Admixtures are materials other than cement, aggregate and water that are added to concrete either before or during its mixing to alter its properties, such as workability, curing temperature range, set time or color. Addition of fibre to concrete makes it tough and fatigue resistant. Such type of admixtures are used extensively in important engineering projects.
Fibre reinforcement of cement and concrete is not really a new concept since we have been using reinforcements like straw in bricks and hair in mortar for a long time. However, there has been a lot of research and development in this area during the last two decades.
Addition of fibre to concrete is a convenient and practical method of improving several properties of the material, for example, toughness, fatigue resistance, impact resistance and flexural strength. It also assists in changing the flow characteristics of the material.
Concrete with fibre reinforcement differs from conventional concrete in several respects. It has higher cement content, lower coarse aggregate and a smaller size of aggregate.
Generally, the fibre content varies from 0.2% to 2%.
Types of Fibres
There are several types of fibres, including steel, alkali-resistant glass and polyethylene.
Fibres can be divided into two types, viz, those with moduli lower than the cement matrix, such as cellulose, nylon and polypropylene, and those with higher moduli, such as asbestos.
Organic Fibres with lower modulus are generally subjected to high creep, which means that if they are used to support permanent high stresses in a cracked composite, considerable elongations or deflections may occur over a period of time.
The intercellular substance or matrix used is important because each has a different degree of alkalinity which affects the durability of glass and steel fibres.
The maximum particle size is also important as it affects the fibre distribution and the quantity of fibres. Concrete should have particles not more than 10mm in size, otherwise it becomes difficult to achieve uniform fibre distribution.
The use of new materials and modern techniques is important in construction activities. Proper use of different kinds of materials and the latest technology becomes imperative to improve quality and cut costs. The life and durability of structure also increases.
Several different types of modern construction materials include blended cements, micro-silicas, fibres for reinforcement, chemicals like epoxy and geo-textiles. Also used are plastics, aluminium prefabricated concrete products, and different cladding materials in walls, doors and windows.
The use of prefabricated concrete has become a common practice in the construction of bridges and flyovers in India. In addition, addictives, curing compounds and steam curing have been used successfully in highways and railways projects in India to ensure fast completion.
The use of new materials like epoxy, polymers, micro-silicas and fibre-reinforced cement mortars has also improved the quality of construction. The use of fibres improves its shear strength and torsional strength, besides improving the energy absorbance, resistance to wear, freeze and thaw damage, friction and skid resistance and durability.
Availability of mineral admixtures marked the opening of a new era for designing concrete mix of higher and higher strengths. However, it was experienced, and hence realized, over a period of time, that it was not only the strength that was important, but also other properties like durability and workability were also vital performance parameters. This has led to research on high performance concrete (HPC). HPC mix is designed with mineral and chemical admixtures along with normal ingredients of concrete, having low water-cementitious ratio.
Mineral admixtures are added to concrete in relatively varying quantities, generally 5% to 40% by weight of cement. Addition of mineral admixtures to concrete reduces heat of hydration due to reduced cement content and increases durability by contributing to pore refinement.
Kaolin is a reactive type of mineral admixture, and has been traditionally used for the manufacture of porcelain. Metakaolin is a sub-type of kaolin which is used in the construction industry.
Applications of Metakaolin
* High performance, High strength and Lightweight concrete
* Precast Concrete for architectural, Civil, Industrial and structural works
* Fibercement and Ferrocement products, Glass Fiber Reinforced Concrete.
* Mortars, Stuccos, Repair Material, Pool plasters.
* Manufactured Repetitive Concrete products
Metakaolin can be used in mortars and concrete for:
* Increased Compressive and Flexural Strengths
* Reduced Permeability & Efflorescence
* Increased resistance to Chemical attacks
* Prevention of ASR
* Reduced Shrinkage
* Improved finishability, colour and appearance.
Metalaolin has a great potential in concrete as cement replacement at lower cost as compared to traditionally used super pozzolans. Concrete produced with metakaolin shows similar behaviour to that with one produced with silica fume. It is expected that use of Concrete admixtures, like metakaolin will grow very fast in cement, mortars and High Performance Concrete.