Polymeric Fibres For Rigid Pavements
Bajaj Reinforcement LLP, Nagpur
In India, construction of rigid pavements using mechanical slip from paver finishers started almost two and a half decades ago. Since the beginning, construction engineers were struggling with the development of huge surface cracks on concrete overlay within a few hours of its placement and on the hardened concrete surface, which was mainly due to plastic shrinkage and drying shrinkage.
Accordingly, the use of membrane curing compound to control loss of moisture from concrete was mandatory as per Ministry of Road Transport & Highways (MORTH) guidelines. However, for the vast exposed concrete surface under the open sun, the curing compound alone was not able to solve the problem for controlling the surface cracks. Further, development of shrinkage cracks is a major problem in rigid pavement construction, especially in tropical climate, as it is well-known that a concrete road surface must perform under dynamic loading, enormous surface abrasion and under varying environmental conditions.
To overcome such problems of crack development, several engineers, scientists and concrete technologist suggested use of fibres in road concrete. After long deliberation, field and laboratory testing on fibre reinforced concrete (FRC), Indian Roads Congress came out with a set of guidelines for design and construction of fibre reinforced concrete pavements: IRC: SP:46-2013. This manual has recommended the use of polymeric fibres in road concrete as a preferred fibre than steel fibre, due to its non-corrosive property. A pioneer in the industry, Bajaj Reinforcement LLP offers PP fibres. Bajaj PP fibres are produced in India and are being used by the constructors in bulk for construction of rigid pavements.
How PP Fibre Helps In Rigid Pavement Constructions
Fibre reinforced concrete is used in construction of rigid pavement for obtaining durable concrete pavements, having improved crack resistance, and to enable reduction in slab thickness due to enhanced mechanical properties in hardened concrete. Generally, it is observed that development of plastic shrinkage cracks in concrete appears within next 1 to 8 hrs of its laying and finishing. It is also observed that fibrillated micro synthetic fibre gives better result than monofilament fibre. Most fine micro fibres (diameter 8-32 micro nm) with high specific fibre surface area when thoroughly mixed and dispersed in concrete are found very effective in controlling plastic shrinkage cracking, as they are closely spaced inside concrete. Further, FRC has better control over plastic shrinkage cracking, and it is resistant to drying shrinkage as observed by the research engineers and scientists.
Bajaj Fibre Guard, a fibrillated micro PP fibre, when used at a dosage of 900 g per cubic metre of road concrete is found very effective for controlling its plastic shrinkage. For controlling drying shrinkage cracks, macro PP fibres like Bajaj Fibre Tuff shall be used at recommended dosage in combination with micro fibres. Further, addition of macro synthetic PP fibre will help obtain enhanced flexural strength and increased toughness of the rigid pavement.
Additionally, the role of fibres in improving the mechanical properties of concrete is well-known. Experiments using the drop weight method that evaluates resistance to blows have shown that concrete specimens with polypropylene fibres at 0.1% to 0.2% by volume have higher impact strength for both first crack and final fracture when compared with plain concrete.
Fibres can be categorized by size, material and function. According to size and function, micro fibres (under 0.3 mm diameter) are used to minimize concrete shrinking-related cracking. They also improve freeze-thaw durability, resistance to impact and abrasion, and help release vapours and reduce spalling during a fire. The typical dosage of micro fibres is 0.91 - 3 kg/m³. On the other hand, the monofilament fibre, i.e., fibrillated fibre provides better anchorage with concrete because of its mesh like structure. Fibrillated fibres are number of fibres, which open up, and offer better anchoring and bonding up characteristics in concrete as deformed rebar anchor performs better with concrete as compared to smooth rebar.
Class I: Micro fibres
Class lb < 0.3 mm diameter, Fibrillated as per EN14889 part II
Bajaj Reinforcement LLP, Nagpur started production of Fibrillated Polypropylene Fibres, i.e., Bajaj Fibre Guard in the year 2006-07. Initially, these fibres were successfully used for canal lining concrete in Maharashtra, but gradually its use extended to Chhattisgarh, MP. However, now it is specified in the respective Government specification to use this fibre in canal lining concrete for obtaining shrinkage crack free concrete surface. Later, Bajaj Fibre Guard was chosen for a pilot concrete road project in Nagpur by CRRI, where the results were found very encouraging. Thereafter, Bajaj Fibre Guard was used in concrete road project in Nagpur, which was constructed by PWD and the Municipal Corporation.
Presently, Bajaj Fibre guard is used in bulk for the regular construction of rigid pavement in India. Inspired with this success, Bajaj Group introduced macro synthetic fibre in the market in 2011-12, with the brand name Bajaj Fibre Tuff, which is a structural PP Fibre. It is used mainly for wet shotcrete in UG construction, with the aim to replace wire mesh in concrete floors, slabs, precast elements and even in rigid pavement, where enhanced mechanical properties are required to achieve for the hardened concrete.
Fig. 1: Bajaj Fibre Guard: Fibrillated Micro PP Fibre used in Road Concrete
The use of Polypropylene Fibres, both micro and macro fibre, has witnessed an increase in concrete construction for infrastructures projects due to cost optimisation and its specific advantage as explained above. Bajaj PP Fibres, i.e., Fibre Guard- a fibrillated micro fibre, is used in bulk by major constructors for concrete road construction in India to resist shrinkage cracks and minimise damages caused by abrasion, whereas Fibre Tuff- the macro structural fibre is used by reputed Indian construction companies for UG construction mainly in wet shotcrete, precast concrete and for other concrete structures. Also, Bajaj PP Fibres are tested as per global and Indian standards with concrete, both in laboratory and in project before its actual use, and as per the results, they conform to the standards.
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