Walls & Roofs Magazine spoke to thought leaders and knowledgeable industry professionals to find out what new research is being done in the cement and concrete industry.
In the past, construction workers and builders would simply take water, cement, sand and stone, mix it together and come up with a workable mixture. After tons of advancements in the industry, cement and concrete offer an entirely new range of options and functionalities.
Cement mixture compositions are continuously being optimised in order to provide design professionals with everything from self-compacting solutions and self-levelling concrete to ultra-thin, super-strong and smooth-as-glass finishes.
Professor Elsabe Kearsley, head of civil engineering at the University of Pretoria, spoke to Walls & Roofs about new trends, recent research trials and new advancements in the industry.
“Today we’re making concrete up to 200MPa in strength. We’re packing concrete really densely so that you’re left with a material that doesn’t have air holes and blow holes. Not only are the mixtures more workable, thanks to dense packaging and spherical particles, but architects are left with more options due to the fact that new types of cement mixtures can be poured into any shape,” says Kearsley.
Another new trend in the industry is the ability to scale down the maximum aggregate size to less than 4mm and 2mm, making it really thin while still retaining the strength of the concrete thanks to thin wire and fibre reinforcement.
This super-thin concrete gives designers and architects more flexibility in terms of cladding options and finishes.
Precision pre-casting is a relatively new idea that will offer a number of architectural uses. It has been researched all over the world and the University of Pretoria has run a number of trial tests over the past few years.
“Precision pre-casting allows us to create movable structures that can be erected on site by only one or two people. The idea is to basically create a system that already has all the needed concrete – with the nuts and bolts already cast into the system,” explains Kearsley.
While architects and designers might be familiar with the precision casting principle, Kearsley believes that it’s not yet being fully utilised in the design community.
“Architects aren’t always on the material manufacturing side of things, so it’s possible that engineers will tell them that a certain thing can’t be done, when this isn’t true. It’s not always the easiest option, but it can be done,” commented Kearsley, before adding that proper quality control, the right factory environment and experienced workmanship are must-haves to make precision casting work.
“Precision casting is really cutting-edge technology and it’s currently being used around the world. Architects should start thinking about more slender concrete structures and more aesthetically pleasing finishes. Exposed surface concrete would be beautiful, but then you really need a quality surface and we are not managing that yet in South Africa,” says Kearsley.
Buying cheap isn’t cheap
Another ongoing problem in South Africa’s construction landscape is that people are still buying cheaper, low-quality cement. A high-strength concrete mixture with very good properties will be more expensive per cubic metre than a low-quality mixture.
“But if you have very good concrete properties, you can use less concrete, so at the end of the day you will end up with a cheaper structure that is better designed and more durable. It’s still a struggle to get people to understand this concept. Rather use an expensive material optimally, because this will be less costly than making large volumes of low-quality concrete,” says Kearsley.
Current research in concrete reinforcement
One of the things that is currently being researched is concrete reinforcement in highly congested areas. This concrete usually consists of a lot of steel.
“In high buildings, it often happens that someone needs to fix the steel – usually the architect wants to put ducting and holes in these buildings in the exact place where reinforcement is needed. We’re currently researching and testing how fibre reinforcement can alleviate this problem,” explains Kearsley.
When steel fibres (instead of stirrups) are used to handle the shear, construction time can be cut down and the reinforcing cages can be removed or made to contain fewer bars thanks to the steel fibres.
Kearsley concludes by saying that the cement and concrete industry needs to continue focussing on optimising the material properties of cement. “We’re stuck in the same way of manufacturing cement the way we did in the 1950s, where everyone thinks that concrete is big, grey, ugly and heavy. Modern technology, however, can make something that looks completely different to traditional concrete. Optimising is the answer,” says Kearsley.
Advanced re-crystallisation concrete: Cyril Attwell, group concrete and research manager at Murray & Roberts
Walls & Roofs Magazine also spoke to Cyril Attwell, group concrete and research manager at Murray & Roberts, to find out more about the research projects that this construction, engineering and mining contracting company is currently undertaking.
“We are currently researching advanced re-crystallisation concrete (ARC) technology. The concept was born in 1999 and it revolves around experimenting with more variables in your standard concrete mix design, such as particle shape, particle size of aggregates and the water-cement ratio,” says Attwell.
Fifteen years ago, one of the junior contracts managers at Murray & Roberts, Anton Botha, requested that the company should try to make a concrete that was able to reach 12MPa in 12 hours at -8 degrees. The challenge with this is that at about 5 degrees, cement stops dissolving in water, so it wasn’t possible to get to the point of super saturation that is needed for crystal growth in concrete.
“Concrete is a big water crystal and the same type of experimentation that can be done with normal crystals, can be applied to concrete. You can go much further in terms of dissolving cement at higher temperatures, so one of the biggest obstacles was being able to get the cement to dissolve at lower temperatures. It was virtually impossible to dissolve at -8 degrees,” explains Attwell.
Several companies were trying to overcome this barrier at the time. Eventually a certain medical company came up with a solution and the patent for this solution expired in 1997, two years before the request was made at Murray & Roberts.
“A material called ethylenediaminetetraacetic acid (EDTA), which is a masking agent, was used by the medical company. I converted this technology to something that focuses on calcium-based systems, which is tetra potassium pyrophosphate (TKPP). So using an extremely small amount – half a litre of this masking agent per 1 000 litres of concrete – I was able to get concrete to set and gain 11,8MPa in 12 hours at -8 degrees,” said Attwell.
The total cost of Attwell’s mix was R297 a cube, less than half of what another concrete technologist was able to achieve in a mix that achieved 2MPa in 12 hours at 5 degrees.
“This was the birth of ARC technology, where we were able to start looking at other variables such as saturation values and how to optimise crystal growth in concrete,” said Attwell.
Virtually all of Attwell’s designs are done via ARC technology and it was used extensively throughout the Gautrain project. “When we were busy with the Gautrain project in 2006, we didn’t have access to high-strength cement because the construction industry was booming. We only had access to Portland Cement, which is generally not regarded as precast cement,” explains Attwell.
The French firm that was a partner on the Gautrain project, Bouyges Travaux Publics, wanted to use steam and other methods to achieve high-strength precast concrete. Instead of opting for this method, Attwell and his team took pulverised flue ash (a waste product from coal-fired power stations), which generally delays the strength in concrete, giving you a lower strength.
Reducing the carbon footprint of cement at the Gautrain project
“In order to get a high-strength concrete from the pulverised flue ash, we applied 28 different variables in order to achieve something that I believe is rather spectacular,” continues Attwell.
Cement manufacturing contributes to 5-8% of all greenhouse gasses. At the beginning of the Gautrain project, it was estimated that approximately 344 000 tons of cement would be needed. The 344 000 tons of cement that was initially estimated would require about 4,5km by 3km of rainforest – alive and growing for 40 years – to counteract the CO2 emissions of this cement production.
“With the introduction of fly-ash, I was able to optimise the chemistry of the Gautrain mix in such a way that we reduced the requirement of cement to 210 000 tons. This shifted the rainforest requirement to 2,5km by 3km, effectively saving 6km² of rainforest for 40 years. Besides the commercial benefits, you can see how ARC technology and chemistry optimisation of concrete can have a huge impact on the sustainability of the project,” says Attwell.
“Nothing is impossible, just highly improbable. You just need to find that small amount of probability. That’s what I love about statistics and applying different variables in concrete manufacturing. I never give up hope because I always have a small probability,” concludes Attwell.
A pioneer of experimental micromechanics, Professor Erik Schlangen from the Delft University of Technology in the Netherlands, spoke to Walls & Roofs about a new research area in the cement and concrete industry, called “self-healing concrete”.
“Self-healing concrete is a term that is used for cement-based materials that repair themselves after the material or structure gets damaged due to some sort of deterioration mechanism,” explains Schlangen.
During the lifetime of a concrete structure, some level of degradation will take place. This degradation needs to be repaired in order to retain the required performance level of the concrete as well as the structure. Self-healing material approaches have been developed over the last decade and these approaches could potentially be applied to repair damaged materials and structures.
“The initial cost of such a self-healing material might be higher, because the self-healing properties have to be included in some way. However, during the lifetime of the structure there are no additional investments needed,” says Schlangen.
With regards to concrete, crack filling and strength regain are important factors to consider.
“Crack filling can be used to stop leakage through a crack or to close the crack to prevent ingress of for instance water and chloride ions. Strength regain can be important when cracks lead to a decrease in structural safety, for instance after an earthquake or overloading of a structure,” says Schlangen.
Cement-based materials have a self-healing property by nature. Cracks in concrete can heal by itself due to ongoing hydration of cement that did not yet hydrate during hardening. These are the self-healing mechanisms that you get for free and which can be enhanced by adding “extra” cement that can be encapsulated or coarsely grinded so that it does not or only partly hydrate during hardening. A completely different approach is the use of bacteria to close cracks.
“In the ongoing project at Delft University on this bacterial concrete, the application of alkali-resistant endospore-forming bacteria to enhance the self-healing capacity of concrete is investigated. In order to predict the financial benefits of self-healing materials, however, the balance between increased material costs and reduced maintenance and user costs is needed. Operating costs, disposal costs and environmental costs need to be considered and further investigation is also needed,” concludes Schlangen.
How the cement industry is changing: Desmond Maharaj, general manager of cement, Lafarge South Africa
Walls & Roofs also spoke to Desmond Maharaj, general manager of cement at Lafarge South Africa, to find out how the cement and concrete industry continues to evolve.
“The industry has evolved to providing solutions for a variety of customers instead of just products,” says Maharaj, before adding that the need to produce more environmentally-friendly products has been a priority for the industry for a few years now.
“Some innovative changes had to be made to the products and processes to ensure a lower carbon footprint. It is safe to say that the industry has considerably reduced its carbon footprint over the past decade by producing cements and concretes that have been re-engineered to produce high-performance characteristics with a minimised impact on the environment,” said Maharaj.
Durability is another need that needs to be addressed by the cement and concrete industry, says Maharaj. Structures need to last longer, which is why cements and concretes are being manufactured to withstand even the harshest environments. “More work is being done on this front to ensure better protection of structures,” adds Maharaj.
Artistic, coloured concretes
Experimenting with aesthetics is also a relatively new trend in the industry. Surface finishes is a critical requirement for any structure to both the clients, engineers and architects. There are many examples in buildings today that show that concrete alone can give an aesthetically pleasing and long-lasting finish. There are also more clients demanding purposefully engineered concretes that are artistic and coloured.
Some of the industry’s new technologies include artistic concretes, coloured concretes, and also self-compacting concretes. These concretes provide excellent and artistic surface finishes whilst still remaining functional.
“Lafarge’s Agilia® is a self-compacting concrete designed to flow under its own weight without the need for vibration. The product’s exceptional fluidity enables it to fill all corners and areas in formwork or moulds effortlessly, while remaining homogeneous. The elimination of some traditional building steps and faster working enhanced productivity and enabled cost optimisation.
Architects have more design freedom than ever before with regards to exposed concrete surfaces. “The wide variety of decorative concretes available to the architect will allow the architect freedom to mix and match a wide variety of surface finishes, textures and colours. The new, innovative products will also allow the architects and engineer to design structures so that the structural elements, as needed by the engineer, are aesthetically pleasing for the architects.
With all these innovations, the use of cement and concrete in exciting projects will continue unabated.
Full thanks and acknowledgement are given to Murray and Roberts, the University of Pretoria, the Delft University of Technology and Lafarge South Africa for the information given to write this article.
Innovations in the cement and concrete industry:
• Ultra-thin, durable concrete mixes.
• Smooth-as-glass concrete finishes.
• More flexibility in terms of cladding options and finishes.
• Precision pre-casting.
• Exposed surface concrete.
• Fibre reinforcement in concrete.
• Optimisation of the material properties of cement.
• Advanced re-crystallisation (ARC) technology.
• Artistic, colours.
• Self-compacting concretes.