Materials Lesson on Fiber-Reinforced Polymers

Fiber-reinforced polymers (FRP) have been used for years in the engineering industry with a variety of applications across different engineering fields. Typically, the engineer designs the FRP while a manufacturing company creates it. In civil engineering, FRP is popular with repairs, renovations, and strengthening projects. FRP does however present both advantages and disadvantages. Some advantages would be that it’s lightweight, durable, stiff, and can be tailored specifically for any application. A few disadvantages are high costs, poor plastic behavior, and the inability to fully replace the strength of steel in concrete. It’s important we discuss what exactly FRP is made of, the advantages and disadvantages of strengthening with this material, and where strengthening with FRP can be applied today.

Diagram of fiber-reinforced polymer

Fiber-reinforced polymers (FRP) are composite materials traditionally consisting of a polymer and a strong fiber. Polymers by definition are either natural or synthetic substances composed of large molecules of the same material bonded together. In the case of FRPs, the polymer material is usually epoxy, vinyl ester resin, or polyester plastic. The fibers can be anything from common fibers such as carbon, glass, aramid, and basalt, to less common alternatives like paper, wood, or even on rare occasions asbestos. The combination of these two materials creates a high-strength, stiff, and lightweight reinforcement system with multiple applications. The sizes can range from small textile squares to long segments.

Strengthening with FRP systems can be seen in the aerospace, automotive, marine, and civil engineering industries due to its rigidity and high strength-to-weight ratios. Aircraft, helicopters, spacecraft, boats, automobiles, chemical equipment, and infrastructure are just a few examples of where FRP can be used. Over the years FRPs have gained popularity in the civil engineering world. The material has proven to be successful in strengthening damaged concrete structures, aging members, and overloaded sections.

A significant advantage of fiber-reinforced polymer systems is their flexural, axial, and shear strength. The material is lightweight adding no additional mass to a structure but still has the capability of increasing that structure's strength. FRP also has an extremely high tensile strength acting similar to the way rebar would for a concrete member. The material is considered non-corrosive, unlike steel rebar, allowing it to be applied in all types of environments.

FRP is also a thin material so there is no spatial impact involved with the installation. It can fit in hard-to-reach spaces that other types of strengthening materials may not be able to fit. The preparation of FRP is less intensive than that of steel rebar and the cost to install it is much less than traditional steel. Most importantly, the structure being repaired or retrofitted does not need to be taken out of service for the FRP system to be installed. The installation of FRP systems is very manageable and the material is compatible with a variety of finishes and protective coatings that are typically present on pre-existing infrastructure. The main consideration during installation would be the temperature and surface moisture of the concrete. The bond between existing concrete and the FRP is influenced by these factors and dictates the materials' performance.

The most significant disadvantage of fiber-reinforced polymer systems is the cost of the material itself. The demand for FRP is not as high as typical steel rebar and therefore costs more to manufacture. Another weakness is its lack of plastic behavior. This type of issue may lead to premature fiber/tendon rupture. Finally, the strength of steel rebars cannot be replaced fully by FRP. Even though both materials provide high tensile strength and stiffness, FRP does not have the same capacities offered by traditional rebar.

FRP application on concrete columns and beams

Another thing to note about FRP is the multiple types. Carbon fibre reinforced polymer is one of the most commonly used in civil engineering repairs.

Types of Fibre Reinforced Polymer (FRP)

Glass Fibre Reinforced Polymer (GFRP) — Basically made by mixing silica sand, limestone, folic acid and other minor ingredients. Glass is generally a good impact resistant fibre but weighs more than carbon or aramid.

Carbon Fibre Reinforced Polymer (CFRP) — Do not absorb water and are resistant to many chemical solutions. They withstand fatigue, corrosion, and don't show any creep or relaxation.

Aramid Fibre Reinforced Polymer (AFRP) — Sensitive to elevated temperatures, moisture and ultraviolet radiation and therefore not widely used in civil engineering applications.

Infrastructure in need of renovations, repairs, and strengthening can use fiber-reinforced polymer systems. Concrete structural renovations are considered one major application of FRPs. Owners of old buildings, such as parking garages and warehouses, can use a strengthening system like FRP to bring their old buildings back to life without the costs of renovation. Rather than replace an entire concrete column because of significant cracks or spalling, the FRP can be sized and wrapped around an existing column increasing its strength and adding to the lifespan of that member. In some cases, the structure's load capacity can even go beyond the original design.

Real-life application of FRP wrap on a reinforced concrete bridge

An FRP system can be used for simple repairs of deteriorated or corroded structural members including beams, slabs, walls, piles, and pier caps. In addition to concrete, timber and masonry can benefit from the strengthening properties of FRP. If a building structure requires seismic upgrades, FRP has the capacity to meet code standards without heavy construction. It can increase the shear strength and displacement of a structural element. In my own experience, I have investigated the benefits of FRP wrap on unreinforced masonry walls to strengthen against lateral flood loads. After studying the material, it proved to be unsuccessful for the project. The 14-foot-high lateral flood loads for my project required a larger and more rigid type of reinforcement and FRP wasn’t meeting the strength requirements. Regardless, my research showed this material to be far more superior than I had initially thought given how thin and lightweight it is versus traditional reinforcement.

To wrap things up, yes we should care about fiber-reinforced polymers. Structures in need of repair or upgrade can turn to FRP for structural reinforcement or concrete strengthening. The stiffness, durability, and lightweight properties of FRP display its ability to be a superior type of material in the engineering world for the foreseeable future.