Steel Fibre Reinforced Concrete/Polymer

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Reinforced Concrete structures are made to efficiently support loads over the expected service life of the structural members. However, due to faults in design, change of usage, poor quality of the materials used, or change in the natural conditions, some of these structural members need to be repaired or strengthened. Repair of these Reinforced Concrete structural elements is imperative, not only for weakened elements but also for strengthening new structural concrete elements; thereby, strengthened structural elements could safely support the design load under different aggressive environmental conditions without excessive damage. Over the last two decades, researchers have created different materials and methods for the repair of deteriorated RC members besides strengthening new concrete structural members. Among the most common of the utilized materials is steel fibre-reinforced polymer (SFRP). Investigations were directed into strengthening structural members with SFRP and came up with many helpful outcomes. In any case, making repairs with SFRP involves deficiencies, which prevent the implementation of SFRP in compression under cyclic loading. This behaviour relies upon the strength of the parent concrete, the CFRP concrete bonds, and their durability. Therefore, a newer material was created and utilized for both the repair and strengthening of damaged or new RC structural members, which is known as ultra high performance fibre-reinforced concrete (UHPFRC). Most research works highlight the two significant features of UHPFRC (durability and strength), which show promising recent outcomes, as reported by many researchers. Studies on the mechanical properties of UHPFRC have shown that the compressive strength could be up to 163 MPa. The results have also demonstrated that increasing the percentage of steel fibres will result in increasing the flexural strength of UHPFRC. Upon investigating the behaviour of UHPFRC under compression and flexure, it was found that the compressive and flexural strengths of UHPFRC could be 2–3 or even 6 times more than high performance concrete (HPC), respectively.
UHPFRC is a highly dense, steel fibre-reinforced cementitious composite material having compressive strength in excess of 170 MPa; a tensile strength of over 8 MPa; and a flexural strength of more than 30 MPa. The high strength of UHPFRC is achieved by improving concreting techniques and materials (the addition of ultrafine pozzolans) and by having a very low water-cement ratio, high quality and higher dosages of superplasticizers, high cementitious material content, and optimum volume of high strength ductile steel fibres. The use of steel fibres is to prevent the growth and interconnectivity of microcracks by absorbing the tensile stresses. The microcracks join together forming macrocracks.
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Why UHPFRC?
1. Low water-cement ratio: As a rule of thumb in concrete technology, the lower the water-cement ratio, the higher will be the strength. A low water-cement ratio helps in reducing pore spaces and further increasing the binding between hydration products and aggregates. In addition to the w/c ratio, other strengthgoverning parameters are properties of the constituents, mixing procedure, mixer type, curing regime, and curing type.
2. Ductility: The higher flexural and tensile strengths of UHPFRC are due to the addition of steel fibres. The steel fibres in the vicinity of the flexural and tensile cracks offer more efficient transfer of stresses. Thereby, reducing the crack propagation rate enhances the ductility of the material.
In case of Tunnels and Underground Structures, water seepage is the most common cause of deterioration of concrete and its structures. However, deficiencies could be the result of substandard design/construction, or the result of unforeseen or changing geologic conditions in the ground that supports the tunnel. Another common reason for repairs is that many tunnels might have outlived their designed life expectancy and therefore the construction materials themselves could start degrading. Due to
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the fact that there are different causes for degradation, the method of repair could vary as per case requirement.
Causes of Tunnel Degradation
Deterioration in tunnels may be caused by any of the various factors listed below:
  • Water Infiltration
  • Cracked and separated joints
  • Lack of tightness
  • Design or construction mistakes
  • Corrosion of embedded metals
  • Thermal load Effects
  • Steep fill slopes above tunnels
  • Changing of geologic conditions
  • Poor Workmanship
  • Deterioration of mortar
  • Degradation in concrete strength
  • Longitudinal loads on tunnels
  • Longitudinal spreading of foundations
  • Longitudinal differential settlement
  • Swelling soil and invert damage
  • Spall of tunnel crown joints.
  • Loss of support due to erosion
  • Seismic load and shape distortion
  • Chemical action on lining
  • Damage to surface finishes
  • Clogging drainage due to fines
  • Cracks in track/road slab
  • Inclined tension cracks at the base
  • Differential movement at crown
  • Ingress of dissolved gases
  • Damage in repair system
The defects must first be evaluated to determine the cause and the severity of the deterioration, in order to select the best repair method. Many concrete linings in highway tunnels have an additional tunnel finish, which may hide the extent of the deterioration. Therefore, a repair analysis has to be taken into account for the replacement or repair of the finish as well. Some practical cases for underground structure repair are described including expansion joint leakage, and the use of advanced composite material or fiber reinforced polymer (FRP) for repair and strengthening. Finally, multi-criteria decision making tools for repair of tunnels are required.
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