What are Fiber Reinforced Polymers?
Fiber reinforced polymers (FRP) are a relatively new, lightweight, high-strength composite material that can be used in a wide variety of applications. Several industries utilize FRP composites for their unique ability to be specifically tailored to their needs. Industries such as consumer goods, marine, automotive, aerospace, and civil engineering all have benefited from their use.
FRP’s are a class of composites in which fibers made of glass, Kevlar™, carbon, boron, etc. are combined with specially formulated polymer resins to form high strength, lightweight composites. The fibers are called reinforcement and provide strength to the composite. The polymer resins, which act as a binder, are known as the matrix. The performance of the various types of FRP composite systems primarily depends on the combination of fibers and resin matrix.
Advanced Composites is a term used to describe ultra-high performance FRP composites. Fibers made of glass, Kevlar™, carbon, boron, et al. are used in combination with a variety of high-performance polymer resins to create extremely strong and lightweight materials. These materials are used to fabricate products for ultra-high performance applications, which were traditionally reserved for the aerospace industry. Their value was appreciated during WWII, when glass fibers were combined with phenolic resins to quickly make complex shaped aircraft parts. More recently, advanced composites were utilized in the development of the Stealth aircraft used by our armed forces, not only for their superior strength and fatigue characteristics, but also for their ability to reduce radar transmission. Another renowned applications of advanced composites is Boeing’s 787 Dreamliner jet aircraft. According to Boeing, 50% of the materials (by weight) used in constructing the 787 consisted of composite materials, with advanced composites being the main material used to construct its airframe. By using advanced composites, Boeing was able to reduce fuel consumption of this long-range jet by 20% over that of similarly sized aircrafts.
Figure 1: The Stealth fighter and Dreamliner aircraft utilize Advanced Composites
FRP reinforcing materials for civil engineering applications are generally manufactured in the form of pre-cured laminates, pre-cured shells or jackets, prepreg sheets, or uncured fabrics. Most fabric materials are around 1 mm (0.04 in.) thick and are made of continuous unidirectional or bidirectional fibers. FRP sheets and fabric materials are impregnated with polymer resin, which also serves as a bonding agent to adhere the material to substrates. Whereas pre-cured laminates are bonded to a substrate using a thin layer of polymer resin, typically epoxy. The performance of the bond at the interface of the FRP composite and substrate is crucial to the effectiveness of FRP composites used in bond-critical strengthening applications.
Figure 2: Flexural and shear strengthening of AASHTO Type III Girder using CFRP
The matrix material for FRP composites generally consists of a polymeric resin and hardener, and can contain additives to enhance or tailor its properties. The matrix serves several key purposes for FRP composites, therefore the proper selection, proportioning, and application of the resin is extremely important. It impregnates and binds the fiber reinforcement together, protects the fibers from the environment, provides a means for load transfer from the substrate to the fibers, and provides a permanent and conforming shape to the final product. Polymeric matrix materials can be classified as thermoplastic or thermoset. Thermoplastic polymers can be repeatedly softened or remolded following initial cure by adding heat. Thermoset polymers do not soften upon application of heat, but instead degrade when exposed to elevated temperatures. However, additives can be mixed with thermosets to improve their performance in moderate to extreme environments. Traditionally, thermoset polymer epoxies have been studied and selected for use as a matrix material for FRP composites in civil engineering applications.
Motivation for Research into Advanced Composites for Civil Engineering Applications
Infrastructure Report Card
In a program that started roughly 25 years ago, The American Society of Civil Engineers (ASCE) aimed to inform the public and government policymakers about the condition and performance of public facilities by publishing an Infrastructure Report Card. Their initial report card (1998) issued an overall grade of D, with an estimated $1.3 Trillion in total investments needed for improvements. Around this same time, the Federal Highway Administration (FHWA), in conjunction with the USDOT, began funding research into advanced composites as a cost effective means to strengthen and rehabilitate deteriorating highway infrastructure. Groundbreaking research was being performed at the Turner-Fairbank Highway Research Center in McLean, VA in an effort to mitigate the enormous costs associated with rehabilitating our nation’s infrastructure.
In 2013, ASCE released their most recent report card, issuing an overall grade of D+ for our infrastructure, with an estimated $3.6 Trillion in total investments needed by 2020 for improvements. The bridge sector received a grade of C+. Approximately 25% of the 607,380 bridges in the US are classified as either structurally deficient or functionally obsolete. According to the FHWA, the investment backlog to repair or replace these deficient bridges is estimated to be $121 billion. Utilizing advanced composites for strengthening and repairing deficient bridges could potentially save millions of dollars.
In addition to the needs associated with rehabilitating infrastructure, the California Northridge Earthquake of 1994 revealed significant design flaws pertaining to building code requirements of reinforced concrete (RC) columns. Several concrete bridge columns failed as a result of this earthquake, which gave rise to the need for structural strengthening of these columns to improve their safety and performance during future seismic events. The Northridge earthquake caused an estimated $20 billion in property damages, making it the costliest earthquake in United States history. This earthquake resulted in considerable scientific and engineering investigations, as well as major revisions to the seismic provisions of building codes. It also was a driving force for research involving strengthening of R/C columns using advanced, lightweight composite materials. Utilizing advanced composites for strengthening deficient R/C columns is a highly cost-effective means for repair, as compared to traditional strengthening methods.
Research on FRP Composites for Civil Engineering Applications
Research on FRP composites for civil engineering applications began in the latter portion of the 1980’s. Original studies found that continuous E-glass fibers combined with epoxy resins were very effective in strengthening existing reinforced concrete (R/C) beams. However, further studies revealed that glass fiber reinforced polymer (GFRP) composites possessed the following shortcomings: a modulus of elasticity that is roughly 1/3 than of steel and therefore does not provide a considerable increase in stiffness, poor abrasion resistance, low fatigue resistance, and a tendency to degrade in alkaline environments. As a result, studies using carbon fiber reinforced polymers (CFRP) soon followed, and a revolutionary advanced composite material was born. Over the last 25 years, advanced composites have been studied and used to successfully strengthen existing concrete bridge decks, bridge girders, columns, beams, masonry walls, underwater bridge piers, utility poles, and large diameter prestressed concrete cylinder pipes (PCCP). Most structural applications utilize CFRP composites.
Why use CFRP composites?
Because they offer tremendous value. Traditional methods of structural strengthening and repair include external post-tensioning, steel jacketing, and steel plate bonding. Although effective, these methods possess several shortcomings. Post-tensioning creates high localized stressed at the anchorage locations and requires corrosion protection, regular inspections and monitoring by the owner. Structural steel plates are very heavy and difficult to install, often causing interruption to services during installation, require corrosion protection in harsh environments, and can also add a significant amount of dead weight to a structure. An increase in dead weight increases the seismic response of the structure, which is undesirable.
In contrast, CFRP composites are lightweight…very lightweight, which minimizes added dead loads and seismic response of a structure. And they are strong…very strong. CFRP composites can be formulated to be seven times (7x) as strong as steel at a fraction of the weight. In terms of strength to weight ratio, 50 ksi steel has a specific strength of about 14,700 while 150 ksi CFRP has a specific strength of about 216,000. Installation of CFRP is greatly simplified over steel plate bonding due to the lightweight nature of the composite, often occurring with minimal in-service interruptions. In the case of fabrics, the thin carbon fiber material is applied to a properly prepared substrate in a manner similar to applying wallpaper. Fabrics also have the ability to conform to a wide variety of shapes. In short, CFRP composites possess the ability to cost-effectively repair structurally deficient structures better than traditional repair methods. In addition, and most importantly, advance composites offer an opportunity to repair a structure that might otherwise need to be replaced.
FRP Code Books and Limitations of Externally Bonded FRP Composites
Although CFPR composites are a revolutionary breakthrough in the world of structural engineering, they do possess limitations. These limitations are a result of the newness of the technology and the nature of the materials. It is important to remember that these composite materials have only been studied for use in civil engineering applications since around 1990. The technology is still in its infancy stage and only limited research has been performed to date.
The American Concrete Institute (ACI) publishes the following code books, which were developed based on prior research conducted using externally bonded FRP composites:
ACI 440.2R-08 Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures
ACI 440.7R-10 Guide for Design & Construction of Externally Bonded FRP Systems for Strengthening Unreinforced Masonry Structures
Both codes have outlined limitations for their use. And both codes require FRP Systems Qualification. FRP systems should be qualified for use on a project on the basis of independent laboratory test data of the FRP constituent materials and the laminates made with them, structural test data for the type of application being considered, and durability data representative of the anticipated environment. Test data provided by the FRP system manufacturer demonstrating the proposed FRP system should meet all mechanical and physical design requirements, including tensile strength, durability, resistance to creep, bond to substrate, and Tg, should be considered for use. FRP composite systems that have not been fully tested should not be considered for use.
The following are some the main limitations associated with ACI 440.2R:
- Strengthening Limits. Limits on overall strength increases are imposed within the code in an effort to prevent collapse in the event of FRP failure.
- Fire and Life Safety. Strict limitations are in place for structures requiring fire protection. FRP composites degrade rapidly when exposed to temperatures above the FRP’s critical temperature, which is known as the glass transition temperature (Tg). FRP composites used in interior building environments are subject to the same fire protection requirements as other structural materials (i.e. concrete and steel). As such, any FRP system subjected to a fire rating shall be capable of maintaining an in service temperature below its Tg for the duration of the fire rating. Typical values of Tg for FRP composites are in the range of 140-180 °F. The intent of most FRP composites research to date has been to address our nation’s deteriorating infrastructure. This research was not intended to address strengthening interior building systems, where strict building code requirements exist for plastics (polymers) used in interior building environments. However, recent research performed by the aerospace industry has revealed the development of an advanced, highly sustainable and cost-effective polymer matrix material for use with FRP composites that can withstand temperatures over 1000 °C.
- Maximum Service Temperature. In dry, hot regions, it is recommended that the in service temperature of an FRP composite system not exceed Tg – 27 °F. In moist environments, the wet glass transition temperature (Tgw) should be used.
- Minimum Concrete Substrate Strength. For bond-critical applications (flexural and shear strengthening), the substrate should possess a tensile strength of at least 200 psi, as determined by a pull-off adhesion test (ICRI 03739/ACI 503R). FRP systems should not be used when the compressive strength of the concrete substrate is less than 2500 psi.
- FRP as Compression Reinforcement. ACI 440.2R applies only to FRP strengthening systems used as additional tensile reinforcement. It is not recommended to use these systems as compressive reinforcement.
- Areas in Need of Future Research. Appendix C of ACI 440.2R provides a summary list of those areas requiring future research to provide information that is currently unclear or lacks sufficient data to validate the performance and behavior of the FRP system. These areas include high strength concrete and lightweight concrete.
The following are some the main limitations associated with ACI 440.7R:
- Unreinforced Masonry (URM). The code is applicable to URM structures made of clay bricks, concrete masonry units, and natural stones using conventional types of mortar.
Strengthening Limits. Limits on strength increases are imposed on masonry walls resisting out-of-plane loads from earth pressure and on masonry shear walls resisting in-plane loads from wind.
- Fire and Life Safety. The same limitations for fire protection of FRP reinforced concrete apply to FRP reinforced masonry.
- Masonry Infill Walls. Infill walls are not included in the code.
- In-Plane Masonry Wall Strengthening (Shear Walls). The use of FRP composites is not recommended for URM walls having the following properties:
a. Hollow unit walls having a thickness of 10-12 inches with grouted cells spaced 60 inches on center or less.
b. Hollow unit walls having a thickness greater than 12 inches.
c. Solid unit walls having a thickness greater than 8 inches.
Despite these limitations, FRP composites can be a cost-effective and viable solution to address a wide variety of structural-related problems. The future of structural engineering will be greatly influenced by the use of advanced composite materials, not only is strengthening and retrofitting existing structures, but also in new construction.