If you think that high-performance concrete equals high-strength concrete, think again. While many might associate high-performance with high-strength properties, this is a common misconception people have about concrete.
To explain why this assumption is not true, and dispel other misconceptions about this common building material, WALLS & ROOFS in Africa magazine tapped the expertise of Professor Hans Beushausen from the Department of Civil Engineering at the University of Cape Town.
Prof Beushausen is also a director of the Concrete Materials & Structural Integrity Research Unit (CoMSIRU), which focuses on infrastructure performance and renewal research, and a specialist industry consultant for condition assessment and the repair of concrete structures.
Myth 1: High-performance concrete = high-strength concrete
“Looking at what is happening in the industry, many engineers assume that adding more cement to a concrete mix or lowering the water-binder ratio will produce higher-strength concrete and therefore also automatically result in higher performance, but this is not necessarily the case,” Prof Beushausen states.
“Depending on the particular application, high-performance concrete can have a wide range of required properties including low strength, low density, high density, high workability, improved durability and more.
“Yes, if you increase the strength of the concrete, certain properties may improve. For example, higher strength generally relates to a higher elastic modulus and lower creep, which for some applications may be beneficial. However, many properties cannot be improved just by increasing the strength of the concrete, most notably durability. In addition, higher strength is also typically related to higher cement contents, with associated detrimental effects related to higher heat of hydration, higher cost, less sustainability, etc.”
Myth 2: High-strength concrete improves durability
Durability is one of the main requirements in concrete construction these days because, according to Prof Beushausen, it is closely linked to the resistance against the ingress of chlorides or carbon dioxide from the environment. But a stronger concrete will not necessarily perform better in this regard.
“The best way to improve durability is to choose the right type of cement for the specific environment it will be exposed to, and this will depend on factors such as humidity and whether you build next to the ocean or inland, or whether it is a hot environment or one with frequent temperature cycles.
“Durability also strongly relates to construction procedures – the correct mixing, batching and placing of concrete, as well as the curing of concrete. It doesn’t relate as much to the strength,” he emphasises.
Myth 3: A higher cement content means a better product
“Absolutely not,” says Prof Beushausen. “A cement content higher than what is actually required is a waste of money, not sustainable and it might give you worse performance instead of an advantage.
“This is because if the concrete is of a higher strength than what it needs to be for structural reasons, it becomes more brittle and more prone to cracking, something that is especially of concern when the concrete dries out due to shrinkage. In turn, these cracks open up pathways for chlorides and carbon dioxide to enter the concrete and attack the reinforcing steel. So you are actually doing more bad than good if you go that route,” he explains.
High-strength concrete should therefore only be specified for structural applications where the structural design requires high strength.
Myth 4: Sticking to a mix-design prescription is best
When considering building standards all over the world, durability is typically linked to mix-design properties such as the water-cement ratio or the cement content in the concrete or even compressive strength.
“But we have moved away from that,” says Prof Beushausen, “because one can’t just assume that you will achieve better performance when adding more cement or using a lower cement-water ratio. Performance needs to be defined and quantified; therefore we need a test method that can measure durability in terms of a number and specify performance which can be verified.
“And South Africa is the first country in the world that has developed and also implemented a useful and practical performance-based approach on a large scale. Similar approaches are being developed in Europe, Japan and North America, but South Africa is leading the field thanks to the South African National Roads Agency Limited that is using this approach across the board.
“In fact, in the field of high-performance concrete, specifically in view of durability, South Africa is probably ahead of most countries in the world. We have a very well-functioning concrete industry, many quality control mechanisms, and excellent producers, engineers and contractors. And we have been very active in adopting the national standards for quality control and modern concrete technology,” he adds.
Myth 5: One can design one concrete mix for a project and then leave it to perform for 50 years
Prof Beushausen uses the example of a car, saying that every part functions as a system and it needs to be serviced and maintained for the whole system to work.
“Concrete, being part of a structure, needs to be designed for its particular application. The stresses and strains vary in different parts of the structure, so instead of mixing just one type of concrete for the whole project, we should engineer each part so that it suits the specific performance requirements in that particular element. This of course needs to be done in the boundaries of what is practical.
“In addition, maintenance plans for concrete structures need to be developed. We can’t just assume that once the concrete has been put into service it will perform perfectly for 50 years. Only proper design, construction and maintenance, all together, will give you good performance for a structure.”
Why is it important to specify high-performance concrete?
According to Prof Beushausen, durability is a really big focus in construction today. “Looking at existing infrastructure, the problem of reinforcement steel corrosion is widely evident and lots of money is being spent on repairs. So when designing new structures for improved durability, it is important to quantify performance in order to specify accurately, optimise design and construction and be able to do quality control.
“If you just assume that durability comes from a mix-design prescription, then you will not achieve durability. The implementation of a performance-based approach is important so that you can actually develop durable infrastructure,” he concludes.
Full thanks and acknowledgement are given to Professor Hans Beushausen for the information given to write this article.
Caption main image:
Problems related to reinforcement corrosion cannot be eliminated simply through using concrete of high strength.
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