FibreTuff: A Make-In-India Solution For PP Fibre Reinforced
Wet Shotcrete In Tunnel Construction

 

Milon Mukhopadhyay
Consultant 
Bajaj Reinforcement LLP, Nagpur

A shotcrete layer in tunnel construction ensures preliminary support and a safe working environment for all workers working inside the tunnel. Further, the addition of fibres in shotcrete layer provides an excellent way of stabilising deformations to mobilise the ground’s inherent strength by transforming the brittle nature of concrete to a ductile behaviour. Fibres are used in concrete as secondary reinforcement, which provides excellent resistance to the post cracking capacity of concrete; the fibres in Fibre Reinforced Wet Shotcrete (FRS) layers are able to take part in the transfer of stress and hence act as primary structural reinforcement, for stabilisation of UG and surface excavation.

 

Fig. 1: Specification for Tunnelling, Third Edition. Recommended use of Fibre Reinforcement in Shotcrete as per Rock Classes

FibreTuff - Macro Synthetic Structural Fibres

FibreTuff are heavy duty synthetic fibres, engineered especially as a replacement of steel fibre in a range of applications, including ground supported slab (both internal and external), sprayed concrete (shotcrete) and precast concrete units. These fibres, supplied loose in plastic bags, are manufactured from polymeric materials, which have a diameter greater than or equal to 0.9 mm ± 0.05 mm as per EN14889-2, and have Young’s modulus between 9-12 GPa and a tensile strength of 500-600 MPa. Further, macro synthetic fibres such as FibreTuff are often fully embossed to achieve mechanical bonding with the concrete; their length varies from 30-60 mm. Also, they have a cross-sectional shape, which is typically circular, rectangular, elliptic, or continuously deformed.

 

Fig. 2: FibreTuff Features a Unique Continuous Embossed Surface, Enabling the Fibre to
Achieve Maximum Bond with the Cement Matrix for Optimum Performance

 

Advantages Using FibreTuff In FRS

Bajaj’s structural macro synthetic fibres, FibreTuff, are commonly used in wet shotcrete due to its specific advantage over plain shotcrete with wire mesh reinforcement. At present, FibreTuff is used by many tunnel builders in India. The other benefits of FRS are mentioned below:

  • Ductility, toughness, energy absorption capacity
  • FRS behaves nicely as crack resistant in hardened shotcrete
  • FRS helps to increase productivity and speed of construction
  • FRS ensures safety as no workmen needs to work under unsupported overburden for fixing a wire mesh
  • FRS adheres to all excavated surface upon proper robotic spraying and consequently, no void is expected, which is common in wire mesh shotcreting as shadow formation
  • Use of FRS can greatly reduce rebound losses as compared to conventional wire mesh reinforced shotcreting in tunnel
  • Use of FRS ensures lower shotcrete consumption as it follows the excavation profile, which is not possible in wire mesh reinforced shotcreting
  • Use of FibreTuff in FRS for transportation tunnel is preferable as in case of fire, fibre may only melt, saving the concrete from crackedor spalling, which is mostly observed when steel fibre reinforced shotcrete is used
  • Due to its less consumption, consumption of macrosynthetic structural fibre in wet shotcrete is less costly than steel fibre to attain similar performance
  • Handling and storage of macrosynthetic fibre is easier than steel fibre. Rusting problem like steel fibre is avoided in long storage while using FibreTuff.

 

   Fig. 3: Commercial Advantages of Using Fibretuff in FRS

 

Commercial Advantages Of Using Fibretuff In FRS For Both Owner And The Constructor

  • Direct cost benefit through saving of materials due to low rebound loss and low shotcrete consumption during spraying and adhesion on rock surface along the excavation profile
  • Cost saving through reduction of reinforcement consumption while changing from wire mesh to fibres
  • Savings through no use of labour cost while using FRS instead of wire mesh, which needs labour to fix it
  • Indirect cost savings through reduction of cycle time when FRS isdirectly sprayed on rock surface, making it a faster and safer operation
  • Lower thickness of FRS is required, as compared to wire mesh reinforced shotcrete, leading to cost savings
  • Bajaj FibreTuff leads to a controlled inventory as it is produced locally at Nagpur, and only the required quantity supplied; thus, bulk import is avoided.

Factors Related To Mix Design And Use

  • Balling can be a problem if the length to diameter ratio is not low, if dosage is too high, or if addition is too fast
  • Fibre length should not exceed 60% of pump line diameter
  • Good distribution of fibres depends on good dosing method, speed, and the mixer
  • Use of fibres may reduce workability; this can be offset using plasticising admixtures

 

For more information,
contact: fibretuff@bajajngp.com