There are various reasons why one would add to the basic ingredients of the standard concrete blend, and even more ways to manipulate the mixture to behave in a certain way.
Concrete essentially constitutes cement, aggregate and water, with portland cement forming the basis of all common cements referred to by SANS 50197-1 and blends that include a cement extender.
There are various reasons why one would change this formula. Bryan Perrie, managing director of the Concrete Institute, explains that admixtures, for example, are used to change the properties of the concrete mix. Fibres are put in to control cracking and to improve strength, while pigments can change the colour. Extenders are added because they improve certain aspects of the concrete and for cost saving.
Another important reason for adding extenders to the mix is to reduce the embodied energy of concrete.
The significance of embodied energy
Speaking at the SASBE 2015 conference at the end of last year, Nozonke Dumani explained that as property owners and builders are working to reduce the operational energy component of buildings, the embodied energy component starts to dominate the total lifecycle energy.
While research has shown that, in conventional buildings, operational energy typically accounts for approximately 80% to 90% of the total lifecycle energy and embodied energy for the remaining 10% to 20%, recently this picture is changing.
For illustration, Dumani referenced a study about an energy-efficient home with a 60% reduction in lifecycle energy over a similar conventional home. As the operational energy decreased from 91% to 74%, the share of embodied energy as part of the total lifecycle energy increased from 9% to 26%. Another study found that the embodied energy of a low-energy house could amount to as much as 60% of the total energy used over the lifecycle of a building.
1. Pre-use phase (initial embodied energy):
2. a. Use phase (operational energy):
– Use of appliances.
2. b. Use phase (recurring embodied energy):
– Replacement of materials.
3. Post-use phase (end-of-life embodied energy)
– Reuse or recycling.
– Disposal at landfill.
The energy use over the life cycle of building consists of the pre-use phase (initial embodied energy), use phase (operational energy and recurring embodied energy) as well as post-use phase (end-of-life embodied energy)
According to Dumani, a number of cradle-to-gate lifecycle analyses have shown that generally, structural materials represent more than 50% of the initial embodied energy of a building.
Although aggregate and water make up about 90% of a concrete mix by volume, the cement component contributes about 80% of the total greenhouse gas emissions of concrete. Dumani pointed out that since clinker production is the most energy-intensive stage in the production of cement, supplementary cementitious materials, such as extenders, could be used to replace a portion of cement in the concrete mix and significantly reduce the greenhouse gas emissions associated with concrete.
Portland cement extenders tend to reduce the rate of early-age strength gain and produces heat at a slower rate, which lessens the likelihood of thermal cracking. In addition, its fine-filler effect brings about a denser and more homogeneous microstructure, resulting in improved strength and impermeability.
According to Perrie, the chief extenders are fly-ash (FA), ground-granulated blast-furnace slag (GGBS) and silica fume (SF). “Fly-ash tends to make concrete more water-tight, so it is normally suggested for water-retaining structures like reservoirs. Both fly-ash and GGBS are recommended for buildings at the coast, where there is a lot of chlorides”, he states.
Standards for portland cement extenders
• SANS 55167: Parts 1 and 2. Ground granulated blast furnace slag for use in concrete, mortar and grout.
• SANS 50450: Parts 1 and 2. Fly ash for concrete.
• SANS 53263: Parts 1 and 2. Silica fume for concrete.
Ground-granulated blast-furnace slag
Ground-granulated blast-furnace slag (GGBS) is a by-product of the iron-making process in which molten slag is quenched and then ground to a fine powder.
The finer fractions of fly-ash (FA) are collected from the exhaust flow of furnaces burning finely ground coal and used as a portland cement extender.
The South African Coal Ash Association (SACAA), which aims to promote the utilisation of coal-combustion products, is continuously working to get fly-ash classified as a raw material instead of waste, as it is in the United States of America (USA).
Dieter Heinichen, former administrator of the SACAA, explains that locally it started with cement manufacturers using fly-ash as an extender, which is still the bulk use for it, but that fly-ash is produced in such a big quantity that Eskom has initiated a drive to offset more of it. Interestingly, it is also used in certain pigments and plastics. Visit the SACAA’s website at www.coalash.co.za for detailed information.
Silica fume (SF) is the condensed vapour by-product of the ferro-silicon smelting process.
According to Perrie, there are many different types of fibres that can be added to strengthen concrete’s tensile strength and to improve toughness in terms of cracking.
Glass-reinforced cement products are using alkali-resistant glass fibres suitable for non-structural uses such as replacement for asbestos fibre in flat sheet, pipes and other precast products, as well as architectural cladding.
Concrete-containing steel fibre has been shown to have substantially greater resistance to impact and improved ductility of failure in compression, flexure and torsion. It is also thought that adding steel fibres to concrete could help to reduce spalling due to thermal shock and thermal gradients, however, in exposed conditions, normal steel fibre’s risk of corrosion is a disadvantage.
These are man-made fibres resulting from research and development in the petrochemical and textile industries. The ones tried in cement concrete mixes are:
Acrylic fibres have been used to replace asbestos fibre in many fibre-reinforced concrete products and to reduce the effects of plastic-shrinkage cracking.
Two-and-a-half times as strong as glass fibres and five times as strong as steel fibres per unit mass, but highly priced, aramid-fibre-reinforced concrete has been primarily used as an asbestos-cement replacement in certain high-strength applications.
Carbon fibres have high tensile strength and good elasticity and a brittle stress-strain characteristic, but are much more expensive than other fibre types and not yet economically viable.
Nylon is heat stable, hydrophilic and particularly effective in imparting impact resistance. It also offers flexural toughness and increases the load-carrying capacity of concrete following first crack.
Polyester fibres have been used at low contents (0,1% by volume) to control plastic-shrinkage cracking in concrete.
At between 2% and 4% of the concrete volume, polyethylene fibres deflect linear flexural loads up to first crack, followed by an apparent transfer of load to the fibres permitting an increase in load until the fibres break.
Despite its many disadvantages such as poor bonding with the cement matrix, polypropylene fibres have been reported to reduce unrestrained plastic and drying shrinkage of concrete at fibre contents of 0,1% to 0,3% by volume.
These can generally be obtained at low cost and low energy use, and are often used in less developed regions.
Unprocessed natural fibres such as sisal have been added to concrete for roof tiles, corrugated sheets, pipes, silos and tanks, while elephant grass has been used to reinforce mortar for low-cost housing projects. However, products made with unprocessed natural fibres such as coconut coir, sisal, sugarcane bagasse, bamboo, jute, wood and vegetable fibres sometimes have issues with long-term durability, and the concrete made with these fibres depends on many factors.
Wood cellulose has relatively good mechanical properties compared to many other man-made fibres, and is the most frequently used processed natural fibre. It has been used to manufacture flat and corrugated sheet and non-pressure pipes.
Perrie explains that the use of admixtures is determined by what the concrete is being used for and will generally be at the discretion of the concrete mix technologist. The majority of readymix concrete already contains an admixture.
These water-reducing agents enable the use of less cement for the same concrete strength. As a rule of thumb, they can reduce the water requirement of a concrete mix by about 10%. They improve workability and pump ability and provide a more even distribution of the binder particles through the mix.
Superplasticizers are used handy to produce flowing concretes.
These agents introduce air in the form of tiny bubbles distributed uniformly throughout the cement paste. They are used to improve the resistance of hardened concrete against damage from freezing and thawing, as well as for improved workability and to reduce bleeding and segregation.
These admixtures speed up the chemical reaction between the cement and water for rapid setting, for example in very cold weather, and early strength gain where rapid turnover of formwork is required, for instance.
When placing concrete in hot weather or hauling it a long distance from the readymix plant, these admixtures are ideal to slow the chemical reaction of the cement and water and prevent cold joints due to delays in placing.
To change the colour of concrete for a specific aesthetic, architects might request pigments to be added to the concrete mix. It seems simple, but Perrie warns that because of their crystal structure, some pigments can make the concrete very sticky and require the use of an admixture.
“Adding the pigment to the many other ingredients that are already part of the mix also means that the concrete is less uniform, which makes it very difficult to get an even colour throughout the project, especially in different batches. To avoid variations, we recommend that all the materials should be on site from the same source at the same time.”
In addition, Perrie advises that when looking for a light-coloured concrete, one needs to opt for a light-coloured aggregate.
Final word of advice
When working with many different ingredients for concrete, it is essential to weigh the batch instead of doing volume batching, to ensure the correct dosages of ingredients.
Then, once the perfect mix is achieved, it is all about good site management to ensure the concrete is properly compacted, cured and protected.
Full thanks and acknowledgement are given to the Concrete Institute, the CSIR and the South African Coal Ash Association for the information given to write this article.