The local cement and concrete industry has experienced many innovations over the last two decades, in terms of design options and sustainability.
As the most commonly used building material on the planet, worldwide, the cement industry contributes to about 5% of man-made CO2 emissions, while in South Africa this figure is less than 1%, according to Bryan Perrie, managing director of The Concrete Institute (TCI). He points out that in terms of cement manufacturing over the years, there have been a number of developments, primarily in the use of cement extenders.
“In the last 20 odd years, the South African cement industry has seen a reduction in CO2 emissions from about 900kg/ton to about 650kg/ton, which has led to the development of low-clinker, high-performance cements.”
He explains that the use of cement extenders such as ground granulated blast-furnace slag (ggbs) and fly-ash, which are secondary products from other industries, is what allows the reduction of the amount of clinker in cement, thus reducing the emissions. The use of these extenders also generally leads to more durable concrete, further enhancing its sustainability.
“It must be noted though that cement is not used on its own, but together with sand and stone in concrete. The CO2 profile of concrete is between 90 and 130kg/ton, depending on strength grade and the amount of extender used, much less than the average figures for cement,” Perrie states.
Dr Jean Bosco Kazirukanyo, managing director of The Advanced Cement Institute (ACTP), adds that the South African cement industry has dedicated immense resources to considerably reduce its energy consumption, CO₂ and other gas emissions. “This has mainly been possible with the introduction of more efficient, integrated cement process control technologies (automation), the vertical roller mill technology and chemical admixtures,” he says.
In terms of concrete, the most significant developments are associated with the admixture industry, resulting in the development of high-performance concrete, self-compacting concrete and many more innovations. “These developments have allowed concrete to be used and placed in situations and methods that were not possible a few years ago,” remarks Perrie.
According to Dr Kazirukanyo, the concrete industry has quickly accepted the ready-mix concrete technology and today the self-compacting technology is progressively conquering the precast concrete industry. The market is also slowly but surely accepting the architectural and decorative concrete.
Architect Niel Crafford indicates that because products such as the decorative concrete is still quite expensive and many contractors are not yet up to the task, at this stage its use is still limited and it is mostly applied on small areas to emphasise certain features of a building.
Innovation put to the test
Experimenting with some of the advancements in concrete, Crafford & Crafford Architects used an unconventional way of waterproofing the flat concrete roof of the Architectural Wing extension on the Tshwane University of Technology’s Pretoria Campus.
A chemical additive, Penetron, was mixed into the concrete in order to seal the roof. This concrete admixture reacts to the presence of moisture and forms a permanent, non-soluble crystalline structure that fills the pores, hairline cracks and capillary tracts in a concrete mass. However, one has to guard against big cracks because the chemical reaction can only span so far and if the cracks are too big, it might not seal properly.
“Being able to work with clean concrete that is waterproofed, without having to apply bitumen and paint to hide the waterproofing, which often results in an outcome not exactly as envisioned, makes it an element that will be used more and more,” states Crafford.
Flexibility in design
“This also makes for very interesting architecture,” highlights Crafford. “One can do a lot with concrete that you cannot do with other materials. From spanning it to being able to vertically build it up quite high without it having to be too thick, one can create very interesting elements with it.”
On the same project, the western wall of the building’s auditorium has been angled at 30 degrees to the north and slanted outwards at 15 degrees to reduce solar heat gains, especially since there is no air-conditioning system installed.
Shell concrete structures
Another exciting application of concrete, thin shell structure technology, is being applied to low-cost housing by Prof Mitchell Gohnert, head of the School of Civil and Environmental Engineering at the University of the Witwatersrand. His students are currently building a prototype structure on campus as part of a NRF-research project.
These buildings are made from a cement-stabilised soil brick, and shell designs are used to replace walls and roof structures. Shell structures are inherently superior in strength, require substantially less materials and are more durable than the customary linear box-shaped buildings. Although the formwork might seem difficult, many innovative methods have been devised to erect shell structures rapidly and efficiently, such as balloon formwork, tracers and sliding formwork.
“Traditional buildings are highly inefficient and extremely wasteful of materials, which is a prominent characteristic of our society. And therein lays the real challenge. South Africa is often reluctant to change and accept innovation, compared to the Northern Hemisphere. However, the price of building materials is escalating and will force our society to consider shapes that are materially efficient, such as shell structures,” states Prof Gohnert.
The Concrete Institute advocates a holistic approach to the sustainability of buildings. “The use phase of a building rather than the materials used is responsible for the largest component of embodied energy,” explains Perrie. “Research has shown that during its lifetime, concrete will absorb a significant proportion of the CO₂ released during the cement manufacturing process.”
Sustainable benefits of concrete:
• Is a local material.
• Lends itself to labour-intensive construction.
• Provides design flexibility as it can be cast into many shapes, with a variety of finishes.
• Is low maintenance and durable.
• Has excellent fire resistance.
• Provides thermal mass that ensures energy-efficiency.
• Has excellent light and heat reflectance.
• Can be used to reduce pollution with titanium dioxide.
• Has excellent acoustic performance.
• Can be retrofitted and reused or recycled.
When buying and using cement and concrete, Themba Sityo from Enhle Consulting warns that there are certain environmental issues that must be considered.
Using local products, for example, will mean less fuel will be used for transport and less carbon dioxide (CO₂) will be emitted to the environment. Concrete rubble can also be recycled, and using this instead of virgin material in the manufacturing process saves energy and reduces prices.
Biodegradable packaging material can be turned into compost after use, instead of ending up in a landfill, as was previously the only option. However, torn bags can cause material to be released into the environment and cause air pollution, so bags must not be hand-stitched. When cement is transported during rainy days, the top bags must be covered so that the integrity of the packaging material isn’t compromised.
Due to the lime component, cement and concrete wastewater has an alkaline pH of between 12 and 13 that can cause severe environmental damage. Therefore it must be contained and disposed of by a waste-accredited company or neutralised by an effluent treatment plant.
“Builders need to seek ways to minimise the generation of waste through waste avoidance, reduction, reuse and the recycling of waste,” she says.
Full thanks and acknowledgement are given to Crafford & Crafford Architects, www.penetron.co.za, The Concrete Institute, Enhle Consultancy, The Advanced Cement Institute (ACTP) and Wits for the information given to write this article.