Concrete is part of our culture

By Eamonn Ryan

Concrete is the single most widely used material in the world — and is frequently pilloried as having a carbon footprint to match. In reality, the reason there’s so much concrete is because it is in fact a very low impact material. If you replace concrete with any other material, it would have a bigger carbon footprint.

Concrete is used in such quantities because it is a remarkably good building material: not just for basic road construction or basements, but also for more glamorous projects. The construction industry has pioneered ultra-strength varieties from which to build earthquake-proof structures in regions such as Japan and California — apartment blocks that form some of the most expensive real estate in the world.

Humans have been using concrete in their architectural designs for millennia. The basic ingredients — sand, aggregate, binding additives such as cement, and water — were used as far back as ancient Egypt, and the Romans were well-documented masters of the material. The Pantheon is still the world’s largest non-reinforced concrete dome at more than 2 000 years old.

Modern concrete was only rediscovered just 200 years ago in the early 19th century with the discovery of Portland cement, the key ingredient used in concretes today.

concrete lab

Concrete is consistently tested in a laboratory.

Making concrete

The process of roasting, and then grinding to a powder, limestone and clay to make ‘artificial stone’ was patented in 1824 by Joseph Aspidin of Leeds, UK, and later refined by his son into a material close to the cement used today.

Mixing water with the cement, sand, and aggregate forms a paste that will bind the materials together until the mix hardens. The strength properties of the concrete are inversely proportional to the water/cement ratio: the more water you use to mix the concrete the weaker the concrete mix.

CoreSlab operational manager Martin Potgieter operates a batch plant at CoreSlab’s precast manufacturing facility in Polokwane, where there are a series of silos above a skip where all materials are blended before being decanted into the mixer. Potgieter explains the importance of moisture to the mix: “The washed river sand, aggregate, chemicals and water go into the skip and then into the mixer, where we have a probe which measures the moisture level at all times and informs whether it matches the calibrated mix. There are parameters within which the precast factory works, and the calibration report forms the basis of acceptance or rejection of a batch, or a basis on which to make an informed decision.”

The plant is fully automated and monitored continually by a computerised system, enabling Potgieter to refine the mix, including the volumes of water and admixtures, to achieve the set parameters. It is a bespoke system “designed by us for us”, says Potgieter. The programmable logistics controller controls the speed of mixing and level of agitation — giving ultimate control over everything that happens inside the mix.

If the mix goes out of sync, it sends an alert of three different escalations of severity, the final one shutting down the plant. The mix is fed into the system, and the correct quantity of raw material is then calculated, decanted into the skip and verified. The system also determines how long the mix must be in the mixer — by presenting a graph which shows the mix is complete when there are no longer variations in the graph, meaning there is no room for human error.

The additives are diluted with water before being administered, using an agricultural doser, says Potgieter — his own innovation “as the equipment requires minimal maintenance”. Despite all efforts, it is a dusty environment that is regularly cleaned.

Innovation is ongoing: the addition of a new silo the next month means fly ash can be added to the mix which reduces the cement and makes provides a durable concrete mix that is also ‘greener’. CoreSlab already adds silica fume to the mix, “the Rolls Royce of additives”, he says. “This makes the micro-structure more dense and impenetrable. It is a process of continual development: our aim is to keep up with European trends, which are far advanced compared to South Africa.”

For quality control purposes, if there is an error there is a paper trail for every mix which can explain where it went wrong. “The more quality assurance we can give the engineers, the fewer questions they will ask.”


Bryan Perrie, managing director of The Concrete Institute.


CoreSlab operational manager, Martin Potgieter.

Speciality concretes

The construction of industrial floors, such as warehousing and factory floors calls for concrete with characteristics entirely different from that required for vertical structures, says Bryan Perrie, managing director of The Concrete Institute, which monthly receives appeals for advice regarding faulty flooring.

Perrie says that the properties required of the concrete for flooring are governed largely by using correct materials and in correct proportions. Material specifications for all the ingredients of the concrete exist — and should be adhered to — as well as South African Codes of Practice for the actual placement of concrete and the finishing of floor slabs. Perrie stresses that consistency is crucial during all the stages of the construction process: receiving the concrete, discharging, placing, consolidating and finishing. The importance of protecting the concrete while it is being placed, and curing thereafter, are also vital factors.  

“An incomplete brief from the client, lack of attention to correct proportioning, handling and finishing, inexperienced contractors and operators — these could all contribute to a floor with poor durability, strength and aesthetics. The flooring process relies on the successful completion of successive phases — understanding what is being done in each phase, using the right concrete and equipment, by the right person at the right time,” he adds.

Weather in the concrete industry is regarded as cold when the ambient temperature falls below 5°C. The effect of concrete freezing at early ages depends on whether the concrete has set, and what strength the concrete had attained when freezing took place. If concrete which has not yet set is allowed to freeze, an increase in the overall volume of the concrete occurs due to the expansion of water, especially in the capillary pores. When thawing takes place, the concrete will set with an enlarged volume of pores which will reduce strength and durability.

If freezing takes place after the concrete has set, but before it has gained sufficient strength (about 3‒5MPa), expansion associated with the formation of ice will cause disruption of the microstructure and irreparable loss of strength and durability.

When the concrete has achieved a compressive strength of at least 3‒5MPa, it can resist a freezing cycle without damage because it has a higher resistance to the pressure of ice and because a large part of the mixing water will either have combined with the cement or will be located in gel pores and therefore unable to freeze.

Perrie says that because of their slower setting and rate of strength gain, the use of highly extended cements or the partial replacement of CEM I cement with significant amounts of either ground-granulated blast furnace slap or fly ash is not recommended. It may be advantageous to use CEM I 42, 5R or 52.5N cement in preference to 42.5N or 32.5 cements.

“Water in aggregate may be prevented from freezing by covering stockpiles with tarpaulins. If aggregates are likely to become frozen or contain ice and snow, they may have to be heated with steam injection or hot air blowers. When using steam heating, adequate draining must be provided. Typically, the aggregate should be heated to between 10 and 20°C. All water pipes must be adequately lagged to prevent supply pipes from freezing, or even bursting.

“The most common and easiest way to heat concrete is to heat the mixing water but care must be taken not to exceed 60‒70°C. At higher temperatures, flash setting of the cement and reduced workability may occur.

“Finally, the main requirement in cold weather concreting is to prevent heat loss of the freshly placed concrete. So, under no circumstances should water-curing methods be used. Heat may be retained by using insulated forms, covering exposed surfaces with insulating materials, or erecting covers with internal heating. Combustion type heating under covers should be avoided,” says Perrie. 

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