The ductility of beams reinforced with FRP bars are the major concern because of the linear elastic behavior up to rupture without yielding of FRP materials.
In structural applications, the FRP bars can be employed in a reliable way provided that ductility conditions of the building are met. Frequently, the ductility of FRP reinforced concrete beams are not satisfied because both concrete and fiber reinforced polymer are brittle, hence methods are required to improve the structural ductility concrete beams reinforced with FRP.
Methods to Improve Ductility of RCC Beams with Fiber Reinforced Polymer Bars
Various techniques which can be utilized to enhance the structural ductility of concrete beams reinforced with fiber reinforced polymer bars are:
- Confinement of FRP reinforced beams by fiber reinforcement
- Confinement of FRP reinforced beams by spirals and/ or stirrups
- Layered tendon and effective prestressed design
- Partial prestressing or hybrid combination of reinforcement
- Unbonded tendons
- Controlled bond failure
- Optimizing sectional ductility through proper reinforcement
Confinement of FRP Reinforced Beams by Fiber Reinforcement
The provision of discontinuous fiber, which could be either polymeric or steel, may increase the compression strain capacity of the concrete and have the same influence as lateral confinement.
The stress strain curve of concrete in compression is affected considerably fibers and consequently the concrete member show more ductile behavior. This could be employed to increase portion of deflection that is not recovered at ultimate and lead to enhance ductility index greatly.
If suitable amount of fibers is utilized, it would certainly improve concrete fracture energy by an amount that ranges from one to two orders of magnitude. Thus, the fracture energy generated when tendon failure happened will be balanced.
In buildings where non-magnetic properties are needed, fibers can be used to achieve that purpose.
It is claimed by Jeong and Namman that, the utilization of fibers in otherwise brittle concrete beams produce ductility indices which are between 2.9 and 5.45 and fracture energy ratios ranging from 3.7 to 9.2.
Fibers can be employed in number of regions of concrete selectively. For example, in the case where failure mechanism is designed in such a way that produces hinges, or in compression zone of concrete elements.
In addition to improving ductility of FRP reinforced concrete beams, FRP bars application lead to enhance shear capacity of concrete matrix, decline cracks with, improves inelastic bond between concrete and reinforcement, and prevent spalling of concrete cover.
Confinement of FRP Reinforced Beams by Spirals and / or Stirrups
Increasing strain capacity of concrete, which can be achieved by confinement of concrete through spirals or stirrup manufactured from FRP or steel reinforcement, lead to increase distribution of plasticity in the compression region of the concrete and consequently enhance ductility. It is proven that, when FRP bars are applied as spirals, its influence is much higher than when it is employed as rectangular or circular stirrups. In the latter case, sharp angles will create large effect on the effectiveness of the FRP bars.
Layered Tendon and Effective Prestressed Design
It is proposed to install the prestressing reinforcement in layers and design the effective prestress in each layer to create progressive failure with increasing deflections.
Partial Prestressing or Hybrid Combination of Reinforcement
Utilizing partially prestressed concrete where prestressed fiber reinforced polymer tendons are employed with ordinary steel reinforcement bars or specifically produced high ductility and low strength FRP bars which permit adequate flexibility and consequently the ductility of concrete members is increased.
From analytical perspective, the application of unbonded tendons whether it is external or internal is substantially attractive because stresses which are developed in the tendons are reaches not their maximum value before concrete failure.
This will permit the utilization of greatest possible ductility from concrete side and the possibility of reinforcement failure is decreased extremely.
Using unbonded tendons necessities introduction of an excellent anchorage which may suffer fatigue loading. Added to that, the use of external tendons might be risky because of its vulnerability to vandalism, and when the external tendons fail, they will produce huge amount of elastic energy which can cause great damage.
This problem might be tackled by designing the element in such a way that transition failure from bonded tendons to unbonded is produced or in another word controlled bond failure is created which will be explained in the following section.
Controlled Bond Failure
To prevent those issues created by the unbonded tendon application, it is recommended to design the bond between concrete and fiber reinforced polymer reinforcements in such a way that when the stress in tendons reach threshold level, the transition failure from bonded to unbonded tendons is achieved.
In this case, bonded FRP tendon arrangement is changed to unbonded FRP tendon arrangement. Lastly, this technique can be provided technologically.
Optimizing sectional ductility through proper reinforcement
Proportioning the reinforcement and designing the section of concrete element to use reinforcement and concrete full strain capacity is substantially significant and is principal design objective.
Provided that all other parameters are equal, sectional ductility could be enhanced through suitably proportioning and positioning the reinforcement in the section and by specifying the effective prestressing in the tendon.
To use the small strain capacity of the reinforcement efficiently, it is suitable to design the section to obtain as low neutral axis as possible at ultimate capacity of the section. And the section which meets such a condition is near to be over reinforced section.