.
  • Supplementary Cementitious Materials (SCMs): materials that can serve as binders, but are by-products of other processes and thereby lead to reduction in CO2 emissions per ton of cement.
  • Fly ash: dusty by-product of burning coal.
  • Slag: glass-like by-product of making iron - ground for use as an SCM.
Concrete

Just add water.... and steel

The magic of modern concrete begins with Portland cement, which can be bought by the bag for a fiver at the local DIY shop or by the ton and stored in huge silos. Concrete is created by mixing dry cement with water and aggregate – the inexpensive, chunky bits that help give concrete bulk.

Dr Leon Black, senior lecturer in Civil Engineering and Director of the Leeds Centre for Ageing of Infrastructure Materials explains:

If you went to Dragon’s Den with concrete, they’d bite your hand off! It has an almost indefinite shelf life, it’s affordable, energy efficient, and can be used in almost limitless applications. And you just add water to activate it.

Cement comes to life when water is added – the chemical reactions begin and the tiny particles in the powder extend even tinier needle-like protuberances, called fibrils, into the space between them. This is how concrete sticks together, with a product from the reaction of the cement and water forming a mesh of more and more fibrils until the concrete becomes solid – not by the evaporation of water, but by locking it in. This reaction can carry on for years, making the concrete stronger with time.

We have concrete that is mouldable when first mixed and solid and strong after the chemical reaction, but what happens if the structure we want needs to bend or withstand a knock? Pouring concrete over a steel skeleton gives us the best of both materials: the characteristics of concrete when compression (crushing) forces are applied, and those of steel when tension (pulling) forces are applied. This is known as reinforced concrete.

Without steel, tiny cracks in concrete join up under the stress of an external forces until the structure rips apart. With the steel in place, the tiny cracks still form, but are prevented from joining together, avoiding calamity and collapse.

Who invented reinforced concrete?

Professor Phil Purnell of the University of Leeds shows how to make reinforced concrete in the George Earle Laboratory in the Civil Engineering department at the University of Leeds

How does reinforced concrete work?

Professor Phil Purnell of the University of Leeds explains how reinforced concrete works

In 1849, Joseph Monier was awarded one of the first patents for reinforced concrete, and its use continued during the late 19th century, primarily for industrial structures. As the 20th century dawned, concrete was used in homes, bridges and roads often disguised as other, more traditional building materials, such as stone or timber. Indeed, the name ‘Portland cement’ arose from a desire to highlight its resemblance to natural Portland stone.

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Lewis’s Department Store on the Headrow in 1933. The Thoresby Society, the Leeds Historical Society

Research in concrete began at the University around 1927, when Civil Engineering comprised one lecturer, R.H. ‘Concrete’ Evans, with an annual research budget of £12 - equivalent to about £700 today. A chance meeting between that lecturer and Sir Edwin Airey (later to be lauded for his prefabricated post-war concrete houses) ended with a contract to build the Lewis’s department store that would be located on the Headrow. A Yorkshire Evening Post article in 1932 said:

The Domed steel cages after being surrounded and encased in cement provide an air space which gives a resiliency and lightness to the floors hitherto unknown in building construction.

Concrete on campus

360 concrete

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The same year the Development Plan was published, Civil Engineering moved into its new purpose-built building – appropriately the first structural concrete building on the University campus. The striking spiral staircase made of concrete in the foyer serves as an unmissable testament to the University’s research at the time.

The cracks begin to show

How did concrete end up splitting opinions? Concrete’s popularity from the 1950s to the 1970s meant that it was used everywhere, but many of the early modern concrete buildings were built without a full understanding of the material – quality control and optimal maintenance practices had yet to be established.

Criticism of the University’s Development Plan predicted concrete’s fall from fashion. In 1970, the Yorkshire Post (who had praised the development just 10 years before) wondered if in 20 years’ time:

the glass-walled aquaria may seem but dowdy relics of a passing fashion of the sixties

They weren’t entirely wrong - from the late 1970s, concrete buildings began dropping out of favour as they came to symbolise urban decay. However, the mistakes of the early buildings have shown us how to properly use concrete in large scale structures.

Professor Purnell says:

We’ve learnt so much from the things that went wrong, we now have concrete that can last almost indefinitely.

Now these modernist concrete developments are being listed to help preserve them from the elements and from the wrecking ball, sometimes after fierce public debate. As the tide of fashion turns back to concrete’s favour, there seems to be a different skeleton emerging from concrete’s closet.

‘There’s no such thing as a green material’

It is said that after water, concrete is the most consumed product on earth - at around 30 billion tons per year (that’s over four tons for every person on the planet), it’s an indisputably huge amount. The sheer quantity of concrete used means it accounts for around 5% of global carbon dioxide emissions. The ecological concerns around the use of so much concrete are compounded when global concrete consumption is growing at a rate of 2.5% a year.

Concrete is often labelled a villain for its carbon footprint, but how does it compare to other building materials? Professor Purnell explains: “Pound for pound, concrete is more environmentally friendly than other materials, but different materials do different jobs.” Can we really compare a pound of concrete to a pound of steel?

The life cycle of concrete provides a range of sustainability issues, from the production and transport of the raw materials to the construction processes, and the performance of the concrete then finally the disposal of the concrete if it becomes unfit for use.

This type of life cycle analysis can be very tricky, take the example of a reusable ceramic coffee cup versus a paper one - it may seem obvious that the ceramic mug is better for the environment since once it’s in your hand, it’s producing less apparent waste than its paper counterpart.

But what about the more energy- and carbon-intensive acquisition of the ceramic materials required to produce the mug versus the paper cup? And the production process? And then all the energy and water required to wash it? And where does the sudsy waste water go once it’s been washed? And where does the mug go when it finally breaks?

The picture becomes a lot less clear when we compare the cups from start to finish, and it is just as murky when trying to compare concrete to other materials. Purnell again:

There’s no such thing as an inherently green material, it depends on the application.

For smaller structures, timber or steel might produce lower CO2 emissions, but often stronger than necessary concrete is used, or elements such as beams or columns are over-specified, meaning excessive emissions. For larger structures, concrete normally offers a clear advantage over other materials in terms of emissions per unit of structural performance. But are there ways we can reduce the carbon footprint even further?

Concrete

Reclaiming concrete

One way to reduce the environmental impact of concrete production is to decrease how much concrete needs to be made overall.

This can be done through ensuring that existing structures are maintained and conserved to provide longer life, or through reclaiming the material once a structure is demolished. As part of the C-VORR project, jointly funded by the National Environment Research Council and the Economic and Social Research Council, Professor Purnell is finding ways to retain the value of concrete.

Purnell’s group is looking at embedding concrete structures with radio frequency identification (RFID) tags to store vital information about the units. These RFID tags, like those used to identify lost pets, could store data such as installation and expiry dates, as well as maintenance history, to ensure optimal performance. It might even be possible to monitor a unit’s surroundings and record a diary of the environmental changes. This information could show where and when any upkeep is necessary and document how the component could be salvaged and reused after the building’s demolition.

Currently, most concrete recycling consists of crushing down the existing components and using the rubble as aggregate in a new mix, but Professor Purnell explains, “The worth of a bottle isn’t in the glass it’s made of, but in its ‘bottle-ness’. There’s a big push to recycle everything, but the worth of the concrete is in its structural function, not in its component materials. We need to understand how we can retain value, and that might mean looking further back in the design process.”

Imagine if a building or bridge were to be demolished and entire individual structural units (eg beams and columns) could be reused, giving dramatic energy savings even compared to recycling. One of the main benefits of concrete, its versatility, is what complicates salvaging it on a large scale, since often structural units are made to measure for each specific purpose. Unless an architect or engineer wants a bridge with similar specifications to the one being torn down, the scavenged parts aren’t likely to be useful.

One solution is to have standardised units and connectors, for example, beams that are always certain dimensions and held together by a standard bolt. Buildings could then be torn down, sort of like LEGO, and any viable components used for another building with transport being the only energy cost. But can we look even further back, to the way the concrete is produced in the first place, to make some carbon savings?

Replacing Portland

The biggest CO2 offender in concrete is the production of Portland cement. For decades, researchers have been looking for ways to replace Portland cement with more environmentally friendly waste materials such as pulverised fuel ash, a by-product of burning coal for power or slag, which remains from the process of making iron. Changing what goes into the cement can have radical effects on the behaviour and strength of concrete and it’s vital to understand how these changes manifest when used in large scale structures.

Dr Black explains:

Our primary concern is how the composition of the concrete affects its microstructure, which in turn affects its performance. So we use analytical chemistry to understand engineering performance.

This approach differs from the conventional methods engineers use to study concrete’s strength and durability, where researchers apply external forces to concrete until it cracks or crumbles - affectionately known as “make it and break it”. Civil Engineers at Leeds seek to understand the very fundamentals of how concrete is formed so they can predict its future behaviour, rather than just record its response to stress.

We think we know everything about cement, but we don’t – and that’s what makes it fascinating

says Dr Black. Due to the variability of concrete, predicting its performance is notoriously challenging. Often a concrete that is stronger than necessary is used to err on the side of caution, and this uses energy and materials unnecessarily.

One way to reduce the carbon footprint of concrete is to ensure the optimal mix for its specific application, something Dr Black has been studying with other researchers at Leeds.

We can reduce the carbon footprint by up to two-thirds by judicious use of mix design. Different ratios of different components can give the same performance, but markedly different levels of CO2 emissions, so optimising the mix is key,

says Dr Black.

New mixes, however, require thorough testing. Engineers at Leeds have been selected to host a national facility with a range of new equipment that will allow them to understand the performance of concrete in ways that were impossible before.

A new home for infrastructure studies

Dr Black will be the lead for the Leeds Centre for Ageing of Infrastructure Materials, part of the National Centre for Infrastructure in collaboration with Imperial College London and the University of Manchester.

Though the performance of many existing structures has been documented for many decades, today’s concrete needs to be able to withstand the climate changes expected in the next 50 to 100 years. New formulations need to be tested in both current and future climate conditions - the Centre will provide the facilities to evaluate the long-term behaviour of past, present and future materials.

The Facility will provide a one-stop-shop for researchers gauging the durability of concrete and other building materials. Its unique combination of resources will include advanced facilities to speed up and record the degradation of materials, large scale chambers for accelerated ageing of structural elements, and an off-campus field site for long-term controlled exposure studies.

These facilities will allow researchers to mimic the effects of natural processes that degrade concrete such as sunlight, rainfall, seismic activity, freezing and thawing, as well as ones that are created by human activity, like traffic pollution.

The Facility aims to have a direct impact on the construction industry to create energy-efficient, sustainable infrastructure now that can be adapted to meet future changes in the environment, technology and user requirements.

Dr Black explains, “What I’m hoping will come out of the facility is the ability to look at concrete from the atoms up. Prediction of performance is difficult, but if we can understand the fundamentals, we can develop better models.”

If it’s vital to understand the small-scale science, how do we peek into what’s happening beneath the deceptively calm appearance of concrete, and document the things too tiny for our naked eyes to see?

Concrete

Sweating the small stuff

Professor Ian Richardson is also concerned with how cement works at the nanoscale, and studies the way the atoms and molecules interact with each other and with their environment.

There are a range of methods that can be used to understand the factors that influence the performance of concrete. A variety of complex reactions take place during concrete formation, and high-resolution microscopes and X-ray analysis give us insight into the chemical and physical processes taking place at the molecular level.

Using a transmission electron microscope (TEM) provides a very high resolution image, but due to difficult preparation and sensitivity to the electron beam, few cement science groups use this technique. Professor Richardson’s group has studied a wide range of cements using TEM. They demonstrated the link between the chemical composition and morphology explaining why Portland cement behaves differently from and blends using different binders.

Richardson has also looked at samples between 15 to 40 years old to study the way they age and document the effects of the environment. Once we know the environment’s influence on the nanoscale, mixes can be tailored to specific and varied applications.

Academics in the School of Civil Engineering are also working together on initiatives to make sure that the little influences the big, and the fundamental guides the practical.

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A transmission electron microscope (TEM) image of Portland cement blended with fuel ash – the fibrils can be seen extending and intertwining – A TEM uses electrons instead of light to get very high resolution pictures of nanoscale objects that cannot be seen with normal light microscopes. Image from

Making the connections

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