Wednesday, September 20, 2017

Mat-Vantage

FIBRE TALK

CONSTRUCTION OPPORTUNITIES offers an overview of some of the most effective fibres used to boost concrete’s properties. 

Concrete in all its dynamic and wide spread use has truly come a long way since its first use in the 18th century. From a simple concoction of cement, sand, aggregates and water, concrete has metamorphosed into an agile, adaptable and by far, the most popular construction material because of its versatility and relatively low cost which is an essential requirement in a construction material meant for large-scale consumption. Though the basic blend remains the same, the research and development into the mix design has sprouted wonders. Just by the inclusion of additional materials, concrete can now be moulded into any shape, pumped to any height. It possesses a versatility of strength ranging from 5-10 MPa in mass concrete to 800 MPa in Reactive Powder Concrete (RPC), and with proper reinforcement (or pre-stressing) it can be designed for required resistance against mechanical loading and desired ductility. In line with the same, fibre reinforcement is the next logical step in the advancement of concrete and concrete technology. Implementation of fibre into construction has not been a new phenomenon. Horsehair and straws were used in mortars and bricks from the time of the Pharaohs. In fact, asbestos fibres were used in concrete till it was discontinued owing to its adverse medical properties. Fibre as a reinforcement is most commonly used to provide toughness and ductility to the otherwise brittle cementitious matrices. Above all it can reduce consumption of steel which can have a significant impact on the cost and time even if in very small quantity. It is crucial to understand that fibres cannot replace or supplement the concrete’s structural strength, but it can severely boost the mechanical properties of cured concrete. Over the years, many fibres have been tested for reinforcement but few have prevailed till the present times. Presenting a roundup of some of the most talked about fibres in the Indian construction industry.

 

Steel Fibre Reinforced Concrete

The concept of steel fibre reinforced concrete was first patented in 1874 and interestingly found its first major use during World War II, where it was used for patching of bomb craters on runways. However, it was during the 1970s that commercial use of this material began to gather momentum, particularly in Europe, Japan and the USA. Steel fibre reinforced concrete (SFRC) finds its application in industrial floors and pavements, and can be used for external paved areas, on shotcrete, composite slabs on steel decking and for precast elements. SFRC is also widely used in the shotcreting of tunnel walls and lining. These fibres are generally used for providing concrete with enhanced toughness and post-crack load carrying capacity. Typically loose or bundled, these fibres are generally made from carbon or stainless steel and are shaped into varying geometries such as crimped, hooked-end or with other mechanical deformations for anchorage in the concrete. Fibre types are classified within ACI 544 as Types I through V and have maximum lengths ranging from 1.5 inch to 3 inch (30 – 80 mm) and can be dosed at 6 to 67 kg/cu. m. The biggest advantage with the use of steel fibres is the ease with which one can implement steel into the concrete. 

 

Glass Fibre Reinforced Concrete  

Another popular fibre that has found wide scale usage is glass fibre, similar to what one would find in household geysers. The glass fibre helps insulate the concrete in addition to making it stronger. Glass fibre also helps prevent the concrete from cracking over time due to mechanical or thermal stress. Unlike steel, glass, fibre helps in reduction of the whole weight too owing to its lower density than steel. Glass fibre reinforced concrete (GFRC) was first introduced in the building industry in the early 1970s in the United Kingdom. Today, it is one of the most popular and innovative building materials used throughout the United States, Europe, Asia and the Middle East. The applications include architectural ornamentation (column covers, cornices, window and door surrounds, etc), terra cotta restoration and replacement, fireplace surrounds, concrete countertops, faux rocks and planters. Without the frame, GFRC will weigh 3-4.5 kg per sq ft. As an engineered material, the properties of GFRC can vary depending upon mix design, glass content and production methods. Glass fibre has a higher tensile strength than steel. As a general rule, the higher the fibre content, the higher the strength. A typical mix with 5 per cent glass fibre has a compressive strength of 6,000 to 8,000 psi. A special mention needs to be made on the variants of glass fibre in concrete. By swapping the glass fibre with an optical fibre, an amazing phenomenon known as Translucent Concrete is achieved. Though this concrete at this point may serve solely cosmetic and decorative purposes, its strong thermal insulative and light weight properties may soon find a wide variant use ahead.

 

Synthetic Fibre Reinforced Concrete

Synthetic fibre-reinforced concrete uses plastic and nylon fibres to improve the concrete's strength. In addition, the synthetic fibres have a number of benefits over the other fibres. While they are not as strong as steel, they do help improve the workability of concrete by keeping it from sticking in the pipes. The synthetic fibres do not expand in heat or contract in the cold which helps prevent cracking. Finally, synthetic fibres help keep the concrete from spalling during impacts or fires. Polypropylene and nylon fibres are used for shrinkage control; they have no structural strength. These fibres play a valuable role during the curing process, but provide no benefit after. They simply stretch too much to provide any resistance to tensile stresses. The two variants under synthetic fibres are micro synthetic and macro synthetic fibres. While the first has gained wide currency as a control for plastic shrinkage, the second offers structural benefits to concrete and is the first viable alternative to crack control mesh in concrete.
 

Micros synthetic fibres: Micro synthetic fibres are 6 to 20 mm long, tens of microns in diameter, have a texture similar to horse hair and are typically dosed at 1 to 2 kg/m3. Micro fibres became widely accepted as a control for plastic shrinkage and for passive fibre protection and anti-spalling.
 

Macro synthetic fibres: Macro synthetic fibres in the 1990s were the first synthetic fibre to offer structural benefits to concrete and were the first viable alternative to crack control mesh in concrete. Macro synthetic fibres are similar in size to steel fibre and provide concrete with the same and often higher level of post crack flexural capacity as steel fibre or mesh. Macro fibres are typically between 30 and 65mm long, dosed between 2.5kg and 10kg m/3 and range between 400 and 700 MPa.

 

PVA Fibres

Like steel, Polyvinyl Alcohol Fibres (PVA) have high tensile strength and a greater modulus of elasticity than regular concrete.  Unlike steel, the high performance PVA fibres develop a molecular and chemical bond with the cement during hydration and curing. This high bond strength makes PVA fibres very tough to pull out during bending or tension.  Studies published by University of Michigan researchers demonstrate PVA engineered cementitious composites with tensile strain capacity of 5 per cent approximately 500 times that of normal concrete or fibre-reinforced concrete. For many years, international manufacturers of facades have chosen PVA over Alkali Resistant (AR) glass fibre.

Some companies recognised over a decade ago the advantages that PVA fibres have for external roof and wall cladding which is its light weight, a strength-to-weight ratio better than AR glass fibre, and the knowledge that PVA will not degrade in highly alkali cement, decade after decade.

 

Natural Fibre (Coir/Cellulose) Reinforced Concrete

Just like for everything Nature has a solution for all. Human hair was a considered for its high tensile strength but it was dropped due to its lack of bonding with concrete due to its smoothness. However, the answer was found among the bamboo and the pines. The cellulose in these plants generate fibres that we otherwise call coir that are not only strong but also form a good bonding with the concrete. It is not just the coir that is beneficial but plant based cellulose derivative are worth a special mention. Agricultural crop residues like bagasse, oil pal, sugar palm, banana, hemp, flax etc are very useful natural reinforced biodegradable polymers used in construction. Natural fibres have found use in low-cost concrete structures as reinforcement materials, especially in tropical earthquake regions. Coir fibre for concrete reinforcement, extracted from the outer shell of the coconut, had become very popular in India and Sri Lanka around 2008-09. The general advantages of coir fibres is that it is moth-proof, resistant to fungi and rot, provide excellent insulation against temperature and sound, flame-retardant, unaffected by moisture and dampness, tough and durable, resilient, and it springs back to shape even after constant use. The use of cellulose fibre as reinforcing agents in composite building materials offers many advantages over glass fibre, such as the possibility to manufacture products with low density and
good biodegradability.

 

Combination blends

A single fibre may not always be enough to provide all solutions, so the shortcomings of one fibre can always be overcome by blending it with another fibre type to yield desirable solutions.  That is where a hybrid composite comes into play. If two or more compatible types of fibres are rationally combined to produce a composite that derives benefits from each of the individual fibres and exhibits a symbiotic response. Concrete can be reinforced with conventional steel bars, and/or blends of steel and/or synthetic and/or cellulose fibres. The reason for using fibre blends is to enhance the properties of concrete by combining the benefits that each particular fibre type can impart. The applications for fibre blends are those where concrete with one type of fibre is not able to fulfil all the design requirements. For example, in flooring slabs or initial linings of shotcrete in underground works, steel fibres could provide the reinforcement needed for the required toughness and crack control of the hardened concrete, but micro synthetic fibres may be required to control plastic cracking and it can also be used to reduce the risk of explosive spalling under fire conditions. This sort of mix–and-match to the concrete mix design has given birth to several unique and amazing concrete all broadly classified under High Performance Fibre Reinforced Concrete. Though there is no single fibre type that can encompass all the desired properties of fresh and hardened concrete in terms of, for example, providing load bearing capacity at cracked sections, crack control, spalling resistance at elevated temperatures, improved abrasion, impact and frost resistance and offer an environmentally friendly solution. However, with sufficient R&D we can come across an appropriate blend of fibres, with or without traditional reinforcing bars which can lead to a synergetic marvel with concrete.

 




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