Getting the mix right

Getting the mix right

By George Evans

Concrete quality is made of as many elements as the material itself. Getting them into perfect balance is critical.

Concrete is a composite material. It is made up of various constituents, among them cement, sand, aggregates, and various chemical compounds. When you know this, you can begin to understand that, when it comes to concrete, quality is equally complex.

The first point of call should be to define what we mean by quality. There is simply no absolute measure: in the final analysis, quality is related to how closely the product meets the need of a specific user. When producing concrete, it is thus critical to understand fully what it is being used for.

One must also understand that measuring quality presents challenges, because concrete is made up of so many different types of material and can be influenced by other factors such as how it is transported and stored. Clearly, the more consistent the quality of the cement and other elements that go into concrete, the more consistent and predictable its results will be — and thus the higher the quality.

When we compare our local product with the rest of the world, I would argue that we stack up well.

In practical terms, then, consistency is a key driver of concrete quality.

While there are many factors that influence concrete quality in South Africa, looking at them in four categories gives us the opportunity to understand where South Africa fares globally.

  • Quality of the raw materials. There are many facets to this category. Obviously, the consistent quality of the cement itself is paramount, but variations in each of the raw materials must be taken into account. For example, the moisture content of the sand would likely vary from load to load. If this is not monitored, the producer or the builder might have to adjust the amount of water used, but this would in turn affect the plasticity and durability of the finished product.
  • The proportions of the mix. It is necessary to control the proportion of each raw material very tightly or the characteristics of the concrete will be affected, and thus its ability to meet the use case.

Raw material quality and proportion mix obviously interact with each other. Together, they have the largest influence on concrete quality.

  • Type of sampling used. An important element in the quality equation is to understand how the sampling to determine quality takes place. Is the sampling representative? Is the testing accurate? At which point of the process did it take place? All of these can affect the results quite substantially. It is perhaps worth noting that the earlier the testing, the cheaper it is to correct any faults.
  • Process faults. Both during production and the construction process, errors could be introduced by faulty equipment or operator error. It must also be borne in mind that the conditions under which ready-mix is transported to a construction site, and then on the site itself, will affect quality.

Concrete quality is thus an extremely complex concept. It is related to its intended use and is affected by a range of variables, as summarised above.

When we compare our local product with the rest of the world, I would argue that we stack up well. South African concrete producers have limited quantities of gravel available and must work with machine-produced aggregate. Although it is harder to work with this material, we have developed techniques to do so, and to measure the potential durability of the mix and its performance on site. These techniques are being slowly adopted elsewhere in the world. In addition, we have national codes of practice, standards, and specifications that are compulsory when using concrete structurally, and these are enforced by consulting engineers.

The proof of the pudding is the fact that we continue to produce concrete structures that bear comparison with the best in the world. PPC’s 125-year heritage in the development of infrastructure in the country and its continued demand globally demonstrate why our commitment to ensuring that quality matters has seen us continue to be an industry favourite.

George Evans is general manager: Product Support Services at PPC.



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