The concept of using fibres in order to reinforce matrices weak in tension is more than 4500 years old. Ancient civilizations used straw fibres in sun-dried mud bricks in order to create a composite with increased toughness, i.e. a matrix with a better resistance to cracking and an improved post cracking response. Since Portland cement concrete started to be used widely as a construction material attempts were made to use fibres for arresting cracks. Engineers had to overcome the major deficiencies of concrete, which were the low tensile strength and the high brittleness. A French engineer, named Joseph Lambot, in 1847 came out with the idea of adding continuous fibres into the concrete, in the form of wires or wire meshes[i]. This led to the development of ferrocement and reinforced concrete as known today. The use of continuous steel reinforcing bars in the tensile zone of concrete undoubtfully helped to overcome the problem of the low tensile strength of concrete. However, the idea of using discontinuous fibres in the concrete was always a challenge.
The development of fibre reinforcement for concrete was very slow before 1960’s. Until then there were some papers describing the basic concept of using fibres for reinforcement in concrete mixes but there was no application. Nevertheless, research on glass fibres had been conducted in USA, UK and Russia in early 1950’s. Actually, in Russia glass fibres were not only under research but were also used in the construction industry. However, this kind of fibres was found to be prone to alkaline attacks. In late 1950’s Portland Cement Association started investigating fibre reinforcement[ii].
Since early 1960’s there has been an increased interest in fibre reinforced concrete (FRC). This period is the turning point for the development of FRC. More rapid modern advances are paralleled by increasing applications. While more new applications were identified a wide range of fibres was introduced. These include:
· Steel Fibres.
· Glass Fibres.
· Carbon Fibres.
· Natural Organic Fibres.
· Polypropylene Fibres.
Generally, the fibres used to reinforce concrete can be characterised as discontinuous, discrete fibres with length less than 50mm and diameter no more than 500mm.
The actual purpose of incorporating fibres in the concrete matrix was the development of a composite with improved strength, both compressive and tensile. By analysing the results of the earliest developments in this field it can be observed that neither the compressive nor the tensile strength were increased by any appreciable amount. The actual benefits of fibre reinforcement were difficult to highlight by the researchers at that time.
Later on, during the modern development of FRC in late 1970’s and early 1980’s, when the testing equipment and analysis procedures became more quantitative and better qualitatively the concept of energy absorption (or fracture toughness) was introduced. This concept enabled the toughness measurement of materials. It was then that the major advantage of FRC was discovered and it was not other than the outstanding property of absorbing large amounts of energy compared to Ordinary Portland Cement Concrete. Even today, after more than three decades of research in this field it can be said that the principal benefit of FRC is the high fracture toughness. However, further research with different types of fibres and admixtures targets the development of a composite with increased tensile and compressive strengths, besides the fracture toughness. These FRC composites are now known as the high performance fibre reinforced concrete (HPFRC).
The production of a cement based material having high tensile and compressive strengths, remarkable energy absorption capacity and which will be homogeneous and isotropic (almost similar to cast iron) is no longer an utopia any more. The incessant research in the field of FRC has led to the production of HPFRC, which shows a combination of amazing properties compared to other cementitious composites.
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