REPAIRING & STRENGTHENING DAMAGED RC BRIDGE GIRDERS USING RHINO CARBON FIBER™ CFRP



Parameters affecting the behaviour of RC girder bridges over time

As the most important parameters affecting the reliability of RC bridges we can name reinforcement, structural steel corrosion as well as traffic load in long term. Amongst them, corrosion has the most negative influence on structural parameters, like the flexural strength of a bridge. According to investigations, the repair cost of corrosion damage to RC bridges in the USA, Australia, Europe and the Middle East had been reported 250~800 million € per year due to the use of de-icing salts alone at the end of the 20th century. This rate in marine regions and in warm weather is higher due to the acceleration of the corrosion process.

Traditional techniques of repairing and strengthening concrete girder bridges

  • Welded steel jackets
  • Internal strand splices 
  • External post-tensioning 
  • Replacement of the damaged girders

Disadvantages of Traditional techniques

  • Labor intensive 
  • Vulnerable to future corrosion
  • Traffic disruption

ADVANTAGES OF STRENGTHENING DAMAGED CONCRETE GIRDER WITH CFRP

  • Large strength/weight ratios
  • Excellent corrosion/fatigue properties
  • Relatively simple to install
  • The ability to quickly restore the structural capacity 
  • Based on field and laboratory tests [4] CFRP reduces beam deflection by as much as 20%
Figure 2- Strengthening damaged concrete bridge girder with CFRP strips

Long-term behavior of Bridge girder strengthened with CFRP

Studies show that the main deterioration factor of the bridge girder is corrosion of the reinforcement due to concrete cracking (Figure2 & 3). According to [5] the steel diameter begins to decrease after the initiation of corrosion at 12.8 years. Between years 12.8 to 20, a rapid reduction in steel diameter has been registered which can be related to the rapid reaction of steel reinforcement by chloride diffusion after cracking.
 
Accordingly, the corrosion of CFRP strips within a period of 100-years serviceability had been approximately 4% [5].

Static deflection and frequency which are other time-dependent properties of CFRP, were relatively stable, including a 5% reduction in the period of 100 years [5].
Figure 2- Schematic illustration of the various steps of the concrete deterioration due to the corrosion of the reinforcement 
Figure 3-  Reinforcement corrosion

Method of strengthening precast RC bridge girders using bonded CFRP strips

To strengthen bridge girders, CFRP strips will be applied in the soffit of the web of the girder. For bonding of the CFRP strips to girders, first of all, a thin layer of RHINO CARBON FIBER™ epoxy adhesive (Figure5) is applied to the surface. After inserting RHINO CARBON FIBER™ CFRP strips (Figure4), the next layer of epoxy adhesive will be applied over the CFRP strips. The main aim of installing CFRP strips to the soffit of girders as an external reinforcement is to increase the flexural capacity of the girders.
To provide an appropriate end anchor for CFRP strips as well as the shear strengthening of the girder, apply CFRP sheets over a specified length at each end of the girder web at an angle of 45°.4.1 DeflectionIt has been seen that by using CFRP laminate strips the tolerable load at the onset of yielding has been subjected to a great increase of about 32~47%. Also, the service load (the load in which under that load the deflection of the beam will stay under allowable deflection of L/360 for Live load and/or L/240 for Dead+Live load) for the strengthened beam has an increase of 45%.

Modes of failure of CFRP-strengthened RC bridge girders

Modes of failure of CFRP-strengthened RC girders include concrete cracking, Flexural cracks, Shear cracks and debonding [2]:
 1-  The first stage is the elastic behavior of the section where CFRP strips and the steel reinforcement are in good coordination with each other.
2-  In The second stage concrete starts to crack and so CFRP strips go under tension load. Tension in CFRP strips increases continuously at a faster speed than the tension in steel reinforcement.
3-  In this stage, the steel reinforcement yields and as the tensile strength of concrete is zero, cracks extend to the neutral axis of the girder. Now the whole tension of the section is resisted by the CFRP strips. Here strain coordination of the strips and the steel reinforcement is entirely lost.
4- In the fourth stage, debonding of the CFRP strips and concrete will start. Due to increasing force, shear flow occurs between the strips and the concrete. Cracked concrete and CFRP strips de-bond from the concrete (Figure6). Finally, CFRP sheets that were holding CFRP strips at both ends of the girder cannot tolerate the imposed tension and shear forces so the final failure occurs when the CFRP sheets tear and can not tolerate the CFRP strips.
Figure 4- Schematic illustration of Failure stages of RC girder flexural-strengthened with CFRP soffit plate or strips

The best  order of CFRP sheets and plates for flexural & shear strengthening of RC bridge girders

As mentioned in stage 4, the main cause of CFRP failure is the extension of cracks in the portion of concrete just below the reinforcement (concrete cover). Thus, to minimize this kind of failure, U-shaped external vertical CFRP strips (Figure7) can be used to:
  • Maintain the strain computability of CFRP strips on the RC girder
  • Prevent concrete cover cracking and failure described earlier
Figure 5- Schematic illustration of proposed arrangement of CFRP for flexural strengthening of RC bridge girders 

Some results of using CFRP on RC girder bridges

  • According to [2] the flexural strength of a double-wrapped CFRP girder increases about 14% in comparison with single-wrapped, and 20% in comparison with a non-strengthened girder.
  • According to previous studies, a wider wrap increases the ductility of the girder by about 40% in comparison with a narrower wrap [2].
  • Due to repeatable traffic loads on bridges, fatigue can be a major problem. The most important effects of fatigue in RC structures can be excessive cracking, excessive deformation and reduced structural stiffness. According to [3], FRP strengthened girders have more fatigue life; of course, it has been mentioned that as CFRP is stiffer than GFRP, for better fatigue strengthening CFRP is preferred. 

Figure 6- Pre-cured carbon fiber straps proved to be an easy and affordable way to strengthen this Ohio bridge
Authors
Parastoo Azad and Dr. Mehrtash Soltani (January 15, 2020)
References
  1. ACI Committee 222. (2001). Protection of metals in concrete against corrosion. ACI 222R-. American Concrete Institute.
  2. Adil K. Al-Tamimi, R. H. (2011). Effects of Ratio of CFRP Plate Length to Shear Span and End Anchorage on Flexural Behavior of SCC RC Beams. Journal of Composites for Construction.
  3. Aidoo, J., Harries, K. A., & Petrou, a. M. (2004, December). Fatigue Behavior of Carbon Fiber Reinforced Polymer-Strengthened Reinforced Concrete Bridge Girders. Journal of Composites for Construction, 8(6).
  4. Klaiber, F. &. (2003). Repair of damaged prestressed concrete bridges using CFRP. 
  5. Yail J. Kim, J.-Y. K.-S.-T. (2017, July). Condition Assessment of Corrosion-damaged Bridge Girders Strengthened with Post-tensioned Composite Strips. Journal of Physics: Conference Series,.