25-1
25Design of FRP
Reinforced andStrengthened Concrete
Lawrence C. Bank, Ph.D., P.E., FASCE*
25.1 Introduction ......................................................................25-125.2 Design of FRP-Reinforced Concrete Members ...............25-2
Introduction Properties of FRP Reinforcing Bars Design Basis for FRP-Reinforced Concrete Design of Flexural Members with FRP Reinforcing Bars
25.3 Design of FRP-Strengthened Concrete Members...........25-9Introduction Properties of FRP Strengthening Systems Design Basis for FRP Strengthening Systems for Concrete Members Design of FRP Flexural Strengthening Systems Design of FRP Shear Strengthening Systems Design of FRP Axial Strengthening Systems
25.4 Summary..........................................................................25-20References ...................................................................................25-20
25.1 Introduction
The design of concrete members either reinforced with FRP reinforcing bars or strengthened with stripsor sheets of FRP laminates or fabrics is discussed in this chapter. The discussion in this chapter followsthe design recommendations of the most current versions of the design guidelines published by theAmerican Concrete Institute (ACI) that are used to design these concrete structures in the United States.The material presented is an updated and expanded version of portions of the chapter Fiber-ReinforcedPolymer Composites, which appeared in the Handbook of Structural Engineering (Bank, 2004) and wasbased on ACI design guidelines in 2003. In addition, this chapter is intended to provide a brief overviewof topics covered in greater detail and accompanied by illustrative examples in Composites for Construction:Structural Design with FRP Materials (Bank, 2006.) Research in the use of FRP reinforcements and FRPstrengthening systems for concrete structures has been the focus of intense international research activitysince the late 1980s. A biannual series of symposia entitled Fiber-Reinforced Plastics in Reinforced ConcreteStructures (FRPRCS) has been the leading venue for reporting and disseminating these research results.The most recent symposium, the seventh in the series dating back to 1993, was held in Patras, Greece,in 2007 (Triantitillou, 2007).
* Professor, Civil and Environmental Engineering, at the University of Wisconsin, Madison; expert in the mechanicsand design of composite material structures with an emphasis on applications to civil engineering.
2008 by Taylor & Francis Group, LLC
25-2 Concrete Construction Engineering Handbook
25.2 Design of FRP-Reinforced Concrete Members
25.2.1 Introduction
Fiber-reinforced polymer (FRP) reinforcing bars and grids have been commercially produced for rein-forcing concrete structures for over 30 years (ACI Committee 440, 1996; Bank, 2006; Nanni, 1993). FRPreinforcing bars have been developed for prestressed and non-prestressed (conventional) concrete rein-forcement. This section considers only non-prestressed reinforcement for concrete structures and followsthe procedures of ACI 440.1R-06, Guide for the Design and Construction of Structural Concrete Reinforcedwith FRP Bars (ACI Committee 440, 2006). Note that ACI 440.1R-06 does not cover reinforcing withprefabricated FRP grids and mats. Recommendations for the design of prestressed FRP-reinforced con-crete can be found in ACI 440.4R-04, Prestressing Concrete with FRP Tendons (ACI Committee 440,2004b). Current FRP reinforcing bars (referred to as FRP rebars in what follows) are commerciallyproduced using thermosetting polymer resins (commonly, polyester and vinylester) and glass, carbon,or aramid reinforcing fibers. The most common bars produced today are glass-fiber-reinforced vinylesterbars. These are recommended for use in reinforcing applications for load-bearing concrete structures.The bars are primarily longitudinally reinforced with volume fractions of fibers in the range of 50 to60%. FRP reinforcing bars are usually produced by a process similar to pultrusion (Starr, 2000) and havea surface deformation or texture to develop the bond to concrete. More information on the historicaldevelopment, constituent materials, and manufacturing processes of FRP rebars can be found in Bank(2006). A photograph of some typical FRP reinforcing bars is provided in Figure 25.1. In addition to theACI design guidelines, a number of other design guides have been published for FRP-reinforced concrete.These include Japanese (BRI, 1995: JSCE, 1997) and Canadian (ISIS, 2001; CSA, 2002) guides.
25.2.2 Properties of FRP Reinforcing Bars
Glass-fiber-reinforced vinylester bars are available from a number of manufacturers in the United States,Europe, and Asia. Bars are typically produced in sizes ranging from 3/8 in. in diameter to 1-1/4 in. indiameter (i.e., #3 to #10 bars.) FRP bars have a non-smooth surface, which is required for bond to theconcrete (see Figure 25.1) and is typically produced by a sand-coated external layer, molded deforma-tions, machined ribs, or a spiral wind. The properties of FRP rebars are intended to be measured andreported by FRP rebar manufacturers in accordance with ACI 440.3R-04, Guide Test Methods for Fiber-Reinforced Polymers (FRP) for Reinforcing or Strengthening Concrete Structures (ACI Committee 440,2004a). A standard product specification for FRP rebars has recently been approved for publication bythe Canadian Standards Organization (ISIS, 2006). The ACI is currently preparing a standard specification
FIGURE 25.1 Typical FRP reinforcing bars for concrete members.
2008 by Taylor & Francis Group, LLC
Design of FRP Reinforced and Strengthened Concrete 25-3
for FRP bars. For design, the key mechanical properties of interest are the longitudinal tensile strengthand longitudinal tensile modulus of the bar. Most FRP bars are brittle and exhibit strongly linear andelastic axial stressstrain or axial load-deformation characteristics up to their failure loads. They do notyield and have no plastic deformation capacity as do steel rebars. It is also important to note that, unlikesteel rebars, the longitudinal strength (but not the longitudinal modulus) of FRP rebars decreases withthe diameter of the bar. This is attributed to the relatively low in-plane shear modulus of FRP rebars(leading to shear lag effects), the additives used to produce larger diameter bars, and a statistical sizeeffect in brittle glass fibers. Designers should always consult the manufacturers published propertiesfor use in design. Typical properties for glass-fiber FRP rebars and carbon-fiber FRP bars are providedin Table 25.1. It should be noted that the carbon-fiber bars are typically used as prestressing tendonsor near-surface-mounted (NSM) strengthening rods and not as conventional reinforcing bars due tocost considerations.
Fiber-reinforced polymer rebars are considered to be transversely isotropic from a mechanics perspec-tive (Bank, 1993). Theoretical equations are available to predict the mechanical and physical propertiesof the FRP rebars from the properties of the fiber and resin constituents; however, for design purposes,measured properties of the as-produced bars must be used. At this time, theoretical methods are not yetavailable to predict the bond properties and the long-term durability characteristics of FRP rebars. Testmethods for determining and reporting the alkali resistance, creep, and fatigue characteristics of FRPrebars are provided in ACI 440.3R-04 (ACI Committee 440, 2004a). FRP rebars containing glass fiberscan fail catastrophically under sustained loads at stresses lower than their tensile strengths, a phenomenonknown as creep rupture or static fatigue. Design guides therefore limit the amount of sustained load onconcrete structures reinforced with FRP rebars.
Fiber-reinforced polymer rebars should only be used at service temperatures below the glass transitiontemperature (Tg) of the polymer resin system used in the bar. For typical vinylester polymers, this isaround 200F. The bond properties have been shown to be highly dependent on the glass transitiontemperature of the polymer. In addition, it is important to note that the coefficients of thermal expansionof FRP rebars are not the same in the transverse (radial) direction as in the longitudinal direction. Thecoefficient of thermal expansion may be close to an order of magnitude higher in the transverse direction
TABLE 25.1 Properties of Typical Commercially Produced FRP Reinforcing Bars
Glass-Reinforced Vinylester Bara,b,c
(0.5-in. Diameter)
Glass-Reinforced Vinylester Bara
(1-in. Diameter)
Carbon-Reinforced Vinylester Bara
(0.375-in. Diameter)
Carbon-Reinforced Epoxy Bar
(0.5-in. Diameter)
Fiber volume (estimated) 5060 5060 5060 5060Fiber architecture Unidirectional Unidirectional Unidirectional Unidirectional
Strength ( 103 psi)Tensile, longitudinal 90100 80 300 327Compressive, longitudinal NR NR NR NRBond strength 1.7 1.7 1.3 NRShear, out-of-plane 2227 22 NR NR
Stiffness ( 106 psi)Tensile, longitudinal 5.96.1 5.9 18 21.3Compressive, longitudinal NR NR NR NRCTE, longitudinal (106/F) 3.74.9 3.7 4.00 0.38CTE, transverse (106/F) 12.218.7 18.7 4158 NRBarcol hardness 60 60 4855 NR24-hour water absorption (% max.) NR NR NR NRDensity (lb/in.3) 0.072 0.072 NR 0.058
a Data for Aslan (Hughes Brothers, Seward, Nebraska).b Data for V-Rod (Pultrall, Quebec, Canada).c Data for Leadline (Mitsubishi, Tokyo, Japan).
Note: CTE, coefficient of thermal expansion; NR, not reported by the manufacturer.
2008 by Taylor & Francis Group, LLC
25-4 Concrete Construction Engineering Handbook
of the bar due to its anisotropic properties (see typical properties in Table 25.1). This may cause longi-
