Advances and innovations are changing concrete as we know it. This article looks at some of the latest developments in concrete mixes – the types of concrete that will shape the buildings of the future.
“Concrete has been around for a while, and it will stay. It is the second-most used construction material after water,” says Joe Odhiambo, chief executive officer of Agrément South Africa and ex-officio member of the Board of Agrément South Africa.
As with most building materials, researchers and manufactures are continuously developing new ways to modify its properties making it better, stronger and aesthetically more interesting.
Some of the latest trends driving new advancements in concrete, as highlighted by Odhiambo, include: ease in placement; early high strength in order to facilitate the early removal of formwork; reduced permeability to reduce water penetration, and volume stability to prevent shrinkage and crack formation during hydration.
“Now one may say that this has been happening for the last 100 years, but what is clear from all the products coming to us for assessment, is that the rate of change is getting faster and the improvements in performance are getting higher as time goes along,” he states.
Over the last decade, many new examples of innovative concrete or advanced mixes have emerged as real solutions. While some of these may still be too expensive to be rolled out on an extensive scale and others still need some refining, these are the concretes that will be built with in the future:
Ultra-high performance concrete (UHPC)
Especially in the readymix industry, Odhiambo points out that people are coming up with higher strength and better performing concrete that conforms to standards above those required for most concrete applications.
“Bridges are much thinner with small-sized columns since one is able to achieve more with less material. We even see ultra-high performance concrete reach strengths in excess of 150 to 250 MPa, six to eight times higher than conventional concrete.”
A National Precast Concrete Association (NPCA) whitepaper on architectural precast UHPC elements explains that the material’s ability to form complex shapes, curves and highly customised texture is thank to its superior compressive and flexural properties. Architectural UHPC is typically reinforced with polyvinyl alcohol (PVA) fibres, while structural UHPC incorporates high-carbon metallic fibres.
“There are various designs and various products from different companies, but this particular type of concrete is quite expensive and, therefore, not used extensively. The challenge for the researchers is to bring the cost down to an affordable level,” adds Odhiambo.
Concrete optimised for the expected use of the structure
Researchers are continuously developing high-performance concrete mixes to achieve specific outcomes by using admixtures such as plasticisers to improve workability, air entrainers to improve the mechanical resistance of concrete and to improve thermal properties, accelerators for early strength concrete and rapid setting, and retarders to slow the chemical reaction in hot weather conditions.
Other outcomes that have been achieved in the last ten years include compaction without segregation, improved long-term mechanical properties, better resistance to severe environments, increased toughness and better density. According to Odhiambo, these can contribute to a better end-product, time savings and increased productivity.
Lafarge is one of the companies doing research in the field of self-levelling concrete, using superplasticisers, also called fluidifiers, to enhance the fluidity of concrete to produce self-placing and self-levelling concretes. These are concretes that flow easily into tight spaces without requiring vibration and as such are ideal for use with complex formwork.
“The main advantages of self-compacting concrete, which flows and levels by itself, is that it reduces on-site labour and results in a truly 100% level surface,” Odhiambo points out. “It also reduces noise and energy on site as the concrete does not have to be vibrated.”
Although the use of this type of concrete is increasing, the cost is still relatively high.
Traditional concrete generally does not have a good resistance to acid and certain other chemicals, but Odhiambo notes that chemical-resistant concrete is coming to the fore.
An article in the International Journal of Sustainable Construction Engineering & Technology (IJSCET) explains that preventive measures are two-fold, involving the improvement of the basic microstructure of concrete and the provision of barriers against acids. Novel techniques like nanotechnology can also help to make concrete acid-resistant.
In addition, many special, high-performance concrete mixes can minimise the risk and effect of an acid attack on the concrete. Mixes that include a combination of silica fume and fly-ash produce denser concrete with less calcium hydroxide, which increases acid resistance; however, silica fume on its own increases the risk since it can produce micro cracks in concrete.
Air entrainment can also increase acid resistance, while blast furnace slag is effective against chloride and sulphate attack.
Some of the most significant research on self-healing concrete is done by the Delft University of Technology in The Netherlands. There researchers have found that by embedding calcite-precipitating bacteria in the concrete mix, the concrete has the capability to repair its own cracks.
This biological solution involves bacillus bacteria, an alkali-tolerant species that can survive the high pH value of concrete. When cracks develop in the concrete, air moisture triggers the bacteria spores to germinate. Once the bacteria feed on the calcium lactate, a chemical reaction causes limestone to form, which fills the cracks.
“Tilt-up construction with precast concrete has been around for quite a while, but it is gaining more and more traction,” states Odhiambo.
This type of construction involves concrete panels being poured on site in close proximity to their final position in a building. These panels are then lifted into position in one crane operation to form the walls of the building. Usually footings or foundation slabs are pre-installed to accommodate the panels. The panels are cast making provision for doors and windows.
Tilt-up concrete allows big commercial and industrial buildings, and even smaller residential projects, to be built quickly and at lower costs since the heavy precast panels don’t have to be transported to the site and the work crews are generally smaller.
Hybrid concrete construction
Another method of construction, highlighted by Odhiambo, which is becoming more popular, integrates the use of precast and cast in-situ concrete for optimum benefit. Hybrid concrete construction takes advantage of the accuracy, safety and quality of precast components as well as the economy and flexibility of cast in-situ concrete.
Fly-ash used as extender
“The use of fly-ash is gaining momentum and there is a lot of research being undertaken in this regard. A lot of bricks and blocks are already being manufactured using geo-polymer based fly-ash,” states Odhiambo.
“In our country we have a surplus of fly-ash since it is produced at all the coal-fired power stations. By using this waste product as part of building or road construction materials, we reduce the impact on the environment.”
One of the latest advances in aesthetic concrete is translucency. Rather than actually being see-through, translucent concrete is made to appear to be transparent by adding optical fibres to the mix. According to the International Journal of Modern Engineering Research (IJMER), the optical fibres transmit light through the concrete, resulting in a certain light pattern showing on the other side.
The light transmission is so effective that there is virtually no loss of light, which means that buildings could use sunlight through translucent concrete to reduce power consumption due to artificial lighting.
At the BAU 2015 trade fair for architecture, materials and systems, Prof Heike Klussman from the University of Kassel in Germany presented experimental concrete with innovative, interactive properties:
An innovative energy-generating concrete, Dyscrete works on the principle of photosynthesis. Using organic dyes to absorb light, it works like chlorophyll in leaves to produce electricity through electrochemical reactions. It may sound far-fetched, but in a few years, Klussman says that a painter may lay cables instead of electricians using this type of conductive dyes, integrating circuitry directly into the concrete matrix.
Opening up new design opportunities, BlingCrete combines the strength, fire resistance and construction capabilities of concrete with reflective properties. Also known as light-reflecting concrete, it can be used for safety markings or guidance systems on stairs, sidewalks, platform edges and tunnels, as well as aesthetically on facades and interiors.
The concrete’s retro-reflecting surface is produced by embedding clear, round micro glass particles in the concrete mix that reflect sunrays or artificial light. The effect changes according to the position of the light as well as the viewer’s perception.
In addition, BlingCrete elements can relay digital information to smartphones and mobile devices, sending out alarms or pre-programmed data.
The next decade is sure to see further advancements in concrete mixes, making this basic building material even more interesting and sustainable. The challenge will be for the people specifying and building with these new types of concrete to use them correctly.
Specifying and building with concrete
John Roxburgh, lecturer at The Concrete Institute’s School of Concrete Technology, emphasises that South African engineers and contractors need detailed knowledge about the properties and construction requirements of concrete to prevent incorrect specifications for building projects and prevent problems during the construction and lifespan of a concrete structure. This is vitally important for working with conventional concrete and even more critical for building with future concretes, for which there isn’t so much knowledge and experience in the field yet.
He points out that in the design and construction of concrete structures, both the plastic and hardened state properties should be considered.
“A contractor is inclined to be more concerned about the plastic state of concrete which, if designed with construction methods in mind, will simplify the job and achieve better results when the shutters are removed. An engineer, on the other hand, often places more emphasis on the hardened properties of concrete.
“But the contractor, engineering consultant and clients can all win if the specifications, whether prescriptive or performance-based, are correct, detailed enough and cover not only the performance of the finished product but also concrete mix design, transport, construction methods and the plastic state of the concrete,” Roxburgh states.
He further explains that the concrete mix design is fundamental in obtaining the correct performance, but many things can go wrong between batching and completion of the hardened product. Therefore the plastic properties of the concrete must suit the transport and construction methods as well as the required finish.
Two other aspects that are often under-specified, according to Roxburgh, are protection and curing. “Curing is not only important for concrete strength, but can also prevent defects such as cracks, surface wearing and quality. Durability is also greatly enhanced with proper curing,” he states.
By far the greatest number of enquiries received by The Concrete Institute deal stem from a lack of knowledge about concrete design, detailing and construction, which is why the institute has developed two one-day training courses specifically dealing with this subject.
“It is essential for both engineers and contractors to fully understand the properties of concrete and construction requirements of different structures to produce appropriately detailed specifications for concrete works,” Roxburgh concludes.
Full thanks and acknowledgement are given to Agrément South Africa, NPCA, IJMER, BAU 2015, www.blingcrete.com and The Concrete Institute for the information given to write this article.
Some of the advancements in concrete:
• Ultra-high performance concrete (UHPC).
• Optimised concrete.
• Self-levelling/self-compacting concrete.
• Chemical-resistant concrete.
• Self-healing concrete.
• Tilt-up concrete.
• Hybrid concrete construction.
• Fly-ash used as extender.
• Translucent concrete.
• Interactive concrete.
CAPTION: The concrete latticework that makes up the exterior of the Jean Bouin Stadium was designed by architect Rudy Ricciotti and constructed with ultra-high performance, fibre-reinforced concrete. The UHPC envelope lets light through while shielding spectators from rain and wind.
Courtesy of lafarge.com