This post is part of a series that looks at the lessons learned from New York City’s restoration of a critical asset in its subway system following Hurricane Sandy in 2012. In the first part, we discussed the preparation and scope of the L train tunnel repair plans. The second part explored the critical role of stakeholder relations in major construction projects. Here, we will look at material innovation – specifically fiber-reinforced polymers – and the impact this advance had on the plan to rebuild the L-train from Brooklyn into Manhattan.
What are Fiber-Reinforced Polymers (FRP)?
Fiber-reinforced polymers (FRP) are one of construction’s new fascinations, appearing in more applications, especially as an innovative solution in structure repairs and as a means to slow down structural deterioration.
FRP materials are composites, comprised of a polymer matrix reinforced with embedded fibers. The fibers provide strength and stiffness to the composite, while the matrix bonds and protects the fibers, and allows stress transfer to occur from fiber-to-fiber through shear stresses. The most common fibers are glass, carbon, and synthetic fibers. FRP composites have very high strength characteristics and are nonconductive, noncorrosive, and lightweight. 
The Value of Fiber-Reinforced Polymers in Construction Projects
A perfect illustration of how an innovation like FRP can impact construction projects can be found in a closer look at the plans to restore New York’s L-Train following Hurricane Sandy.
In the original plan, a critical tunnel would be shut down for the entire length of the line’s repair – an estimated 15-18 months. In large part, the lengthy timeline was because the benchwalls, where power and communications cables are housed in order to protect them from water damage or the occasional hungry rodent, needed to be removed and replaced.
L Train Concrete Benchwall in Poor Condition:
With the innovation of fiber-reinforced polymers, the costly and time-intensive process of demolishing and rebuilding the concrete benchwalls, which served as the tunnel’s walkways and housed the electrical cables, was reimagined. Rather than chip away the existing walls in order to replace the embedded cabling, then reforming and pouring new concrete, a team of engineers proposed an alternative: since advances in cabling now eliminate the need to encase them in concrete, it would be possible to abandon the existing cables in place and add new cables in cable trays above the benchwalls.
Under the new plan, the existing cables would continue to power train service until the new cables were installed and a switchover could take place.
The plan clearly had great value, especially because it would allow traffic to continue through the tunnel while repairs were being made. However, the deterioration of the benchwalls still had to be addressed. This challenge is where FRP comes into play, providing a viable solution for encasing sections of the benchwalls.
Uses of Fiber-Reinforced Polymers
In various applications, FRPs are used for adding strength to structures, as they can be bonded to beams, columns, and slabs. Applying FRP to slabs has similar results, adding tensile strength. When “wrapping” a beam (around the side-faces and bottom of the beam), shear resistance can be increased.
FRP Strengthening of Reinforced-concrete Floor:
FRP can also be utilized for structural steel members. As in the case of the L-Train restoration, some repairs which would be time-consuming using traditional repair methods can be expedited using FRP. Another example: an FRP bridge deck could replace a deteriorated bridge deck in a fraction of the time of conventional methods, allowing traffic on the bridge within hours. 
FRP on RFK Bridge:
Advantages of Fiber-Reinforced Polymers
With other advantageous properties, such as corrosion resistance, its lightweight and high strength-to-weight ratio, ease of application, and more, FRPs have other applications, such as utility poles, pipelines, power plant chimney liners, and even modular housing. 
Disadvantages of Fiber-Reinforced Polymers
FRP does have its disadvantages, which include being cost-prohibitive in some applications, low shear strength, a lack of experienced constructors for its application, lack of design standards, limited history of performance, durability, and ductility. FRP is continuing to make strides and advances, and issues like its costs are expected to improve as use becomes more widespread.
More from this series
-  American Concrete Institute – Fiber Reinforced Polymer
-  Tang, B., and Podolny, W. “A Successful Beginning for Fiber Reinforced Polymer (FRP) Composite Materials in Bridge Applications.” Proceedings of International Conference on Corrosion and Rehabilitation of Reinforced Concrete Structures, Dec. 7-11, 1998.
-  Advanced Fiber Reinforced Polymer Composites for Sustainable Civil Infrastructures