For retrofitting reinforced concrete beams to increase flexural capacity, which FRP is most commonly used?
- AGlass FRP
- BCarbon FRPCorrect
- CAramid FRP
- DBasalt FRP
Solution
Concept
Flexural retrofitting with Carbon FRP
Formula
NONE
Step-by-step solution
1. Flexural retrofitting of concrete beams involves adhering composite laminates or sheets to the tension soffit. 2. To enhance bending moment capacity and control crack propagation, the composite must possess high tensile strength and high elastic modulus compatible with steel reinforcement. 3. Carbon FRP (CFRP) features an elastic modulus of 200 to 300 GPa, closely matching or exceeding structural steel. 4. In contrast, Glass FRP has a much lower modulus (around 70 GPa), resulting in excessive beam deflections. Answer: B
Explanation
Carbon Fibre Reinforced Polymer (CFRP) is the standard composite chosen for flexural retrofitting of concrete beams due to its extremely high tensile strength and high elastic modulus. The modulus of CFRP matches or exceeds that of reinforcing steel, allowing effective stress transfer and crack control on the tension face of beams. It also offers superior resistance to creep rupture and fatigue.
Why the other options are wrong
A) Glass FRP has lower modulus of elasticity, permitting excessive deflections before developing required tensile stresses. C) Aramid FRP displays high impact resistance but undergoes higher long-term creep rupture under sustained loads. D) Basalt FRP has lower commercial availability and design standardization compared to CFRP.
Shortcut
Flexural strengthening / high stiffness = CFRP; Shear confinement = CFRP or GFRP.
Common mistake
Selecting Glass FRP on basis of cost while disregarding its lower modulus which makes it inefficient in flexure.
Correct answer: B) Carbon FRP