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Concrete durability – The role of materials in influencing durability

by Darren
Concrete-durability

AfriSam explains the factors that contribute to higher ultimate compressive strength and improve durability in concrete.

 

Infrastructure development has been identified as one of the key economic drivers in the development of the continent. In South Africa, the government has identified this path as one that will transform the economy and in the process, drive job creation. As one of the most utilised materials in the world, the role of concrete in fulfilling this mandate and as a vital ingredient in the construction of many structures is undeniable.

Regardless of the type of structure that’s being built, durability should be the top priority for any construction company. Defects and failures in any project are not only costly in terms of re-work but slow down the project and have the potential to damage the reputation of the project’s designers, builders, specifiers and contractors. When structures fail, the quality of workmanship and materials used are called into question. 

The quality of materials used during a project determines its visual appeal and the durability and strength of the structure and should therefore be a major consideration during the planning and specifying stage. Construction materials contribute significantly to the longevity of the structure and determine ongoing maintenance costs.

Maintenance costs, however, should be a long-term consideration. While low material costs may sound like an attractive option during the building process, this may not necessarily translate into long-term cost savings. If the quality of the material drops in order to accommodate initial building budgets, this could lead to structure failures and building collapses, which are much more costly.

How aggregate materials in concrete affect quality and strength
Concrete has been used in many projects around the world, thanks to its strength and durability. The making of quality concrete, however, also requires quality ingredients.

One of the key ingredients in concrete is aggregate materials. This material makes up a large proportion of the total volume of concrete and serves to enhance its overall strength. Aggregate material properties tend to differ, depending on the parent geological rock.

When choosing the correct aggregate for an application, the technical details regarding the influence of its various characteristics need to be well understood and their effects in concrete fully appreciated. Aggregate materials should be free of deleterious materials as these can have a negative effect on the concrete mix.

When choosing the correct concrete aggregate for an application, consider the following:
• Climate.
• Dust content.
• Flakiness index.
• Water absorption.
• Geological properties
• Physical properties
• Fines content
• Grading limits
• Quality
• Apparent relative density
• Aggregate Crushing Value (ACV)
• Polished Stone Value (PSV)• 

In addition to making sure that aggregate materials are suitable for the intended application, they also need to comply with the relevant statutory and regulatory requirements and be sourced from sustainable sources.

The role of cement in concrete
Similarly, as the glue that binds other materials and one of the main ingredients in concrete, cement needs to offer the strength and resilience required.  Developments in the industry have led to more sustainable methods of production in the making of what has come to be known as composite cements. The use of mineral components in cement not only lowers the carbon footprint, but also offers significant technical advantages over traditional cements.

As one of the composite cement manufacturing leaders in the industry, AfriSam cements with C-Tech, have a significantly reduced carbon footprint and offer various advantages over pure cements. Generally, composite cements are known to create more durable concrete compared to pure cements and are particularly suited for harsh environments. The use of fly-ash, slag and limestone in this mixture plays a huge role in improving durability.

How fly-ash acts as ball bearings when added to the cement mix
Fly-ash particles are spherical in shape and finer than cement particles. When added to the concrete mix, fly-ash particles find themselves between the cement particles, displacing water that would normally occupy this space. The process helps the fly-ash to act as ball bearings, allowing the other materials in the mix to pass more easily across each other without the need to use too much water.

This cement mix results in a cohesive and workable mixture that bleeds less and gives an aesthetically pleasing finish. This mix is also less permeable, preventing water and sulphate penetration, which could subsequently lead to a loss of strength in the concrete. Where substantial concrete structures are cast using pure cement, the concrete tends to generate heat which cannot escape easily. Fly-ash has a low heat of hydration and therefore prevents thermal cracking in concrete made using composite cements.

How to prevent steel reinforcement weakening the concrete

It is common practice to use steel reinforcement during construction to further enhance the tensile strength of the structure. Steel rebar is prone to chloride ingress when exposed to water or other chemicals. This can lead to corrosion, which can also weaken the concrete.

The use of slag in composite cements is known to capture the chloride ions that cause corrosion to steel reinforcements. As a result, slag is resistant to various forms of chemical attacks such as sulphate-acid attack and alkali-silica reaction. Like fly-ash, slag also has a low heat of hydration, significantly reducing the risk of thermal stresses. These factors contribute to the increased durability offered by composite cements.

The role of Limestone in composite cements
During the grinding process, limestone grinds preferentially as it is often softer than clinker and able to produce very small particles. Again these particles displace water between the cement particles and provide many more nucleation sites for the cement gel to form on, thereby improving durability.

The properties of concrete, especially compressive strength, bond strength and abrasion resistance, can also be improved when silica fume is added to the mixture. Silica fume’s fine particles improve cohesiveness of fresh concrete, allowing for improved handling characteristics. In the process, this limits bleeding of freshly laid concrete, allowing the curing process to start early. Silica fume is also effective for use in applications that require a “fine filling agent” as its fine particles fill spaces between the clinker grains, thereby producing a denser, less permeable paste.

With composite cements’ reduced permeability properties, the combined finer particles in GGBFS, fly-ash, silica fume and limestone offer the ideal solution to water and sulphate penetration, subsequently preventing the concrete from weakening. When used in pumping, concrete containing mineral components reduces pump pressure and flows easily through the pipes, allowing for easy placing even in those hard-to-reach places.

Due to its less refined pore structure, the traditional pure Portland cement lends itself to attack by sulphates which infiltrate the concrete, while thermal cracking as a result of heat generated is a real possibility. These factors can lead to rapid deterioration of the concrete, considerably affecting the durability of the structure. 

Composite cements go a long way to enhancing the strength characteristics of cement. All these factors combine to give the concrete higher ultimate compressive strength and improve durability, even under harsh environmental conditions. In addition, composite cement-based concretes continue to gain strength over time as they contain a higher proportion of strength-enhancing calcium silicate hydrates than pure cements. 

AfriSam (Pty) Ltd
Tel: 011 670 5500
Email: victor.bouguenon@za.afrisam.com
Website: www.afrisam.co.za

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