Use Of Macro Synthetic Bajaj Fibre Tuff In Precast Concrete Construction

 

Milon Mukhopadhyay

Consultant
Bajaj Reinforcements LLP

Fibre Reinforced Concrete is a Composite material made of Hydraulic Cement, water, WRA,  Coarse & Fine Aggregate, may be silica fume and uniformly dispersed Fibres. In general Fibres are used in Precast concrete at higher volume varying from 0.3% to 5%. Volume Fraction is dependent on type of application and ease of mixing. At significantly higher dosages, in addition to post crack toughening, FRCs can also exhibit strain hardening; that is, the composite can support stresses beyond the strength of the matrix. Multiple cracking is often noted in these pseudo-ductile composites, and significant energy absorption is achieved.

Fibres Used In Precast Concrete

ASTM C1116/C1116M2 describes four types of Fibres used in Precast concrete, out of which Type III is synthetic-fibre-reinforced concrete (SynFRC). Polypropylene Produced from the homo polymer polypropylene resin, this fibre has a low modulus of elasticity and a low melting point. The low melting point may be beneficial in producing refractory products or products with a high fire resistance like Tunnel Lining using Precast Concrete segment because the fibre is expected to melt and provide a system of relief channels to dissipate internal pressure. Macro Synthetic Fibre like Bajaj Make Fibre Tuff a structural PP fibre conforming to EN 14889-2 Class II certification can used in Precast concrete as structural Reinforcement and its dosage will be @ 8 to 10 kg per Cubic metre of Concrete depending on specific requirement.

Use FIBRE TUFF In Precast Concrete For Higher Mechanical Properties Of Concrete

Experiments shows that precast concrete specimens with polypropylene fibres at 0.3% to 0.5% by volume have higher impact strength for both first crack and final fracture compared with plain concrete.  Polypropylene itself is more strain-rate sensitive than steel. However, at significantly higher dosage (about 2% by volume) these fibres imparted a substantial increase in the Flexural strength under impact loading, which indicates fibre bridging and crack arrest during subcritical crack growth. The nature of the cementitious system also plays a significant role in how the system will respond to higher rates of loading when reinforced with fibres. A stronger matrix will be stiffer but less resilient.

 

 

FIBRE TUFF Helps In Shrinkage Crack Control In Precast Concrete

Fibres are known to significantly reduce the free shrinkage and other early-age properties of Precast concrete. Experiment shows that use of polypropylene fibres (1% by volume) reduced free plastic shrinkage by about 30%. And a more uniform settlement in concrete. Addition of Fibres in Concrete increased the number of large pores in cement paste, thereby changing the bleeding behaviour and reducing the free shrinkage. Other than reducing free shrinkage, the effect of fibres on restrained shrinkage has also been studied using various techniques. The presence of fibre is expected to infuence both the lengths and the widths of shrinkage-induced cracks under restrained conditions. Fibres with extensive geometric deformations such as fibrillations impart greater efficiency in bonding between Cement paste and Fibre, which ultimately ensure higher mechanical properties.

Fibre Reinforced Precast Concrete Performs Better In Achieving Water Tightness And Enhance Durability

Precast concrete products are susceptible to degradation as a result of sulphate attack, freeze-thaw cycling, alkali-silica reaction, and corrosion of embedded reinforcing bars, if present. In all these cases, permeability to water plays an important part. Durability of precast concrete products is therefore infuenced by the rate at which water may enter. Again, it is known fact that permeability, depends largely on cracking & Porosity in concrete. Porosity is dependent on concrete density. Crack development can be controlled by Fibre. Any increase in the crack width will produce a highly permeable concrete. Fibre reinforcement improves crack resistance, increases the surface roughness of cracks & thereby significantly reducing the permeability of concrete in service. In case of stresses and stress-induced cracks, experiment shows that cracks dramatically increase the permeability of plain concrete, while the permeability of fibre-reinforced concrete remains far below the plain precast concrete.

 

 

Segmental Tunnel Lining Using PP Fibre Reinforced Concrete

Segmental tunnel linings are unique structures to design because of the many different loads they must resist. The segments are exposed to bending within a few hours of casting when they are removed from the production moulds and stacked in curing chambers. Within 12 hours after steam/hot air curing, the segments are stacked one after another in matched rings on top of each other for storage.  The segments are then transported to the jobsite after its complete wet curing, lowered into the tunnel, and placed into position with the erector placed in tunnel-boring machine. Once the segments are in place, the tunnel-boring machine pushes off the segments to advance the tunnel boring, creating high localized bearing and splitting forces. The final step is to inject grout into the annular space behind the segments to ensure full contact with the surrounding earth. These segments are then left to hold open the hole that was bored underground, which imposes high compressive stresses and moderate bending stresses in the lining. Many segments are reinforced with reinforcing bar. Use of Bajaj Fibre Tuff in Precast concrete segment casting in addition to rebar will help to take care of the large moments, and will reduce shrinkage and improve fire resistance.

Conclusion

Use of Fibres in Precast concrete is now designed and used globally. In India also its use is seen in Metro Railway construction. ITAtech Guidance for Precast Fibre Reinforced Concrete Segment, Vol:1, Design Aspect is available for practicing Tunnel Engineers.

 

For more information, contact: milon.mukhopadhyay@bajajngp.com